Magnetic nanocomposite photocatalyst, preparation method and application thereof

Abstract

The application discloses a kind of magnetic nanometer composite photocatalyst and its preparation method and application, comprising: preparation heteropoly acid precursor alpha-SiW9;Using heteropoly acid precursor alpha-SiW9 Preparation SiW9Mo3;Preparation Co3O4;Preparation Co3O4 / SiW9Mo3.Compared with prior art, the magnetic nanometer composite photocatalyst of the application has good photocatalytic activity, under the experimental conditions that the concentration of organic dye is 20mg / L, respectively in reaction system 0.6g / L magnetic nanometer composite photocatalyst is added, adjust the pH of reaction system to 7, additional magnetic field is 200Gs and carries out photocatalytic degradation organic dye, has the best photocatalytic degradation activity and efficiency, and has good stability, reusable, is a kind of excellent performance magnetic auxiliary photocatalyst.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of photocatalysts, in particular to a magnetic nano-composite photocatalyst and a preparation method and application thereof. BACKGROUND

[0002] With the development of industry and the improvement of people's living standards, people's demand for resources has increased significantly, which has also caused a series of negative effects, especially water pollution. Long-term contact with wastewater can cause a series of hazards such as anemia, nervous system diseases, and carcinogenesis. Therefore, efficient treatment of domestic wastewater and industrial wastewater becomes a top priority. At present, various water remediation technologies, such as biological method, sedimentation method, coagulation method, flocculation method, ion exchange method, membrane filtration method and reverse osmosis method, have been developed for purifying wastewater, but some organic pollutants are difficult to remove.

[0003] As a non-metallic semiconductor photocatalyst, polyoxometalate (POMs) is considered to be an ideal photocatalyst in the process of solar light conversion due to its thermal stability and chemical stability. When POMs is used as a photocatalyst alone, it is easily dissolved in water, which makes it difficult to separate after the reaction and makes it difficult to reuse, thus greatly limiting its application

[12] . At present, a large number of POMs are used to synthesize new composite materials by compounding with nanomaterials. SiW 12 and PW 12 are uniformly dispersed into the silica phase by sol-gel method to synthesize microporous silica immobilized POMs photocatalyst. The prepared POMs / SiO2 photocatalyst has high water insolubility and high stability

[13] . In addition, the microporous structure has a high specific surface area, and shows high practicability for removing hexachlorocyclohexane. By using a gradual coating method, a magnetic POMs-based photocatalyst with core-shell structure is prepared by taking magnetite Fe3O4 as the core and then successively coating Ag and H3PW 12 O 40 . Under visible light irradiation, the photocatalytic degradation ability of the double-coated photocatalyst on rhodamine B (RhB) is significantly enhanced compared with pure POMs.

[0004] The promotion of magnetic field on catalytic performance has attracted widespread attention and has made great progress in some catalytic fields. External magnetic field adjustment method, as a non-contact and environmentally friendly method, has been widely discussed and effectively promoted sustainable development. In this sense, it is very attractive to improve the performance in the process of photocatalysis by using external magnetic field. Most of the researches on Fe3O4 in the field of photocatalysis are to compound it with the main body of the catalyst. In order to achieve this excellent recycling effect, it is necessary to avoid the light shielding effect and surface defects caused by the addition of a large amount of Fe3O4, and the core-shell structure can retain better photocatalytic performance.

[0005] Therefore, it is urgent to develop a magnetic nanocomposite photocatalyst. SUMMARY

[0006] To solve the above technical problems, the application provides a magnetic nanocomposite photocatalyst, a preparation method and application thereof.

[0007] To achieve the above-mentioned purpose, the application is implemented according to the following technical solutions:

[0008] The first object of the application is to provide a preparation method of a magnetic nanocomposite photocatalyst, comprising the following steps:

[0009] S1, preparing a heteropoly acid precursor α-SiW9;

[0010] S2, preparing SiW9Mo3 by using the heteropoly acid precursor α-SiW9;

[0011] S3, preparing Co3O4;

[0012] S4, preparing Co3O4 / SiW9Mo3: 1g of SiW9Mo3 is dissolved in 10mL of water, 1-9% of Co3O4 of the mass of SiW9Mo3 is added, and stirring is performed for 30min to make it completely mixed; 10mL of 0.1mol / L tetrabutylammonium bromide aqueous solution is added dropwise; stirring is continued for 5-6 hours, after the reaction is completed, the sample is centrifuged and washed with deionized water and ethanol for multiple times, and the washed sample is dried at 60℃ for 6h to obtain the magnetic nanocomposite photocatalyst Co3O4 / SiW9Mo3.

[0013] Further, the step S1 specifically comprises:

[0014] 18.20g of sodium tungstate and 1.10g of sodium silicate are weighed and dissolved in 20mL of 80℃-100℃ hot water, and stirring is performed for mixing; then 13mL of a hydrochloric acid solution with a concentration of 6M is added dropwise, and when the solution boils to a volume of 30mL, centrifugal filtration is performed to remove unreacted Si, and the filtrate is collected; then 5.00g of anhydrous sodium carbonate is dissolved in 15mL of water, and then slowly added to the filtrate, and the precipitate is slowly formed, and after the precipitate is completely formed, the solid is separated by filtration, and the separated solid is mixed with 100mL of a sodium chloride solution with a concentration of 4M and stirring is performed, and then filtration is performed again; finally, the impurities are removed by washing with anhydrous ethanol, crystallization is accelerated, and vacuum drying is performed to obtain the heteropoly acid precursor α-SiW9.

[0015] Further, the step S2 specifically comprises:

[0016] Take 2.50g heteropoly acid precursor alpha-SiW9, 0.70g Na2MoO4 mixed and dissolved in 10mL water, after stirring well, add dropwise 3M concentration hydrochloric acid solution until the solid is completely dissolved and adjust pH=1, heat in water bath at 80℃ for 15min, cool to room temperature, and filter and dry, finally get yellow-green solid, which is SiW9Mo3.

[0017] Further, the step S3 specifically comprises:

[0018] Take 0.8g Co(CH3COO)2·6H2O and add it to 16mL water and 24mL ethanol mixed solution and stir for 1h, then add 4mL 25% mass fraction ammonia water, continue to stir the mixture in air for 20min, then transfer the liquid to 100mL high pressure reactor, react at 150℃ for 3h, after the autoclave is naturally cooled, wash the obtained liquid with deionized water for several times, dry at 60℃ for 4h, get black solid Co3O4.

[0019] Preferably, in the step S4, Co3O4 with mass 7% of SiW9Mo3 is added.

[0020] The second object of the present application is to provide a magnetic nanocomposite photocatalyst prepared by the above method.

[0021] The third object of the present application is to provide an application of the magnetic nanocomposite photocatalyst in photocatalytic degradation of organic dyes, under the experimental conditions of 20mg / L of organic dye concentration, 0.6g / L of magnetic nanocomposite photocatalyst added in the reaction system, pH of the reaction system adjusted to 7, and 100Gs-300Gs of external magnetic field, photocatalytic degradation of organic dyes is carried out.

[0022] Preferably, the external magnetic field is 200Gs.

[0023] Compared with the prior art, the magnetic nanocomposite photocatalyst has good photocatalytic activity, under the experimental conditions of 20mg / L of organic dye concentration, 0.6g / L of magnetic nanocomposite photocatalyst added in the reaction system, pH of the reaction system adjusted to 7, and 200Gs of external magnetic field, photocatalytic degradation of organic dyes has the best photocatalytic degradation activity and efficiency, and has good stability and can be reused, which is a magnetic-assisted photocatalyst with excellent performance. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 It is the infrared spectrum of alpha-SiW9-POM.

[0025] Figure 2 It is the infrared spectrum of SiW9Mo3-POM.

[0026] Figure 3 X-ray powder diffraction (XRD) pattern of SiW9Mo3.

[0027] Figure 4 UV-Vis spectra (a) and band gap energy (b) of SiW9Mo3.

[0028] Figure 5 Scanning electron microscope (SEM) image of SiW9Mo3-POM.

[0029] Figure 6 X-ray powder diffraction pattern of Co3O4.

[0030] Figure 7 Transmission electron microscope (TEM) image (a) and EDS elemental mapping of Co3O4 sample surface (b) of Co3O4.

[0031] Figure 8 UV-Vis spectra (a) and band gap energy (b) of Co3O4.

[0032] Figure 9 Infrared spectra of Co3O4 / SiW9Mo3 composites with different ratios.

[0033] Figure 10 X-ray powder diffraction pattern of Co3O4 / SiW9Mo3 composite.

[0034] Figure 11 XPS survey spectrum (a) and Si 2p (b), W 4f (c), Mo 3d (d), Co 2p (e), O 1s (f) XPS spectra of Co3O4 / SiW9Mo3.

[0035] Figure 12 UV spectra of Co3O4 / SiW9Mo3 composites with different ratios.

[0036] Figure 13 Nyquist (a) and Bode (b) plots of CoMo6 / Fe3O4@C composites with different ratios.

[0037] Figure 14 Fluorescence spectra of Co3O4 / SiW9Mo3 composites with different ratios.

[0038] Figure 15 Electrochemical impedance spectroscopy (a) and Bode (b) plots of Co3O4 / SiW9Mo3 composites with different ratios under magnetic field.

[0039] Figure 16 Standard curve of methylene blue solution.

[0040] Figure 17 Direct photolysis curve of methylene blue.

[0041] Figure 18 Photocatalytic data graph (a) and rate graph (b) of composite Co3O4 / SiW9Mo3 under different dye concentrations.

[0042] Figure 19 Photocatalytic data graph (a) and rate graph (b) of composite Co3O4 / SiW9Mo3 under different dye concentrations.

[0043] Figure 20 Photocatalytic data graph (a) and rate graph (b) of composite Co3O4 / SiW9Mo3 under different dye concentrations.

[0044] Figure 21 Photocatalytic data graph (a) and rate graph (b) of composite Co3O4 / SiW9Mo3 under different dye concentrations.

[0045] Figure 22 Photocatalytic data graph (a) and rate graph (b) of composite Co3O4 / SiW9Mo3 under different dye concentrations.

[0046] Figure 23 Photocatalytic data graph (a) and rate graph (b) of composite Co3O4 / SiW9Mo3 under different dye concentrations.

[0047] Figure 24 Cycle experiment C / C0time change graph (a) and infrared comparison graph (b) of cycle experiment sample of composite.

[0048] Figure 25 Masking agent test graph of composite Co3O4 / SiW9Mo3.

[0049] Figure 26 Mott-Schottky graph (a) of SiW9Mo3-POM and Mott-Schottky graph (b) of Co3O4.

[0050] Figure 27 Photocatalytic mechanism graph of composite. DETAILED DESCRIPTION

[0051] In order to make the purpose, technical scheme and advantages of the present application more clear, the present application is further described in detail below in combination with examples. The specific examples described herein are only used to explain the present application, and are not used to limit the present application.

[0052] Example 1

[0053] S1, Preparation of heteropoly acid precursor a-SiW:

[0054] Weigh 18.20g of sodium tungstate and 1.10g of sodium silicate into 20mL of 80-100℃ hot water, stir the mixture; then add 13mL of 6M concentration hydrochloric acid solution drop by drop, when the solution boils to 30mL, filter and centrifuge to remove unreacted Si, collect the filtrate; then dissolve 5.00g of anhydrous sodium carbonate in 15mL of water, and then slowly add it to the filtrate, the precipitate slowly forms, after the precipitate is completely separated, filter out the solid, and then mix and stir the separated solid with 100mL of 4M concentration sodium chloride solution, and then filter again; finally, wash with anhydrous ethanol to remove impurities, accelerate crystallization, and vacuum dry to obtain the heteropoly acid precursor a-SiW9;

[0055] To determine the molecular structure and chemical composition, the measured sample was characterized by infrared spectroscopy (FI-IR), as shown in Figure 1 The characteristic absorption peaks of the infrared spectrum mainly appear at 981cm -1 , 930cm -1 , 864cm -1 and 807cm -1 . Among them, 981cm -1 belongs to the characteristic absorption peak of W=O d bond, 930cm -1 is attributed to the characteristic absorption peak of Si-O a bond, 864cm -1 and 807cm -1 belong to the characteristic absorption peak of W-O-W bond. By comparison with the literature, it can be basically determined that this is the heteropoly acid precursor a-SiW9.

[0056] S2, Preparation of SiW9Mo3 using heteropoly acid precursor a-SiW9:

[0057] Take 2.50g of heteropoly acid precursor a-SiW9 and 0.70g of Na2MoO4, mix and dissolve in 10mL of water, add 3M concentration hydrochloric acid solution drop by drop after stirring, until the solid is completely dissolved and the pH is adjusted to 1, heat in water bath at 80℃ for 15min, cool to room temperature, and filter and dry, finally obtain yellow-green solid, which is SiW9Mo3;

[0058] Figure 2 The infrared spectrum of SiW9Mo3 is shown in the figure, and its main characteristic absorption peaks are 978cm -1 , 922cm -1 , 879cm -1 and 777cm -1 , which are respectively the characteristic absorption peaks of W=O d bond, Si-Oa Characteristic absorption peaks of the bond, WO b -W key and WO c The stretching vibration absorption peak of the -W bond is shifted compared to the absorption peak in the infrared spectrum of α-SiW9. This is because the Mo=O bond is formed after the introduction of Mo. d Bond, Si-O a Key, Mo-O b -Mo key and Mo-O c The characteristic absorption peak of the -Mo bond causes the characteristic absorption peak in the infrared image to shift.

[0059] like Figure 3 The X-ray powder diffraction (XRD) pattern of SiW9Mo3-POM shown indicates that the synthesized SiW9Mo3-POM exhibits characteristic diffraction peaks at 2θ angles of 16-22° and 25-30°, which correspond one-to-one with the characteristic diffraction peaks reported in the literature for Keggin-type POMs. This proves that the synthesized material has a stable Keggin structure.

[0060] The band structure of POMs is a crucial factor determining their photocatalytic activity. Optical analysis of the synthesized SiW9Mo3-POM was performed using ultraviolet-vis spectroscopy (UV-vis). Figure 4 As shown in (a), we can observe that the light absorption range of SiW9Mo3-POM is 200-480 nm, while that of SiW 12 The light absorption range is 200-400 nm, and the mixed-type heteropolyacid SiW9Mo3-POM can significantly extend the light absorption range. The band gap of SiW9Mo3-POM was calculated to be 2.61 eV using the Kubelka-Munk equation from the measured UV spectrum data. (See...) Figure 4 (b)

[0061] The morphological characteristics of a material have a significant impact on its properties. For example... Figure 5 This is a scanning electron microscope image of SiW9Mo3-POM, from... Figure 5 It can be clearly seen that the POM is a short rod-shaped structure with a size of approximately 600 nm.

[0062] S3, Preparation of Co3O4:

[0063] Co(CH3COO)2·6H2O was added to a mixture of 16 mL of water and 24 mL of ethanol and stirred for 1 h, then 4 mL of 25% ammonia water was added, and the mixture was continuously stirred in air for 20 min. Then the liquid was transferred to a 100 mL high-pressure reactor, and reacted at 150 °C for 3 h. After the high-pressure reactor was naturally cooled, the obtained liquid was washed with deionized water for several times, and dried at 60 °C for 4 h to obtain a black solid Co3O4.

[0064] The crystal phase structure and composition change of the obtained sample were analyzed by x-ray diffraction (XRD), and the results are shown in Figure 6 For the pure Co3O4 sample, the diffraction peaks at 19.0°, 31.3°, 36.8°, 44.8°, 59.3° and 65.2° correspond to the (111), (220), (311), (400), (511) and (440) crystal planes of Co3O4 respectively (PDF 74-2120), and no other impurity peaks appeared.

[0065] SEM was used to observe the shape and surface shape of the Co3O4 sample, as shown in Figure 7 (a). The sample was in the form of nanoparticles, and different degrees of agglomeration phenomenon existed. The EDS element mapping test diagram of the Co3O4 sample is shown in Figure 7 (b), and Co and O elements can be clearly observed in the sample, and the distribution is uniform.

[0066] Optical analysis of the synthesized Co3O4 was carried out by ultraviolet visible spectroscopy (UV-vis). As shown in Figure 8 , we can observe the absorption of ultraviolet visible light at 200-400 nm and 600-800 nm. The measured UV spectrum data was calculated by Kubelka-Munk equation to obtain the band gap width of Co3O4 as 1.58 eV, as shown in Figure 8 (b).

[0067] S4, preparation of Co3O4 / SiW9Mo3: 1 g of SiW9Mo3 was dissolved in 10 mL of water, and 1% of Co3O4 by mass of SiW9Mo3 was added, and stirred for 30 min to make it completely mixed; 10 mL of 0.1 mol / L tetrabutylammonium bromide aqueous solution was added dropwise; continue to stir for 5-6 hours, after the reaction is completed, centrifugal separation sample, and washed with deionized water and ethanol respectively for several times, the washed sample was dried at 60 °C for 6 h, to obtain a magnetic nanocomposite photocatalyst Co3O4 / SiW9Mo3, recorded as Co3O4 / SiW9Mo3-1%.

[0068] Example 2

[0069] The difference between the embodiment 1 is that Co3O4 of 3% mass of SiW9Mo3 is added in step S4, and the obtained magnetic nano-composite photocatalyst is recorded as Co3O4 / SiW9Mo3-3%.

[0070] Embodiment 3

[0071] The difference between the embodiment 1 is that Co3O4 of 5% mass of SiW9Mo3 is added in step S4, and the obtained magnetic nano-composite photocatalyst is recorded as Co3O4 / SiW9Mo3-5%.

[0072] Embodiment 4

[0073] The difference between the embodiment 1 is that Co3O4 of 7% mass of SiW9Mo3 is added in step S4, and the obtained magnetic nano-composite photocatalyst is recorded as Co3O4 / SiW9Mo3-7%.

[0074] Embodiment 5

[0075] The difference between the embodiment 1 is that Co3O4 of 9% mass of SiW9Mo3 is added in step S4, and the obtained magnetic nano-composite photocatalyst is recorded as Co3O4 / SiW9Mo3-9%.

[0076] FI-IR spectra of the magnetic nano-composite photocatalysts prepared in the embodiments 1-5 with different proportions are shown in Figure 9 , in which the proportions of Co3O4 are 1%, 3%, 5%, 7%, and 9%, respectively. The characteristic absorption peaks of W=O -1 , 922cm -1 , 879cm -1 , and 777cm -1 are respectively the characteristic absorption peaks of Si-O d , W-O a , W-O b , and W-O c , which confirm the structural integrity of the Keggin unit in Co3O4 / SiW9Mo3.

[0077] The crystal structure and chemical composition of Co3O4 / SiW9Mo3 are characterized by XRD test, as shown in Figure 10 . By compounding SiW9Mo3-POM and Co3O4, it is found that the diffraction peaks of SiW9Mo3-POM and Co3O4 can still be found in the XRD spectrum of the composite material after loading, which shows that the loading of Co3O4 does not affect the crystal structure of SiW9Mo3, and the Keggin unit structure is complete. And the diffraction peak position of the XRD of the composite material Co3O4 / SiW9Mo3 with different proportions is the same, and the peak type is consistent.

[0078] The surface chemical state of the prepared Co304 / SiW9Mo3 composite sample was analyzed by X-ray photoelectron spectroscopy (XPS) with C 1s (283.4 eV) as the reference. Figure 11 The total spectrum of (a) shows that Co, O, N, C, Mo, Si and W elements exist in the Co304 / SiW9Mo3 composite sample, indicating that Co304and SiW9Mo3 are successfully composited. From Figure 11 (b) can be seen that there is an XPS peak of Si at 2p at 102.09 eV. Figure 11 (c) gives the nuclear energy level spectrum of W 4f spin orbit. The characteristic peaks centered at 35.7 eV and 37.9 eV correspond to W 6+ 4f 5 / 2 and 4f 7 / 2 of W, respectively. Figure 11 (d) is the nuclear energy level spectrum of Mo 3d spin orbit. The characteristic peaks centered at 232.8 eV and 235.9 eV correspond to Mo 6+ 3d 5 / 2 and 3d 3 / 2 of Mo, respectively. Figure 11 (e) shows the nuclear energy level spectrum of Co 2p spin orbit. Since the Co 2p 3 / 2 and Co 2p 1 / 2 in Co304, two strong signals are detected in the Co 2p spectrum at 780.7 eV and 795.9 eV. Figure 11 (f) is the nuclear energy level spectrum of O 1s spin orbit

[34] . Two peaks at 530.4 and 532.4 eV, respectively, are attributed to the lattice oxygen (O latt ) inside the sample and H2O molecules, respectively.

[0079] Figure 12 UV-vis spectra of Co304 / SiW9Mo3 composite materials (different Co304loading amounts). Co304 / SiW9Mo3 produces very strong characteristic absorption peaks at 300 nm and 700 nm, the absorption peak at 250 nm can be attributed to the pπ-dπ charge transfer transition of O t → W, the absorption peak at 340 nm can be attributed to the pπ-dπ charge transfer transition of O b,c → W, and the absorption peak at 700 nm is due to the d-d metal transition of Co 3+ -Co 2+ in Co304. It can be proved that SiW9Mo3 is successfully composited with Co304.

[0080] To verify the relationship between the separation rate of photo-generated charge carriers and the photocatalytic performance, we tested the electrochemical impedance spectroscopy (EIS) and fluorescence spectroscopy (PL).

[0081] The separation efficiency of photo-generated carriers in the prepared catalysts can be explored by EIS, Figure 13 (a) Nyquist plots of the electrodes of the composite materials with Co3O4 mass ratios of 1%, 3%, 5%, 7%, and 9%, respectively. It is generally believed that the Nyquist plot is divided into two parts, the high-frequency region and the low-frequency region. The high-frequency region usually shows a semicircle, and the low-frequency region shows a linear change curve. The radius of the semicircle in the high-frequency region is positively correlated with the charge transfer resistance (Rct) of the sample, and the smaller the radius, the smaller the charge transfer resistance. The low-frequency region of the Nyquist plot is positively correlated with the slope of the curve, and the larger the slope, the faster the electron diffusion rate. As can be seen from the figure, when the Co3O4 loading ratio is different, the composite material has certain differences in the radius of the semicircle in the high-frequency region and the slope in the low-frequency region. The composite sample with a loading ratio of 7% has the smallest radius of the semicircle and the largest slope, i.e., the smallest charge transfer resistance and the largest electron diffusion rate. This indicates that the separation efficiency of photo-generated carriers is higher, and the efficient transfer on the electrode / electrolyte interface, thus facilitating the photocatalytic reaction. As shown in Figure 13 (b), the lower the modulus value in the Bode plot, the faster the photo-generated carriers are generated. From Figure X It can be judged from the above that the composite material Co3O4 / SiW9Mo3-7% produces electron-hole pairs at the fastest speed, which is conducive to the photocatalytic reaction and has the highest theoretical photocatalytic activity.

[0082] The recombination rate and transfer efficiency of photo-generated electron-hole pairs in the prepared catalysts were investigated by fluorescence spectroscopy (PL), which is also considered to be a major key factor in improving photocatalytic efficiency. After the catalyst is irradiated by light, electron-hole pairs are generated by excitation, and electrons jump from the valence band to the conduction band, generating an equal number of holes in the conduction band. Only when the electron-hole pairs recombine and release in the form of light, will the luminescence phenomenon occur. The higher the fluorescence intensity, the easier the material electron-hole pairs recombine, and the lower the photocatalytic activity. Under an excitation wavelength of 397 nm, the fluorescence emission spectrum of Co3O4 / SiW9Mo3 was obtained, as shown in Figure 14 All samples show similar fluorescence emission peaks at 592 nm. The fluorescence intensity of different composite materials is different, so the separation degree and transfer effect of photo-generated electron-hole pairs are also different. Compared with the composite materials with a loading ratio of 1%, 3%, 5%, and 9%, Co3O4 / SiW9Mo3-7% has a lower fluorescence peak intensity, i.e., the lowest carrier recombination rate and higher photocatalytic activity.

[0083] The composite material Co3O4 / SiW9Mo3, with Co3O4 content of 1%, 3%, 5%, 7%, and 9% respectively, was fully magnetized, and electrochemical impedance spectroscopy (EIS) was performed under a magnetic field to investigate the separation efficiency of photogenerated carriers after magnetization. Figure 15 Under a magnetic field, the composite material with the smallest radius of curvature, Co3O4 / SiW9Mo3-7%, exhibits the lowest impedance. This indicates that the composite material with a 7% Co3O4 content has the smallest radius of curvature. The Bode plot modulus also confirms that the Co3O4 / SiW9Mo3-7% composite material theoretically has higher photocatalytic activity. Therefore, Co3O4 / SiW9Mo3-7% was subsequently used for the applied magnetic field experiments.

[0084] Application Examples

[0085] Using Co3O4 / SiW9Mo3 nanocomposite materials as photocatalysts, photocatalytic degradation experiments on methylene blue solution were conducted under simulated sunlight. The effects of different experimental factors (catalyst dosage, pollutant solution concentration, and catalyst ratio) on photocatalytic degradation were systematically analyzed. Photocatalytic experiments were performed with a certain amount of photocatalyst and a certain concentration of dye solution to evaluate the photocatalytic activity of the composite material. Simultaneously, the photocatalytic activity and stability of the composite photocatalyst were assessed through a study on the degradation cycle performance of methylene blue.

[0086] By adding methanol (h) to the methylene blue dye solution respectively + Quencher), isopropanol (·OH scavenger), and p-benzoquinone (·O2) - Using quenchers and combined with photocatalytic data, this study investigated the main active species in the photocatalytic degradation of methylene blue by Co3O4 / SiW9Mo3 composite materials.

[0087] (1) Standard Curve

[0088] 100 mg of methylene blue was dissolved in 500 mL of deionized water to prepare a 200 mg / L methylene blue standard solution. Different volumes of this standard solution were pipetted into 15 mL colorimetric tubes and diluted with deionized water to 10 mL to obtain methylene blue solutions of various concentrations (25, 20, 15, 10, 5, and 2 mg / L). The absorbance was measured at 664 nm to obtain a linear relationship between absorbance and concentration.

[0089] Several methylene blue standard solutions of different concentrations were prepared, and their absorbance at a wavelength of 664 nm was measured using a UV-Vis spectrophotometer. A standard curve was plotted with concentration (C) on the ordinate and absorbance (A) on the abscissa, as shown below. Figure 16As shown. The standard curve equation generated after fitting is y = 0.1173x + 0.1912, with a correlation coefficient of R0. 2 =0.9981. The results show that, within a certain wavelength range, absorbance and concentration exhibit a good linear relationship.

[0090] (2) Direct photolysis experiment of methylene blue dye

[0091] In the direct photocatalytic degradation experiment of methylene blue dye, no photocatalyst was added to the reaction system; instead, direct light irradiation was performed. Samples were taken at regular intervals and tested, and the results are as follows: Figure 17 As shown, the concentration of the methylene blue solution remained essentially unchanged after a certain period of illumination. The results indicate that even without a photocatalyst, methylene blue undergoes some degree of direct photolysis under illumination, but the degree of photolysis is small, with a decolorization rate of only about 10.1%.

[0092] (3) Effect of different initial concentrations on photocatalysis

[0093] Figure 18 The figure shows the degradation trend of methylene blue solution by photocatalytic decomposition of methylene blue solution by Co3O4 / SiW9Mo3 when the concentration of methylene blue solution is 15 mg / L, 20 mg / L, 25 mg / L and 30 mg / L.

[0094] from Figure 18 It can be seen that the catalyst has a strong degradation ability for the dye at dye concentrations of 15 mg / L and 20 mg / L. At a dye concentration of 15 mg / L, although the degradation effect is high, it is almost complete within 30 minutes, and the degradation rate reaches over 80% even in dark light adsorption, indicating a relatively weak photocatalytic effect. However, at a dye concentration of 20 mg / L, the degradation rate is around 50% in the dark light adsorption stage, and after photocatalysis, it reaches a degradation rate of 88.96%, demonstrating a very significant photocatalytic effect. However, as the initial concentration of methylene blue increases, the photocatalytic degradation efficiency gradually decreases. At dye concentrations of 25 mg / L and 30 mg / L, the overall photocatalytic effect is not very good, around 55.99% and 28.10% respectively, and the degradation rate is also relatively slow. This is because with increasing concentration, the amount of catalyst added decreases relatively, and the number of active sites on the catalyst surface also decreases relatively, affecting the photocatalytic reaction. Therefore, a dye concentration of 20 mg / L can be selected for subsequent experiments.

[0095] (4) Effect of different catalyst addition amounts on photocatalysis

[0096] The effect of different amounts of catalyst on the photocatalytic reaction was explored by adding 0.4 g / L, 0.5 g / L, 0.6 g / L, and 0.7 g / L of catalyst in the reaction system under the experimental conditions of a methylene blue concentration of 20 mg / L. The experimental results are shown in Figure 19 .

[0097] From Figure 19 (a), it can be seen that as the amount of catalyst increases, the degradation rate of methylene blue increases from 53.62% to 88.33%. This is because as the number of catalyst particles increases, more active species are generated under light, accelerating the degradation of the dye. When the amount of catalyst reaches 0.6 g / L, the degradation rate no longer increases, Figure 19 (b) can also show that the reaction rate will decrease. The reason is that when the amount of catalyst is too large, the turbidity of the suspension system will be large, which will reflect and scatter the visible light, thereby affecting the catalytic rate.

[0098] (5) Effect of different proportions on photocatalysis

[0099] Under the conditions of a catalyst addition amount of 0.6 g / L and a methylene blue concentration of 20 mg / L, the effect of different proportions of catalyst (the proportion of Co3O4 in Co3O4 / SiW9Mo3 is 1%, 3%, 5%, 7%, and 9%) on the photocatalytic degradation of dye was studied, and the results are shown in Figure 20 .

[0100] From Figure 20 (a), it can be seen that the Co3O4 / SiW9Mo3-7% composite material has the best photocatalytic degradation effect on methylene blue, indicating that the amount of 7%wt-Co3O4 composite material is the optimal point for degradation, reaching 89.37%. When the proportion is 1%, 3%, 5%, and 9%, the final degradation rate of photocatalysis is 59.1%, 57.13%, 72.31%, and 66.72%, respectively, and the degradation efficiency decreases. Comparing it with Figure 20 (b) the methylene blue solution degradation rate graph, it can be clearly seen that Co3O4 / SiW9Mo3-7% has the highest decomposition rate for 20 mg / L methylene blue and the best catalytic activity.

[0101] (1) Effect of different chloride ion concentrations on photocatalysis

[0102] Under the conditions of a catalyst addition amount of 0.6 g / L, a methylene blue concentration of 20 mg / L, and a composite ratio of 7%, the effect of different chloride ion concentrations on photocatalysis was explored, with chloride ion concentrations of 100 mg / L, 300 mg / L, 500 mg / L, and 700 mg / L.

[0103] FromFigure 21 (a) As can be seen, in the first 20 min, the photocatalytic degradation rate is significantly improved after the addition of chloride ions, which is due to the occurrence of (·O2 - +Cl - →OCl - ) and (OCl - +H2O→HOCl+OH - ) as the concentration of Cl - increases. The generated HOCl has strong oxidizing properties, thereby increasing the degradation efficiency. After 20 min, as the reaction continues, the amount of HOCl produced increases, and (HOCl+·O2 - →O2+Cl - ) occurs, consuming superoxide radicals and HOCl, thereby leading to a decrease in photocatalytic degradation efficiency. The results are shown in Figure 21 (b) The degradation rate of methylene blue solution can also be clearly seen from the comparison that Co3O4 / SiW9Mo3-7% has the highest methylene blue decomposition rate without adding chloride ions, and the catalytic activity is the best.

[0104] (2) Effect of different pH on photocatalysis

[0105] To explore the chemical stability of the photocatalyst in an acidic environment and the effect of different pH on photocatalysis under the conditions of catalyst addition amount of 0.6 g / L, methylene blue concentration of 20 mg / L, and composite ratio of 7%, the pH is 1, 3, 5, and 7.

[0106] Figure 22 (a) As shown, the photocatalytic rate of methylene blue shows different trends at different pH values. It can be clearly seen that in all the adsorption data, the dark adsorption data is relatively stable, mainly concentrated in the interval of 10%-20%. This indicates that the composite material has stability, and the presence of hydrogen ions will not continue to change the stability of the composite material, so the adsorption of the composite material tends to be stable. As can be seen from the figure, when pH=7, the photocatalytic efficiency is the highest, which can reach 89.37%. Figure 22 (b) It can be clearly seen that the photocatalytic degradation rate under the condition of pH=7 is much higher than that of the other groups, which is because as the concentration of H+ increases, (·O2 - +2H + →H2O2) occurs, and H2O2 has strong oxidizing properties, which will accelerate the degradation of pollutants. As the reaction proceeds, the amount of H2O2 produced increases, and H2O2 will occur (2H2O2→2H2O+O2), thereby consuming ·O2 - , thereby leading to a decrease in photocatalytic degradation efficiency. Ultimately, it is concluded that the photocatalytic effect is best when the pH is 7.

[0107] (6) Effect of magnetic field on photocatalytic performance

[0108] Under the conditions of the best initial concentration of dye and the most suitable catalyst addition amount of Co3O4 / SiW9Mo3-7% catalyst, experiments of different magnetic field intensities were carried out to photocatalytically degrade methylene blue, considering the effect of magnetic field on the efficiency of photocatalyst. As shown in Figure 23 , the photocatalytic reaction performance and rate under the conditions of adding 100Gs-300Gs magnetic field outside the catalytic device were compared with those under the condition of not adding magnetic field. When the magnetic field intensity was 200Gs, the degradation rate was best, reaching 85.97%, and the degradation rate was also faster. Under the condition of 500Gs magnetic field intensity, as can be known from the rate graph on the right, the catalytic rate reached 0.0281, proving at the experimental level that a certain magnetic field intensity is beneficial to the photocatalytic reaction.

[0109] The addition of external magnetic field can further enhance the photocatalytic activity of Co3O4 / SiW9Mo3-7%, providing a suitable magnetic field for the separation and migration of photoinduced carriers under the action of Lorentz force. The improvement of carrier separation efficiency can make more electrons and holes reach the surface of the catalyst, thereby participating in the subsequent redox reaction, thus improving the catalytic ability and catalytic efficiency.

[0110] (7) Photocatalytic cycle experiment of composite Co3O4 / SiW9Mo3

[0111] The ideal photocatalytic performance of photocatalyst plays an extremely important role in practical application, and its stability also occupies the same position. The photocatalytic cycle experiment is to judge whether the photocatalyst is stable and reusable through the continuous degradation of the cycle experiment of eosin B. Figure 24 (a) shows that after two cycle experiments, the photocatalytic activity of Co3O4 / SiW9Mo3 slightly decreased from 89.37% initially to 85.69% after three cycles. The second cycle experiment shows that Co3O4 / SiW9Mo3 has sufficient stable photocatalytic degradation activity. And the structure before and after the cycle experiment does not change, still maintaining the complete Keggin structure, and the activity loss is small, as shown in Figure 24 (b) infrared spectrum.

[0112] (8) Kinetics and photocatalytic mechanism test

[0113] In order to better understand the mechanism of Co3O4 / SiW9Mo3 catalytic degradation of methylene blue, we carried out free radical and hole capture experiments. In this study, we chose methanol, isopropyl alcohol and ascorbic acid as photo-generated holes (h + ), hydroxyl radicals (·OH) and superoxide radicals (·O2 -) species. As Figure 25 The results show that in the presence of ascorbic acid, ·O2 - was masked, and the photocatalytic result of the composite material was reduced to about 77.83%, proving that ·O2 - is an important factor in the photocatalytic decomposition of methylene blue. In the presence of methanol and isopropanol, the photocatalytic decomposition rate and the final catalytic result are almost unchanged, proving that h + , ·OH is not the main factor limiting the photocatalytic activity. In summary, we speculate that in the experiment of photocatalytic decomposition of methylene blue, the photocatalytic process is completed by the superoxide radical.

[0114] In order to further clarify the band structure of SiW9Mo3 and Co3O4, we carried out Mott-Schottky (M-S) to analyze and determine the flat band potential (E cb )

[37] As Figure 26 shown, according to the M-S equation, the conduction band position of SiW9Mo3 and Co3O4 is 0.37 eV and -0.37 eV. Then, by converting the ultraviolet-visible spectrum of SiW9Mo3 and Co3O4 through the K-M equation, the width of the gap between the conduction band and the valence band is 2.61 eV and 1.58 eV, and then the position of the valence band (E (HOMO)(2.98V) =E (LUMO)(0.37V) +Eg (2.61V) ) and (E (VB)(1.21V) =E (CB)(-0.37V) +Eg (1.58V) ) is calculated. According to the conduction band, the valence band and the width of the band gap, a photocatalytic mechanism diagram of the composite material is drawn, as shown in Figure 27 .

[0115] The technical scheme of the present application is not limited to the above specific embodiments, and any technical modification made according to the technical scheme of the present application falls within the protection scope of the present application.

Claims

1. A method for preparing a magnetic nanocomposite photocatalyst, characterized by, The method comprises the following steps: S1, preparing a heteropoly acid precursor α-SiW9; S2, preparing SiW9Mo3 by using the heteropoly acid precursor α-SiW9: 2.50 g of the heteropoly acid precursor α-SiW9 and 0.70 g of Na2MoO4 are mixed and dissolved in 10 mL of water, and then a hydrochloric acid solution with a concentration of 3 M is added dropwise until the solid is completely dissolved and the pH is adjusted to 1; after being heated in a water bath at 80 ℃ for 15 min, the solution is cooled to room temperature and filtered and dried; finally, a yellow-green solid is obtained, which is SiW9Mo3; S3, preparing Co3O4; S4, preparing Co3O4 / SiW9Mo3: 1 g of SiW9Mo3 is dissolved in 10 mL of water, and Co3O4 with a mass fraction of 1-9% of SiW9Mo3 is added and stirred for 30 min to make them fully mixed; 10 mL of a tetrabutylammonium bromide aqueous solution with a concentration of 0.1 mol / L is added dropwise; the stirring is continued for 5-6 hours, and after the reaction is completed, the sample is centrifugally separated and washed with deionized water and ethanol for multiple times; the washed sample is dried at 60 ℃ for 6 h to obtain a magnetic nano-composite photocatalyst Co3O4 / SiW9Mo3.

2. The method for preparing the magnetic nanocomposite photocatalyst according to claim 1, characterized in that, The step S1 specifically comprises the following steps: 18.20 g of sodium tungstate and 1.10 g of sodium silicate are weighed and dissolved in 20 mL of hot water with a temperature of 80-100 ℃ and stirred and mixed; then 13 mL of a hydrochloric acid solution with a concentration of 6 M is added dropwise, and when the solution boils and the volume becomes 30 mL, the unreacted Si is removed by filtration and centrifugation to collect the filtrate; then 5.00 g of anhydrous sodium carbonate is dissolved in 15 mL of water and then slowly added to the filtrate, and the precipitate is slowly formed; after the precipitate is completely formed, the solid is separated by filtration and mixed with 100 mL of a sodium chloride solution with a concentration of 4 M and stirred, and then filtered again; finally, the separated solid is washed with anhydrous ethanol to remove impurities and accelerate crystallization, and vacuum dried to obtain the heteropoly acid precursor α-SiW9.

3. The method for preparing the magnetic nanocomposite photocatalyst according to claim 1, characterized in that, The step S3 specifically comprises the following steps: 0.8 g of Co(CH3COO)2·6H2O is added to a mixed solution of 16 mL of water and 24 mL of ethanol and stirred for 1 h, and then 4 mL of ammonia water with a mass fraction of 25% is added; the mixture is continuously stirred in air for 20 min, and then the liquid is transferred to a 100 mL high-pressure reaction kettle and reacted at 150 ℃ for 3 h; after the high-pressure kettle is naturally cooled, the obtained solid is washed with deionized water for multiple times and dried at 60 ℃ for 4 h to obtain black solid Co3O4.

4. The method for preparing the magnetic nanocomposite photocatalyst according to claim 1, characterized in that, In the step S4, Co3O4 with a mass fraction of 7% of SiW9Mo3 is added.

5. A magnetic nano-composite photocatalyst prepared by the method according to any one of claims 1-4.

6. Application of the magnetic nano-composite photocatalyst according to claim 5 in photocatalytic degradation of organic dyes.

7. Use of the magnetic nanocomposite photocatalyst according to claim 6 for photocatalytic degradation of organic dyes, characterized in that, Under the experimental condition of 20 mg / L of organic dye concentration, 0.6 g / L of the magnetic nanocomposite photocatalyst is added into the reaction system, the pH of the reaction system is adjusted to 7, and the photocatalytic degradation of the organic dye is carried out under the external magnetic field of 100 Gs-300 Gs.

8. Use of the magnetic nanocomposite photocatalyst according to claim 7 for photocatalytic degradation of organic dyes, characterized in that, The external magnetic field is 200 Gs.

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