Spinel-based composite photocatalysts, their preparation methods, and their applications in catalyzing CO2 conversion.

Abstract

The application relates to a spinel-based composite photocatalyst, a preparation method and application of the spinel-based composite photocatalyst in catalyzing CO2 conversion, and comprises the following steps: mixing a copper source, an indium source, a tin source and a sulfur source with water to form a first solution; adding a silver source into the first solution and uniformly mixing to obtain a second solution; and performing hydrothermal reaction on the second solution, performing solid-liquid separation, washing and drying to obtain the spinel-based composite photocatalyst. The spinel-based composite photocatalyst is prepared through a one-step hydrothermal method, and the production process is simple; Ag + is selected in the application to perform substitutional doping, the valence band energy level of CuInSnS4 is significantly improved, the band gap width is reduced, the adsorption capacity of CO2 is improved, the light absorption capacity is enhanced, and the catalytic activity and selectivity of the photocatalysis of CO2 into CH4 can be effectively improved.

Description

Technical Field

[0001] This invention relates to the field of photocatalyst materials, specifically to a spinel-based composite photocatalyst, its preparation method, and its application in catalyzing CO2 conversion. Background Technology

[0002] Reducing CO2 into high-value-added chemicals and renewable fuels such as methane through photocatalysis not only provides a new solution to the energy shortage problem, but is also a key path to achieving carbon emission reduction.

[0003] However, the photocatalytic reduction of CO2 to methane faces numerous challenges in practical applications. Firstly, the CO2 molecule possesses extremely high thermodynamic stability, with a C=O bond dissociation energy as high as 750 kJ / mol, making it difficult to initiate the CO2-to-methane conversion reaction. Secondly, traditional semiconductor photocatalysts have wide band gaps, absorbing only visible light and failing to effectively utilize approximately 50% of the near-infrared light in the solar spectrum, resulting in insufficient light energy utilization. Thirdly, photogenerated electrons and holes in photocatalysts readily recombine, leading to unsatisfactory selectivity and conversion efficiency in the CO2-to-methane reduction. To overcome these bottlenecks, researchers are focusing on exploring novel photocatalyst materials, striving to develop catalysts with narrower band gaps that can absorb a wider range of solar spectra and enhance electron-hole separation efficiency, thereby improving the performance of the CO2-to-methane reduction reaction.

[0004] CuInSnS4 (CISS) is a polymetallic sulfide semiconductor material that can selectively photocatalyze the reduction of CO2 to methane; however, the high activation barrier for CO2 adsorption on its surface results in poor reduction efficiency and product selectivity. Summary of the Invention

[0005] The purpose of this invention is to overcome the above-mentioned technical deficiencies and provide a spinel-based composite photocatalyst, its preparation method, and its application in catalyzing CO2 conversion, thereby solving the technical problems of insufficient light energy utilization, poor reduction efficiency, and poor product selectivity of photocatalysts in the prior art.

[0006] To achieve the above-mentioned technical objectives, the technical solution provided by this invention is as follows: In a first aspect, the present invention provides a method for preparing a spinel-based composite photocatalyst, comprising the following steps: mixing a copper source, an indium source, a tin source and a sulfur source with water to form a first solution; adding a silver source to the first solution and mixing them to obtain a second solution; subjecting the second solution to a hydrothermal reaction, solid-liquid separation, washing and drying to obtain a spinel-based composite photocatalyst.

[0007] Secondly, the present invention provides a spinel-based composite photocatalyst prepared by the above-mentioned preparation method.

[0008] Thirdly, the present invention provides an application of the above-mentioned spinel-based composite photocatalyst in photocatalytic CO2 conversion.

[0009] Compared with the prior art, the beneficial effects of the present invention include: This invention prepares a spinel-based composite photocatalyst via a one-step hydrothermal method by mixing copper, indium, tin, and sulfur sources with water, and then adding a silver source as a dopant ion source. The preparation method of this invention has advantages such as simple production process, high product yield, and few by-products. This invention uses Cu... + Ag has the same valence state and valence layer but a higher d orbital energy level + Substitutional doping significantly improves the valence band level of CuInSnS4, thereby reducing its band gap width, enhancing its CO2 adsorption capacity, and strengthening its light absorption capacity. This effectively improves the catalytic activity and selectivity of photocatalytic CO2 to CH4 conversion. Under full-spectrum light irradiation or visible light irradiation, the yield and selectivity of the spinel-based composite photocatalyst for CO2 reduction to CH4 are significantly higher than those of CuInSnS4. Attached Figure Description

[0010] Figure 1 The images show the XRD and SEM images of different photocatalyst samples, where (a) is the XRD pattern and magnified view of the photocatalysts obtained in Examples 1-3 and Comparative Example 1; (b) is the SEM image of CISS obtained in Comparative Example 1; and (c) is the SEM image of 1Ag-CISS obtained in Example 1. Figure 2 The images show the Raman spectra of the photocatalysts obtained in Examples 1-3 and Comparative Example 1, where (a) is the overall image and (b) is a magnified view of a portion thereof; Figure 3 The images shown are test results for the photocatalyst 1Ag-CISS obtained in Example 1, where (a) is a TEM image with a scale bar of 200 nm; (b) is a TEM image with a scale bar of 10 nm; (c) is an HRTEM image; (d) is a SEAD pattern; (e) is a HAADF image; and (fj) is an EDS elemental distribution map, including Ag, Cu, In, Sn, and S. Figure 4 The images show the test results of photocatalyst 2.5Ag-CISS from Example 2 and photocatalyst CISS from Comparative Example 1, where (a) is the UV-Vis-NIR DRS spectrum of CISS and 2.5Ag-CISS; (b) is the band gap diagram of CISS and 2.5Ag-CISS; Mott-Schottky curves of the samples: (c) CISS; (d) 2.5Ag-CISS; (e) XPS-VB plots of CISS and 2.5Ag-CISS; (f) schematic diagram of the band structure of CISS and 2.5Ag-CISS. Figure 5 The results show the application performance of different photocatalyst samples, where (a) is the average gas yield after 4 h of visible light irradiation; (b) is the average gas yield after 4 h of full-spectrum irradiation; (c) is the CH4 product selectivity under visible light; and (d) is the CH4 product selectivity under full-spectrum irradiation. Detailed Implementation

[0011] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0012] To address the shortcomings of existing photocatalysts CuInSnS4, such as its susceptibility to photocorrosion and the high activation barrier for CO2 adsorption on its surface, which leads to poor reduction efficiency and product selectivity, this invention selects Cu... + Ag has the same valence state and valence layer but a higher d orbital energy level + Substitutional doping was performed to address the current limitations of CuInSnS4 materials and overcome the aforementioned shortcomings.

[0013] In a first aspect, the present invention provides a method for preparing a spinel-based composite photocatalyst, comprising the following steps: The copper source, indium source, tin source and sulfur source are mixed evenly with water to form the first solution; Add a silver source to the first solution and mix thoroughly to obtain the second solution; The second solution was subjected to hydrothermal reaction, solid-liquid separation, washing, and drying to obtain a spinel-based composite photocatalyst.

[0014] This invention uses Cu + Ag has the same valence state and valence layer but a higher d orbital energy level + Substitutional doping is necessary because of the special structure of CISS, where Cu is tetrahedral coordinated with a +1 valence, In is trivalent with a +3 valence, and Sn is quadrivalent with a +4 valence. Only Ag is suitable for substitutional doping of Cu, and other metals such as Ni, Co, and Fe are not effective.

[0015] Preferably, the copper source includes CuCl; the indium source includes InCl3·4H2O; the tin source includes SnCl4·5H2O; and the sulfur source includes thioacetamide.

[0016] Preferably, the molar ratio of copper source, indium source, tin source, and sulfur source is 1:1:1:(4-10). Specifically, CuCl:SnCl4·5H2O:InCl3·4H2O:TAA is 1:1:1:(4-10). In this invention, an excess of sulfur source can be used to ensure that all metal sources participate in the reaction. If the ratio of Cu, In, Sn, and S is changed beyond the above range, it will lead to the formation of a mixture of other metal sulfides and result in impure materials, leading to poor results.

[0017] Preferably, the ratio of copper source to water is 2 mmol: (60-100) mL.

[0018] It is understood that the volume of water used to dissolve the raw materials in this invention is 1 / 3 to 2 / 3 of the volume of the closed system.

[0019] Preferably, the first solution is obtained by first adding copper source, indium source and tin source to deionized water and stirring until uniform, and then adding sulfur source and stirring to dissolve; the stirring rate is 1000-1500 r / min.

[0020] Preferably, the silver source includes AgNO3 or AgCl.

[0021] Preferably, the molar amount of silver added accounts for 0.5% to 7.5% of the molar amount of CuInSnS4; more preferably, it is 0.5% to 2.5%.

[0022] In this invention, the 2.5% Ag-doped sample exhibits the best performance, but other Ag-doped samples also show varying degrees of improvement in yield and selectivity compared to the original CISS (with 0% doping).

[0023] Preferably, the hydrothermal reaction conditions include a temperature of 160℃~200℃ and a time of 18~24h.

[0024] Preferably, solid-liquid separation is performed by centrifugation.

[0025] More preferably, the centrifugation speed is 8000-10000 rpm, even more preferably 9000-10000 rpm, and the centrifugation time is 3-10 min.

[0026] Preferably, the washing is performed using deionized water or ethanol; the drying is performed at 40–80°C for 12–20 hours.

[0027] Secondly, the present invention provides a spinel-based composite photocatalyst prepared by the above-mentioned preparation method.

[0028] Thirdly, the present invention provides an application of the above-mentioned spinel-based composite photocatalyst in photocatalytic CO2 conversion.

[0029] This invention prepares an Ag-doped CuInSnS4 spinel-based composite photocatalyst via a one-step hydrothermal method. It possesses Ag-doped active sites, promotes near-infrared light absorption, and exhibits tunable carrier mobility and high carrier density. The main mechanism of action and advantages of this invention are as follows: (1) In this invention, CuCl is used as the copper source; InCl3·4H2O is used as the indium source; SnCl4·5H2O is used as the tin source; TAA (thioacetamide) is used as the sulfur source; and AgNO3 is used as the doping ion source. Ag-doped CuInSnS4 spinel-based composite photocatalyst was prepared by a one-step hydrothermal method. The preparation method of this invention has the advantages of simple production process and suitability for industrial production.

[0030] (2) The introduction of Ag significantly increases the valence band level of CISS, thereby reducing its band gap and enhancing the light absorption capacity of the catalyst. Furthermore, Ag doping increases the charge density around S atoms on the catalyst surface, enhancing the Lewis basicity of the catalyst surface and improving its CO2 adsorption capacity. In addition, Ag doping improves the electron occupancy of antibonding states below the Fermi level of CISS, increasing the Cu-S bond level and thus enhancing the catalytic stability of CISS. (3) DFT calculations and in-situ DRIFTS show that Ag-CISS can effectively reduce the product of the rate-determining step ( ) and key reaction intermediates ( The Gibbs free energy of formation accelerates the conversion of CO2 to CH4, thereby improving the catalytic activity and selectivity of the catalyst for the product CH4, resulting in high average yield of catalytic products and few by-products. (4) The spinel-based composite photocatalyst of the present invention exhibits good photocatalytic performance. Among samples with different doping ratios, 2.5Ag-CISS, in particular, shows significantly better photocatalytic CO2 reduction activity than pure CISS. Under full-spectrum light irradiation and visible light irradiation, the average yield and selectivity of CH4 in the 2.5Ag-CISS system are the highest.

[0031] The present invention will be further described in detail below through specific embodiments.

[0032] Comparative Example 1 Preparation of CuInSnS4 in its original form: CuInSnS4 photocatalyst was prepared using a one-step hydrothermal method. The specific operation procedure is as follows: (1) First, add 2 mmol CuCl, 2 mmol SnCl4·5H2O and 2 mmol InCl3·4H2O to 80 mL of deionized water and stir vigorously to form a mixed solution; then dissolve 10 mmol thioacetamide (TAA) in the above mixed solution and stir at room temperature for 30 min to obtain the first solution; (2) The first solution was transferred to the inner liner of the reactor and sealed in a stainless steel container for hydrothermal reaction. The hydrothermal temperature was controlled at 180 °C for 24 hours. After the reaction was completed, the product was collected, washed three times with deionized water, and dried under vacuum at 60 °C for 12 hours, and recorded as the original CISS.

[0033] Example 1 A method for preparing a spinel-based composite photocatalyst includes the following steps: (1) First, add 2 mmol CuCl, 2 mmol SnCl4·5H2O and 2 mmol InCl3·4H2O to 80 mL of deionized water and stir vigorously to form a mixed solution; then dissolve 10 mmol thioacetamide (TAA) in the above mixed solution and stir at room temperature for 20 min to obtain the first solution; (2) Add AgCl, which accounts for 1% of the molar fraction of CuInSnS4, to the first solution and stir at room temperature for 10 min to obtain the second solution; (3) The second solution was transferred to the inner liner of the reactor and sealed in a stainless steel container for hydrothermal reaction. The hydrothermal temperature was controlled at 180 °C for 24 hours. After the reaction was completed, the product was collected, washed three times with deionized water, and dried under vacuum at 60 °C for 12 hours to obtain the spinel-based composite photocatalyst, denoted as 1Ag-CISS.

[0034] Example 2 The only difference from Example 1 is that the amount of AgCl in step (2) is adjusted to 2.5% of the molar fraction of CuInSnS4. The other steps and conditions are the same as in Example 1. The resulting spinel-based composite photocatalyst is denoted as 2.5Ag-CISS.

[0035] Example 3 The only difference from Example 1 is that the amount of AgCl in step (2) is adjusted to 4% of the molar fraction of CuInSnS4. The other steps and conditions are the same as in Example 1. The resulting spinel-based composite photocatalyst is denoted as 4Ag-CISS.

[0036] Application Example 1 The application of a spinel-based composite photocatalyst in the photocatalytic conversion of CO2 to CH4 includes the following steps: First, 30 mg of catalyst was dispersed in 5 ml of deionized water and sonicated for 10 min. Then, the catalyst was dropped into the reaction vessel and dried at 60 °C to facilitate dispersion. Next, the dried catalyst was placed in a quartz reactor, and high-purity CO2 gas was introduced into the reactor, ensuring consistent pressure inside and outside the reactor. Finally, 500 μL of deionized water was injected into the reactor through a rubber stopper as a reducing agent and proton source.

[0037] The reactor was placed under a 300 W xenon lamp to carry out the photocatalytic reaction.

[0038] Performance testing (1) XRD analysis was performed on the photocatalysts obtained in Examples 1-3 and Comparative Example 1, and SEM scanning was performed on the photocatalysts obtained in Example 1 and Comparative Example 1; the results are as follows: Figure 1 As shown.

[0039] Depend on Figure 1 As shown in (a) of the diagram, the XRD patterns of the Ag-CISS obtained in Examples 1-3 did not show any new peaks compared to the original CISS, indicating that Ag doping did not change the spinel structure of CISS. It is noteworthy that with the increase of Ag doping concentration, the main peak (311) and the high-proportion tetrahedral coordination center plane (440) showed a significant low-angle shift, preliminarily proving that Ag doping... + Replaced Cu in the CISS lattice + Site. Figure 1 The (bc) SEM test results showed that the introduction of Ag caused slight agglomeration of the catalyst, but did not significantly change its morphological characteristics.

[0040] (2) The Raman spectra of CISS obtained in Comparative Example 1 and Ag-CISS obtained in Examples 1-3 are as follows: Figure 2 As shown.

[0041] Depend on Figure 2 It can be seen that it is located between 50-150 cm -1 T band 2g The Raman peaks are attributed to Cu-S bond vibrations at the CISS tetrahedral sites. It was observed that with increasing Ag doping concentration, Ag-CISS compared to the original sample at T... 2g The increased half-peak width of the peak indicates that the addition of Ag disrupts the symmetry of the tetrahedral coordination structure in CISS, causing a certain degree of lattice distortion.

[0042] (3) The photocatalyst 1Ag-CISS obtained in Example 1 was subjected to transmission electron microscopy and elemental analysis, and the results are as follows: Figure 3 As shown.

[0043] Figure 3 In the study, (ab) it was observed that the photocatalyst 1Ag-CISS has a two-dimensional urchin-like morphology composed of nanosheets. Figure 3 Image (ej) shows a high-angle annular dark-field scanning transmission electron microscope (HAADF) image and elemental mapping (EDS) plot of Ag-CISS, observing that Cu, In, Sn, S, and the dopant element Ag are uniformly distributed on the sample surface (the large area distribution of Cu is due to the copper mesh). Figure 3As shown in the high-resolution transmission electron microscope (HRTEM) image in (c), with

[422] as the zone axis, there are two types of lattice fringes with spacings of 0.367 nm and 0.315 nm, corresponding to the (220) and (311) planes, respectively. The angle between the two planes is calculated to be 31.05°, which is consistent with... Figure 3 The reciprocal lattice vector angles shown by the (d) selected area electron diffraction (SEAD) are highly self-consistent, which also indicates that the exposed crystal plane of Ag-CISS is (422).

[0044] (4) The light absorption characteristics and band gap size of the prepared samples were studied using ultraviolet-visible-near-infrared diffuse reflectance spectroscopy (UV-Vis-NIR DRS), and the results are as follows: Figure 4 As shown.

[0045] Depend on Figure 4 As shown in (a), both CISS and the 2.5Ag-CISS obtained in Example 2 exhibit excellent light absorption capabilities across a broad spectral range below 1000 nm. Notably, after Ag doping, the light absorption band edge shows a certain degree of redshift, indicating that Ag doping narrows the band gap of CISS, effectively broadening the material's light absorption range. Furthermore, the light absorption intensity of Ag-CISS in the visible light band is significantly better than that of CISS. This result may be due to the introduction of impurity ions disrupting the lattice periodicity, enhancing light scattering, reducing light transmittance, and thus improving the material's light absorption capability. This enhanced light absorption capability helps promote the generation of photogenerated carriers, improving the catalytic activity of the photocatalyst.

[0046] The bandgap of the material was calculated using the Kubelka-Munk formula, and the corresponding Tauc plot was plotted. According to the research of P. Ojo et al., CISS is an indirect bandgap semiconductor, with n taken as 1 / 2, and the results are as follows... Figure 4 As shown in (b), the optical band gaps of CISS and 2.5Ag-CISS are 1.51 and 1.40 eV, respectively.

[0047] The flat-band potential (Φ) of CISS and 2.5Ag-CISS was measured using the Mott-Schottky method. FB ) was measured, such as Figure 4 As shown in Figure (cd), the MS curves exhibit a positive slope, indicating that both CISS and 2.5Ag-CISS are n-type semiconductors. Test results show that their flat-band potentials are -0.82 V and -0.80 V (reference electrode vs. Ag / AgCl), respectively, which translates to -0.60 V and -0.58 V at the standard hydrogen electrode (NHE). Based on the general rule that the conduction band potential of n-type semiconductors is typically about 0.10 V negative than the flat-band potential, ... Figure 4The test results in (e) show that the highest occupied state energy levels of CISS and Ag-CISS are 0.66 and 0.43 eV below the Fermi level, respectively. Finally, the Φ values ​​of CISS and 2.5Ag-CISS are calculated. CB They are -0.70 V and -0.68 V, respectively. The formula Φ is used to calculate the valence band potential. VB =Φ CB +Eg / e, further calculations show that the valence band potentials of CISS and 2.5Ag-CISS are 0.81V and 0.72V, respectively.

[0048] Based on the above analysis of the band gap and band structure of the photocatalysts, the band structures of CISS and 2.5Ag-CISS were finally determined. Figure 4 As shown in (f), the valence band position of 2.5Ag-CISS is more negative and the optical band gap is smaller.

[0049] (5) Photocatalytic performance test: After 4 hours of illumination following the photocatalytic reaction in Example 1, 500 μL of gas was extracted from the reactor using a 500 μL microsyringe and injected into a gas chromatograph for product analysis. CH4 was detected using a flame ionization detector (FID); CO was detected by FID after nickel-catalyzed reforming. Qualitative and quantitative analyses of the products were performed using the retention time method and standard curve method. Results are as follows... Figure 5 As shown.

[0050] Figure 5 The photocatalytic CO2 reduction performance of the prepared sample under full-spectrum light and visible light (>420 nm) irradiation was demonstrated. Figure 5 As shown in Figures (b) and (d), after 4 h of full-spectrum light irradiation without the addition of any sacrificial agent or photosensitizer, CISS exhibited a low level of photocatalytic CO2 reduction activity as is, with average production rates of the reduction products CH4 and CO at 4.6 and 2.1 μmol g, respectively, over 4 h. -1 h -1 However, doping CISS with Ag significantly improved the CO2 to CH4 conversion rate. Specifically, the average rates of CO2 reduction to CH4 and CO using 2.5Ag-CISS photocatalytic reduction reached 10.9 and 1.8 μmol g, respectively. -1 h -1Compared to CISS, the yield of CH4 increased by 1.4 times. Under the full spectrum, Ag-CISS achieved a selectivity of 85.83% for CH4, 1.3 times that of the original sample. These results indicate that the introduction of Ag improved the catalytic activity and selectivity of CISS for CH4. Notably, the catalytic activity of the doped samples for CH4 increased with increasing Ag doping concentration, with 2.5Ag-CISS exhibiting the highest yield and selectivity. However, when the doping concentration was further increased, the photocatalytic activity decreased. This may be because excessive Ag doping introduces too many impurity defects into the crystal, reducing carrier mobility and thus weakening the separation efficiency of photogenerated charges. Furthermore, catalytic activity experiments were conducted under visible light irradiation. Figure 5 As shown in (a) and (c), under visible light irradiation in the wavelength range greater than 420 nm, 2.5Ag-CISS also exhibits superior photocatalytic activity compared to CISS, with a CH4 yield and selectivity of 7.2 μmol g. -1 h -1 The percentages were 85.71%, which were 2.5 and 1.4 times that of the original sample, respectively. Combined with the previous analysis, Ag doping significantly improved the photocatalyst's response to visible light by narrowing the optical band gap and increasing the light scattering rate. This result further confirms the significant enhancing effect of Ag doping on photocatalytic performance.

[0051] The specific embodiments of the present invention described above do not constitute a limitation on the scope of protection of the present invention. Any other corresponding changes and modifications made in accordance with the technical concept of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. The application of a spinel-based composite photocatalyst in photocatalytic CO2 conversion, characterized in that, The preparation method of the spinel-based composite photocatalyst includes the following steps: The copper source, indium source, tin source and sulfur source are mixed evenly with water to form the first solution; Add a silver source to the first solution and mix thoroughly to obtain a second solution; The second solution was subjected to hydrothermal reaction, solid-liquid separation, washing and drying to obtain a spinel-based composite photocatalyst; The copper source includes CuCl; and / or, The indium source includes InCl3·4H2O; and / or, The tin source includes SnCl4·5H2O; and / or, The sulfur source includes thioacetamide; The first solution is obtained by first adding copper source, indium source and tin source to deionized water and stirring until uniform, and then adding sulfur source and stirring until uniform. The molar amount of silver added accounts for 0.5% to 7.5% of the molar amount of CuInSnS4.

2. The application according to claim 1, characterized in that, The molar ratio of the copper source, indium source, tin source and sulfur source is 1:1:1:(4-10).

3. The application according to claim 1, characterized in that, The ratio of copper source to water is 2 mmol: (60-100) mL.

4. The application according to claim 1, characterized in that, The silver source includes AgNO3 or AgCl.

5. The application according to claim 1, characterized in that, The conditions for the hydrothermal reaction include: a temperature of 160℃ to 200℃ and a time of 18 to 24 hours.

6. The application according to claim 1, characterized in that, The solid-liquid separation is performed by centrifugation; the drying is carried out at 40–80°C for 12–20 hours.

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