Preparation method and application of lignin carbon quantum dot photocatalyst

Abstract

The application relates to a preparation method and application of a lignin carbon quantum dot photocatalyst, and relates to the field of photocatalytic materials, in particular to a preparation method and application of a lignin carbon quantum dot photocatalyst. The application aims at solving the problem of low photocatalytic activity of the existing BiVO4 photocatalyst. The method comprises the following steps: 1, preparing a carbon quantum dot solution by using alkali lignin; 2, dissolving CuSO4.5H2O in distilled water to obtain a solution A; dissolving BiVO4 in the solution A to obtain a solution B; uniformly dropping a NaBH4 solution into the solution B to obtain a solution C; uniformly stirring and then performing ultrasonic treatment; dissolving the carbon quantum dot solution in the solution C and performing ultrasonic treatment; after the reaction is completed, centrifugal separation is performed on the green powder, the green powder is washed and dried, and the lignin carbon quantum dot photocatalyst is obtained. The application is used for preparing the photocatalyst to improve the CO2 photocatalytic reduction performance.

Description

Technical Field

[0001] This invention relates to the field of photocatalytic materials, and more particularly to a method for preparing and applying a lignin-carbon quantum dot photocatalyst. Background Technology

[0002] CO2 is a major component of greenhouse gases, and its emissions are gradually increasing. Many methods have been devised to reduce CO2 levels in the atmosphere, such as converting CO2 into energy. Among these methods, heterogeneous catalysis and electrochemical reactions using non-biological processes have been extensively studied. However, non-biological reactions require stringent conditions, typically high temperature, high pressure, or high potential. These conditions consume significant amounts of energy, making them economically impractical. Solar energy, on the other hand, is a green energy source that does not pollute the environment. If, under normal temperature and pressure, a catalyst is used to photocatalytically reduce CO2 in aqueous solution to organic compounds such as formic acid and methanol under sunlight, a new method utilizing solar energy can be developed, aligning with the current trend of green chemistry. In artificially simulated photosynthesis, CO2 cannot absorb all visible and ultraviolet light. Therefore, developing suitable photochemical sensitizers to enable CO2 to utilize energy between 200 and 900 nm during artificial photocatalytic reduction is crucial. Since most photosensitive agents are composed of semiconductor materials, selecting a suitable catalyst is the primary issue in the photocatalytic reduction of CO2. To date, a large number of semiconductor CO2 photoreduction catalysts have been developed, such as TiO2, Bi2WO6, g-C3N4, MoS2, Mo2C and BiOX (X = F, Cl, Br and I).

[0003] However, due to their inherent drawbacks, including relatively poor visible light absorption, low photoexcitation carrier separation efficiency, and poor photocatalytic performance, their photocatalytic performance is far from meeting the basic requirements of industrial applications, as well as limited surface reaction kinetics. Therefore, exploring effective ways to overcome these shortcomings and improve the photocatalytic CO2 conversion efficiency is essential and urgently needed.

[0004] BiVO4 is a visible-light-driven n-type semiconductor photocatalyst with excellent properties such as low cost, low toxicity, high chemical stability, resistance to photocorrosion, narrow bandgap (2.4 eV), and visible spectral effects. As a visible-light-responsive photocatalyst, BiVO4 can both split water to produce hydrogen and degrade organic pollutants (such as methylene blue (MB) and the antibiotic TC). However, the rapid recombination of photogenerated electrons and holes in BiVO4, induced by light, limits its photocatalytic activity. Summary of the Invention

[0005] The present invention aims to address the problem of low photocatalytic activity of existing BiVO4 photocatalysts by providing a method for preparing and applying a lignin-carbon quantum dot photocatalyst.

[0006] The preparation method of the lignin-carbon quantum dot photocatalyst of the present invention includes the following steps:

[0007] I. Preparation of carbon quantum dot solution using alkali lignin;

[0008] II. Preparation of lignin-carbon quantum dot photocatalysts Cu-BiVO4-CQDs

[0009] CuSO4·5H2O was dissolved in distilled water and stirred at room temperature to obtain solution A. BiVO4 was dissolved in solution A and stirred evenly at room temperature to obtain solution B. NaBH4 solution was added dropwise to solution B at a uniform rate and stirred at room temperature. After stirring evenly, the solution was ultrasonically treated to obtain solution C. Carbon quantum dot solution was dissolved in solution C and ultrasonically treated. The ultrasonically dispersed solution was placed in a constant temperature water bath shaker for reaction. After the reaction was completed, the green powder was separated by centrifugation, washed, and dried to obtain the lignin carbon quantum dot photocatalyst Cu-BiVO4-CQDs.

[0010] Furthermore, the preparation method of the carbon quantum dot solution in step one is as follows:

[0011] Alkali lignin was dissolved in deionized water and stirred at room temperature. Then, it was sonicated. After sonication, it was stirred at room temperature. During stirring, ethylenediamine was added, and hydrogen peroxide was added dropwise and stirred. The uniformly dispersed solution was then transferred to a polytetrafluoroethylene liner, sealed in a stainless steel autoclave, and placed in a constant temperature drying oven for reaction. After the reaction was completed, the autoclave was allowed to cool naturally to room temperature. The autoclave was then opened to obtain a brownish-yellow solution.

[0012] The brownish-yellow solution was centrifuged and then filtered through a glass frit filter with a 0.22 μm aqueous filter membrane. Subsequently, it was dialyzed in deionized water for 48 h using a 3000 Da dialysis bag, with the water changed every 12 h. After dialysis, the solution was centrifuged to obtain a carbon quantum dot solution, which was stored in a brown plastic reagent bottle at 4 °C.

[0013] Furthermore, in step two, the mass ratio of CuSO4·5H2O to the volume ratio of distilled water is 0.5g:(80-100)mL.

[0014] Furthermore, in step two, the mass ratio of CuSO4·5H2O to BiVO4 is 1:1.

[0015] Furthermore, in step two, the concentration of the NaBH4 solution is 1 mol / L.

[0016] Furthermore, in step two, the mass ratio of CuSO4·5H2O to the volume ratio of NaBH4 solution is 0.5g:(5-10)mL.

[0017] Furthermore, in step two, the volume ratio of the carbon quantum dot solution to solution B is 1:(4-5).

[0018] Furthermore, in step two, the ultrasonic treatment temperature is 0℃ and the ultrasonic time is 30min.

[0019] Furthermore, in step two, the reaction is carried out in a constant temperature water bath shaker for 2 hours.

[0020] This invention also provides the application of lignin carbon quantum dot photocatalysts in improving CO2 photoreduction performance.

[0021] The beneficial effects of this invention are:

[0022] This invention uses alkaline lignin as a precursor to prepare fluorescent CQDs from the bottom up via a hydrothermal method. These CQDs are then composited with Cu-BiVO4 to prepare a Cu-BiVO4-CQDs composite material, and the materials are characterized and analyzed. The prepared Cu-BiVO4-CQDs composite material was used in a photocatalytic reduction of CO2 under simulated sunlight. The photocatalytic reduction effect was compared with that of Cu-BiVO4, showing a significant improvement in the photocatalytic performance of the composite material.

[0023] This invention prepares Cu-BiVO4-CQDs as a photocatalyst to improve the photoreduction performance of CO2. The Cu-BiVO4 prepared in this invention can selectively reduce CO2 to methane. BiVO4 itself can reduce CO2 to methane. Under visible light, BiVO4 is activated, generating electrons in the conduction band and holes in the valence band. Subsequently, the photoexcited electrons are captured by surface oxygen vacancies, promoting the separation of photoexcited charge carriers in BiVO4, thus enabling BiVO4 to possess photocatalytic performance. Cu nanoparticles exhibit visible light absorption through surface plasmon resonance and generate hot electron-hole pairs. Since the Fermi level of Cu is positive relative to the conduction band and surface oxygen vacancies of BiVO4, Cu nanoparticles can also act as traps to absorb photoexcited electrons from the conduction band of BiVO4. Therefore, by combining Cu with BiVO4, the synergistic effect between BiVO4, surface oxygen vacancies, and Cu nanoparticles can improve the separation and transport of photoexcited charge carriers, thereby enhancing the photocatalytic activity of the composite material.

[0024] The combination of Cu-BiVO4 and CQDs significantly improved the methane yield. The optimal amount of CQDs introduced was 50 mL, and the maximum yield of CH4 by Cu-BiVO4-CQDs was 9.166 μmol / g. -1 h -1The introduction of CQDs reduces the band gap of Cu-BiVO4, which helps to separate photogenerated electrons and holes, thereby improving the photocatalytic efficiency of Cu-BiVO4. However, too many CQDs may cover the active catalytic sites of Cu-BiVO4, thus reducing the photocatalytic performance of Cu-BiVO4.

[0025] The method of this invention uses lignin as a raw material to maximize its value utilization. After lignin carbon quantum dots are combined with bismuth vanadate and elemental Cu, carbon dioxide can be converted into methane in a short time, thereby realizing the conversion of carbon dioxide into high-value-added products.

[0026] The Cu-BiVO4-CQDs composite material prepared by this invention has broad application prospects as a photoreduction CO2 catalyst. Attached Figure Description

[0027] Figure 1a XRD patterns of carbon quantum dots (CQDs);

[0028] Figure 1b XRD patterns of BiVO4, Cu-BiVO4 and Cu-BiVO4-CQDs composites;

[0029] Figure 1c XRD patterns of Cu-BiVO4 and Cu-BiVO4-CQDs-50;

[0030] Figure 2a SEM images of BiVO4;

[0031] Figure 2b SEM image of Cu-BiVO4;

[0032] Figure 2c SEM images of Cu-BiVO4-CQDs;

[0033] Figure 3a TEM images of CQDs;

[0034] Figure 3b Particle size distribution of CQDs;

[0035] Figure 3c TEM image of BiVO4;

[0036] Figure 3d TEM image of Cu-BiVO4;

[0037] Figure 3e TEM image of Cu-BiVO4-CQDs;

[0038] Figure 4aThe measured spectrum of Cu-BiVO4-CQDs nanocomposite material;

[0039] Figure 4b XPS spectrum of Bi 4f:

[0040] Figure 4c XPS spectra for V2p:

[0041] Figure 4d XPS spectrum of O1s:

[0042] Figure 4e XPS spectrum of Cu 2p:

[0043] Figure 4f XPS spectrum of C1s:

[0044] Figure 4g XPS spectrum of N1s:

[0045] Figure 5a Raman spectra of CQDs;

[0046] Figure 5b Raman spectra of BiVO4, Cu-BiVO4, and Cu-BiVO4-CQDs;

[0047] Figure 6a To prepare the UV-Vis DRS spectrum of the sample

[0048] Figure 6b (αhv) 2 The curve corresponding to hv (containing carbon quantum dots);

[0049] Figure 6c (αhv) 2 The curve corresponding to hv (excluding carbon quantum dots);

[0050] Figure 7a The graph shows the CH4 yield of different photocatalysts;

[0051] Figure 7b This is a graph showing the average rate of CH4. Detailed Implementation

[0052] The technical solution of the present invention is not limited to the specific embodiments listed below, but also includes any combination of the specific embodiments.

[0053] Specific Implementation Method 1: The preparation method of the lignin carbon quantum dot photocatalyst in this implementation method includes the following steps:

[0054] I. Preparation of carbon quantum dot solution using alkali lignin;

[0055] II. Preparation of lignin-carbon quantum dot photocatalysts Cu-BiVO4-CQDs

[0056] CuSO4·5H2O was dissolved in distilled water and stirred at room temperature to obtain solution A. BiVO4 was dissolved in solution A and stirred evenly at room temperature to obtain solution B. NaBH4 solution was added dropwise to solution B at a uniform rate and stirred at room temperature. After stirring evenly, the solution was ultrasonically treated to obtain solution C. Carbon quantum dot solution was dissolved in solution C and ultrasonically treated. The ultrasonically dispersed solution was placed in a constant temperature water bath shaker for reaction. After the reaction was completed, the green powder was separated by centrifugation, washed, and dried to obtain the lignin carbon quantum dot photocatalyst Cu-BiVO4-CQDs.

[0057] This method induces structural changes by doping with specific metal ions (Cu) to promote orbital electron transitions, thereby suppressing the rapid recombination of photogenerated carriers and achieving higher interfacial charge separation and transfer efficiency, thus enhancing visible light photocatalytic activity. It effectively improves the efficient utilization of visible light and, to some extent, promotes the electron conduction mechanism of composite materials, enhancing the separation and transfer capability of visible light-driven charge carriers, thereby effectively improving its photoreduction efficiency of CO2 under visible light irradiation.

[0058] The combination of Cu-BiVO4 and CQDs significantly improves methane yield. The introduction of CQDs reduces the band gap of Cu-BiVO4, which helps with the separation of photogenerated electrons and holes, thus improving the photocatalytic efficiency of Cu-BiVO4. However, excessive CQDs may cover the active catalytic sites of Cu-BiVO4, thereby reducing the photocatalytic performance of Cu-BiVO4.

[0059] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the preparation method of the carbon quantum dot solution in step one is as follows:

[0060] Alkali lignin was dissolved in deionized water and stirred at room temperature. Then, it was sonicated. After sonication, it was stirred at room temperature. During stirring, ethylenediamine was added, and hydrogen peroxide was added dropwise and stirred. The uniformly dispersed solution was then transferred to a polytetrafluoroethylene liner, sealed in a stainless steel autoclave, and placed in a constant temperature drying oven for reaction. After the reaction was completed, the autoclave was allowed to cool naturally to room temperature. The autoclave was then opened to obtain a brownish-yellow solution.

[0061] The brownish-yellow solution was centrifuged, then filtered through a glass frit filter with a 0.22 μm aqueous filter membrane. Subsequently, it was dialyzed in deionized water for 48 hours using a 3000 Da dialysis bag, with the water changed every 12 hours. After dialysis, the solution was centrifuged to obtain a carbon quantum dot solution, which was stored at 4°C in a brown plastic reagent bottle. Other procedures were the same as in Specific Embodiment 1.

[0062] Specific Implementation Method 3: This implementation method differs from Specific Implementation Method 1 or 2 in that the mass ratio of CuSO4·5H2O to the volume ratio of distilled water in step 2 is 0.5g:(80-100)mL. Everything else is the same as in Specific Implementation Method 1 or 2.

[0063] Specific Implementation Method Four: This implementation method differs from Specific Implementation Methods One to Three in that the mass ratio of CuSO4·5H2O and BiVO4 in step two is 1:1. Everything else is the same as in Specific Implementation Methods One to Three.

[0064] Specific Implementation Method Five: This implementation method differs from Specific Implementation Methods One to Four in that the concentration of the NaBH4 solution in step two is 1 mol / L. Everything else is the same as in Specific Implementation Methods One to Four.

[0065] Specific Implementation Method Six: This implementation method differs from Specific Implementation Methods One to Five in that the mass ratio of CuSO4·5H2O to the volume ratio of NaBH4 solution in step two is 0.5g:(5-10)mL. Everything else is the same as in Specific Implementation Methods One to Five.

[0066] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Methods One to Six in that the volume ratio of carbon quantum dot solution to C solution in step two is 1:(4-5). Everything else is the same as in Specific Implementation Methods One to Six.

[0067] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Methods One to Seven in that the ultrasonic treatment temperature in step two is 0℃ and the ultrasonic treatment time is 30-60 minutes. Everything else is the same as in Specific Implementation Methods One to Seven.

[0068] Specific Implementation Method Nine: This implementation method differs from Specific Implementation Methods One to Eight in that the reaction in step two is carried out in a constant temperature water bath shaker for 2 hours. Everything else is the same as in Specific Implementation Methods One to Eight.

[0069] Specific Implementation Method 10: Application of lignin carbon quantum dot photocatalyst in improving CO2 photoreduction performance.

[0070] The embodiments of the present invention will be described in detail below. The following embodiments are implemented based on the technical solution of the present invention, and detailed implementation schemes and specific operation processes are given. However, the protection scope of the present invention is not limited to the following embodiments.

[0071] Example 1:

[0072] I. Preparation of lignin carbon quantum dots

[0073] Weigh 0.5g of alkali lignin and dissolve it in 50mL of deionized water. Stir at room temperature for 30min at a stirring speed of 120r / min, then sonicate at 0℃ for 30min. After sonication, stir at room temperature, adding 3mL of ethylenediamine and then slowly adding 10mL of hydrogen peroxide. Stir magnetically at 120r / min for 30min. Transfer the well-dispersed solution to a 100mL polytetrafluoroethylene-lined container, seal it in a stainless steel autoclave, and place it in a 190℃ constant temperature drying oven for 12h. After the reaction, allow the autoclave to cool naturally to room temperature. Open the autoclave to obtain a brownish-yellow solution.

[0074] The brownish-yellow solution was centrifuged at 9000 r / min for 10 min to remove unreacted large particles of alkali lignin. Then, the large carbon particles generated during the reaction were filtered out using a glass frit filter with a 0.22 μm aqueous filter membrane. Subsequently, the solution was dialyzed in deionized water for 48 h using a 3000 Da dialysis bag, with the water changed every 12 h. After dialysis, the solution was centrifuged at 9000 r / min for 10 min to remove the precipitate and obtain a carbon quantum dot solution, which was stored in a brown plastic reagent bottle at 4 °C.

[0075] II. Preparation of BiVO4

[0076] 12 mmol of Bi(NO3)3·5H2O precursor was dissolved in 64 mL of 1 mol / L HNO3 aqueous solution and magnetically stirred at 120 r / min for 30 min at room temperature. Then, 12 mmol of NH4VO3 was added to the solution and magnetically stirred at 120 r / min for 1 h to obtain a mixed solution. 6.0 g of urea was then slowly dispersed into the mixed solution and stirred at room temperature for 30 min. The uniformly dispersed solution was transferred to a 100 mL polytetrafluoroethylene-lined container, sealed in a stainless steel autoclave, and placed in a 180℃ constant temperature drying oven for 12 h. After the reaction, the autoclave was allowed to cool naturally to room temperature, and centrifuged at 9000 r / min for 10 min to separate the bright yellow powder. The powder was washed 5 times with ultrapure water and dried at 60℃ for 24 h to obtain BiVO4.

[0077] III. Preparation of Cu-BiVO4

[0078] 0.05 g of CuSO4·5H2O was dissolved in 100 mL of distilled water and magnetically stirred at 120 r / min for 30 min at room temperature to obtain solution A. 0.5 g of BiVO4 was dissolved in solution A and magnetically stirred until homogeneous at room temperature to obtain solution B. 10 mL of 1 mol / L NaBH4 was added dropwise to solution B at a uniform rate, and the mixture was stirred at room temperature for 30 min. The dispersed solution was placed in a constant temperature water bath shaker and reacted at 20 °C for 4 h. After the reaction was complete, the mixture was centrifuged at 9000 r / min for 10 min to separate the green powder. The powder was washed five times with ultrapure water and dried at 60 °C for 24 h to obtain Cu-BiVO4.

[0079] IV. Preparation of Cu-BiVO4-CQDs

[0080] 0.05 g of CuSO4·5H2O was dissolved in 100 mL of distilled water and magnetically stirred at 120 r / min for 30 min at room temperature to obtain solution A. 0.5 g of BiVO4 was dissolved in solution A and magnetically stirred until homogeneous at room temperature to obtain solution B. 10 mL of 1 mol / L NaBH4 was added dropwise to solution B at a uniform rate and stirred at room temperature for 30 min to obtain solution C. Different volumes of carbon quantum dot solution were added to solution C and stirred at room temperature for 30 min. The dispersed solution was placed in a constant temperature water bath shaker and reacted at 20 °C for 4 h. After the reaction was completed, the solution was centrifuged at 9000 r / min for 10 min to separate the green powder, which was washed five times with ultrapure water and dried at 60 °C for 24 h. Furthermore, by changing the volume of the carbon quantum dot solution (30, 50, and 70 mL), Cu-BiVO4-CQDs composite materials with different CQDs cocatalyst contents can also be obtained, and the samples are denoted as Cu-BiVO4-CQDs-x (x represents the volume of the carbon quantum dot solution).

[0081] The effects of the composite material prepared in this embodiment were verified as follows:

[0082] (I) Characterization of composite materials

[0083] The morphology of the prepared samples was analyzed using a transmission electron microscope (TEM) with an accelerating voltage of 200 kV and a scanning electron microscope (SEM) on a Hitachi S-4800. The crystal structure of the synthesized photocatalyst was characterized using a Cu-K-α radiation X-ray diffractometer (XRD, Bruker D8 Advance X-ray diffractometer). The 2θ range was 10°–80°, with a step size of 0.04° and a scan rate of 8° / min. The surface chemical composition and chemical state of the prepared samples were investigated using X-ray photoelectron spectroscopy (XPS, PHI-5702, Physical Electronics). The structural characteristics of the samples were analyzed using a DXR microconfocal laser Raman spectrometer (USA) with an excitation wavelength of 514 nm.

[0084] 1. Structural Analysis

[0085] The crystal structures of CQDs, BiVO4, Cu-BiVO4, and Cu-BiVO4-CQDs were characterized by XRD analysis. Figure 1a As shown, the XRD diffraction pattern of the CQDs sample has fewer impurity peaks, indicating that the prepared carbon dots are relatively pure. Furthermore, the broad peak centered at 20.3° in the spectrum corresponds to the (002) crystal plane of graphitic carbon, while the small peak at 43.5° belongs to the (100) crystal plane of graphitic carbon, indicating that the CQDs sample has a good degree of graphitization conversion. Figure 1b The X-ray powder diffraction (XRD) spectra of different samples are shown. It can be seen from the figure that the prepared BiVO4 is monoclinic scheelite type (JCPDS14-0688). The diffraction peaks of the crystal planes (020), (110), (011), (121), (040), (002), (211), (150), (123), (240), (024), (202), (161), (321), and (132) are 15.23°, 18.64°, 19.02°, 30.50°, 34.46°, 35.16°, 39.50°, 43.25°, 45.42°, 46.02°, 47.55°, 50.34°, 53.22°, 56.20°, 57.94°, and 59.30°, respectively. The Cu composite had almost no effect on the crystal phase, crystal form, or degree of crystallinity of BiVO4, and no characteristic diffraction peaks attributed to Cu were detected, which may be related to the low Cu concentration in the preparation. No additional peaks for Cu and CQDs were observed in the XRD pattern, indicating that detecting low concentrations of Cu and CQDs is difficult. Furthermore, it can be seen that the introduction of different composite ratios of CQDs had almost no effect on the crystal structure of the Cu-BiVO4 nanocomposite material. Figure 1cAs shown, compared with Cu-BiVO4, the positions of all diffraction peaks in the Cu-BiVO4-CQDs-50 composite material did not shift. These results further indicate that the low CQDs content loading on Cu-BiVO4-CQDs has no effect on the crystal structure of Cu-BiVO4.

[0086] 2. Microscopic morphology analysis

[0087] from Figure 2a The smallest BiVO4 particles exhibit a decahedral structure with a smooth surface and sharp edges, ranging in size from 0.9 μm to 1.5 μm and a thickness of approximately 0.38 μm, showing some aggregation. From... Figure 2b It can be clearly seen that elemental Cu is orderly coated on the BiVO4 surface, forming a Cu-BiVO4 structure. Compared to Cu-BiVO4, the morphology of the Cu-BiVO4-CQDs composite catalyst did not show significant changes after CQDs modification, which is evident from... Figure 2b The results of XRD can also confirm this.

[0088] The particle size, distribution, and graphitization degree of the prepared carbon dots were observed using transmission electron microscopy (TEM and HRTEM). As shown in Figure 3(a), the CQDs sample mainly consisted of well-dispersed nanoparticles with a spherical structure. A few larger, darker-colored particles were also observed. These larger nanoparticles, due to their small carbon dot size, have a large specific surface area, resulting in high surface energy and an unstable energy state, making them prone to aggregation and "agglomeration." In the HRTEM image, the lattice fringe spacing of the graphitic carbon (002) crystal plane was clearly observed to be 0.26 nm, indicating a high degree of graphitization in the CQDs prepared under optimal process conditions. Furthermore, statistical analysis of the carbon dot particle size was performed using Nano Measurer software. Figure 3b As shown, the carbon dots have a particle size distribution of 1–5.5 nm, mainly concentrated in the 2–4 nm range. Figure 3c The image shown is a TEM image of pure BiVO4. BiVO4 has a blocky structure, and... Figure 2a It matches. Figure 3d As can be seen, elemental Cu is orderly precipitated on the surface of BiVO4. TEM images of the composite material are shown below. Figure 3e As shown, numerous CQDs with diameters less than 10 nm are uniformly and tightly attached to Cu-BiVO4.

[0089] 3. Elemental composition analysis

[0090] To reveal the interfacial bonding between CQDs and Cu-BiVO4, X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical environment and chemical state of the elements. Figure 4a The XPS spectrum of the Cu-BiVO4-CQDs nanocomposite is shown. Spectral investigation indicates that the Cu-BiVO4-CQDs nanocatalyst contains Bi, V, C, O, Cu, and N, with no other impurities. The N element content is minimal, and the peaks are not prominent in the full spectrum. The XPS peak positions were calibrated using c 1s at 284.60 eV. In the fine spectrum of Bi 4f (… Figure 4b Two distinct peaks were observed: 158.5 eV for Bi₄f₇ / ₂ and 163.9 eV for Bi₄f₅ / ₂, with a splitting energy of 5.3 eV, corresponding to the trivalent Bi state of monoclinic BiVO₄. 3+ .exist Figure 4c In the figure, the peak values ​​at 516.0 eV and 523.5 eV belong to V 2p3 / 2 and V 2p1 / 2 of BiVO4. This indicates that vanadium in Cu-BiVO4-CQDs is located at V 2p3 / 2 and V 2p1 / 2, respectively. 5+ The chemical state. From O1s( Figure 4d The spectrum shows that the fitting peak at 529.5 eV can be attributed to the HO bond, while the shoulder peaks at 531.5 eV and 533.7 eV belong to CO and C=O, respectively, indicating that the Cu-BiVO4-CQDs sample contains abundant oxygen-containing functional groups (-OH, -COOH, etc.). Figure 4e As shown, Cu 2p3 / 2 and Cu 2p1 / 2 of Cu 2p are located at two binding energy peaks at 935.0 eV and 956.4 eV, respectively, verifying the presence of Cu. Simultaneously, two weaker binding peaks are located at 944.62 eV and 962.5 eV, indicating the presence of Cu. 2+ This indicates that trace amounts of Cu are formed. From this... Figure 4f The C1s spectrum shows that the main peak at 284.6 eV is primarily composed of graphitic carbon (CC), while the two shoulder peaks at 285.9 eV and 287.97 eV can be attributed to COC and OC=O bonds, respectively. For N1s (… Figure 4g Fitting the fine spectrum of the Cu-BiVO4-CQDs sample revealed that the fitting peaks at 398.53 e V, 399.26 e V, and 400.67 e V correspond to the characteristic peaks of pyridine nitrogen, pyrrole nitrogen, and graphitic nitrogen, respectively. Therefore, the analysis of these results further confirms the successful preparation of the Cu-BiVO4-CQDs composite material.

[0091] 4. Fourier transform Raman spectroscopy analysis

[0092] Raman spectroscopy was used to further identify the structural features of the sample. For example... Figure 5a The image shows CQDs, 1350cm. -1(D band) and 1581cm -1 The peaks in the G band correspond to vibrations of disordered sp3 hybridized carbon atoms and ordered sp2 hybridized carbon atoms. Furthermore, surface defects in CQDs samples can be calculated using the intensities of the D band (ID) and G band (IG). The ID / IG ratios of CQDs are 0.85. Figure 5b Electronic structure information of BiVO4 samples, with the 858 cm⁻¹ as the focal point. -1 The strongest peak centered on the 763cm peak is related to the symmetrical stretching mode, while the 763cm peak is related to the symmetrical stretching mode. -1 The weak shoulder at 370cm can be attributed to an anti-symmetrical stretching pattern. -1 and 250cm -1 The Raman peaks observed at the locations correspond to the symmetric bending mode δs(VO4) and the external vibration mode, respectively. No peak displacement was observed in pure BiVO4 and in Cu-BiVO4 and Cu-BiVO4-CQDs samples supported with CQDs and Cu catalysts. This indicates that CQDs and Cu are deposited only on the surface of BiVO4, rather than bound to the BiVO4 lattice, which is consistent with the results obtained from XRD in Figure 1.

[0093] 5. Optical performance analysis

[0094] To characterize the optical properties of the photocatalyst, ultraviolet-visible diffuse reflectance measurements were performed. Figure 6a In the study, the absorption band edge of pure BiVO4 is located at 515 nm, representing a band-to-band transition. CQDs show no obvious absorption edge. After combining BiVO4 with Cu to form a composite catalyst, a significant red shift in the absorption band edge of Cu-BiVO4 is clearly visible, indicating the successful introduction of Cu and broadening the visible light absorption range of BiVO4. When CQDs are introduced into Cu-BiVO4, Cu-BiVO4-CQDs exhibit a strong absorption band with a steep edge in the visible light region, indicating the interband transition nature of the Cu-BiVO4-CQDs material. Furthermore, an absorption edge of approximately 540 nm can be observed, showing a red shift of about 10 nm compared to Cu-BiVO4, further demonstrating the successful introduction of CQDs and broadening the visible light absorption range of Cu-BiVO4.

[0095] The bandgap value (Eg) is calculated using the Tauc formula shown below:

[0096] (αhv) 2 =A(hv-Eg)

[0097] Eg, hv, α, and A represent the band gap energy, photon energy, absorption coefficient, and parameters related to the transition probability, respectively. Figure 6b and Figure 6cThe calculated band gaps for BiVO4, Cu-BiVO4, Cu-BiVO4-CQDs-30, Cu-BiVO4-CQDs-50, and Cu-BiVO4-CQDs-70 are 2.52, 2.38, 2.32, 2.30, and 2.36 eV, respectively. These results indicate that the optical absorption range increases after adding CQDs to construct a heterojunction with Cu-BiVO4. CQDs significantly affect the optical properties and band structure of Cu-BiVO4 microspheres, which is attributed to the distortion of intrinsic electrical density, leading to changes in the band structure within Cu-BiVO4.

[0098] (II) Photocatalytic CO2 Reduction Performance Test

[0099] The photocatalytic carbon dioxide reduction reactor consists of a double-layer quartz reactor (with condensate flowing through the outer layer), a gas cylinder, a gas chromatograph, and a xenon lamp. Specific testing method: 0.06g of catalyst is dispersed in 3mL of 1mol / L NaOH, sonicated for 30min, and the mixed solution is transferred to a petri dish and dried in a vacuum drying oven (60℃). The dried petri dish is placed in the reactor, and high-purity CO2 is then introduced. When the CO2 and H2O adsorbed on the catalyst surface reach adsorption equilibrium, the reaction is initiated under light irradiation. The light source is a 300W xenon lamp with a filter (λ≥420). During irradiation, 1mL of gas is extracted from the reactor at regular intervals and analyzed online using gas chromatography. The gas chromatographs used are a GC-7920 manufactured by Beijing Zhongjiao Jinyuan, equipped with a TCD and FID detector, and a GC-7900 manufactured by Beijing Bofeilai, equipped with a TCD detector. (Molecular sieve packed column).

[0100] The experimental conditions for photocatalytic reduction of CO2 were as follows: the sacrificial agent for the CO2 reduction reaction was 3 mL of 1 mol / L NaOH, the simulated solar light source was a xenon lamp with a wavelength greater than 420 nm, and the amount of test sample was uniformly 60 mg.

[0101] Figure 7a and Figure 7b The activity of different photocatalysts for the photocatalytic reduction of CO2 was demonstrated. In this photocatalytic system, the product is methane. Figure 7a The graph shows the CH4 production from CO2 reduction in different samples over 3 hours under visible light irradiation; Figure 7b The value represents the rate of CH4 generation from CO2 photocatalytic reduction under visible light irradiation. Figure 7aThe results show that the CH4 yields of BiVO4, Cu-BiVO4, Cu-BiVO4-CQDs-30, Cu-BiVO4-CQDs-50, and Cu-BiVO4-CQDs-70 were recorded during the 3-hour reaction. The CH4 yield of BiVO4 was 1.27 μmol / g·h. The methane yield of pure BiVO4 was 3.83 μmol / g, while CQDs and Cu alone showed no photocatalytic CO2 reduction performance. After loading CQDs, the photocatalytic performance of BiVO4 in reducing CH4 from CO2 was improved to some extent, with the CH4 yield increasing to 4.33 μmol / g, indicating that the addition of CQDs can improve the photocatalytic performance of bismuth vanadate. After loading Cu, the photocatalytic performance of BiVO4 in reducing CH4 from CO2 was improved to some extent, with the CH4 formation rate increasing to 8.71 μmol / g, indicating that the addition of Cu can improve the photocatalytic CO2 reduction performance of BiVO4. The CO2 reduction performance of Cu-BiVO4-CQDs was significantly improved after combining CQDs with Cu-BiVO4, indicating excellent loading effect of CQDs. The CH4 yield of Cu-BiVO4-CQDs-30 was 16.188 μmol / g, which was 1.85 times that of Cu-BiVO4. The CH4 yield of Cu-BiVO4-CQDs-50 increased to 27.5 μmol / g, which was 3.15 times that of Cu-BiVO4. The CH4 yield of Cu-BiVO4-CQDs-70 increased to 18.45 μmol / g, which was 2.15 times that of Cu-BiVO4. The CH4 yield of BiVO4, Cu-BiVO4, Cu-BiVO4-CQDs-30, Cu-BiVO4-CQDs-50 and Cu-BiVO4-CQDs-70 were calculated during the 3-hour reaction (e.g., Figure 7b The yield of CH4 from BiVO4 was 1.27 μmol / g·h. The yield of CH4 from Cu-BiVO4-CQDs-50 increased to 9.166 μmol / g·h. When the optimal loading of CQDs was 50 mL, the CO2 photoreduction performance of the sample first increased and then decreased, indicating that excessive CQDs nanoparticles covering the Cu-BiVO4 surface may have inhibited interfacial charge transfer. The maximum CO2 photoreduction efficiency of CH4 was 9.166 μmol / g·h.

Claims

1. The application of lignin-carbon quantum dot photocatalysts in the photoreduction of CO2 to methane, characterized in that, The preparation method of the lignin carbon quantum dot photocatalyst includes the following steps: I. Preparation of carbon quantum dot solution using alkali lignin; 2. Dissolve CuSO4·5H2O in distilled water and stir at room temperature to obtain solution A; dissolve BiVO4 in solution A and stir evenly at room temperature to obtain solution B; add NaBH4 solution dropwise to solution B at a uniform rate and stir at room temperature. After stirring evenly, perform ultrasonic treatment to obtain solution C. Dissolve carbon quantum dot solution in solution C and perform ultrasonic treatment. Place the ultrasonically dispersed solution in a constant temperature water bath shaker for reaction. After the reaction, centrifuge to separate the green powder, wash, and dry to obtain the lignin carbon quantum dot photocatalyst Cu-BiVO4-CQDs; the volume ratio of carbon quantum dot solution to solution C in step 2 is 1:(4-5).

2. The application according to claim 1, characterized in that, The preparation method of the carbon quantum dot solution in step one is as follows: Alkali lignin was dissolved in deionized water and stirred at room temperature. Then, it was sonicated. After sonication, it was stirred at room temperature. During stirring, ethylenediamine was added, and hydrogen peroxide was added dropwise and stirred. The uniformly dispersed solution was then transferred to a polytetrafluoroethylene liner, sealed in a stainless steel autoclave, and placed in a constant temperature drying oven for reaction. After the reaction was completed, the autoclave was allowed to cool naturally to room temperature. The autoclave was then opened to obtain a brownish-yellow solution. The brownish-yellow solution was centrifuged and then filtered through a glass frit filter with a 0.22 μm aqueous filter membrane. Subsequently, it was dialyzed in deionized water for 48 h using a 3000 Da dialysis bag, with the water changed every 12 h. After dialysis, the solution was centrifuged to obtain a carbon quantum dot solution, which was stored in a brown plastic reagent bottle at 4 °C.

3. The application according to claim 1 or 2, characterized in that, In step two, the mass ratio of CuSO4·5H2O to the volume ratio of distilled water is 0.5g:(80-100)mL.

4. The application according to claim 3, characterized in that, In step two, the mass ratio of CuSO4·5H2O to BiVO4 is 1:

1.

5. The application according to claim 4, characterized in that, In step two, the concentration of the NaBH4 solution is 1 mol / L.

6. The application according to claim 5, characterized in that, In step two, the mass ratio of CuSO4·5H2O to the volume ratio of NaBH4 solution is 0.5g:(5-10)mL.

7. The application according to claim 6, characterized in that, In step two, the ultrasonic treatment temperature is 0℃ and the ultrasonic time is 30-60 min.

8. The application according to claim 7, characterized in that, In step two, the reaction is carried out in a constant temperature water bath shaker for 2 hours.

Images