A method for preparing a core-shell structured photocatalyst for Cu-Ag alloy confined in MOFs and its application.

Abstract

This invention discloses a method for preparing a core-shell structured photocatalyst of Cu-Ag alloy confined in MOFs and its application. The purpose of this invention is to address the problems of low photocatalyst activity and poor C2 product selectivity in traditional photocatalytic CO2 reduction technology, as well as the issues of metal nanocluster leaching, low carbon dioxide mass transfer efficiency, and unclear spatial distribution of active sites in MOF-based photocatalysts. This invention employs an oil bath method to prepare Cu-Ag alloys. Then, an electrostatic self-assembly method is used to encapsulate ultrasmall Cu-Ag alloys within a visible-light-responsive Zr-MOF to prepare a core-shell structured photocatalyst (Cu-Ag / UiO66-NH2). This catalyst is applied to the photocatalytic conversion of CO2 to high-value-added C2 products.

Description

Technical Field

[0001] This invention relates to a method for preparing a core-shell structured photocatalyst of a Cu-Ag alloy confined in MOFs and its application. Background Technology

[0002] Environmental governance and energy conversion are crucial issues related to social development. The greenhouse effect caused by high carbon dioxide emissions is a serious problem that cannot be ignored. At the same time, the development of non-fossil fuels is also vital for sustainable human development. It is worth noting that carbon dioxide is not only a greenhouse gas but also a key carbon source. Converting carbon dioxide into high-value-added products (such as CO, CH4, CH3OH, C2H4, and CH3CH2OH) is considered an effective strategy for realizing carbon resource utilization.

[0003] Semiconductor-based photocatalysis technology boasts advantages such as environmental friendliness, ease of operation, and low cost. The photocatalytic process directly uses sunlight as an excitation source to drive the CO2 reduction reaction (CO2RR). Therefore, implementing photocatalytic CO2 emission reduction technology can convert atmospheric carbon dioxide into high-value-added chemicals, effectively addressing the energy and environmental crisis. However, currently, the products of photocatalytic CO2 reduction reactions are mainly limited to C1 products (such as CO, CH4, and HCOOH). The yield and selectivity for generating high-value C2 products (such as ethylene and ethanol) are very low, remaining a significant challenge. This is because the CO2 photoreduction process involves complex multi-electron reactions and C-C coupling processes, leading to high energy barriers, slow kinetics, and low efficiency in C2 product formation.

[0004] Previous studies have shown that metal nanoclusters with atomically precise structures have great potential in the catalytic conversion of carbon dioxide. To date, Cu-based metal nanoclusters are considered the most promising catalysts for the photocatalytic conversion of CO2 to C2 products. This is because the strong bonding between Cu active sites and *CO intermediates favors the C=C coupling reaction of the two *CO intermediates, thus achieving efficient C2 product formation. In recent years, various strategies (such as morphology control, surface modification, and metal alloying) have been developed to improve the conversion efficiency and carbon reduction product selectivity of Cu-based metal nanoclusters. Many researchers have prepared copper-based bimetallic alloys (such as Cu-Sn alloys and Au-Cu alloys) to increase the binding strength of reaction intermediates by altering the electronic structure and chemical properties of the photocatalyst, thereby increasing the C2 product selectivity. However, due to the instability of the alloy structure, auto-oxidation and aggregation often occur during photocatalysis.

[0005] Metal-organic frameworks (MOFs) possess high porosity, high chemical stability, and highly tunable physical / chemical properties. They can effectively suppress alloy migration and aggregation through nanoconfinement effects, and improve the lifetime and migration efficiency of photogenerated carriers. Using MOFs as supports for alloy encapsulation offers a favorable opportunity to stabilize alloys and construct efficient photocatalytic systems. Therefore, we have reason to believe that encapsulating Cu-Ag alloys in MOFs to prepare core-shell structured photocatalysts, by improving the bonding strength of key CO intermediates, holds promise for achieving highly selective C2 product generation. However, some MOF-based photocatalysts still face challenges such as metal nanocluster leaching, low carbon dioxide mass transfer efficiency, and unclear spatial distribution of active sites. Summary of the Invention

[0006] The purpose of this invention is to address the problems of low photocatalyst activity and poor C2 product selectivity in traditional photocatalytic CO2 reduction technology, as well as the issues of metal nanocluster leaching, low carbon dioxide mass transfer efficiency, and unclear spatial distribution of active sites in MOF-based photocatalysts. This invention provides a method for preparing a core-shell structured photocatalyst of Cu-Ag alloy confined in MOFs and its application.

[0007] This invention discloses a method for preparing a core-shell structured photocatalyst of Cu-Ag alloy confined in MOFs, comprising the following steps:

[0008] I. Synthesis of Cu-Ag alloy: AgNO3, Cu(OAc)2·H2O and polyvinylpyrrolidone were dissolved in ethylene glycol, and then N2 was introduced for degassing. After degassing, the mixture was sealed and transferred to an oil bath for heating. After heating, the mixture was cooled, centrifuged, and the solid product was collected. It was then washed with anhydrous ethanol, dried under vacuum, and redispersed with pure water to obtain an aqueous solution of Cu-Ag alloy.

[0009] II. Synthesis of Zr6-oxo cluster: ZrCl4 was added to a mixture of glacial acetic acid and isopropanol, heated and stirred at 120°C, and the solid product was collected by centrifugation and filtration. The product was washed twice with acetone and dried under vacuum at room temperature to obtain Zr6-oxo cluster.

[0010] III. Synthesis of Cu-Ag / UiO66-NH2: Zr6-oxo clusters and glacial acetic acid were mixed and stirred continuously until the Zr6-oxo clusters were completely dispersed. Then, an aqueous solution of Cu-Ag alloy was added and stirring was continued. 2-Aminoterephthalic acid and anhydrous ethanol were added and stirred for 72 h. The solid product was collected by centrifugation, washed three times, and dried under vacuum to obtain the core-shell structured photocatalyst Cu-Ag / UiO66-NH2 of MOF-confined Cu-Ag alloy.

[0011] This invention discloses a method for preparing a Cu-Ag / UiO66-NH2 core-shell structured photocatalyst using a "dual-bottleneck" strategy, and constructs a photocatalytic system based on this catalyst for the photocatalytic reduction of CO2 to C2 products. First, a Cu-Ag alloy was prepared using an oil bath method. Then, to avoid damage to the alloy structure during MOF synthesis, an electrostatic self-assembly method at room temperature was employed to prepare the Cu-Ag / UiO66-NH2 core-shell structured photocatalyst. Based on the reverse charge between the Cu-Ag alloy and UiO66-NH2, the Cu-Ag alloy can be electrostatically attracted to Zr6-oxo clusters. The Zr6-oxo clusters then further grow to achieve high dispersion of the Cu-Ag alloy in UiO66-NH2. Many individual Cu-Ag alloys were successfully encapsulated in UiO66-NH2, and the nano-confinement effect of UiO66-NH2 effectively prevented the aggregation of Cu-Ag alloys, thereby significantly improving the catalytic activity of Cu-Ag / UiO66-NH2. The Cu-Ag / UiO66-NH2 core-shell photocatalyst prepared in this invention exhibits a significant increase in electron transfer from UiO66-NH2 to the Cu-Ag alloy under visible light irradiation, resulting in improved separation efficiency and extended lifetime of photogenerated carriers. Under simulated visible light irradiation, the main products of Cu-Ag / UiO66-NH2 photocatalytic CO2 reduction are C2H5OH and C2H4 (with a C2 product selectivity of 91.67%), along with small amounts of CH4, CH3OH, and H2. The photocatalytic yields of C2H5OH and C2H4 are 38.64 and 64.72 μmol·g, respectively. -1 ·h -1 After 5 cycles, the yields of C2H5OH and C2H4 remained as high as 25.2 and 50.4 μmol·g, respectively. -1 ·h -1 Meanwhile, the Cu-Ag / UiO66-NH2 core-shell structured photocatalyst exhibits durability exceeding 30 hours. The strong bond strength between the Cu catalytic active sites and the *CO intermediate in Cu-Ag / UiO66-NH2 promotes C=C coupling reactions to generate C2 products. Furthermore, Cu-Ag / UiO66-NH2 also demonstrates strong adsorption of the *H intermediate, effectively inhibiting the hydrogen evolution reaction. Attached Figure Description

[0012] Figure 1 The images show TEM, HAADF-STEM, and EDS images of Cu-Ag / UiO66-NH2; where a is TEM, b is HAADF-STEM, and c1 to c4 are EDS.

[0013] Figure 2The UV-Vis diffuse reflectance (UV-vis DRS) spectra of UiO66-NH2 and Cu-Ag / UiO66-NH2 are shown.

[0014] Figure 3 X-ray diffraction (XRD) of UiO66-NH2 and Cu-Ag / UiO66-NH2;

[0015] Figure 4 X-ray photoelectron spectroscopy (XPS) and in-situ X-ray photoelectron spectroscopy SI-XPS (Zr 3d) spectra of UiO66-NH2 and Cu-Ag / UiO66-NH2;

[0016] Figure 5 X-ray photoelectron spectroscopy (XPS) and in-situ X-ray photoelectron spectroscopy (SI-XPS) (Cu 2p) for Cu-Ag / UiO66-NH2;

[0017] Figure 6 X-ray photoelectron spectroscopy (XPS) and in-situ X-ray photoelectron spectroscopy (SI-XPS) (Ag 3d) for Cu-Ag / UiO66-NH2 are shown.

[0018] Figure 7 The Tauc plot curve of UiO66-NH2;

[0019] Figure 8 Tauc plot of Cu-Ag / UiO66-NH2;

[0020] Figure 9 The Mott-Schottky curve for UiO66-NH2;

[0021] Figure 10 The Mott-Schottky curve for Cu-Ag / UiO66-NH2 is shown.

[0022] Figure 11 The photoluminescence (PL) spectra of UiO66-NH2 and Cu-Ag / UiO66-NH2 are shown.

[0023] Figure 12 Electrochemical impedance spectroscopy (EIS) for UiO66-NH2 and Cu-Ag / UiO66-NH2;

[0024] Figure 13 Transient photocurrent response (TPR) spectra of UiO66-NH2 and Cu-Ag / UiO66-NH2;

[0025] Figure 14Time-resolved photoluminescence (TRPL) spectra of UiO66-NH2 and Cu-Ag / UiO66-NH2;

[0026] Figure 15 The femtosecond transient absorption (fs-TA) spectrum of UiO66-NH2;

[0027] Figure 16 The femtosecond transient absorption (fs-TA) spectrum of Cu-Ag / UiO66-NH2;

[0028] Figure 17 The yields and selectivity of CO, CH4, CH3OH, H2, C2H4 and C2H5OH in the photocatalytic process of UiO66-NH2 and Cu-Ag / UiO66-NH2;

[0029] Figure 18 The yield of reduced carbon products of UiO66-NH2 under different conditions;

[0030] Figure 19 The yield of reduced carbon products of Cu-Ag / UiO66-NH2 under different conditions;

[0031] Figure 20 Cyclic stability of Cu-Ag / UiO66-NH2 photocatalytic CO2 reduction;

[0032] Figure 21 Durability of Cu-Ag / UiO66-NH2 photocatalytic CO2 reduction;

[0033] Figure 22 The apparent quantum yield (AQY) of Cu-Ag / UiO66-NH2 photocatalyst;

[0034] Figure 23 The in-situ Raman spectrum of Cu-Ag / UiO66-NH2;

[0035] Figure 24 The in-situ infrared spectrum of Cu-Ag / UiO66-NH2;

[0036] Figure 25 The CO2RR step diagram for Cu-Ag / UiO66-NH2;

[0037] Figure 26 The water photolysis step diagram of Cu-Ag / UiO66-NH2;

[0038] Figure 27 This is a schematic diagram of the photocatalytic CO2 reduction reaction pathway of Cu-Ag / UiO66-NH2. Detailed Implementation

[0039] 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.

[0040] Specific Implementation Method 1: This implementation method describes a method for preparing a core-shell structured photocatalyst of a Cu-Ag alloy confined in MOFs, which is carried out according to the following steps:

[0041] I. Synthesis of Cu-Ag alloy: AgNO3, Cu(OAc)2·H2O and polyvinylpyrrolidone were dissolved in ethylene glycol, and then N2 was introduced for degassing. After degassing, the mixture was sealed and transferred to an oil bath for heating. After heating, the mixture was cooled, centrifuged, and the solid product was collected. It was then washed with anhydrous ethanol, dried under vacuum, and redispersed with pure water to obtain an aqueous solution of Cu-Ag alloy.

[0042] II. Synthesis of Zr6-oxo cluster: ZrCl4 was added to a mixture of glacial acetic acid and isopropanol, heated and stirred at 120°C, and the solid product was collected by centrifugation and filtration. The product was washed twice with acetone and dried under vacuum at room temperature to obtain Zr6-oxo cluster.

[0043] III. Synthesis of Cu-Ag / UiO66-NH2: Zr6-oxo clusters and glacial acetic acid were mixed and stirred continuously until the Zr6-oxo clusters were completely dispersed. Then, an aqueous solution of Cu-Ag alloy was added and stirring was continued. 2-Aminoterephthalic acid and anhydrous ethanol were added and stirred for 72 h. The solid product was collected by centrifugation, washed three times, and dried under vacuum to obtain the core-shell structured photocatalyst Cu-Ag / UiO66-NH2 of MOF-confined Cu-Ag alloy.

[0044] Specific Implementation Method Two: This implementation method differs from Specific Implementation Method One in that the mass ratio of AgNO3, Cu(OAc)2·H2O, and polyvinylpyrrolidone in step one is 32-35:118-121:1050-1070. Its other features are the same as in Specific Implementation Method One.

[0045] Specific Implementation Method 3: This implementation method differs from Specific Implementation Method 1 or 2 in that the mass-to-volume ratio of polyvinylpyrrolidone to ethylene glycol in step 1 is 1.05–1.1 g: 40–60 mL. Everything else is the same as in Specific Implementation Method 1 or 2.

[0046] Specific Implementation Method Four: This implementation method differs from Specific Implementation Methods One to Three in that: in step one, heating is performed in an oil bath at 180°C for 15 minutes. Everything else is the same as in Specific Implementation Methods One to Three.

[0047] Specific Implementation Method Five: This implementation method differs from Specific Implementation Methods One to Four in that the mass-to-volume ratio of ZrCl4, glacial acetic acid, and isopropanol is 1.0–1.2 g: 15–20 mL: 25–30 mL. Everything else is the same as in Specific Implementation Methods One to Four.

[0048] Specific Implementation Method Six: This implementation method differs from Specific Implementation Methods One to Five in that the mass-to-volume ratio of Zr6-oxo cluster, glacial acetic acid, and Cu-Ag alloy aqueous solution is 1.0–1.2 g : 10–15 mL : 15–20 mL. Everything else is the same as in Specific Implementation Methods One to Five.

[0049] Specific Implementation Method Seven: This implementation method differs from Specific Implementation Methods One to Six in that the mass-to-volume ratio of Zr6-oxo cluster, 2-aminoterephthalic acid, and anhydrous ethanol is 1200–1250 mg: 400–450 mg: 20–30 mL. Everything else is the same as in Specific Implementation Methods One to Six.

[0050] Specific Implementation Method Eight: This implementation method differs from Specific Implementation Methods One to Seven in that the vacuum drying in steps one and three is carried out overnight in a vacuum oven at 60°C. Everything else is the same as in Specific Implementation Methods One to Seven.

[0051] Specific Implementation Method Nine: This implementation method describes the application of a core-shell structured photocatalyst of a Cu-Ag alloy confined in MOFs in photocatalytic CO2 reduction.

[0052] Specific Implementation Method Ten: This implementation method differs from Specific Implementation Method Nine in that it involves photocatalytic reduction of CO2 to C2 products. Everything else is the same as in Specific Implementation Method Nine.

[0053] The beneficial effects of the present invention are verified using the following embodiments:

[0054] Example 1: A method for preparing a core-shell structured photocatalyst of Cu-Ag alloy confined in MOFs, comprising the following steps: I. Synthesis of Cu-Ag alloy aqueous solution: First, 33.74 mg AgNO3, 119.65 mg Cu(OAc)2·H2O, and 1.06 g polyvinylpyrrolidone were dissolved in 50 mL ethylene glycol to obtain a mixed solution, which was then poured into a round-bottom flask. A gas tube carrying N2 was introduced into the mixed solution, and degassing was stopped after 20 minutes. The round-bottom flask was quickly sealed with a glass stopper and the mouth was wrapped with sealing film to prevent leakage. The round-bottom flask was transferred to an oil bath at 180 °C and fixed, heated for 15 minutes, then quickly transferred to a cold water bath at -15 to 0 °C for rapid cooling. The solid product was collected by centrifugation (8000 rpm, 8 minutes) and washed twice with anhydrous ethanol. The obtained solid product was transferred to a vacuum oven and vacuum dried overnight at 60 °C. The obtained product was redispersed with 25 mL of purified water and labeled as Cu-Ag alloy aqueous solution.

[0055] II. Synthesis of Zr6-oxo clusters: 1.2 g ZrCl4 was added to a mixture of 15 mL glacial acetic acid and 25 mL isopropanol, and the mixture was heated and stirred at 120 °C for 60 minutes. The solid products were collected sequentially by centrifugation and filtration. Finally, the collected white solid was washed twice with acetone and dried under vacuum at room temperature to obtain Zr6-oxo clusters.

[0056] III. Synthesis of Cu-Ag / UiO66-NH2: 1.2 g of Zr6-oxo clusters and 10 mL of glacial acetic acid were poured into a 100 mL beaker and stirred continuously until the Zr6-oxo clusters were completely dispersed. 15 mL of Cu-Ag alloy aqueous solution was poured into the beaker, and stirring was continued for 10 minutes. Then, 400 mg of 2-aminoterephthalic acid and 20 mL of anhydrous ethanol were added to the beaker, and the mixture was stirred for 72 hours. The solid product was collected by centrifugation (8000 rpm, 8 minutes) and washed three times with purified water. Finally, the solid product was dried overnight in a vacuum oven at 60 °C, and the product was labeled as Cu-Ag / UiO66-NH2 photocatalyst.

[0057] UiO66-NH2 was synthesized using the Zr6-oxo cluster prepared in step two as a control. The synthesis method was as follows: 300 mg of Zr6-oxo cluster was dissolved in 1.5 mL of acetic acid and 6 mL of H2O. Then, 100 mg of 2-aminoterephthalic acid and 5 mL of ethanol were added, and the reaction was stirred vigorously at room temperature for 24 hours. Finally, the solid product was collected by centrifugation, washed three times with methanol, and dried under vacuum at 60 °C overnight to obtain UiO66-NH2 (Zr-MOF).

[0058] The morphology of the Cu-Ag / UiO66-NH2 photocatalyst was analyzed by TEM and HAADF. Figure 1 As shown in Figure a, the Cu-Ag / UiO66-NH2 photocatalyst exhibits a hexahedral microcrystalline morphology with a particle size ranging from 50 to 55 nm. The contact region between the two different crystal planes of the Cu-Ag alloy and UiO66-NH2 indicates that the Cu-Ag alloy is uniformly distributed within UiO66-NH2. The HAADF-STEM image results show ( Figure 1 b) The bright Cu-Ag alloy is uniformly encapsulated in UiO66-NH2, confirming the successful implementation of the "dual-ship bottling" strategy. EDS results demonstrate the coexistence of Cu, Ag, and Zr in Cu-Ag / UiO66-NH2. Figure 1 (c1~c4). However, the elemental contents of Cu and Ag in Cu-Ag / UiO66-NH2 are relatively low, which is related to the fact that the Cu-Ag alloy is encased in a thicker shell (UiO66-NH2). Even with a stronger electron beam inserted from outside the material, it is still impossible to excite more secondary excited electrons in Cu and Ag in Cu-Ag / UiO66-NH2.

[0059] The agglomeration of the alloy was assessed using UV-Vis diffuse reflectance spectroscopy. Figure 2 The results show that the DRS of Cu-Ag / UiO66-NH2 is similar to that of UiO66-NH2, with no obvious agglomeration peaks in the 450–550 nm range, confirming the uniform distribution of Cu-Ag alloy in UiO66-NH2. These results indicate that the strong spatial confinement effect of UiO66-NH2 is beneficial for maintaining the particle morphology of small-sized alloys during synthesis, thus effectively preventing the aggregation of Cu-Ag alloys.

[0060] X-ray diffraction (XRD) results Figure 3 The peaks at 3.24°, 5.11°, and 18.72° represent the (111), (200), and (006) crystal planes of UiO66-NH2, respectively. In Cu-Ag / UiO66-NH2, the peaks at 37.26°, 46.13°, and 75.92° represent the (111), (200), and (220) crystal planes of Ag, while the peak at 42.73° represents the (111) crystal plane of Cu. Furthermore, the sharp diffraction peaks indicate that the synthesized Cu-Ag / UiO66-NH2 photocatalyst possesses good crystallinity.

[0061] To verify the charge transfer pathway in the Cu-Ag / UiO66-NH2 photocatalyst, XPS and SI-XPS techniques were used to detect the changes in electron density of various metal atoms in the excited state. In the absence of light illumination, Cu-Ag / UiO66-NH2 exhibited a higher binding energy transition than UiO66-NH2. Figure 4This may be related to the transfer of electrons from UiO66-NH2 to the Cu-Ag alloy. Under illumination, the binding energy of Zr 3d in Cu-Ag / UiO66-NH2 shows a positive shift ( Figure 4 This indicates a decrease in the electron density of UiO66-NH2. Simultaneously, the binding energies of Cu 2p and Ag 3d in Cu-Ag / UiO66-NH2 exhibit a negative shift. Figure 5 The result (6) indicates an increase in the electron density of the Cu-Ag alloy. After the illumination ends, the binding energy of Cu 2p and Ag 3d in Cu-Ag / UiO66-NH2 returns to its original position ( Figures 4-6 The change in binding energy indicates the interfacial migration path of photogenerated carriers from UiO66-NH2 to the Cu-Ag alloy, confirming the reversibility of photoinduced electron transfer.

[0062] Figure 7 and Figure 8 The Tauc plots of UiO66-NH2 and Cu-Ag / UiO66-NH2 are given. The E values ​​of UiO66-NH2 and Cu-Ag / UiO66-NH2 are calculated from the Tauc plots. g The values ​​were 2.87 eV and 1.57 eV, respectively. After encapsulation with Cu-Ag alloy, the EV of the Cu-Ag / UiO66-NH2 photocatalyst... g The value decreased significantly from 2.87 eV to 1.57 eV, indicating that the encapsulation of the alloy is beneficial to the rapid separation of photogenerated carriers.

[0063] Next, the flat-band potential (E) of the semiconductor is determined using the Mott-Schottky (MS) plot. fb In MS charts, the longest straight line segment is used as the tangent (X-axis intercept). For example... Figure 9 and 10 As shown, the E values ​​of UiO66-NH2 and Cu-Ag / UiO66-NH2 are... fb The values ​​are -0.81 eV and -0.83 eV, respectively (relative to Ag / AgCl, pH = 7). In the MS plot, both UiO66-NH2 and Cu-Ag / UiO66-NH2 exhibit positive slopes, therefore they are N-type semiconductors. Typically, the EV of N-type semiconductors... fb Value ratio E CB The value is positive 0.1–0.3 eV. Therefore, the conduction band (E) of UiO66-NH2 and Cu-Ag / UiO66-NH2 can be calculated. CB The values ​​were -0.61 eV and -0.63 eV (relative to NHE, pH = 0), respectively. Finally, the valence bands (E values) of UiO66-NH2 and Cu-Ag / UiO66-NH2 were calculated. VBThe values ​​were 2.26 eV and 0.94 eV (relative to NHE, pH = 0), respectively. Clearly, the valence and conduction band edges of Cu-Ag / UiO66-NH2 cross well through the redox potentials of CO2 / C2H4 = -0.38 eV and CO2 / C2H5OH = -0.35 eV, confirming the thermodynamic feasibility of using Cu-Ag / UiO66-NH2 for photocatalytic CO2RR.

[0064] The photoexcited charge carrier recombination behavior of the photocatalyst was studied using photoluminescence (PL) spectroscopy. Generally, a stronger emission peak in the PL spectrum indicates a higher recombination efficiency of photogenerated charge carriers. Figure 11 As shown, UiO66-NH2 exhibits a strong photoluminescence (PL) emission peak near 430 nm, while the PL peak intensity of Cu-Ag / UiO66-NH2 decreases sharply. This result indicates that photoexcited charge carrier recombination is effectively suppressed in Cu-Ag / UiO66-NH2.

[0065] The intrinsic interfacial resistance of charge carriers was analyzed using electrochemical impedance spectroscopy (EIS). Generally, a smaller arc radius in the EIS spectrum indicates a lower interfacial charge transfer resistance. The EIS spectra were obtained through equivalent Randle circuit modeling. Figure 12 As shown, the arc radius of the Cu-Ag / UiO66-NH2 photocatalyst is much larger than that of UiO66-NH2, indicating that the Cu-Ag / UiO66-NH2 photocatalyst has a higher charge transfer resistance, which can promote the migration of charge carriers and thus improve photocatalytic activity.

[0066] Next, the recombination capacity of electrons and holes in the photocatalyst was analyzed using transient photocurrent response (TPR). Lower transient photocurrent intensities indicate rapid electron-hole recombination. For example... Figure 13 As shown, UiO66-NH2 (0.19mA / cm 2 The photocurrent density of Cu-Ag / UiO66-NH2 is much lower than that of Cu-Ag / UiO66-NH2 (0.33 mA / cm). 2 The above results indicate that Cu-Ag / UiO66-NH2 can significantly improve the separation efficiency of electrons and holes in photocatalysts.

[0067] Finally, the real-time charge separation kinetics were analyzed using time-resolved photoluminescence (TRPL) spectroscopy. Generally, a longer fluorescence lifetime in a material indicates easier recombination of electrons and holes. Figure 14 As shown, the average carrier lifetime (τ) of UiO66-NH2 and Cu-Ag / UiO66-NH2 is... avgThe mean lifetimes are 3.206 ns and 2.098 ns, respectively. These results indicate that Cu-Ag / UiO66-NH2 has the shortest average carrier lifetime, suggesting that its charge carrier diffusion length is the longest, effectively suppressing electron-hole recombination.

[0068] Finally, the photogenerated charge carrier dynamics of UiO66-NH2 and Cu-Ag / UiO66-NH2 were investigated using femtosecond transient absorption (fs-TA) spectroscopy. Figure 15 and 16 As shown, the photoinduced excited-state absorption band of UiO66-NH2 exhibits positive absorption in the range of 475–800 nm (XA1). The absorption spectrum of Cu-Ag / UiO66-NH2 shows a negative broadband bleaching signal in the range of 450–500 nm (XB), and a significant positive absorption band at 650 nm (XA2). Figure 13 This is attributed to the exciton transition from a low-energy state to a high-energy state and the ground-state bleaching signal (GSB). Compared to UiO66-NH2, the XA2 band intensity of Cu-Ag / UiO66-NH2 is significantly reduced near 650 nm, confirming that the introduction of the Cu-Ag alloy promotes the separation of photogenerated carriers. These results indicate that Cu, as an active site, can trap photoelectrons, thereby promoting the separation of photogenerated carriers.

[0069] The photocatalytic performance of UiO66-NH2 and Cu-Ag / UiO66-NH2 photocatalysts in aqueous media under visible light irradiation (λ>400nm) was investigated. Figure 17 As shown, the CO generation rate of the UiO66-NH2 photocatalytic system is 57.25 μmol·g. -1 ·h -1 This indicates that the system is conducive to the conversion of CO2 to CO. The Cu-Ag / UiO66-NH2 photocatalyst exhibits a high C2H5OH content (38.64 μmol·g). -1 ·h -1 ) and C2H4 (64.72 μmol·g -1 ·h -1 Yield. The above results show that the prepared Cu-Ag / UiO66-NH2 photocatalyst can still maintain high catalytic activity for CO2 reduction without external additives (such as sacrificial agents and carbonate solutions). Besides other gaseous products, only trace amounts of H2 (1.13 μmol·g⁻¹) were measured on the Cu-Ag / UiO66-NH2 photocatalyst. -1 ·h -1The results indicate that the hydrogen evolution reaction was well suppressed during the photocatalytic process. Therefore, the Cu-Ag / UiO66-NH2 photocatalyst exhibits high selectivity for the C2-(C2H4,C2H5OH) (91.67%) product.

[0070] To further explore the generation pathways of reduction products in the UiO66-NH2 and Cu-Ag / UiO66-NH2 photocatalytic systems, six control experiments were conducted under different conditions. For example... Figure 18 and 19 As shown, under dark conditions, the yields of carbon reduction products in the UiO66-NH2 and Cu-Ag / UiO66-NH2 systems are very low (<8 μmol·g). -1 ·h -1 This indicates that UiO66-NH2 and Cu-Ag / UiO66-NH2 exhibit almost no catalytic activity in the absence of light. Even with light but without a catalyst, the yields of carbon reduction products in the UiO66-NH2 and Cu-Ag / UiO66-NH2 photocatalytic systems are extremely low (<3 μmol·g⁻¹). -1 ·h -1 The above results confirm that the light source and photocatalyst play a decisive role in photocatalytic CO2 reduction. Under N2 or Ar atmospheres, the yield of reduced carbon products in the UiO66-NH2 and Cu-Ag / UiO66-NH2 photocatalytic systems is zero, meaning that the generation of reduced carbon products originates from the CO2 reduction reaction (CO2RR). Finally, with the addition of an additional sacrificial agent (triethanolamine, TEOA), the yield of reduced carbon products in the UiO66-NH2 and Cu-Ag / UiO66-NH2 photocatalytic systems is slightly higher than that under standard conditions, which may be related to the small amount of gas generated by the self-decomposition of TEOA. This indicates that Cu-Ag / UiO66-NH2 can achieve efficient separation of charge carriers without the addition of traditional sacrificial agents, thus maintaining high photocatalytic activity.

[0071] Depend on Figure 20 It can be seen that during the cyclic photocatalytic CO2 reduction process (each cycle lasting 4 hours), the Cu-Ag / UiO66-NH2 photocatalyst consistently maintained high yields of C2H4 and C2H5OH. After the first CO2RR reaction, the yields of C2H4 and C2H5OH in the system reached as high as 63.1 and 37.5 μmol·g, respectively. -1 ·h -1 After five cycles of photocatalytic CO2 reduction, the yields of C2H4 and C2H5OH in the system remained as high as 50.4 and 25.2 μmol·g, respectively. -1 ·h -1 This indicates that Cu-Ag / UiO66-NH2 has excellent photocatalytic cycle stability.

[0072] Figure 21 The results showed that even after prolonged photocatalytic treatment (30 hours), the yields of C2H4 and C2H5OH in the Cu-Ag / UiO66-NH2 system still reached 1926 and 1152 μmol·g, respectively. -1 The above results demonstrate that the prepared Cu-Ag / UiO66-NH2 photocatalyst exhibits excellent durability for CO2 photocatalytic reduction.

[0073] Figure 22 The apparent quantum yield (AQY) of the Cu-Ag / UiO66-NH2 photocatalyst at different visible wavelengths is shown. The AQY value of the Cu-Ag / UiO66-NH2 photocatalyst is 10.1% (385 nm). Furthermore, the AQY (orange box) of the Cu-Ag / UiO66-NH2 photocatalyst matches well with the shape of the DRS absorption spectrum (orange line), further demonstrating the high photocatalytic activity of the Cu-Ag / UiO66-NH2 photocatalyst under visible light irradiation.

[0074] The intermediates of Cu-Ag / UiO66-NH2 in the photocatalytic CO2RR process were determined by in-situ Raman spectroscopy under visible light irradiation. With increasing irradiation time, C≡O was observed at 2091 cm⁻¹ in Cu-Ag / UiO66-NH2. -1 The gradually increasing tensile vibration peaks confirm the presence of the *CO intermediate. Figure 23 ).

[0075] Then, through in-situ infrared (DRIFTS) spectroscopy ( Figure 24 The generation and transformation of intermediates in the photocatalytic CO2RR process of Cu-Ag / UiO66-NH2 were further determined. In the in-situ DRIFTTS spectrum of Cu-Ag / UiO66-NH2, the intermediates at 1600–1800 cm⁻¹ were observed. -1 H2O signal within range and 1513cm -1 The presence of a CO2 signal at a certain depth confirms the adsorption of H2O and CO2 on the Cu-Ag / UiO66-NH2 surface. The in-situ DRIFTS spectrum of Cu-Ag / UiO66-NH2 shows a concentration in the 1100–1300 cm⁻¹ range. -1 The presence of *COOH signal confirms the conversion of *CO2 to *COOH. (At 2000–2100 cm⁻¹) -1The appearance of a C≡O stretching vibration peak at 1541 cm⁻¹ confirms the formation of the *CO intermediate on Cu-Ag / UiO₆₆-NH₂. Furthermore, with increasing irradiation time, the infrared signal of C≡O gradually increases (i.e., the Stark effect) and shifts to lower wavenumbers, indicating the accumulation of the *CO intermediate on Cu-Ag / UiO₆₆-NH₂. After a period of irradiation, the accumulated *CO is transformed into other reduced species (*COCO and other hydroxyl-containing intermediates). -1 The presence of C=C infrared signals is related to the formation of the *COCO intermediate, which determines the high selectivity of the C2-(C2H4, C2H5OH) product.

[0076] The photocatalytic CO2 reduction mechanism of Cu-Ag / UiO66-NH2 was further elucidated through Gibbs free energy calculations. Figure 25 As shown, CO2 molecules adsorbed on Cu-Ag / UiO66-NH2 are first reduced by two electrons to the *CO intermediate. Then, the *CO intermediate readily desorbs from the Ag site on Cu-Ag / UiO66-NH2 to generate CO. The generated CO migrates to nearby Cu sites, forming a stable *CO intermediate, which promotes the C=C coupling reaction. Therefore, the formation of *OC-CHO is a downhill reaction with a low energy barrier. The energy barrier (ΔG) for the formation of *OHC-CHO on Cu-Ag / UiO66-NH2 is... *OHC-CHO =0.81 eV) is much lower than that of the *HOC-CHO and *OC-CH2O intermediates. Subsequently, the *HOHC-CH2O intermediate is generated through a series of hydrogenation reactions of *OHC-CHO. Further hydrogenation and dehydroxylation reactions of *HOHC-CH2O generate the *HC-CH2O and *HO2HC-CH2O intermediates. Finally, the *HC-CH2O and *HO2HC-CH2O intermediates are converted to C2H5OH and C2H4, respectively.

[0077] like Figure 26 As shown, the reaction energy barrier of H2O photolysis in Cu-Ag / UiO66-NH2 under alkaline conditions was investigated. The construction of the alkaline HER reaction pathway includes the early stages of hydrolysis to generate H* (Volmer step) and H2 generation (Tafel step or Heyrovsky step). The results show that the strong negative H* adsorption barrier of Cu-Ag / UiO66-NH2 hinders H2 generation, leading to a slower HER kinetics. Therefore, the H2 yield of the Cu-Ag / UiO66-NH2 system is low ( Figure 17 ).

[0078] Based on the above analysis, a Cu-Ag / UiO66-NH2 photocatalytic CO2 reduction reaction pathway is proposed ( Figure 27First, CO2 is adsorbed onto Cu-Ag / UiO66-NH2 to generate *CO2, which is then protonated to produce the *COOH intermediate. During this process, a *CO intermediate is generated via a proton-coupled electron transfer (PCET) step. Two *CO intermediates couple to form the *COCO intermediate, which is then hydrogenated to generate the *COCHO intermediate. The *COCHO intermediate then undergoes a PCET step to generate *CH2OCHOH and *CH2OCH2O intermediates, respectively. The *CH2OCHOH intermediate undergoes a dehydroxylation reaction to generate the *CH2OCH intermediate, which is then hydrogenated to generate the *CH2OCH2 intermediate, and finally deoxygenated to generate C2H4. The CH2OCH2O intermediate undergoes hydrogenation to generate the CH2OCH2OH intermediate, which is then deoxygenated to generate CH2OCH3, and finally hydrogenated to generate C2H5OH.

Claims

1. A method for preparing a core-shell structure photocatalyst of MOFs confined Cu-Ag alloy, characterized in that, The preparation method is carried out according to the following steps: I. Synthesis of Cu-Ag alloy: AgNO3, Cu(OAc)2·H2O and polyvinylpyrrolidone are dissolved in ethylene glycol, then N2 is introduced for degassing, after degassing is completed, it is sealed, moved into an oil bath pot for heating, after heating is completed, it is cooled, centrifuged, the solid product is collected, washed with anhydrous ethanol, then vacuum dried, redispersed with pure water to obtain a Cu-Ag alloy aqueous solution; wherein the mass ratio of AgNO3, Cu(OAc)2·H2O and polyvinylpyrrolidone is 32-35:118-121:1050-1070, and the mass-volume ratio of polyvinylpyrrolidone and ethylene glycol is 1.05-1.1 g:40-60 mL; II. Synthesis of Zr6-oxo cluster: ZrCl4 is added to a mixture of glacial acetic acid and isopropyl alcohol, heated and stirred at 120℃, then the solid product is collected by centrifugation and suction filtration, washed with acetone twice, and vacuum dried at room temperature to obtain Zr6-oxo cluster; the mass-volume ratio of ZrCl4, glacial acetic acid and isopropyl alcohol is 1.0-1.2 g:15-20 mL:25-30 mL; III. Synthesis of Cu-Ag / UiO66-NH2: Zr6-oxo cluster and glacial acetic acid are mixed and continuously stirred until the Zr6-oxo cluster is completely dispersed, then the Cu-Ag alloy aqueous solution is added, continuously stirred, then 2-amino terephthalic acid and anhydrous ethanol are added, stirred for 72h, the solid product is collected by centrifugation, washed three times, and vacuum dried to obtain the MOFs confined Cu-Ag alloy core-shell structure photocatalyst Cu-Ag / UiO66-NH2; wherein the mass-volume ratio of Zr6-oxo cluster, glacial acetic acid and Cu-Ag alloy aqueous solution is 1.0-1.2 g:10-15 mL:15-20 mL; the mass-volume ratio of Zr6-oxo cluster, 2-amino terephthalic acid and anhydrous ethanol is 1200-1250 mg:400-450 mg:20-30 mL.

2. The preparation method of the core-shell structure photocatalyst of MOFs confined Cu-Ag alloy according to claim 1, characterized in that, In step one, heating is carried out in an oil bath pot at 180℃ for 15 minutes.

3. The method for preparing a core-shell structure photocatalyst of MOFs confined Cu-Ag alloy according to claim 1, characterized in that, The vacuum drying in steps one and three is vacuum drying overnight at 60℃ in a vacuum oven.

4. Application of the MOFs confined Cu-Ag alloy core-shell structure photocatalyst obtained by the preparation method of claim 1 in photocatalytic reduction of CO2.

5. Use according to claim 4, characterized in that Photocatalytic reduction of CO2 into C2 products.

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