Preparation method and application of a lignin-degradable heterojunction photocatalyst

Abstract

The application relates to a preparation method and application of a degradable lignin heterojunction photocatalyst, relates to the field of photocatalysts, and aims to solve the problem of low photocatalytic reaction efficiency of existing metal sulfide catalysts. Method: I. disperse alkali lignin into deionized water, add ethylenediamine and magnesium hydroxide in sequence to obtain a mixed solution A; cool to room temperature after reaction, perform filtration and dialysis, and dry to obtain Mg, N-CQDs; II. mix zinc acetate dihydrate, indium chloride trihydrate, Mg, N-CQDs and deionized water, add thioacetamide, and perform condensation reflux reaction; centrifuge the reaction solution to obtain a product precursor, perform washing, centrifugation and drying, and calcine to obtain a heterojunction photocatalyst. The application narrows the band gap of the composite photocatalyst, makes the material better respond to visible light, and thus improves the photoelectron reduction capacity, electron-hole pair separation efficiency and electron transfer capacity. The application is used for preparing a photocatalyst and photocatalytically degrading lignin.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of photocatalysts, in particular to a preparation method and application of a lignin-degradable heterojunction photocatalyst. BACKGROUND

[0002] Photocatalysis is a green technology that can use light energy to mineralize water pollutants into CO2 and H2O, and has the advantages of low energy consumption, simple reaction conditions, no waste and harmful by-products, and is therefore considered to be a promising water treatment technology.

[0003] Although photocatalysis technology has been applied in the fields of water splitting for hydrogen production, reduction of carbon dioxide and degradation of organic pollutants, single photocatalysts generally have the problems of narrow light response range, high electron and hole recombination rate, and low electron transfer rate, which greatly limits the application of catalytic technology.

[0004] Metal sulfide catalysts (CdS, ZnS, In2S3) have a small band gap and can be excited by visible light in sunlight, and the light response region even extends to the near-infrared region. Zn4In2S7 is a classic n-type semiconductor with visible light response, but the inherent shortcomings of n-type semiconductors result in a large number of photo-generated electrons accumulating in the conduction band of the catalyst, reducing the efficiency of photocatalytic reaction. SUMMARY

[0005] The present application aims to solve the problem of low photocatalytic reaction efficiency of existing metal sulfide catalysts, and provides a preparation method and application of a lignin-degradable heterojunction photocatalyst.

[0006] The preparation method of the lignin-degradable heterojunction photocatalyst of the present application comprises the following steps:

[0007] I. Preparation of Mg, N-CQDs from alkali lignin

[0008] The alkali lignin is dispersed in deionized water and subjected to ultrasonic treatment, and then continuous stirring is carried out, during which ethylenediamine and magnesium hydroxide are added in sequence to obtain a mixed solution A; then the mixed solution A is placed in a reaction kettle and heated at 190-200℃ for 12-14h, and after the reaction is completed, it is cooled to room temperature to obtain a suspension; the yellow-brown filtrate is obtained by suction filtration; the yellow-brown filtrate is subjected to dialysis and dried to obtain Mg, N-CQDs;

[0009] II. Preparation of Zn4In2S7 / Mg, N-CQDs photocatalyst

[0010] Firstly, zinc acetate dihydrate, indium chloride tetrahydrate, Mg, N-CQDs and deionized water are mixed to obtain a mixed solution B; the mixed solution B is stirred, then thioacetamide is added into the mixed solution B and continues to be stirred, and then the condensation reflux reaction is carried out at a temperature of 100-110 DEG C for 4-5 hours; after the reaction is completed, the reaction solution is cooled to room temperature, the reaction solution is centrifuged to obtain a product precursor, the product precursor is washed and centrifuged with deionized water and anhydrous ethanol respectively to obtain a yellow / yellow-black solid, the obtained yellow / yellow-black solid is dried, and then calcination is carried out under a nitrogen atmosphere to obtain a heterojunction photocatalyst.

[0011] Further, the mass ratio of the alkali lignin to deionized water in step one is (0.5-0.6):66.

[0012] Further, the mass ratio of the alkali lignin to ethylenediamine in step one is (0.5-0.6) g:0.33 mL.

[0013] Further, the mass ratio of the alkali lignin to magnesium hydroxide in step one is (0.5-0.6):0.22.

[0014] Further, the stirring time in step one is 30-40 min.

[0015] Further, the mass ratio of the zinc acetate dihydrate to indium chloride tetrahydrate in step two is (0.61-0.71):0.44.

[0016] Further, the mass ratio of the zinc acetate dihydrate to thioacetamide in step two is (0.61-0.71):0.39.

[0017] Further, the mass ratio of the zinc acetate dihydrate to deionized water in step two is (0.61-0.71):75.

[0018] Further, the mass ratio of the zinc acetate dihydrate to Mg, N-CQDs in step two is (0.61-0.71) g:(1.61-6.44) mg.

[0019] Further, the stirring time in step two is 30-40 min.

[0020] Further, the calcination condition in step two is that the temperature is 350-370 DEG C and the time is 2-2.5 hours.

[0021] The application of the heterojunction photocatalyst in the degradation of lignin.

[0022] The application has the following beneficial effects:

[0023] The method of the application prepares Mg, N-CQDs spherical particles of nanometer level through a simple hydrothermal reaction, and then the Zn4In2S7 is compounded with the Mg, N-CQDs through a condensation reflux method, and further calcination increases the interface bonding force of the catalyst, and the Zn4In2S7 / Mg, N-CQDs photocatalyst is prepared.

[0024] The Zn4In2S7 / Mg, N-CQDs constructed by the heterojunction strategy of the application reduces the band gap of the composite photocatalyst to some extent, so that the material responds better to visible light, and the conduction band potential of the composite photocatalyst can also be slightly reduced, so that the reduction capacity of photo-generated electrons, the separation efficiency of electron-hole pairs and the electron transfer capacity are improved. The surface of the CQDs is rich in functional groups such as carboxyl groups, which can be doped with elements to improve the electron-hole migration rate of the catalyst. The Mg and N doping can further prevent the recombination of photo-generated electrons and holes of the photocatalyst. The ZIS-6 350℃ photocatalyst has the optimal photocatalytic performance.

[0025] The application provides a new idea for improving the performance of metal sulfide photocatalysts and recycling lignin. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 The XRD spectrum of Mg, N-CQDs is shown in the figure;

[0027] Figure 2 The XRD spectrum of Zn4In2S7 / Mg, N-CQDs is shown in the figure;

[0028] Figure 3 The FT-IR spectrum of Mg, N-CQDs is shown in the figure;

[0029] Figure 4 The FT-IR spectrum of Zn4In2S7 / Mg, N-CQDs is shown in the figure;

[0030] Figure 5 The TEM image of Mg, N-CQDs is shown in the figure;

[0031] Figure 6 The size distribution histogram of Mg, N-CQDs is shown in the figure;

[0032] Figure 7 The HRTEM image of Mg, N-CQDs is shown in the figure;

[0033] Figure 8 The SEM image of the photocatalyst ZIS is shown in the figure;

[0034] Figure 9 SEM image of photocatalyst ZIS-3 at 350 °C;

[0035] Figure 10 SEM image of photocatalyst ZIS-3 at 350 °C;

[0036] Figure 11 SEM image of photocatalyst ZIS-6 at 350 °C;

[0037] Figure 12 SEM image of photocatalyst ZIS-12 at 350 °C;

[0038] Figure 13 TEM image of photocatalyst ZIS-6 at 350 °C;

[0039] Figure 14 XPS survey spectrum of photocatalyst ZIS-6 at 350 °C;

[0040] Figure 15 Zn 2p fine spectrum of photocatalyst ZIS-6 at 350 °C;

[0041] Figure 16 In 3d fine spectrum of photocatalyst ZIS-6 at 350 °C;

[0042] Figure 17 S2p fine spectrum of photocatalyst ZIS-6 at 350 °C;

[0043] Figure 18 C1s fine spectrum of photocatalyst ZIS-6 at 350 °C;

[0044] Figure 19 O1s fine spectrum of photocatalyst ZIS-6 at 350 °C;

[0045] Figure 20 N1s fine spectrum of photocatalyst ZIS-6 at 350 °C;

[0046] Figure 21 Mg 2p fine spectrum of photocatalyst ZIS-6 at 350 °C;

[0047] Figure 22 N2 adsorption-desorption isotherm of photocatalyst;

[0048] Figure 23 Pore size distribution of photocatalyst;

[0049] Figure 24 Decolorization curve of photocatalytic depolymerization of sodium lignosulfonate;

[0050] Figure 25The photocatalyst is a quasi-first-order kinetics fitting curve;

[0051] Figure 26 The photocatalyst ZIS-6 350℃ is a UV-visible spectrum of depolymerizing sodium lignosulfonate in the 190-500nm wave band;

[0052] Figure 27 The photocatalyst is a UV-visible diffuse reflectance spectrum;

[0053] Figure 28 The photocatalyst is (αhv) 2 -hv;

[0054] Figure 29 The photocatalyst ZIS-6 350℃ is a Mott-Schottky curve;

[0055] Figure 30 The photocatalyst ZIS 350℃ is a Mott-Schottky curve;

[0056] Figure 31 The photocatalyst is a transient photocurrent;

[0057] Figure 32 The photocatalyst is an electrochemical impedance diagram. DETAILED DESCRIPTION

[0058] The technical solution of the present application is not limited to the following specific embodiments, and also includes any combination of the specific embodiments.

[0059] Specific embodiment one: the preparation method of the heterojunction photocatalyst for degrading lignin, comprising the following steps:

[0060] I. Alkaline lignin preparation Mg, N-CQDs

[0061] The alkaline lignin is dispersed into deionized water, ultrasonic treatment is carried out, and then continuous stirring is carried out. Ethylenediamine and magnesium hydroxide are sequentially added during the stirring process to obtain a mixed solution A. Then, the mixed solution A is placed in a reaction kettle and heated at 190-200℃ for 12-14h. After the reaction is completed, it is cooled to room temperature to obtain a suspension. Yellow-brown filtrate is obtained by suction filtration. The yellow-brown filtrate is dialyzed and dried to obtain Mg, N-CQDs.

[0062] II. Preparation of Zn4In2S7 / Mg, N-CQDs photocatalyst

[0063] First, zinc acetate dihydrate, indium chloride tetrahydrate, Mg, N-CQDs and deionized water are mixed to obtain a mixed solution B; the mixed solution B is stirred, then thioacetamide is added to the mixed solution B and continues to be stirred, and then a condensation reflux reaction is carried out at a temperature of 100-110°C for 4-5h; after the reaction is completed, the reaction solution is cooled to room temperature, the reaction solution is centrifuged to obtain a product precursor, the product precursor is washed with deionized water and anhydrous ethanol respectively, centrifuged to obtain a yellow / yellow-black solid, the obtained yellow / yellow-black solid is dried, and then calcined under a nitrogen atmosphere to obtain a photocatalyst.

[0064] Specific embodiment two: different from the specific embodiment one, the mass ratio of alkali lignin to deionized water in step one is (0.5-0.6):66. The others are the same as the specific embodiment one.

[0065] Specific embodiment three: different from the specific embodiment one or two, the mass ratio of alkali lignin to ethylenediamine in step one is (0.5-0.6)g:0.33mL. The others are the same as the specific embodiment one or two.

[0066] Specific embodiment four: different from one of the specific embodiments one to three, the mass ratio of alkali lignin to magnesium hydroxide in step one is (0.5-0.6):0.22. The others are the same as one of the specific embodiments one to three.

[0067] Specific embodiment five: different from one of the specific embodiments one to four, the stirring time in step one is 30-40min. The others are the same as one of the specific embodiments one to three.

[0068] Specific embodiment six: different from one of the specific embodiments one to five, the mass ratio of zinc acetate dihydrate to indium chloride tetrahydrate in step two is (0.61-0.71):0.44. The others are the same as one of the specific embodiments one to five.

[0069] Specific embodiment seven: different from one of the specific embodiments one to six, the mass ratio of zinc acetate dihydrate to thioacetamide in step two is (0.61-0.71):0.39. The others are the same as one of the specific embodiments one to six.

[0070] Specific embodiment eight: different from one of the specific embodiments one to seven, the mass ratio of zinc acetate dihydrate to deionized water in step two is (0.61-0.71):75. The others are the same as one of the specific embodiments one to seven.

[0071] Specific implementation nine: the difference between this implementation and one of the specific implementations one to eight is that: in step two, the mass ratio of zinc acetate dihydrate to Mg, N-CQDs is (0.61-0.71) g:(1.61-6.44) mg. The others are the same as one of the specific implementations one to eight.

[0072] Specific implementation ten: the difference between this implementation and one of the specific implementations one to nine is that: in step two, the stirring time is 30-40 min each time. The others are the same as one of the specific implementations one to nine.

[0073] Specific implementation eleven: the difference between this implementation and one of the specific implementations one to ten is that: in step two, the calcination conditions are: temperature 350-370℃, time 2-2.5h. The others are the same as one of the specific implementations one to ten.

[0074] The following detailed description of the embodiments of the present application is based on the premise that the technical solutions of the present application are implemented, and detailed implementation schemes and specific operation processes are given, but the protection scope of the present application is not limited to the following examples.

[0075] Example 1:

[0076] The preparation method of the Zn4In2S7 / Mg, N-CQDs heterojunction catalyst of this example includes the following steps:

[0077] I. Preparation of Mg, N-CQDs from alkali lignin

[0078] 0.55g of alkali lignin was dispersed in 66mL of deionized water, then ultrasonic cleaning was carried out under the power of 250W for 30min, then continuous stirring was carried out at the magnetic stirring speed of 500rpm for 30min, and 0.33mL of ethylenediamine and 0.22g of magnesium hydroxide were added in sequence during the stirring process to obtain a mixed solution; then the mixed solution was placed in a 100mL polytetrafluoroethylene reaction liner, and the reaction kettle was heated at 190℃ for 12h; after the reaction was completed, it was cooled to room temperature to obtain a suspension; the suspension was filtered with a 0.45μm water-based filter membrane to obtain a yellow-brown filtrate; the obtained yellow-brown filtrate was transferred to a 500Da dialysis bag, and dialysis was carried out with deionized water for 72h, and then the dialyzed filtrate was placed in a freeze dryer to dry, to obtain Mg, N-CQDs.

[0079] II. Preparation of Zn4In2S7 / Mg, N-CQDs photocatalyst

[0080] First, 0.66 g of zinc acetate dihydrate, 0.4395 g of indium trichloride and Mg, N-CQDs were added into a round bottom flask containing 75 mL of deionized water in sequence to obtain a mixed solution. The amount of Mg, N-CQDs added in each experiment was 0 mg, 1.609 mg, 3.218 mg and 6.437 mg, respectively. The mixed solution was stirred for 30 min, then 0.3945 g of thioacetamide was added into the mixed solution and stirred for another 30 min. The round bottom flask was then placed in a temperature of 100 °C for 4 h of condensation reflux reaction. After the reaction was completed, the reaction solution was cooled to room temperature. The reaction solution was centrifuged to obtain a product precursor. The product precursor was washed and centrifuged with deionized water and anhydrous ethanol three times to obtain a yellow / yellow-black solid. The yellow / yellow-black solid was placed in a vacuum dryer at 75 °C for 24 h, and then calcined at 350 °C in a nitrogen atmosphere for 2 h to obtain a photocatalyst. The photocatalyst was labeled as ZIS-x 350 °C. ZIS 350 °C, ZIS-3 350 °C, ZIS-6 350 °C and ZIS-12 350 °C represent the amount of Mg, N-CQDs added as 0 mg, 1.609 mg, 3.218 mg and 6.437 mg, respectively. ZIS represents pure Zn4In2S7, and ZIS 350 °C represents Zn4In2S7 calcined at 350 °C (nitrogen atmosphere).

[0081] The photocatalysts prepared in the above examples were subjected to the following experiments:

[0082] (1) Crystal form test and analysis

[0083] The materials were tested and analyzed by X-ray powder diffractometer (XRD). The XRD pattern of Mg, N-CQDs is shown in Figure 1 It was observed that there was a broad diffraction peak of 17°-24°, which was attributed to amorphous carbon, indicating that there was disordered carbon in Mg, N-CQDs. Figure 2For the XRD pattern of Zn4In2S7 / Mg,N-CQDs composite, the characteristic diffraction peaks of the synthesized Zn4In2S7 series materials are consistent with the standard card (JCPDS-72-0733) of hexagonal Zn4In2S7, in which the characteristic signal peaks at 21.7°, 28.1°, 47.3°, 52.8° and 56.8° correspond to (006), (102), (110), (116) and (204) crystal faces, respectively. The half-peak width of the diffraction peak at 21.7° expands with the increase of the amount of Mg,N-CQDs, indicating that there is a close contact between Mg,N-CQDs and Zn4In2S7. In addition, the composite of Mg,N-CQDs does not cause obvious changes in the diffraction peaks of the catalyst, indicating that the addition of Mg,N-CQDs does not change the structure of Zn4In2S7. In addition, there is no characteristic diffraction peak of Mg,N-CQDs in the XRD pattern, because the loading amount of Mg,N-CQDs is not much, and it exists in amorphous structure.

[0084] (2) Group analysis

[0085] The FT-IR spectrum of Mg,N-CQDs is shown in Figure 3 , in which the peak at 1517 cm -1 is attributed to the stretching vibration of aromatic functional group C-C group, and the absorption peak at 1599 cm -1 is the stretching vibration of C=C bond of aromatic skeleton compound, indicating that Mg,N-CQDs retains the aromatic functional group skeleton of alkali lignin. The absorption peak at 1124 cm -1 is attributed to the stretching vibration of C-O-C bond of alkali lignin (AL), and this absorption peak basically disappears in Mg,N-CQDs, indicating that the hydrothermal reaction of alkali lignin is very sufficient. The infrared absorption peak at 1382 cm -1 is attributed to the stretching vibration of C-N group, indicating that there is a nitrogen group functional group in Mg,N-CQDs. The infrared absorption peaks at 574 cm -1 and 3741 cm -1 are respectively attributed to the metal ligand bending vibration and lattice vibration of magnesium 2u , confirming that magnesium is successfully doped into Mg,N-CQDs. In the infrared spectrum of Mg,N-CQDs, the peak related to the stretching vibration of -OH group at 3400 cm -1 is weakened, indicating that a large amount of phenolic hydroxyl reaction occurs between alkali lignin and Mg(OH)2 and EDA during the hydrothermal reaction. The FT-IR spectrum of Zn4In2S7 / Mg,N-CQDs composite is shown in Figure 4As shown, it can be seen that the infrared absorption peaks of the Zn4In2S7 / Mg,N-CQDs composite material are basically similar to those of Zn4In2S7, and the peak related to the -OH group (3400 cm -1 ) is weakened after calcination, but no obvious infrared absorption peak of Mg,N-CQDs in the composite material is observed, which is most likely due to the too low doping amount of Mg,N-CQDs.

[0086] (3) Surface morphology analysis

[0087] Figure 5 、 Figure 6 and Figure 7 are TEM, size distribution histogram and HRTEM images of Mg,N-CQDs, respectively. From Figure 5 and Figure 6 , the spherical morphology of Mg,N-CQDs nanoparticles can be clearly observed, and the average diameter is 5.53 nm ± 1.72 nm, indicating that Mg,N-CQDs (≤10 nm) have been successfully synthesized. Figure 7 The high-resolution TEM (HRTEM) image of

[0088] Figures 8 to 13 are SEM images of ZIS, ZIS 350℃, ZIS-3 350℃, ZIS-6 350℃ and ZIS-12 350℃. From Figure 8 and Figure 9 , it can be seen that pure ZIS is composed of a large number of nanoparticles, and calcination at 350℃ cannot significantly change the morphology of ZIS. With the increase of the loading content of Mg,N-CQDs, the aggregation state of ZIS gradually collapses and finally forms a layered structure stacked by many nanoparticles but more dispersed. Figure 13 is an HRTEM image of ZIS-6 350℃, and the crystal structure with a lattice spacing of 0.218 nm corresponding to the (006) crystal plane of ZIS can be seen, and amorphous Mg,N-CQDs can be seen at the edge of the material, confirming that Mg,N-CQDs are loaded on ZIS.

[0089] (4) XPS analysis

[0090] Figures 14 to 21 is the XPS test result of the photocatalyst ZIS-6 350℃. Figure 14 It is shown that the photocatalyst ZIS-6 350℃ is composed of Zn, In, S, C, O, N and Mg elements. Figure 15indicates that the Zn 2p signal peak can be fitted into Zn 2p at 1044.75 eV 1 / 2 and Zn 2p at 1021.75 eV 3 / 2 peak. Figure 16 indicates that the In 3d peak can be decomposed into In 3d at 452.81 eV 3 / 2 and In 3d at 445.28 eV 5 / 2 peak. In addition, Figure 17 the S2p peak in can also be decomposed into S2p peak at 163.09 eV 1 / 2 and S2p peak at 161.99 eV 3 / 2 The three peaks of C 1s can also be assigned Figure 18 ). Among them, the signal peak at 284.8 eV is attributed to the amorphous carbon on the surface of the photocatalyst contaminated by the external environment and the C-C and C=C functional group signals existing in Mg, N-CQDs itself; the peak at 286.30 eV is attributed to the C-O and C-N functional group signals in Mg, N-CQDs; the peak at 288.73 eV is attributed to the C=O functional group signals in Mg, N-CQDs. Figure 19 The O1s peak in can also be decomposed into two peaks, where the peak at 531.49 eV is attributed to the C=O functional group signals in Mg, N-CQDs; the peak at 532.83 eV is attributed to the C-O functional group signals in Mg, N-CQDs, which proves that Mg, N-CQDs have been compounded to ZIS. Next is the element of doped Mg, N-CQDs. Figure 20 The N1s peak in can also be decomposed into three peaks, where the peaks at 400.40 eV, 401.47 eV and 402.58 eV are respectively attributed to the N-H, C-N-C and N-(C)3 functional group signals in Mg, N-CQDs. Figure 21 The Mg 2P peak in can also be decomposed into three peaks, where the peaks at 49.57 eV and 50.65 eV are respectively attributed to the Mg-O and Mg-N functional group signals in Mg, N-CQDs, which confirms that magnesium and nitrogen elements have been doped into Mg, N-CQDs. The above characterization analysis proves that the ZIS-6 350°C photocatalyst is composed of ZIS and Mg, N-CQDs.

[0091] (5) Analysis of pore structure and specific surface area of composite material

[0092] Figure 22 indicates that the specific surface area of the photocatalysts ZIS, ZIS 350°C and ZIS-6 350°C estimated by BET method is 153.8, 139.6 and 164.2 m 2g. It can be seen that the specific surface area of ZIS 350℃ is smaller than that of ZIS, which is due to the partial agglomeration of the catalyst caused by calcination, and the specific surface area of the photocatalyst ZIS-6 350℃ is the largest, and the pore size is mostly mesoporous (0.6-2 nm) Figure 23 ), which is conducive to the adsorption of the photocatalyst to the sodium lignosulfonate substrate, thereby enhancing its photocatalytic depolymerization activity. At the same time, considering that the nanometer-sized pore size helps to reduce the transmission distance of electrons in the catalyst, improve the efficiency of electron transmission, which is beneficial to promote the ZIS-6 350℃ photo-generated charge separation and reduce electron-hole recombination, and the surface effect and photon resonance effect accompanying the nanometer size can lead to more efficient absorption of light, thereby improving the photocatalytic performance of ZIS-6 350℃.

[0093] (6) Catalytic activity test

[0094] Figure 24 The decolorization curves of all photocatalysts for depolymerizing sodium lignosulfonate are shown in the figure. It shows that the decolorization ability of the photocatalyst to sodium lignosulfonate presents a trend of first increasing and then decreasing with the loading amount of Mg,N-CQDs, which may be due to the fact that Mg,N-CQDs can rapidly separate the photo-generated electrons at the Zn4In2S7 conduction band, thereby promoting the separation of the photo-generated electron-hole pairs. Among them, when the composite amount of Mg,N-CQDs is 0.6wt%, the composite photocatalyst ZIS-6 350℃ has the strongest decolorization ability to sodium lignosulfonate. When the composite amount of Mg,N-CQDs gradually increases to 1.2wt%, the response ability of ZIS-12350℃ catalyst to simulated sunlight is improved, but it is not conducive to the excitation of photo-generated electrons on the Zn4In2S7 valence band, and the separation of photo-generated charges is not as good as ZIS-6 350℃, so the photocatalytic performance is reduced. Under the condition of only light, the analysis of the ultraviolet diffuse reflectance spectrum of sodium lignosulfonate shows that pure light can only cause slight decolorization of lignin (the decolorization rate is only 5%), which may be due to the decolorization of the quinone functional group of lignin. Through the excitation of simulated sunlight, the decolorization ability of sodium lignosulfonate by the photocatalyst ZIS-6 350℃ after 3h of catalysis is 64.4%, while the decolorization ability of sodium lignosulfonate by pure ZIS under the same conditions is 43.2%, which shows that Mg,N-CQDs can indeed improve the photocatalytic performance of ZIS 350℃.

[0095] In addition, the kinetic process of decolorizing sodium lignosulfonate by all photocatalysts is also studied and analyzed in this experiment. As shown in Figure 25 , the fitted kinetic process of all photocatalysts conforms to the pseudo-first-order kinetic fitting curve. However, for the ZIS-12 350℃ photocatalyst, the linear correlation coefficient R 20.8728 (Table 1), which can be due to the fact that the concentration of sodium lignosulfonate is constantly changing during the photocatalytic reaction, and the concentration of free radicals is also changing at all times.

[0096] Figure 26 is the 190-500 nm band decolorization curve of ZIS-6 350℃ photocatalyst for sodium lignosulfonate. From the graph, it can be seen that the ultraviolet absorption peak at 275 nm decreases first and then remains unchanged with the extension of reaction time. Referring to the previous literature, the absorption peak at 275 nm is caused by the n→π* electron transition of the etherified hydroxyl and hydroxyl functional groups in sodium lignosulfonate. The absorption peak at 197 nm is caused by the π→π* electron transition of sodium lignosulfonate. Therefore, the decrease of the absorption peak at 275 nm indicates that the etherified hydroxyl and hydroxyl functional groups in sodium lignosulfonate are destroyed. The absorption peak at 197 nm first decreases and then remains stable, which indicates that the aromatic functional groups in sodium lignosulfonate are not decomposed. These results show that ZIS-6 350℃ does not completely mineralize sodium lignosulfonate into CO2 and H2O, but breaks the connecting bonds in sodium lignosulfonate, retaining the basic structure of the aromatic ring.

[0097] Table 1 Correlation parameters of pseudo-first-order kinetics

[0098]

[0099] (7) Optical properties

[0100] In order to obtain the optical absorption properties of the synthesized catalyst, the optical absorption properties of the synthesized catalyst were analyzed by ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS) with an integrating sphere, and the results are shown in Figure 27 . It shows that all the photocatalysts have similar response curves to light, and have visible and ultraviolet light absorption ability.

[0101] Figure 28 is the K-M equation calculation and conversion of the ultraviolet-visible diffuse reflectance absorption spectrum of the catalyst (αhv) 2 -hv curve. It shows that the band gap of ZIS-6 350℃ and ZIS 350℃ is 2.39 eV and 2.59 eV, respectively, indicating that ZIS-6 350℃ has better optical absorption ability.

[0102] (8) Photoelectric properties

[0103] Figure 29 and Figure 30Mott-Schottky curves of ZIS-6 350℃ and ZIS 350℃. The slope of the curves is positive, which indicates that the catalysts are n-type semiconductors. It can be seen from the figure that with the increase of Mg, N-CQDs loading, the flat band potential of the photocatalyst moves to a more negative direction, indicating that Mg, N-CQDs reduces the conduction band potential of the photocatalyst, thus can produce photoelectrons with stronger reducing ability. Ag / AgCl was used as the reference electrode in the experiment, and the conduction band and valence band potential values of ZIS-6 350℃ and ZIS 350℃ photocatalysts can be calculated according to the formula E g = E VB -E CB

[0104] Table 2 Band gap energy level values of the synthesized samples

[0105]

[0106] Figure 31 is the transient photocurrent curve test spectrum of all photocatalysts. The figure confirms that all photocatalysts have photocurrent response and ZIS-6 350℃ has the best photocurrent response, indicating that the photo-generated carrier separation and transfer efficiency of photocatalyst ZIS-6 350℃ is the highest, which is conducive to the migration of photo-generated electrons, thereby improving the photocatalyst to accelerate the conversion of sodium lignosulfonate. Among them, the relative intensity of the transient photocurrent response of ZIS-3 350℃ and ZIS-12 350℃ is not much different, which also indicates that the catalytic activity of ZIS-3 350℃ and ZIS-12 350℃ photocatalysts is not much different in photocatalytic activity test.

[0107] In order to explore the resistance of photo-generated electrons in the transmission process, the electrochemical impedance spectra of all catalysts were tested. Generally speaking, the charge transfer resistance of the catalyst is equivalent to the diameter of the semicircle part of the Nyquist diagram, and the smaller the radius, the lower the charge transfer resistance. It can be seen from Figure 32 Compared with ZIS 350℃, ZIS-6 350℃ has a smaller circular arc radius, which confirms that ZIS-6 350℃ has a smaller interface electron transfer resistance. ZIS-6 350℃ has the largest transient photocurrent, the second smallest circular arc radius and the best decolorization curve, which also confirms that ZIS-6 350℃ has the best photo-generated electron-hole separation ability and the second smallest electron transfer resistance. These characteristics promote ZIS-6 350℃ to have the best photocatalytic activity.​

Claims

1. A method for preparing a degradable lignin-based heterojunction photocatalyst, characterized in that, A method for preparing a heterojunction photocatalyst, comprising the following steps: I. Alkaline lignin is dispersed in deionized water, and ultrasonic treatment is performed, followed by continuous stirring. Ethylenediamine and magnesium hydroxide are added in sequence during the stirring process to obtain a mixed solution A. Then, the mixed solution A is placed in a reaction kettle and heated at 190-200℃ for 12-14h. After the reaction is completed, the reaction is cooled to room temperature to obtain a suspension. The suspension is extracted by suction filtration to obtain a yellow-brown filtrate. The yellow-brown filtrate is subjected to dialysis and dried to obtain Mg,N-CQDs. II. First, zinc acetate dihydrate, indium trichloride tetrahydrate, Mg,N-CQDs and deionized water are mixed to obtain a mixed solution B. The mixed solution B is stirred, and then thioacetamide is added to the mixed solution B and continues to be stirred. Then, a condensation reflux reaction is performed at a temperature of 100-110℃ for 4-5h. After the reaction is completed, the reaction liquid is cooled to room temperature, and the product precursor is obtained by centrifugation. The product precursor is washed and centrifuged with deionized water and anhydrous ethanol to obtain a yellow / yellow-black solid. The yellow / yellow-black solid is dried and then calcined under a nitrogen atmosphere to obtain a Zn4In2S7 / Mg,N-CQDs heterojunction photocatalyst.

2. The method for preparing a degradable lignin-based heterojunction photocatalyst according to claim 1, characterized in that, In step I, the mass ratio of alkaline lignin to deionized water is (0.5-0.6):

66.

3. The method for preparing a degradable lignin-based heterojunction photocatalyst according to claim 1 or 2, characterized in that, In step I, the mass of alkaline lignin to the volume of ethylenediamine is (0.5-0.6)g:0.33mL.

4. The method for preparing a heterojunction photocatalyst for degrading lignin according to claim 3, characterized in that, In step I, the mass ratio of alkaline lignin to magnesium hydroxide is (0.5-0.6):0.

22.

5. The method for preparing a heterojunction photocatalyst for degrading lignin according to claim 1, characterized in that, In step II, the mass ratio of zinc acetate dihydrate to indium trichloride tetrahydrate is (0.61-0.71):0.

44.

6. The method for preparing a degradable lignin-based heterojunction photocatalyst according to claim 1 or 5, characterized in that, In step II, the mass ratio of zinc acetate dihydrate to thioacetamide is (0.61-0.71):0.

39.

7. The method for preparing a heterojunction photocatalyst for degrading lignin according to claim 6, characterized in that, In step II, the mass ratio of zinc acetate dihydrate to deionized water is (0.61-0.71):

75.

8. The method for preparing a heterojunction photocatalyst for degrading lignin according to claim 7, characterized in that, In step II, the mass ratio of zinc acetate dihydrate to Mg,N-CQDs is (0.61-0.71)g:(1.61-6.44)mg.

9. The method for preparing a heterojunction photocatalyst for degrading lignin according to claim 1, characterized in that, In step II, the calcination conditions are: temperature 350-370℃, time 2-2.5h.

10. Use of the heterojunction photocatalyst prepared by the method of claim 1 in the degradation of lignin.

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