CN116078410A - Fe/WO 3 BiOBr monoatomic catalyst, preparation method thereof and application of catalyst in visible light catalytic degradation of ciprofloxacin - Google Patents

Fe/WO 3 BiOBr monoatomic catalyst, preparation method thereof and application of catalyst in visible light catalytic degradation of ciprofloxacin Download PDF

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CN116078410A
CN116078410A CN202211105472.4A CN202211105472A CN116078410A CN 116078410 A CN116078410 A CN 116078410A CN 202211105472 A CN202211105472 A CN 202211105472A CN 116078410 A CN116078410 A CN 116078410A
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fesa
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CN116078410B (en
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戴友芝
于芹芹
张柱
肖业勇
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Xiangtan University
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Abstract

The invention belongs to the field of environmental protection and water treatment, in particular to Fe/WO 3 BiOBr monoatomic catalyst, its preparation method and application in visible light catalytic degradation of ciprofloxacin. The FeSA/WB-0.5 monoatomic catalyst prepared by the invention selects CIP as a treatment object and is combined with pure WO under the irradiation of visible light 3 Compared with FeSA/WB-0.5 composite material, the photocatalytic activity for degrading CIP is obviously improved, the complete degradation of 10mg/L CIP can be achieved within 30min, and the degradation rate of 20mg/L CIP reaches more than 98.5% within 120min. In the process of repeatingAfter five times of use, the FeSA/WB-0.5 composite material still has good photocatalytic activity and stability. The FeSA/WB-0.5 monoatomic catalyst is a very promising photocatalytic material, and provides a valuable way for the treatment design of refractory organic pollutants in wastewater.

Description

Fe/WO 3 BiOBr monoatomic catalyst, preparation method thereof and application of catalyst in visible light catalytic degradation of ciprofloxacin
Technical Field
The invention belongs to the field of environmental protection and water treatment, in particular to Fe/WO 3 BiOBr monologA sub-catalyst, a preparation method thereof and application of the sub-catalyst in visible light catalytic degradation of ciprofloxacin.
Background
CIP wastewater has the characteristics of high toxicity, high durability and difficult degradation, the CIP is difficult to degrade efficiently by the traditional wastewater treatment method, and the photocatalytic oxidation technology has potential advantages because the reaction condition is mild, the method is rapid and efficient, and secondary pollutants are not generated.
BiOBr and WO 3 Is a catalyst capable of utilizing visible light, and has a forbidden bandwidth of 2.6-2.7 and eV, wherein BiOBr has strong absorption to visible light with wavelength less than 440 nm (Y. Wang, Y. Long, Z. Yang, et al, A-level of ion-exchange strategy for the fabrication of high strong BiOI/BiOBr heterostructure film coated metal wire mesh with tunable visible-light-driven photocatalytic reactivity [ J)]. J. Hazard. Mater., 2018, 351: 11-19.；[] GAO Z, YAO B, YANG F, et al. Preparation of BiOBr-Bi heterojunction composites with enhanced photocatalytic properties on BiOBr surface by in-situ reduction[J]. Mater Sci Semicond Process, 2020, 108: 104882.），WO 3 Valence band potential (E) VB ) About 2.7V (vs. NHE) (P. Dong, G. Hou, X. Xi, et al, WO) 3 -based photocatalysts: morphology control, activity enhancement and multifunctional applications [J]Environ, sci, nano, 2017, 4:539-557.) the resulting photogenerated holes (h + ) Has a chemical structure with TiO 2 Similar strong oxidizing power, but both have the problem of easy recombination of photogenerated electrons and holes. High E VB And visible light response characteristics give WO 3 The potential for constructing high performance heterojunction, so BiOBr and WO can be used 3 And a heterojunction is built in a composite mode, separation of photo-generated carriers is promoted, and photocatalysis efficiency is improved. WO synthesized by Junlei Zhang et al 3 BiOBr has ideal chlorophenol degradation and methyl orange effect but complex preparation (Zhang Junlei, zhangLisha, shenXiaofeng, et al Synthesis of BiOBr/WO) 3 p-n heterojunctions with enhanced visible light photocatalytic activity [J]CrystEngComm, 2016,18 (21): 3856-3865.). Wherein, WO prepared by simple solvothermal method 3 The degradation rate of the BiOBr to CIP reaches 83.1 percent, which is attributed to the fact thatThe large specific surface area increases the reactive sites, and the proper energy band structure facilitates separation of photogenerated electron-hole pairs. The metal monoatomic modification can further improve the photocatalytic performance due to the increase of the specific surface area and the reaction site, and monoatoms promote electron transfer and inhibit the photon-generated carrier recombination. Han Xu et al prepared a catalyst with Fe anchored to nitrogen doped carbon single atom, and a degradation rate of rhodamine B of 96.8% (30 min) due to specific surface area, highly dispersed Fe-N x Species and Gao OH yield. The Co/Fe-N-C single-atom catalyst prepared by Deng Fangxin is 100% degraded to bisphenol A and phenol at 20min. The organic covalent framework-supported monoatomic iron catalyst prepared by Yunjin Yao et al was 100% degraded by dye at 45min (YAO Y, YIN H, GAO M, et al Electronic structure modulation of covalent organic frameworks by single-atom Fe doping for enhanced oxidation of aqueous contaminants [ J)]Chemical Engineering Science,2019,209 (23): 115211.). Fe prepared by Lizhi Huang et al x Mo 1-x The degradation rate of the S monoatomic catalyst to the aniline and propranolol reaches 90% at 20min (HUANG L-Z, ZHOU C, SHEN M, et al Persulfate activation by two-dimensional MoS2 confining single Fe atoms: performance, mechanism and DFT calculations [ J)]. Journal ofHazardous Materials,2020,389(9): 122137.； HUANG L-Z,WEI X,GAO E, et al.Single Fe atoms confined in two-dimensional MoS2 forsulfite activation: A biomimetic approach towards efficient radical generation[J]AppliedCatalysis B Environmental, 2020,268 (13): 118459.). Currently, biOBr or WO 3 The research of constructing heterojunction with other semiconductor materials to improve the photocatalytic performance is common, but the research of introducing single atoms is rare.
Disclosure of Invention
The Fe monoatomic composite material is prepared, the crystal structure, the microcosmic appearance, the chemical valence state and the photoelectric property of the Fe monoatomic composite material are analyzed by combining various characterization means, the activity and the stability of the photocatalytic degradation CIP are researched, and the degradation mechanism is explored. The researches have theoretical and practical significance for developing novel photocatalysts and ciprofloxacin degradation methods.
The invention firstly provides a Fe/WO 3 A process for preparing a BiOBr monoatomic catalyst comprising the steps of:
1) Na is mixed with 2 WO 4 ·2H 2 O and NaHSO 4 ·H 2 O is dissolved in ultrapure water and transferred into a reaction kettle for hydrothermal reaction; cooling to room temperature, centrifuging to collect precipitate, washing with ultrapure water and absolute ethanol, and drying to obtain solid WO 3 A bundle of nanotubes;
2) Dissolving 1-hexadecyl-3-methyl imidazole bromide in ethylene glycol methyl ether, adding WO 3 A nano tube bundle is subjected to ultrasonic dispersion to form a suspension A; bi (NO) 3 ) 3 •5H 2 O is thoroughly dissolved in ethylene glycol methyl ether to obtain a solution B; mixing the suspension A and the solution B, stirring uniformly, and transferring into a reaction kettle for reaction; after cooling to room temperature, centrifugally collecting the generated precipitate, washing with ultrapure water and absolute ethyl alcohol, and drying to obtain a solid, namely the tungsten trioxide nano tube/bismuth oxybromide nano sheet composite catalyst, namely the WB composite catalyst;
3) Ultrasonically dispersing WB-0.5 in methanol to obtain suspension C; fe (NO) 3 ) 3 ·9H 2 O is dissolved in ultrapure water to obtain solution D; mixing the suspension C and the solution D, introducing nitrogen to remove oxygen after ultrasonic dispersion, and irradiating under a xenon lamp; and centrifugally collecting the solid, cleaning the solid by using ultrapure water and absolute ethyl alcohol, and drying to obtain the solid which is the FeSA/WB-0.5 iron monoatom/tungsten trioxide nano tube/bismuth oxybromide nano sheet composite catalyst.
In a preferred mode, in the step 1), na 2 WO 4 ·2H 2 O and NaHSO 4 ·H 2 The molar ratio of O is 1:2-4, preferably 1:3; the reaction kettle is a polytetrafluoroethylene lining stainless steel reaction kettle; the hydrothermal reaction is carried out at 140-220 ℃, preferably 160-200 ℃, more preferably 175-185 ℃ and most preferably 180 ℃ for 18-30 hours, preferably 20-28 hours, more preferably 22-26 hours and most preferably 24 hours; the drying is carried out in a vacuum drying oven at 50-70deg.C, preferably 55-65deg.C, most preferably 60deg.C for 8-16 hr, preferably 10-14 hr, most preferably 12 hr.
In said step 2), 1-hexadecyl-3-methyl imidazole bromide is dissolved in ethylene glycol methyl ether at a concentration of 0.02-0.06mol/L, preferably 0.04 mol/L, 1-hexadecyl-3-methyl imidazole bromide and WO 3 The molar ratio of the dosage of the nano tube bundles is 1-3:1, preferably 2:1;
Bi(NO 3 ) 3 •5H 2 the concentration of O dissolved in glycol methyl ether is 0.05-0.15mol/L, preferably 0.1 mol/L, and the O is thoroughly dissolved by magnetic stirring;
the mixing ratio of suspension A and solution B is such that WO 3 Nanotube bundles and Bi (NO) 3 ) 3 •5H 2 The molar ratio of O is 1:1-3, preferably 1:1.5-2.5, more preferably 1:2;
the reaction in the reaction kettle is that a polytetrafluoroethylene lining stainless steel water heating kettle is put into an electrothermal blowing drying oven after sealing tightly, the reaction temperature is set to be 120-200 ℃, preferably 140-180 ℃, preferably 155-165 ℃, most preferably 160 ℃, and the reaction time is 0.5-3.5h, preferably 1-3h, more preferably 1.5-2.5h;
the drying is carried out in a vacuum drying oven at 50-70deg.C, preferably 55-65deg.C, most preferably 60deg.C for 8-16 hr, preferably 10-14 hr, most preferably 12 hr.
In step 3), the WB complex catalyst is used in an amount of 0.05 to 0.15mol/L, preferably 0.0875 mol/L, in suspension C; fe (NO) 3 ) 3 ·9H 2 The molar ratio of O to WB-0.5 is 1:3-4, preferably 1:3.78; the xenon lamp irradiation is carried out at an illumination intensity of 400 to 600W, preferably 450 to 550W, most preferably 500W; the drying is carried out in a vacuum drying oven at 50-70deg.C, preferably 55-65deg.C, most preferably 60deg.C for 8-16 hr, preferably 10-14 hr, most preferably 12 hr.
The invention provides Fe/WO obtained by the preparation method 3 BiOBr monoatomic catalyst. Preferably, the mass percentage of iron is 3-15-wt%, more preferably 5-8wt%.
The invention provides the Fe/WO 3 Use of a BiOBr monoatomic catalyst for degrading organic contaminants, preferably for degrading CIP.
The present invention provides a method of degrading CIP comprisingSaid Fe/WO 3 The BiOBr monoatomic catalyst is added into the CIP-containing liquid to be treated, and the dark reaction and the photoreaction are carried out. Preferably, at the beginning of the reaction, ph=5-12, preferably ph=8-10, more preferably ph=9; the initial concentration of CIP is adjusted to not more than 30mg/L, preferably not more than 20mg/L, more preferably not more than 10mg/L, such as 5-10mg/L. The addition amount of the monoatomic catalyst is 0.4-1.2g/L, preferably 0.6-1.0g/L, more preferably 0.8 g/L; the illumination intensity during the light reaction is 300-800W, preferably 400-600W, more preferably 500-W. Preferably, the dark reaction is carried out for 20-60 min (preferably 35-45 min), and the light reaction is carried out for 80-160min, preferably 100-140min, more preferably 110-130min, more preferably 120min. It is further preferred that the dark reaction is carried out for 35 to 45 minutes and the light reaction is carried out for 100 to 140 minutes, more preferably 110 to 130 minutes, still more preferably 120 minutes.
The FeSA/WB-0.5 monoatomic catalyst with different loading ratios is prepared by adopting a simple hydrothermal method, a solvothermal method and a photo-reduction method. CIP was selected as the treatment target, and was irradiated with visible light, in combination with pure WO 3 Compared with FeSA/WB-0.5 composite material, the photocatalytic activity of CIP degradation is obviously improved, the complete degradation of 10mg/L CIP can be achieved within 30min, and the degradation rate of 20mg/L CIP reaches more than 98.5% within 120min. After five times of repeated use, the FeSA/WB-0.5 composite material still has good catalytic activity and stability. Radical trapping experiments show that O 2 - 、h + The composite material is a main active species for reaction, and simultaneously, a photocatalysis mechanism for efficiently degrading CIP is provided. The FeSA/WB-0.5 monoatomic catalyst is a very promising photocatalytic material, and provides a valuable way for designing a wastewater organic pollutant degradation system.
Drawings
XRD patterns of the photocatalyst prepared in FIG. 1.
In FIG. 2 (a) is WO 3 Nanotube bundles, (b) WB-0.5 and (c) SEM images of FeSA/WB-0.5 composites.
FIG. 3 elemental distribution and energy spectrum of FeSA/WB-0.5 composite.
In FIG. 4, (a) - (d) are TEM images of FeSA/WB-0.5 composite material, and (e) - (f) are SAED spectra.
FIG. 5 shows XPS spectra of the prepared photocatalyst.
FIG. 6 (a) is a normalized iron K-edge XANES spectrum; (b) Fourier Transform (FT) plots for the iron K-edge EXAFS spectrum and (c) - (f) are Wavelet Transform (WT) plots.
In fig. 7, (a) is an ultraviolet-visible diffuse reflection absorption spectrum and (b) is a Tauc curve.
FIG. 8 comparison of the activities of different catalysts to photo-catalytically degrade CIP.
FIG. 9 effect of different factors on CIP degradation effects.
FIG. 10 CIP mineralization rate and photocatalytic stability experiments.
The fluorescence spectrum of the photocatalyst prepared in FIG. 11 (excitation wavelength 325 nm).
In fig. 12, (a) and (b) are mott-schottky diagrams of the prepared photocatalyst, (c) is an energy distribution diagram of a conduction band and a valence band, (d) is a transient photocurrent response curve, and (e) is an alternating current impedance spectrum.
FIGS. 13 (a) and (b) are the effect of different sacrificial agents on FeSA/WB-0.5 CIP degradation efficiency; (c) OH, (d) is O 2 - ，(e) 1 O 2 And (f) is h + EPR test of (C).
FIG. 14 FeSA/WB-0.5 shows the transfer path of the photogenerated carriers.
Detailed Description
The invention will be further illustrated by the following detailed description in order to provide a better understanding of the invention, but without limiting the invention.
The main reagents used in the examples: sodium tungstate dihydrate, sodium bisulfate monohydrate, 1-hexadecyl-3-methyl imidazole bromide, ferric nitrate nonahydrate (Tianjin, denko chemical industries, ltd.); absolute methanol, absolute ethanol, ethylene glycol methyl ether (Tianjin optical complex technology development Co., ltd.); ciprofloxacin (a Ding Shiji company). The experimental medicines are all analytically pure, and the experimental water is ultrapure water.
The main instrument used in the examples: x-ray diffractometer (XRD, D/Max 2500, japanese Rigaku Corporation Co.); a field emission scanning electron microscope (SEM, TESCAN MIRA, JEOL ltd.) and an energy chromatograph (EDS, ULTIM MAX 40, uk Oxford Instruments); high resolution transmission electron microscope (HRTEM, JEM-2100, japan JEOL ltd.); x-ray photoelectron spectroscopy (XPS, ESCAlab 250Xi, usa company Thermo Scientific); ultraviolet-visible spectrophotometers (UV-Vis DRS, UV-2550, shimadzu Co., ltd.).
Example one, preparation of materials
Preparation of WO by hydrothermal method 3 Nanotube bundles: 1.98g of Na 2 WO 4 ·2H 2 O and 2.38g NaHSO 4 ·H 2 O is dissolved in 80mL of ultrapure water to form a precursor solution, then the precursor solution is transferred into a polytetrafluoroethylene-lined stainless steel reaction kettle with the capacity of 100mL, the reaction kettle is sealed, and the precursor solution is put into an electrothermal blowing drying oven to be subjected to hydrothermal reaction for 24 hours at 180 ℃; after the hydrothermal reaction is finished, naturally cooling the reaction kettle to room temperature, centrifuging to collect the generated precipitate, sequentially cleaning with ultrapure water and absolute ethyl alcohol for 3-5 times to remove possible impurities, then placing the precipitate into a vacuum drying oven, drying at 60 ℃ for 12 hours, and obtaining solid after drying, namely WO 3 Bundles of nanotubes.
WO is prepared by a solvothermal method 3 BiOBr composite: 0.775g of 1-hexadecyl-3-methyl imidazole bromide was dissolved in 50mL of ethylene glycol methyl ether, and 0.232g of WO was added 3 A nano tube bundle is ultrasonically dispersed for 30min to form a suspension A; 0.963g Bi (NO) 3 ) 3 •5H 2 O is dissolved in 20mL glycol methyl ether, and solution B is obtained after magnetic stirring until the O is thoroughly dissolved; mixing the suspension A and the solution B, stirring for 30min by a magnetic stirrer to fully mix, transferring into a polytetrafluoroethylene lining stainless steel water heating kettle with the capacity of 100mL, sealing tightly, placing into an electrothermal blowing drying oven, setting the reaction temperature to 160 ℃, and reacting for 2h; after the solvothermal reaction is finished, naturally cooling the reaction kettle to room temperature, centrifugally collecting the generated precipitate, alternately washing the precipitate with ultrapure water and absolute ethyl alcohol for 3-5 times to remove impurities, then placing the precipitate into a vacuum drying oven, and drying at 60 ℃ for 12 hours to obtain a solid, namely the tungsten trioxide nano-particlesThe molar ratio of the tube to the bismuth oxybromide nano-sheet composite catalyst is 1:2, and is marked as WB-0.5.
Preparation of Fe/WO by photochemical method 3 BiOBr composite: placing 0.982g of WB-0.5 composite material in a 100mL beaker, adding 40mL of absolute methanol, and performing ultrasonic dispersion for 3min to obtain a suspension C; 0.374g of Fe (NO 3 ) 3 ·9H 2 Placing O in a 50mL beaker, adding 20mL of ultrapure water, and magnetically stirring until the O is thoroughly dissolved to obtain a solution D; mixing the suspension C and the solution D, and performing ultrasonic dispersion for 5min. Then degassing for 40min by using nitrogen to remove oxygen in the reaction system; after the degassing is completed, placing the beaker under a 500W xenon lamp for irradiation for 20min; and centrifugally collecting solids, cleaning the solids with ultrapure water for 3-5 times, cleaning the solids with absolute ethyl alcohol for 2-3 times, then placing the cleaned solids into a vacuum drying oven, and vacuum drying the dried solids at 60 ℃ for 12 hours, wherein the dried solids are the iron monoatom/tungsten trioxide nano tube/bismuth oxybromide nano sheet composite catalyst 5-FeSA/WB-0.5. By varying Fe (NO) 3 ) 3 ·9H 2 O was 0.22, 0.374, 0.618, 0.79, 1.254 and g, respectively, to prepare a series of composite photocatalysts containing iron in different mass percentages, labeled as x-FeSA/WB-0.5 (where x=3, 5, 8, 10, 15 wt%).
Example two characterization of catalyst
1) Analysis of phase and Crystal Structure (XRD)
The crystal structure of the sample was determined by X-ray diffractometer technique (XRD), as shown in fig. 1. A high and sharp diffraction peak indicates that the sample has good crystallinity, and no other diffraction peak indicates no impurity. Pure WO 3 The diffraction peak of (2) is attributed to hexagonal phase (JCPLDS 85-2460); the diffraction peak of pure BiOBr is attributed to pure tetragonal phase (JCPDS 73-2061); WB-0.5 has in addition to WO 3 Also, there are low and broad characteristic diffraction peaks of BiOBr at 2θ=30-35℃and 45-50℃indicating that BiOBr was successful with WO 3 Compounding; feSA/WB-0.5 with WO 3 Characteristic peaks of BiOBr and FeOOH (JCPDS 73-2326) indicate successful recombination of Fe with WB-0.5.
2) Microcosmic morphology and elemental composition distribution analysis (SEM, EDS)
By Scanning Electron Microscopy (SEM)) And energy spectrum (EDS) characterizes the microscopic morphology of the material and analyzes the elemental composition and distribution state of the material. As shown in FIG. 2, WO 3 The nanotube bundles are formed by self-assembly of nanotubes; biOBr ultra-thin nanosheets in WO 3 The surfaces of the nanotube bundles grow and form a three-dimensional layered structure, and the nano sheets are staggered, so that more heterojunction can be exposed, and the absorption of light is enhanced; fe on the surface of FeSA/WB-0.5 can accelerate electron transfer and improve catalytic degradation efficiency. As shown in FIG. 3, the EDS spectrum shows that Fe is uniformly distributed on WB-0.5, the FeSA/WB-0.5 composite material contains Fe, O, bi, br and W elements, and no other impurities exist, so that the prepared sample has higher purity. This is consistent with the XRD test results.
3) Analysis of crystal plane and lattice spacing (TEM, SAED)
Crystal properties such as crystal planes, lattice distances and the like of the materials are analyzed by Transmission Electron Microscopy (TEM) and selective area diffraction (SAED). As shown in FIG. 4, the BiOBr ultra-thin nanoplatelets are tightly adhered to WO 3 The surface of the nanotube bundles showed a strong bonding force between them. WO (WO) 3 The lattice spacing of 0.280 and 0.278 nm is attributed to the (012) and (110) crystal planes of BiOBr and the lattice spacing of 0.316 nm is attributed to WO 3 The lattice spacing of 0.313 nm is attributed to the (040) crystal plane of FeOOH, which is consistent with previous XRD results. The SAED pattern diffraction pattern on the surface of the composite material is a regularly arranged lattice, which shows that BiOBr belongs to single crystals; the coexistence of diffraction spots and diffraction rings at the composite interface suggests that the composite has a mixed single and polycrystalline crystal character.
4) Elemental valence and electron transfer direction analysis (XPS)
The surface element composition, atomic chemistry and electron transfer direction of the samples were tested and analyzed by X-ray photoelectron spectroscopy (XPS). As shown in fig. 5 (a), the full scan spectrum of the sample showed that the sample contained W, bi, br, O and Fe, and the presence of other elements was not detected, indicating that the purity of the sample was high. WO in WB-0.5 composites was studied using high resolution XPS 3 Binding to BiOBr. As shown in FIG. 5 (b), pure WO 3 W4 f of (2) 7/2 And W4 f 5/2 The binding energies of the orbitals were 35.90 and 38.00 eV, with the W4 f peak of the WB-0.5 composite shifted toward higher binding energies and the W4 f peak of the FeSA/WB-0.5 composite shifted toward lower binding energies. As shown in FIGS. 5 (c) and (d), bi 4f of pure BiOBr 7/2 And Bi 4f 5/2 The peaks of (2) are located at 159.68 and 165 eV, respectively, br 3d 5/2 And Br 3d 3/2 Peaks at 68.75 and 69.76 eV, respectively; while the peaks of Bi 4f and Br 3d for WB-0.5 and FeSA/WB-0.5 are slightly shifted to lower binding energies. The shift in these binding energies clearly indicates that WO 3 Some of the electrons in (a) are transferred to the BiOBr after hybridization. The reason for this result is that the p-type photocatalyst BiOBr has a lower Fermi level than that of the n-type WO 3 Resulting in electrons from WO 3 Migration to BiOBr until the fermi levels of the two are consistent; and the uniformly distributed monoatomic Fe is used as a good electron acceptor and an electron transfer medium, so that the transfer of photo-generated electrons can be accelerated, and the photo-catalytic reaction is accelerated. In FIG. 5 (e), the peaks with binding energies 710.80, 713.70 and 724.30 eV are associated with Fe 2p 3/2 And Fe 2p 1/2 Correspondingly, the presence of these peaks confirms that FeSA/WB-0.5 contains Fe 2+ And Fe (Fe) 3+ The presence of multiple valence iron species further demonstrates that monoatomic iron is involved in the electron transfer process. As shown in FIG. 5 (f), pure WO 3 The O1 s peaks at 530.70 and 532.50 eV correspond to their lattice oxygen (W-O) and Hydroxyl (OH), respectively, the O1 s peaks at 530.50 and 531.80 eV for pure BiOBr correspond to their lattice oxygen (Bi-O) and Hydroxyl (OH), respectively, the O1 s spectra of WB-0.5 can be fit to three peaks of 530.30 (Bi-O), 531.40 (W-O) and 532.50 (OH) eV, and the O1 s spectra of FeSA/WB-0.5 can be fit to four peaks of 529.60 (Fe-O), 530.05 (Bi-O), 531.45 (W-O) and 532.70 (OH) eV. The binding energies of Bi-O and W-O show negative and positive shifts, respectively, which also indicate that there is an electron from WO in the complexes WB-0.5 and FeSA/WB-0.5 3 Migration to bitbr. In summary, these shifts in binding energy in the FeSA/WB-0.5 composite indicate strong chemical bond interactions between their interfaces, and it can be inferred that heterojunction rather than simple physical mixing is formed between them.
5) Single atomic state verification of materials (XANES, EXAFS)
The chemical valence and coordination form of the Fe atom in FeSA was further explored using X-ray absorbing near edge structures (XANES) and extended X-ray absorbing fine structures (EXAFS). The standard samples adopted in the figure are respectively elemental iron (Fe Foil), iron oxide (FeO, fe) 2 O 3 Fe (Fe) 3 O 4 ). As can be seen from the XANES spectrum (FIG. 6 (a)), the absorption lines of FeSA materials are between FeO and Fe in the near-edge absorption region of the iron K edge 2 O 3 The average valence state of the iron atoms in the FeSA material is demonstrated to be between +2 and +3, indicating that iron species of +2 and +3 are present in the FeSA material at the same time, consistent with the results of the previous XPS analysis. The K of the Fe K edge EXAFS (FIG. 6 (b)) spectrum 3 The weighted Fourier Transform (FT) spectrum shows that the FeSA material has obvious Fe-O signals at about 1.5A, and no absorption peak appears at the position of the absorption peak of the elemental iron, which proves that Fe-Fe bonds do not exist in the FeSA, so the material can be judged as a monoatomic catalyst. The coordination form of the FeSA sample was further determined by wavelet transformation, based on FeO and Fe 2 O 3 At 5.5 a -1 And 7.0 a -1 The left and right Wavelet Transform (WT) maxima may correspond to the Fe-O bonds of the FeSA sample (FIGS. 6 (c) - (f)), which are in accordance with the k of EXAFS 3 The weighted fourier transform results are consistent.
6) Analysis of light absorption Properties (UV-Vis DRS)
Samples were tested and analyzed for light absorption properties by ultraviolet-visible diffuse reflectance absorption spectroscopy (UV-Vis DRS). As shown in FIG. 7, WO 3 The light absorption boundaries of BiOBr, WB-0.5 and FeSA/WB-0.5 are 459, 438, 447 and 536 nm, respectively, which can absorb and utilize visible light. The band gap energy is estimated according to the following formula: αhv=a (hv-E g ) n/2 Wherein for indirect semiconductors WO 3 Or BiOBr, n is 4, alpha, A, hv and E g Respectively absorption coefficient, proportionality constant, photon energy and band gap energy. Utilization (ahv) 1/2 By plotting hv and then extrapolating the straight line portion to the intersection of the abscissa, the forbidden band width value (band gap energy) can be obtained, as can be seen from the graph, WO 3 And the bandgap energy of the BiOBr is about 2, respectively.63 and 2.67 eV.
Example III, effect of different systems on CIP degradation
1. Experiment and analysis method
100mL of 20mg/L CIP solution is taken and placed in a colorimetric tube, 0.05g of the catalyst prepared in the first embodiment is added, and the mixture is placed in a photocatalytic reactor for carrying out dark reaction for 40min and then carrying out photoreaction for 120min. At each set time point, 5mL of the solution was sampled with a 5mL syringe, and the supernatant was obtained by filtration through a 0.22 μm aqueous filter, and the residual concentration of CIP was measured with an ultraviolet spectrophotometer.
2. Effect of different systems on CIP degradation
The effect of the different catalysts prepared in example one on degradation of CIP under dark and visible light irradiation is shown in figure 8. In the absence of catalyst, the CIP concentration hardly changed after 120min of visible light irradiation, which indicates that CIP is stable under visible light and the self-decomposition effect is negligible. The activities of the prepared catalyst for degrading CIP through photocatalysis are arranged as follows in order of magnitude: feSA/WB-0.5>WB-0.5>WO 3 Wherein FeSA/WB-0.5 with Fe loading of 5 and 8.8 wt% has CIP degradation rate of more than 98.5% after 120min illumination, and WO 3 The degradation rate of CIP is only 25.8% under the same conditions. The reason for this large difference in catalytic activity is mainly WO 3 The photo-generated carriers of (2) are easy to be combined, however, feSA/WB-0.5 has greatly reduced combination rate of photo-generated carriers due to the heterojunction and existence of single-atom Fe, which is similar to WO in FIG. 11 3 The experimental results of the highest fluorescence intensity and the lowest FeSA/WB-0.5 fluorescence intensity are consistent.
3. Influence of different factors on CIP degradation effect
The effect of different factors on CIP degradation effects is shown in FIG. 9. The effect of the initial pH on the degradation rate of CIP is shown in (a) of FIG. 9, the degradation effect of the catalytic system on CIP is continuously enhanced along with the gradual increase of the pH, and the degradation effect is best when the pH is 9, and the degradation effect reaches more than 99.5% in 120min.
The effect of catalyst dosage on CIP degradation rate is shown in FIG. 9 (b), and when the catalyst dosage is increased from 0.1g/L to 1.0g/L, the CIP removal rate is increased from 42.4% to 100%. When the catalyst dosage is low, enough reactive sites cannot be provided for CIP degradation; with the increase of the catalyst, the active sites are more and more, and the photocatalytic effect is gradually enhanced; when the active sites are enough, the catalyst is added again, and the CIP degradation rate is not improved any more.
The effect of the initial CIP concentration on CIP degradation rate is shown in FIG. 9 (c). The lower the CIP initial concentration, the higher the photocatalytic degradation rate. When the initial concentration of CIP is within 20mg/L, the degradation rate of CIP can reach more than 98% after 120min of degradation; and when the initial concentration of CIP is within 10mg/L, the time for reaching the same degradation rate is not more than 1h.
The effect of light intensity on CIP degradation rate is shown in FIG. 9 (d), and the higher the light intensity, the more electron-hole pairs on the surface of the composite photocatalyst, and the higher the degradation efficiency. When the illumination intensity is 300W, the CIP degradation rate is less than 60 percent; when the illumination intensity is 400W, the CIP degradation rate can reach more than 80 percent; when the illumination intensity is 500W, the CIP degradation rate can reach more than 90 percent.
The effect of inorganic anions on CIP degradation rate is shown in FIGS. 9 (e) and (f), cl - And HCO 3 - With slight obstruction, probably due to Cl - Easy to combine with CIP, HCO 3 - Can be used as an OH scavenger; NO (NO) 3 - And SO 4 2- Is slightly more inhibitory, possibly due to NO 3 - Occupy the active site of the photocatalyst, SO 4 2- Inhibit adsorption of CIP and can quench h + And OH. The extent of influence follows the following sequence: NO (NO) 3 - ＞SO 4 2- ＞HCO 3 - ＞Cl - However, the CIP degradation rate is still higher than 85%. This result shows that these anions, while having some effect on CIP degradation, have little overall effect.
4. Mineralization rate and stability of photocatalytic materials
The mineralization degree of the photocatalytic material to CIP under the irradiation of visible light and the stability of the photocatalytic material to degrade CIP are shown in FIG. 10. As can be seen from FIGS. 10 (a) and (b), after 40min dark reaction and 120min light reaction, the mineralization rate of FeSA/WB-0.5 to CIP can reach 55.4%, indicating that more than 55.4% of CIP is completely converted into carbon dioxide and water. The results of 5 cycles of CIP degradation and XRD patterns before and after use are compared as shown in fig. 10 (c) and (d). After 5 times of cyclic degradation experiments, the degradation rate of FeSA/WB-0.5 to CIP is still kept at 88.8%; the XRD pattern did not change significantly before and after cycling. The result shows that FeSA/WB-0.5 has relatively high light stability and reusability.
5. Photocatalytic mechanism analysis
1) Photogenerated carrier separation efficiency analysis
FIG. 11 shows WO 3 Fluorescence spectra (PL) obtained at excitation wavelength of 325nm for BiOBr, WB-0.5 and FeSA/WB-0.5. The low PL intensity indicates a low probability of photo-generated electrons combining with holes, i.e., a high photo-generated carrier separation efficiency. From the figure, the PL intensity of WB-0.5 was significantly lower than that of WO 3 And BiOBr, feSA/WB-0.5, and PL intensity was lower than WB-0.5, indicating WO in composite WB-0.5 3 The combination with BiOBr greatly compensates the defect that the photo-generated carriers of a single component material are easy to be compounded, because of WO 3 The heterojunction structure favorable for separating photo-generated carriers is successfully constructed by being compounded with BiOBr; in the composite material FeSA/WB-0.5, the existence of Fe monoatoms can further enhance the separation of photo-generated carriers, because the Fe monoatoms can serve as both an electron acceptor and an electron donor, the Fe monoatoms can serve as an electron transfer medium, and the electron flow of the whole composite material system is accelerated, so that the separation of the photo-generated carriers is accelerated. Taken together with the above analysis, the PL test results further confirm that WO in FeSA/WB-0.5 composite material 3 Successful construction of the BiOBr heterojunction and electron transfer of monatomic Fe.
2) Analysis of photovoltaic Properties
In fig. 12, (a) and (b) are mott-schottky diagrams (M-S), and the type of the semiconductor photocatalyst can be determined from the M-S diagrams. The slope of the M-S curve for an n-type photocatalyst is positive and the slope of the M-S curve for a p-type photocatalyst is negative. It can be inferred that WO 3 Is an n-type photocatalyst and BiOBr is a p-type photocatalyst. As also shown in the interpolated panels, WO 3 Is negative (anodic current) under illumination, also described in WO 3 Is n-type photocatalysisAn agent; the photocurrent of the bitbr shows a positive current (cathode current), and similarly, the bitcr is a p-type photocatalyst. WO can also be obtained from the M-S diagram 3 The position of the conduction band and the BiOBr valence band. Flat band potential of semiconductor (E fb ) The potential at the intersection is a flat band potential by extrapolating the linear portion of the M-S plot to the potential x-axis. General E fb Is about equal to the conduction band electricity E of the semiconductor CB (for n-type semiconductors) or valence band potential E VB (for p-type semiconductors). As shown in fig. 12 (a) and (b), WO 3 And BiOBr E fb The values are about-0.57 and 1.53V, respectively, and thus WO 3 E of (2) CB And BiOBr E VB -0.57 and 1.53V, respectively. Reference electrode Hg/Hg 2 Cl 2 The potential of (SCE) and the potential transition between the Reversible Hydrogen Electrode (RHE) is given by the formula E (RHE) =e (SCE) +0.0591ph+0.24v (ph=7). Available, WO 3 E of (2) CB E of about 0.08V, biOBr VB About 2.18V (vs. RHE). E (E) CB And E is VB Calculated from the following formula: e (E) VB = E CB + E g Wherein E is g Is WO 3 And a band gap energy of BiOBr of 2.63 and 2.67 eV, respectively. Thus, we can calculate WO 3 E of (2) VB E of BiOBr at 2.71V CB is-0.49V (vs. RHE). WO (WO) 3 And BiOBr E g 、E CB And E is VB The values are detailed in FIG. 12 (c).
In fig. 12, (d) is a transient photocurrent response curve (i-t), and for ease of comparison, the current is equalized to a negative value. The current in the system is basically 0 under the dark condition, the current is increased suddenly at the moment of illumination, the current intensity is basically kept unchanged during illumination, the current is reduced in a shaking way at the moment of no illumination, and the current is repeated for a plurality of times, which indicates that the current is generated by a visible light excited light-emitting catalyst. The intensity of the photocurrent of FeSA/WB-0.5 is maximum, WB-0.5 times, biOBr is lower, and WO 3 The reason for the lowest photocurrent intensity is that the photo-generated carriers have different photoelectron concentrations under irradiation of visible light, and the photo-generated carriers have high separation efficiency due to the high photocurrent density of the photo-generated carriers, i.e., the large number of photo-generated electrons and holes.
In FIG. 12 (e) is an ACThe smaller the radius of the graph, the smaller the charge transfer resistance, the faster the separation and migration speed of photogenerated electrons and holes, and the smallest the arc radius of the FeSA/WB-0.5 AC impedance graph, indicating the fastest rate of interfacial charge transfer. The above photoelectric property analysis can explain the reason that FeSA/WB-0.5 has optimal photocatalytic activity, namely, the separation efficiency of photon-generated carriers is high, which is the same as that of n-type photocatalyst WO 3 Is closely related to the heterojunction structure constructed by the p-type photocatalyst BiOBr and the existence of single-atom iron.
3) Active species analysis
Free radical and non-free radical capture experiments were performed on the active species of FeSA/WB-0.5 during photocatalytic degradation CIP, and the active species were detected using an electron paramagnetic resonance spectrometer (EPR). In general, OH, O 2 - And h + Is the main active substance for photocatalytic degradation of organic pollutants. In addition, due to photo-generated electrons e - And cavity h + Often exist in pairs, and singlet oxygen 1 O 2 Can be obtained by reacting peroxy ions with various locally halogenated intermediates. Thus, in the experiment, p-benzoquinone (PBQ), t-butanol (TBA), ammonium Oxalate (AO), silver nitrate (AgNO) 3 ) And L-histidine (L-His) as O, respectively 2 - 、·OH、h + 、e - And 1 O 2 to identify active substances that play a role in CIP degradation reactions. As shown in fig. 13 (a) and (b), when PBQ is present in the CIP solution, significant inhibition of CIP degradation is observed; CIP degradation is also strongly inhibited in the presence of AO; presence of AgNO 3 And L-His, CIP degradation is inhibited to some extent; in the presence of TBA, CIP degradation is less subject to printing. This indicates that during degradation of CIP by FeSA/WB-0.5, O 2 - 、·OH、h + 、e - And 1 O 2 all are functioning, but the main function is O 2 - And h + . To further determine the presence of OH, O in the system 2 - 、 1 O 2 And h + Respectively under visible lightEPR test with 5, 5-dimethyl-1-pyrroline-N-oxide (DMPO), 2, 6-tetramethylpiperidine oxide (TEMPO) and 2, 6-Tetramethylpiperidine (TEMP) as capture agents. As shown in FIGS. 13 (c) - (f), no peak was observed in the dark state, but four characteristic OH peaks with a peak intensity ratio of 1:2:2:1 were observed under irradiation with visible light, respectively, and four peak intensities were two-four-high O 2 - Characteristic peaks, three peak intensity ratios of 1:1:1 equal height 1 O 2 Characteristic peak sum h of (2) + Characteristic peaks (g=2.005), these being DMPO-OH, DMPO-O, respectively 2 - 、TEMP- 1 O 2 And TEMPO-h + Typical spectra of the compounds show that the FeSA/WB-0.5 composite material generates OH and O under the irradiation of visible light 2 - 、 1 O 2 And h + Consistent with the results of the capture experiments. In summary, in the reaction of FeSA/WB-0.5 photocatalytic degradation of CIP, & OH, & O 2 - 、 1 O 2 And h + Are all active species, and O 2 - And h + Is the main active species.
4) Analysis of degradation mechanism
Based on the above results and analysis, a FeSA/WB-0.5 photocatalytic mechanism was proposed, as shown in FIG. 14. n-type photocatalyst WO 3 The fermi level of the p-type photocatalyst bisbr is close to the conduction band and the fermi level of the p-type photocatalyst bisbr is close to the valence band, and when the two are combined, the fermi levels tend to coincide (fig. 14 (a)). If the photogenerated carriers follow the conventional heterojunction transport mechanism (fig. 14 (b)), the photogenerated electrons on the bilbr conduction band will be transferred (with or without the Fe monoatoms) to WO 3 On the guide belt due to WO 3 Has a higher potential than the conduction band of BiOBr; and WO 3 The photogenerated holes on the valence band will flow to the valence band of BiOBr because the valence band potential of BiOBr is higher than that of WO 3 Is lower in (c). O (O) 2 /·O 2 - And OH (OH) - Standard oxidation/reduction potentials of/(vs. RHE, pH=7) of-0.33 and 2.29. 2.29V, respectively, WO 3 Electrons on the conductive tape cannot drive O 2 Reduction to O 2 - The holes on the BiOBr valence band cannot convert OH - Oxidation to ·OH due to WO 3 Potential of conduction band (0.05V) to O 2 /·O 2 - The standard oxidation/reduction potential (-0.33V) is high and the potential of the BiOBr valence band (2.18V) is higher than OH - The standard oxidation/reduction potential of/(OH) (2.29V) is low. Thus, the heterojunction mechanism per p-n will only have h + Plays a main role. However, the active species experiments and EPR experiments showed that O 2 - And h + All play a major role (fig. 13), so it is inferred that photogenerated carriers in FeSA/WB-0.5 migrate according to another new pathway. As shown in FIG. 14 (c), WO at a higher potential 3 The photo-generated electrons on the conduction band and the holes on the BiOBr valence band at lower potential are directly (or through Fe monoatoms at the junction) recombined at the junction, so that the photo-generated electrons with strong reducibility and the photo-generated holes with strong oxidizability are effectively separated and accumulated in the BiOBr conduction band at lower potential and the WO at higher potential respectively 3 On the price band. BiOBr conduction band potential of-0.49V, ratio of O 2 /·O 2 - More negative electrode potential (-0.33V), so that the photo-generated electrons on the BiOBr guide band can adsorb O on the catalyst surface 2 Reduction to O 2 - ；WO 3 Valence band potential (2.71V) to OH - The electrode potential of/(OH) (2.29, V) was higher, indicating WO 3 H on the price tape + Can oxidize OH - And generating OH. Due to WO 3 The valence band potential is high, so that the photo-generated holes on the valence band have strong oxidizing property, and besides a large amount of oxidizing free radicals are generated, the photo-generated holes play an important role in CIP degradation. In addition, monoatomic Fe gives WO 3 Is reduced to Fe 2+ ，Fe 2+ Is oxidized into Fe under the action of active oxygen 3+ Thereby realizing Fe 3+ ↔Fe 2+ The valence state circulation of the catalyst accelerates the flow of electrons and the reaction of the system. For in the system 1 O 2 Is attributable to the self-disproportionation of OH, H 2 O pair O 2 - Is a disproportionation induced by (a) O 2 - And various partially halogenated intermediates. Thus, it can be concluded that: simultaneous presence of O in the system 2 - 、·OH、h + And 1 O 2 this is in complete agreement with the results of the active species experiments; the photogenerated electrons and holes in FeSA/WB-0.5 are separated according to a Z-type mechanism, probably because of WO 3 There are numerous defects between the contact interface with the BiOBr that allow charge to readily pass through and act as charge recombination centers; in addition, the valence state of the iron at the active site of the catalyst metal is cycled (Fe 2+ ↔Fe 3+ ) The separation of photogenerated electrons and holes is also accelerated. From the analysis, the degradation mechanism of the system on organic pollutants comprises a Z-type heterojunction mechanism and a valence state circulation mechanism of Fe, and the degradation pathway simultaneously comprises a free radical pathway and a non-free radical pathway.

Claims (10)

1. Fe/WO 3 The preparation method of the BiOBr monoatomic catalyst is characterized by comprising the following steps:
1) Na is mixed with 2 WO 4 ·2H 2 O and NaHSO 4 ·H 2 O is dissolved in ultrapure water and transferred into a reaction kettle for hydrothermal reaction; cooling to room temperature, centrifuging to collect precipitate, washing with ultrapure water and absolute ethanol, and drying to obtain solid WO 3 A bundle of nanotubes;
2) Dissolving 1-hexadecyl-3-methyl imidazole bromide in ethylene glycol methyl ether, adding WO 3 A nano tube bundle is subjected to ultrasonic dispersion to form a suspension A; bi (NO) 3 ) 3 •5H 2 O is thoroughly dissolved in ethylene glycol methyl ether to obtain a solution B; mixing the suspension A and the solution B, stirring uniformly, and transferring into a reaction kettle for reaction; after cooling to room temperature, centrifugally collecting the generated precipitate, washing with ultrapure water and absolute ethyl alcohol, and drying to obtain a solid, namely the tungsten trioxide nano tube/bismuth oxybromide nano sheet composite catalyst, namely the WB composite catalyst;
3) Ultrasonically dispersing WB-0.5 in methanol to obtain suspension C; fe (NO) 3 ) 3 ·9H 2 O is dissolved in ultrapure water to obtain solution D; mixing the suspension C and the solution D, introducing nitrogen to remove oxygen after ultrasonic dispersion, and irradiating under a xenon lamp; centrifuging to collect solid, washing with ultrapure water and absolute ethanol, and drying to obtain solidFeSA/WB-0.5 of iron monoatom/tungsten trioxide nano-tube/bismuth oxybromide nano-sheet composite catalyst.
2. The process according to claim 1, wherein in step 1),
Na 2 WO 4 ·2H 2 o and NaHSO 4 ·H 2 The molar ratio of O is 1:2-4, preferably 1:3;
the reaction kettle is a polytetrafluoroethylene lining stainless steel reaction kettle;
the hydrothermal reaction is carried out at 140-220 ℃, preferably 160-200 ℃, more preferably 175-185 ℃ and most preferably 180 ℃ for 18-30 hours, preferably 20-28 hours, more preferably 22-26 hours and most preferably 24 hours;
the drying is carried out in a vacuum drying oven at 50-70deg.C, preferably 55-65deg.C, most preferably 60deg.C for 8-16 hr, preferably 10-14 hr, most preferably 12 hr.
3. The process according to claim 1, wherein in step 2),
the concentration of the 1-hexadecyl-3-methyl imidazole bromide dissolved in ethylene glycol methyl ether is 0.02-0.06mol/L, preferably 0.04 mol/L, and the 1-hexadecyl-3-methyl imidazole bromide and WO 3 The molar ratio of the dosage of the nano tube bundles is 1-3:1, preferably 2:1;
Bi(NO 3 ) 3 •5H 2 the concentration of O dissolved in glycol methyl ether is 0.05-0.15mol/L, preferably 0.1 mol/L, and the O is thoroughly dissolved by magnetic stirring;
the mixing ratio of suspension A and solution B is such that WO 3 Nanotube bundles and Bi (NO) 3 ) 3 •5H 2 The molar ratio of O is 1:1-3, preferably 1:1.5-2.5, more preferably 1:2;
the reaction in the reaction kettle is that a polytetrafluoroethylene lining stainless steel water heating kettle is put into an electrothermal blowing drying oven after sealing tightly, the reaction temperature is set to be 120-200 ℃, preferably 140-180 ℃, preferably 155-165 ℃, most preferably 160 ℃, and the reaction time is 0.5-3.5h, preferably 1-3h, more preferably 1.5-2.5h;
the drying is carried out in a vacuum drying oven at 50-70deg.C, preferably 55-65deg.C, most preferably 60deg.C for 8-16 hr, preferably 10-14 hr, most preferably 12 hr.
4. The process according to claim 1, wherein in step 3),
the WB complex catalyst is used in the suspension C in an amount of 0.05 to 0.15mol/L, preferably 0.0875 mol/L;
Fe(NO 3 ) 3 ·9H 2 the molar ratio of O to WB-0.5 is 1:3-4, preferably 1:3.78;
the irradiation under a xenon lamp is carried out at an illumination intensity of 400 to 600W, preferably 450 to 550W, most preferably 500W;
the drying is carried out in a vacuum drying oven at 50-70deg.C, preferably 55-65deg.C, most preferably 60deg.C for 8-16 hr, preferably 10-14 hr, most preferably 12 hr.
5. The Fe/WO obtained by the process of claim 1 3 The mass percent of the BiOBr monoatomic catalyst is preferably 3-15-wt%, preferably 5-8% by weight.
6. The Fe/WO according to claim 5 3 Use of a BiOBr monoatomic catalyst for degrading organic contaminants, preferably for degrading CIP.
7. A process for degrading CIP comprising adding the Fe/WO of claim 5 3 Adding a BiOBr monoatomic catalyst into the CIP-containing solution to be treated, and carrying out dark reaction and light reaction;
preferably, at the beginning of the reaction, ph=5-12, preferably ph=8-10, more preferably ph=9;
the initial concentration of CIP is adjusted to not more than 30mg/L, preferably not more than 20mg/L, more preferably not more than 10mg/L, such as 5-10mg/L.
8. The method for degrading CIP according to claim 7, wherein the monoatomic catalyst is added in an amount of 0.4-1.2g/L, preferably 0.6-1.0g/L, more preferably 0.8 g/L; the illumination intensity during the light reaction is 300-800W, preferably 400-600W, more preferably 500-W.
9. The method of degrading CIP according to claim 7, wherein the reaction is performed for 20-60 min (preferably 35-45 min) followed by a photoreaction for 80-160min, preferably 100-140min, more preferably 110-130min, more preferably 120min.
10. The method of degrading CIP according to claim 7, wherein the darkening reaction is carried out for 35-45min, followed by the photoreaction for 100-140min, more preferably 110-130min, more preferably 120min.
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