Alpha-FeO / CdS / MXene heterojunction photoreduction carbon dioxide catalyst based on surface oxygen vacancy regulation and preparation method

Abstract

The invention relates to an alpha-Fe2O3 / CdS / MXene heterojunction photoreduction carbon dioxide catalyst based on surface oxygen vacancy regulation and a preparation method of the alpha-Fe2O3 / CdS / MXene heterojunction photoreduction carbon dioxide catalyst. According to the catalyst, a tetramer oxygen vacancy cluster (the density reaches 2.1 * 10 < 15 > spins / g) is constructed on an alpha-Fe2O3 (001) crystal face, the tetramer oxygen vacancy cluster and CdS form an S-type heterojunction, and meanwhile, the tetramer oxygen vacancy cluster and Mo2CTxMXene form a Schottky junction. Under visible light, the space time yield of the catalyst for reducing CO2 into methanol reaches 9.8 mu mol.g <-1 >. H <-1 >, and the selectivity exceeds 85%. In a microgravity environment, the CO2 conversion rate is increased to 18.7%, and the methanol yield reaches 18.7 [mu] mol.g <-1 >. H <-1 >; the design of the catalyst breaks through the bottleneck of the traditional CO2 photocatalytic reduction technology. Through precise construction of oxygen vacancy clusters, the CO2 adsorption and activation capability is significantly enhanced, the reaction activation energy barrier is reduced, and the carrier life is prolonged. Under the synergistic effect of the S-type heterojunction and the Schottky-type heterojunction, the separation and transmission efficiency of photon-generated carriers is further improved.

Description

Technical Field

[0001] The present invention relates to the technical field of photocatalytic materials, and specifically to an α-Fe2O3 / CdS / MXene heterojunction photoreduction carbon dioxide catalyst based on surface oxygen vacancy regulation and a preparation method thereof. Background Art

[0002] 1. Bottlenecks of CO2 photoreduction technology Photocatalytic reduction of CO2 is a technology that uses solar energy to convert carbon dioxide into high-value-added chemicals (such as methanol and methane), which has important environmental and energy significance. However, current photocatalytic reduction technology faces the following major bottlenecks: High activation energy barrier: The CO2 molecule has a linear symmetrical structure, and the C=O bond energy is as high as 750 kJ / mol. Traditional catalysts (such as TiO2 and g-C3N4) need to overcome an activation energy barrier of about 1.90 eV, resulting in a low reaction rate and difficulty in achieving efficient CO2 conversion.

[0003] Severe photogenerated carrier recombination: During the photocatalytic process, the recombination of photogenerated electrons and holes leads to the waste of light energy and reduces catalytic efficiency. For example, bulk oxygen vacancies (OVs) in Bi2O2CO3 can become carrier recombination centers, shortening the carrier lifetime to 2.1 ns, seriously affecting photocatalytic performance.

[0004] Low interfacial charge separation efficiency: While conventional Z-type heterojunctions can facilitate the separation of photogenerated carriers, they weaken redox capacity by 20-40%. Furthermore, the interfacial contact area of ​​MXene-based catalysts is typically less than 30%, limiting electron transfer efficiency.

[0005] Low reaction selectivity: During the photocatalytic reduction of CO2, the water reduction potential (0 V vs. RHE) is lower than the CO2 reduction potential (-0.24 V vs. RHE), resulting in a competitive hydrogen evolution reaction (HER), which makes the selectivity of target products such as methanol usually less than 60%.

[0006] 2. Breakthroughs and limitations of oxygen vacancy regulation Oxygen vacancies (OVs) play an important role in semiconductor materials and can significantly affect the electronic structure and surface properties of the materials. However, traditional oxygen vacancy regulation has the following problems: Negative effects of bulk oxygen vacancies: Bulk oxygen vacancies often become recombination centers for photogenerated carriers, resulting in a shortened carrier lifetime and reduced photocatalytic performance.

[0007] The distribution of oxygen vacancies is difficult to precisely control: it is difficult to achieve precise distribution of oxygen vacancies on the catalyst surface, resulting in limited promotion of the photocatalytic reaction.

[0008] 3. Shortcomings of existing heterojunction technology Limitations of traditional Z-type heterojunction: Although traditional Z-type heterojunction can promote the separation of photogenerated carriers, it weakens the redox capacity by 20-40%, limiting the efficiency of the photocatalytic reaction.

[0009] Interface contact problems of MXene-based catalysts: The interface contact area of ​​MXene-based catalysts is usually less than 30%, which limits the electron transfer efficiency and affects the photocatalytic performance.

[0010] Therefore, those skilled in the art provide an α-Fe2O3 / CdS / MXene heterojunction photoreduction carbon dioxide catalyst and a preparation method based on surface oxygen vacancy regulation. Summary of the Invention

[0011] The purpose of the present invention is to provide a catalyst for photoreduction of carbon dioxide based on the regulation of surface oxygen vacancies in an α-Fe2O3 / CdS / MXene heterojunction and a preparation method thereof, so as to solve the problems raised in the above background technology.

[0012] To achieve the above object, the present invention provides the following technical solutions: High activation energy barrier of CO2 molecules: CO2 molecules have a linear symmetrical structure, and the C=O bond energy is as high as 750 kJ / mol. Traditional catalysts such as TiO2 and g-C3N4 need to overcome an activation energy barrier of about 1.90 eV, which makes the reaction rate low and difficult to achieve efficient CO2 conversion.

[0013] Severe photogenerated carrier recombination: During the photocatalytic process, the recombination of photogenerated electrons and holes leads to the waste of light energy and reduces catalytic efficiency. For example, bulk oxygen vacancies (OVs) in Bi2O2CO3 can become carrier recombination centers, shortening the carrier lifetime to 2.1 ns, seriously affecting photocatalytic performance.

[0014] Low interfacial charge separation efficiency: While conventional Z-type heterojunctions can facilitate the separation of photogenerated carriers, they weaken the redox capacity by 20-40%. Furthermore, the interfacial contact area of ​​MXene-based catalysts is typically less than 30%, limiting electron transfer efficiency.

[0015] Low reaction selectivity: During the photocatalytic reduction of CO2, the water reduction potential (0 V vs. RHE) is lower than the CO2 reduction potential (-0.24 V vs. RHE), resulting in a competitive hydrogen evolution reaction (HER), which makes the selectivity of target products such as methanol usually less than 60%.

[0016] (3) Research motivation of this invention To address these challenges, the present invention aims to develop a highly efficient, stable, and suitable catalyst for photocatalytic CO2 reduction in complex environments through innovative catalyst design and preparation methods. By manipulating surface oxygen vacancies and designing heterojunctions, the present invention addresses the challenges inherent in conventional catalysts, improving the efficiency and selectivity of photocatalytic CO2 reduction and providing a novel technical approach to achieving these goals.

[0017] 2. Catalyst Design Principles (1) Oxygen vacancy regulation mechanism Oxygen vacancies (OVs) play an important role in semiconductor materials, significantly affecting their electronic structure and surface properties. This study achieves precise control of the catalyst surface electronic structure by constructing tetrameric oxygen vacancy clusters on the α-Fe2O3 (001) crystal plane. Specifically, the introduction of oxygen vacancy clusters has the following advantages: 1. Enhance CO2 adsorption and activation capabilities Oxygen vacancy clusters can generate a localized electric field, polarizing the C=O bond of CO2 molecules, reducing the CO2 adsorption energy and the formation barrier of the *COOH intermediate. DFT calculations show that oxygen vacancy clusters reduce the CO2 adsorption energy from -0.21 eV to -0.67 eV and the formation barrier of the *COOH intermediate by 0.52 eV. The formation of this localized electric field can significantly improve the CO2 adsorption and activation efficiency, promoting the photocatalytic reaction.

[0018] 2. Extend carrier lifetime In traditional catalysts, bulk oxygen vacancies often serve as recombination centers for photogenerated carriers, shortening their lifetime. This new method, using a NaBH4 gradient reduction method, precisely constructs oxygen vacancies on the surface, avoiding the formation of bulk oxygen vacancies. Experiments have shown that surface oxygen vacancy clusters can significantly extend the carrier lifetime from 2.1 ns in traditional materials to 9.8 ns, improving the efficiency of photogenerated carrier utilization.

[0019] 3. Optimizing surface active sites Oxygen vacancy clusters can provide abundant active sites, such as Fe 2+ These active sites can effectively adsorb and activate CO2 molecules, promoting the photocatalytic reaction. EPR spectroscopy shows that the density of oxygen vacancy clusters in the present invention reaches 2.1×10¹ 5 spins / g, providing sufficient active sites for the adsorption and activation of CO2.

[0020] (2) Double heterojunction synergistic mechanism The present invention designs S-type α-Fe2O3 / CdS heterojunction and Schottky-type CdS / Mo2CT xHeterojunction, through the synergistic effect of double heterojunction, achieves efficient separation and transmission of photogenerated carriers: 1. S-type α-Fe2O3 / CdS heterojunction An S-type heterojunction is formed between α-Fe₂O₃ and CdS, with a built-in electric field strength of 8.7 kV / cm. This heterojunction retains the strong redox potential of α-Fe₂O₃ (+2.4 eV in the valence band) and the high photogenerated carrier mobility of CdS (-0.9 eV in the conduction band). This effectively promotes the separation of photogenerated electrons and holes, avoiding the weakened redox capacity found in traditional Z-type heterojunctions.

[0021] 2. Schottky-type CdS / Mo2CT x heterojunction Using Mo2CT x The high work function (5.2 eV) of MXene forms a Schottky barrier on the CdS surface, further promoting the separation and transport of photogenerated carriers. MXene's two-dimensional structure and high surface area effectively adsorb CO2 molecules, while its surface functional groups (such as -OH and -F) provide additional active sites, enhancing CO2 adsorption and activation.

[0022] (3) Adaptability of Space Applications In a microgravity environment, traditional photocatalytic reaction systems are often limited by the low gas-liquid-solid three-phase mass transfer efficiency. This invention significantly improves the mass transfer efficiency in a microgravity environment by optimizing the catalyst interface structure and reaction system design.

[0023] Specifically: 1. Three-phase interface optimization By constructing a sandwich-layered heterojunction structure, the contact area and mass transfer path of the gas-liquid-solid three phases were optimized, which increased the CO2 mass transfer efficiency by 50% in a microgravity environment.

[0024] 2. Reaction system design The continuous flow photothermal reactor, combined with a photothermal synergistic temperature control module, achieved efficient operation of the reaction system. In a microgravity environment, the CO2 conversion rate reached 18.7% and the methanol yield reached 18.7 μmol·g -1 ·h -1 , far higher than the performance in the earth environment.

[0025] 3. Catalyst Preparation Method The present invention provides a simple and efficient method for preparing a catalyst, the specific steps of which are as follows: (I) Synthesis of oxygen vacancy cluster-modified α-Fe2O3 1. Raw material preparation FeCl₃·6H₂O and NaBH₄ (15 ± 5 g / L) were dissolved in an ethanol / water mixture and stirred to ensure thorough dispersion. The NaBH₄ concentration is critical. When the concentration is below 10 g / L, oxygen vacancy clusters cannot form. However, when the concentration is above 20 g / L, bulk oxygen vacancies increase, leading to an increase in the carrier recombination rate.

[0026] 2. Hydrothermal reaction The mixed solution was transferred to a polytetrafluoroethylene-lined hydrothermal reactor and reacted at 180°C for 12 hours. During the reaction, the temperature was recorded every 2 hours to ensure that the reaction temperature remained stable within the range of 180°C ± 2°C.

[0027] 3. Product processing After the reaction, the mixture was cooled to room temperature and washed three times with ethanol and deionized water, each time centrifuged at 8000 rpm for 10 minutes to remove unreacted raw materials and impurities. Finally, the product was dried in a vacuum oven at 60°C for 12 hours to obtain an α-Fe2O3 sample modified with oxygen vacancy clusters.

[0028] (2) CdS in situ growth and interface bonding 1. Addition of interfacial coupling agent The oxygen vacancy cluster modified α-Fe2O3 sample was dispersed in deionized water, and an appropriate amount of L-cysteine ​​was added as an interface coupling agent. The -SH group of L-cysteine ​​can react with Fe 3+ Forming Fe-S bonds with a bond energy of 2.3 eV, reducing the interfacial potential; Compared with the prior art, the present invention has the following beneficial effects: 1. Improved catalytic performance High activity and selectivity: space-time yield reaches 9.8 μmol·g -1 ·h -1 , selectivity >85%; in microgravity environment, CO2 conversion rate increased to 18.7%, and methanol yield reached 18.7μmol·g -1 ·h -1 .

[0029] Improved carrier separation and transmission efficiency: The S-type heterojunction and Schottky-type heterojunction work synergistically to extend the carrier lifetime to 9.8 ns, significantly improving the separation and transmission efficiency.

[0030] 2. Structure and Stability Precise control of oxygen vacancy clusters: The surface distribution of oxygen vacancies is achieved through the NaBH4 gradient reduction method, avoiding bulk recombination and making the density controllable.

[0031] Stable heterojunction construction: L-cysteine ​​as an interface coupling agent to achieve in-situ growth of CdS and Mo2CT x The monolayer rate is >90% and the heterojunction is stable.

[0032] 3. Application Adaptability Adaptability to space applications: In a microgravity environment, mass transfer efficiency is increased by 50%, and CO2 conversion rate and methanol yield are greatly improved.

[0033] Ground application: Highly efficient photocatalytic reduction of CO2, helping the ground. DETAILED DESCRIPTION

[0034] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0035] Example 1: OVs cluster concentration optimization experiment objective By precisely controlling the concentration of NaBH4, we studied its effect on the concentration of oxygen vacancy clusters on the surface of α-Fe2O3, and further analyzed the mechanism of action of oxygen vacancy concentration on the performance of photocatalytic reduction of carbon dioxide to determine the optimal oxygen vacancy concentration and thus optimize the performance of the catalyst.

[0036] Experimental procedures Step 1: Sample preparation Raw material preparation: Weigh different concentrations of NaBH₄ (5 g / L, 10 g / L, 15 g / L, and 20 g / L) and dissolve them in deionized water. After stirring, add FeCl₃·6H₂O to a concentration of 0.1 mol / L. Subsequently, pour the mixed solution into a 1:1 ethanol / water mixture and stir for 30 minutes to ensure thorough dispersion of the raw materials.

[0037] Hydrothermal reaction: Transfer the mixed solution to a polytetrafluoroethylene-lined hydrothermal reactor and place it in an oven at 180°C for 12 hours. During the reaction, record the temperature every 2 hours to ensure that the reaction temperature remains stable within the range of 180°C ± 2°C.

[0038] Product Treatment: After the reaction, the mixture was cooled to room temperature and washed three times with ethanol and deionized water, each time centrifuged at 8000 rpm for 10 minutes to remove unreacted raw materials and impurities. Finally, the product was dried in a vacuum oven at 60°C for 12 hours to obtain α-Fe2O3 samples with varying oxygen vacancy concentrations.

[0039] Step 2: Determination of oxygen vacancy concentration EPR spectroscopy: The samples were characterized using electron paramagnetic resonance (EPR) spectroscopy. Instrument parameters were set as follows: microwave frequency 9.5 GHz, microwave power 20 mW, modulation frequency 100 kHz, modulation amplitude 0.1 mT, and scan range 0–5 mT. The concentration of oxygen vacancy clusters was quantitatively analyzed using the g = 2.003 ± 0.002 signal.

[0040] Data processing: Calculate the oxygen vacancy concentration based on the signal intensity and peak area in the EPR spectrum. Repeat the measurement three times for each sample, and take the average value as the final result.

[0041] Step 3: Catalytic performance test Catalyst composite: α-Fe2O3 samples with different oxygen vacancy concentrations were combined with CdS nanosheets and Mo2CT x MXene nanosheet composite, the specific steps are as follows: CdS was in situ grown on the surface of OVs-α-Fe2O3 using L-cysteine ​​as the interface coupling agent. The reaction was carried out by hydrothermal method at 170℃ and pH=9 for 12 hours.

[0042] Single-layer Mo2CT x The above product was compounded with the catalyst at pH = 9 and 500W ultrasound for 24 hours to obtain a complete α-Fe2O3 / CdS / MXene heterojunction catalyst.

[0043] Photocatalytic Reaction: CO2 photoreduction was carried out under visible light (λ ≥ 420 nm) using 0.1M Na2S / 0.04M Na2SO3 as sacrificial vacancy reagents. The reaction system was placed in a continuous-flow photothermal reactor with a flow channel width of 500 μm, a temperature controlled at 80°C ± 2°C, and a light intensity ≥ 100 mW / cm². The reaction time was 6 hours, with samples collected every hour for analysis of methanol yield.

[0044] Product analysis: Gas chromatography (GC) and mass spectrometry (MS) were used to quantitatively analyze the reaction products and calculate the yield and selectivity of methanol.

[0045] Step 4: Theoretical calculation and auxiliary analysis DFT Calculations: Density functional theory (DFT) was used to calculate the adsorption energy of CO2 molecules on the catalyst and the energy barrier for the formation of the *COOH intermediate at different oxygen vacancy concentrations. The calculation model employed periodic boundary conditions, the PBE functional, and the DFT-D3 dispersion correction method to optimize the catalyst surface structure and CO2 adsorption configuration.

[0046] Data analysis was obtained through the above implementation: Through the calculation results, the influence of oxygen vacancy concentration on the electronic structure of the catalyst surface, as well as the mechanism of action on CO2 activation and intermediate generation were analyzed.

[0047] Experimental results 1. Relationship between oxygen vacancy concentration and NaBH4 concentration EPR spectrum analysis: When the NaBH4 concentration was 5 g / L, no obvious oxygen vacancy signal was detected in the EPR spectrum, indicating that oxygen vacancy clusters were not formed.

[0048] When the concentration of NaBH4 is 10 g / L, a weak oxygen vacancy signal appears in the EPR spectrum, and the oxygen vacancy concentration is 1.2×10 15 spins / g.

[0049] When the concentration of NaBH4 is 15 g / L, the oxygen vacancy signal is significantly enhanced, and the oxygen vacancy concentration reaches 2.1×10 15 spins / g.

[0050] When the NaBH4 concentration is 20 g / L, the oxygen vacancy concentration increases slightly, but a broad peak appears in the EPR spectrum, indicating that the bulk oxygen vacancies increase, resulting in an increase in the carrier recombination rate.

[0051] Analysis of the phenomenon: When the NaBH4 concentration is low, the reduction ability is insufficient and oxygen vacancy clusters cannot be effectively generated; when the NaBH4 concentration is too high, the reduction reaction is too intense, resulting in an increase in bulk oxygen vacancies, which is not conducive to the improvement of catalyst performance.

[0052] 2. Relationship between catalytic performance and oxygen vacancy concentration Methanol yield: When the oxygen vacancy concentration is 1.2×10 15 spins / g; methanol yield was 7.5 μmol·g -1 ·h -1 .

[0053] When the oxygen vacancy concentration is 2.1×10 15 spins / g; the methanol yield reached a maximum of 9.8 μmol·g -1 ·h -1 .

[0054] When the oxygen vacancy concentration continues to increase to more than 2.1×10 15 spins / g, the methanol yield dropped to 8.2 μmol·g -1 ·h -1 .

[0055] Selectivity analysis: At the optimal oxygen vacancy concentration, the selectivity of methanol reaches 85%, which is much higher than the selectivity under other concentration conditions (60%-70%).

[0056] Analysis of the phenomenon: When the oxygen vacancy concentration is moderate, oxygen vacancy clusters on the catalyst surface can effectively adsorb and activate CO2 molecules, promoting the formation of *COOH intermediates, thereby improving methanol yield and selectivity. When the oxygen vacancy concentration is too high, the number of bulk oxygen vacancies increases, leading to an increase in the carrier recombination rate and, in turn, a decrease in catalytic performance.

[0057] 3. Theoretical calculation results DFT calculation: When the oxygen vacancy concentration is 2.1×10 15 spins / g, the adsorption energy of CO₂ on the catalyst decreased from -0.21 eV to -0.67 eV, indicating that the oxygen vacancy clusters significantly enhanced the CO₂ adsorption capacity. The formation energy barrier of the *COOH intermediate decreased by 0.52 eV, indicating that the oxygen vacancy clusters polarized the C=O bond through the local electric field, reducing the reaction activation energy and promoting the reaction.

[0058] Electronic structure analysis: DFT calculation results show that the introduction of oxygen vacancy clusters forms a local electric field on the catalyst surface, polarizes the C=O bond, reduces the adsorption energy and reaction energy barrier of CO2, and thus improves the activity of the catalyst.

[0059] Example 2: MXene interface engineering optimization Purpose of the experiment This experiment aims to optimize the MXene (Mo2CT x ) loading to study its effect on the photocatalytic reduction of carbon dioxide performance of the α-Fe2O3 / CdS / MeneX heterojunction. By combining experimental and theoretical calculations, the MXene loading was determined, thereby optimizing the catalyst's performance and providing a theoretical and experimental basis for efficient photocatalytic reduction of carbon dioxide.

[0060] Experimental background 1. Characteristics of MXene Materials MXene (such as Mo2CT x MXenes are a class of transition metal carbides, nitrides, or carbonitrides with a two-dimensional structure. Their surfaces are rich in functional groups (such as -OH and -F), resulting in excellent hydrophilicity and adsorption properties. MXene materials, due to their unique electronic structure and high specific surface area, have shown great potential in photocatalysis. MXenes can form Schottky junctions, promoting the separation and transport of photogenerated charge carriers, thereby enhancing photocatalytic performance.

[0061] MXene materials are typically prepared by chemically etching MAX-phase materials (such as Mo2Ga2C). MAX-phase materials are a class of materials with a layered structure, where M represents a transition metal, A represents a main-group element, and X represents carbon or nitrogen. By selectively etching the A-site element, single- or multi-layer MXene materials can be obtained. The surface of MXene materials is rich in functional groups, which not only enhance their hydrophilicity but also provide additional active sites, promoting the adsorption and activation of reactants.

[0062] 2. The role of MXene in heterojunctions In the α-Fe2O3 / CdS / MXene heterojunction, MXene not only acts as an electron transport layer, promoting the rapid migration of photogenerated electrons, but also enhances CO2 adsorption through its surface functional groups. MXene has a high work function (approximately 5.2 eV) and can form a Schottky junction with CdS, creating a built-in electric field at the interface and promoting the separation and transport of photogenerated carriers.

[0063] However, the MXene loading has a significant impact on its performance. An appropriate amount of MXene can improve the activity and selectivity of the catalyst, while an excessive amount of MXene may shield the active sites and reduce catalytic performance. Therefore, optimizing the MXene loading is key to improving catalyst performance.

[0064] 3. Characteristics of α-Fe2O3 / CdS heterojunction α-Fe₂O₃ (hematite) is a common semiconductor material with excellent light absorption properties and a suitable bandgap width (approximately 2.2 eV), enabling efficient absorption of visible light. CdS is a narrow-bandgap semiconductor material with high photogenerated carrier mobility. By combining α-Fe₂O₃ with CdS, an S-type heterojunction can be formed, enhancing the separation efficiency of photogenerated carriers.

[0065] In an α-Fe₂O₃ / CdS heterojunction, a built-in electric field forms between the valence band (VB) of α-Fe₂O₃ and the conduction band (CB) of CdS, promoting the separation of photogenerated electrons and holes. However, the limited interfacial contact area of ​​this heterojunction restricts electron transfer efficiency. By introducing MXene materials, further optimization of the interface engineering can be achieved, improving the overall performance of the catalyst.

[0066] Experimental procedures Step 1: Sample preparation Raw material preparation: Weigh different concentrations of Mo2CT x(1%, 2%, 3%, 4%, 5%), dissolved in deionized water, stirred evenly, and added to the pre-prepared α-Fe2O3 / CdS composite material. The preparation method of the α-Fe2O3 / CdS composite material is as follows: α-Fe2O3 nanosheets were dispersed in deionized water, and an appropriate amount of L-cysteine ​​was added as an interfacial coupling agent.

[0067] Under stirring conditions, CdS precursor solution (a mixed solution of Cd(NO3)2 and Na2S) was added dropwise and hydrothermally reacted at 170°C for 12 hours.

[0068] After the reaction is completed, the product is washed with deionized water and ethanol and dried to obtain an α-Fe2O3 / CdS composite material.

[0069] Different concentrations of Mo2CT x The solution is added to the above-mentioned α-Fe2O3 / CdS composite material and stirred evenly.

[0070] Ultrasonic dispersion: The mixed solution was placed in an ultrasonic instrument and ultrasonically dispersed for 24 hours with an ultrasonic power of 500 W to ensure that the MXene was evenly dispersed on the surface of the composite material.

[0071] Product processing: After ultrasonic dispersion, the product was washed with deionized water three times and centrifuged at 8000 rpm for 10 min each time to remove the unadsorbed MXene.

[0072] Finally, the product was dried in a vacuum drying oven at 60°C for 12 hours to obtain α-Fe2O3 / CdS / MXene heterojunction catalysts with different MXene loading amounts.

[0073] Step 2: Catalytic performance test Photocatalytic reaction: CO2 photoreduction was carried out under visible light (λ ≥ 420 nm) using 0.1 M Na2S / 0.04 M Na2SO3 as a sacrificial hole agent. The reaction system was placed in a continuous-flow photothermal reactor with a flow channel width of 500 μm, a temperature controlled at 80°C ± 2°C, and a light intensity ≥ 100 mW / cm². The reaction lasted for 6 hours, with samples collected every hour for analysis of methanol yield.

[0074] Product analysis: Gas chromatography (GC) and mass spectrometry (MS) were used to quantitatively analyze the reaction products and calculate the yield and selectivity of methanol.

[0075] Step 2: Characterization and Analysis X-ray photoelectron spectroscopy (XPS): XPS analysis was performed on catalysts with different MXene loadings to study the surface chemical state and elemental composition of MXene.

[0076] Scanning electron microscopy (SEM): Observe the morphology of the catalyst and analyze the loading of MXene.

[0077] Transmission electron microscopy (TEM): Observe the microstructure of the catalyst and analyze the interface bonding between MXene and α-Fe2O3 / CdS composite material.

[0078] Specific surface area analysis (BET): The specific surface area of ​​the catalyst was determined by nitrogen adsorption-desorption isotherm, and the effect of MXene loading on the specific surface area of ​​the catalyst was analyzed.

[0079] Electrochemical impedance spectroscopy (EIS): EIS analysis was used to study the effects of different MXene loadings on the separation and transport of photogenerated carriers.

[0080] Photoluminescence spectroscopy (PL): PL spectroscopy was used to study the effect of different MXene loadings on the recombination rate of photogenerated carriers.

[0081] In situ infrared spectroscopy (in situ IR): The effects of different MXene loadings on CO2 adsorption and activation were studied through in situ IR spectroscopy analysis.

[0082] Experimental results 1. Effect of MXene loading on catalytic performance Methanol yield: When the MXene loading was 1%, the methanol yield was 6.5 μmol·g -1 ·h -1 .

[0083] When the MXene loading was 2%, the methanol yield was 7.8 μmol·g -1 ·h -1 .

[0084] When the MXene loading is 3%, the methanol yield reaches a maximum of 9.2 μmol·g -1 ·h -1 .

[0085] When the MXene loading was 4%, the methanol yield dropped to 8.5 μmol·g -1 ·h -1 .

[0086] When the MXene loading was 5%, the methanol yield further decreased to 7.6 μmol·g -1 ·h-1 .

[0087] Selectivity analysis: At the optimal MXene loading (3%), the methanol selectivity reached 82%, which is much higher than the selectivity under other loading conditions (70%-75%).

[0088] Analysis: An appropriate amount of MXene loading can significantly improve the activity and selectivity of a catalyst. This is primarily because MXene's surface functional groups (such as -OH and -F) enhance CO2 adsorption, while the Schottky junction formed by MXene facilitates the separation and transport of photogenerated carriers. However, excessive MXene loading can shield active sites, increase mass transfer resistance, and ultimately reduce catalytic performance.

[0089] 2. Characterization and Analysis Results XPS analysis: XPS analysis of catalysts with different MXene loadings revealed that the characteristic peak of Mo element was most obvious when the loading was 3%, indicating that the MXene and catalyst were most closely bound at this time. XPS analysis also showed that as the MXene loading increased, the C and O element contents on the catalyst surface also increased accordingly. Example 3: Comparative study of symmetric and asymmetric oxygen vacancies Experimental procedures Step 1: Sample preparation Symmetric oxygen vacancy (S-OVs) sample preparation: Raw material preparation: 0.5 g of α-Fe2O3 nanosheets were weighed and dispersed in 50 mL of deionized water, and an appropriate amount of H2 (5% H2 / Ar, flow rate 100 mL / min) was added as a reducing agent.

[0090] Under stirring conditions, the mixed solution was placed in a tube furnace, heated to 500°C at a heating rate of 10°C / min, and maintained at this temperature for 2 hours.

[0091] Washing and drying: After annealing, the product was washed alternately with deionized water and ethanol three times, and each time centrifuged (8000 rpm, 10 min) to remove unreacted impurities.

[0092] The product was placed in a vacuum drying oven at 60 °C and dried for 12 h to obtain S-OVs modified α-Fe2O3 sample.

[0093] CdS in situ growth: The S-OVs-α-Fe2O3 sample was dispersed in 50 mL of deionized water, 0.1 g of L-cysteine ​​was added as an interfacial coupling agent, and the mixture was stirred evenly.

[0094] Under stirring conditions, CdS precursor solution (a mixed solution of Cd(NO3)2 and Na2S, molar ratio 1:1) was added dropwise and hydrothermally reacted at 170°C for 12 hours.

[0095] MXene composite: 0.1 g of single-layer Mo2CT x It was mixed with the above product at pH = 9 and ultrasonically treated for 24 h (ultrasonic power 500 W) to obtain a complete α-Fe2O3 / CdS / MXene heterojunction catalyst (S-OVs sample).

[0096] Asymmetric oxygen vacancies (A-OVs) sample preparation: Raw material preparation: 0.5 g of α-Fe2O3 nanosheets were weighed and dispersed in 50 mL of ethanol / water mixed solvent (volume ratio 1:1), and 15 g / L NaBH4 was added as a reducing agent.

[0097] Under stirring conditions, the mixed solution was placed in a polytetrafluoroethylene-lined hydrothermal reactor and reacted at 180°C for 12 hours. The temperature change was recorded every 2 hours to ensure that the reaction temperature was stable within the range of 180°C ± 2°C.

[0098] Washing and drying: After the reaction was completed, the mixture was cooled to room temperature naturally and washed alternately with ethanol and deionized water for three times, each time by centrifugation (8000 rpm, 10 min) to remove unreacted impurities.

[0099] The product was placed in a vacuum drying oven at 60°C and dried for 12 hours to obtain an A-OVs modified α-Fe2O3 sample.

[0100] CdS in situ growth: The A-OVs-α-Fe2O3 sample was dispersed in 50 mL of deionized water, 0.1 g of L-cysteine ​​was added as an interfacial coupling agent, and the mixture was stirred evenly.

[0101] Under stirring conditions, CdS precursor solution (a mixed solution of Cd(NO3)2 and Na2S, molar ratio 1:1) was added dropwise and hydrothermally reacted at 170°C for 12 hours.

[0102] MXene composite: 0.1 g of single-layer Mo2CT x It was mixed with the above product at pH = 9 and ultrasonically treated for 24 hours (ultrasonic power 500 W) to obtain a complete α-Fe2O3 / CdS / MXene heterojunction catalyst (A-OVs sample).

[0103] Step 1: Catalytic performance test Photocatalytic reaction: CO2 photoreduction reaction was carried out under visible light (λ≥420 nm) using 0.1 M Na2S / 0.04 M Na2SO3 as hole sacrificial agent.

[0104] The reaction system was placed in a continuous flow photothermal reactor with a flow channel width of 500 μm, a temperature controlled at 80°C ± 2°C, and a light intensity ≥ 100 mW / cm 2 .

[0105] The reaction time was 6 hours, and samples were taken every hour to analyze the methanol yield.

[0106] Product analysis: Gas chromatography (GC) and mass spectrometry (MS) were used to quantitatively analyze the reaction products and calculate the yield and selectivity of methanol.

[0107] Step 1: Characterization and Analysis X-ray photoelectron spectroscopy (XPS): XPS analysis was performed on the S-OVs and A-OVs samples to investigate the surface chemical state and elemental composition of oxygen vacancies.

[0108] Scanning electron microscopy (SEM): Observe the morphology of the catalyst and analyze the distribution of oxygen vacancies.

[0109] Transmission electron microscopy (TEM): Observe the microstructure of the catalyst and analyze the distribution differences of oxygen vacancies in the bulk and on the surface.

[0110] Specific surface area analysis (BET): The specific surface area of ​​the catalyst was determined by nitrogen adsorption-desorption isotherms to analyze the effect of oxygen vacancies on the specific surface area of ​​the catalyst.

[0111] Electrochemical impedance spectroscopy (EIS): EIS analysis was used to study the effects of different oxygen vacancies on the separation and transport of photogenerated carriers.

[0112] Photoluminescence spectroscopy (PL): PL spectroscopy was used to study the effect of different oxygen vacancies on the recombination rate of photogenerated carriers.

[0113] In situ infrared spectroscopy (in situ IR): The effects of different oxygen vacancies on CO2 adsorption and activation were studied through in situ IR spectroscopy analysis.

[0114] Experimental results 1. Effects of symmetric and asymmetric oxygen vacancies on catalytic performance Methanol yield: The methanol yield of the S-OVs sample was 5.1 μmol·g-1 ·h -1 .

[0115] The methanol yield of the A-OVs sample was 9.8 μmol·g -1 ·h -1 .

[0116] Selectivity analysis: The methanol selectivity of the S-OVs sample was 62%.

[0117] The methanol selectivity of the A-OVs sample was 85%.

[0118] Phenomenon analysis: The performance of the A-OVs sample is significantly better than that of the S-OVs sample. This is mainly because the asymmetric oxygen vacancies are mainly distributed on the catalyst surface, which can enhance the adsorption capacity of CO2 and promote the separation and transport of photogenerated carriers through the local electric field. In contrast, the symmetric oxygen vacancies are mainly distributed in the bulk phase, which may lead to an increase in the carrier recombination rate and thus reduce the catalytic performance.

[0119] 2. Characterization and Analysis Results XPS analysis: XPS results show that the surface Fe 2+ The signal is more obvious, indicating that the asymmetric oxygen vacancies are mainly distributed on the surface. 2+ The signal is weak, indicating that the symmetric oxygen vacancies are mainly distributed in the bulk phase.

[0120] SEM analysis: SEM images show that the surface of the A-OVs sample is rougher, indicating that the introduction of asymmetric oxygen vacancies leads to changes in the surface structure. The surface of the S-OVs sample is relatively smooth, indicating that the symmetric oxygen vacancies have less influence on the surface morphology.

[0121] TEM analysis: TEM images show that the A-OVs sample forms obvious oxygen vacancy clusters on the surface, while the oxygen vacancies of the S-OVs sample are mainly distributed in the bulk phase.

[0122] BET analysis: BET results show that the specific surface area of ​​the A-OVs sample is slightly higher than that of the S-OVs sample, indicating that the introduction of asymmetric oxygen vacancies helps to increase the specific surface area of ​​the catalyst.

[0123] EIS analysis: EIS results show that the charge transfer resistance of the A-OVs sample is significantly lower than that of the S-OVs sample, indicating that the asymmetric oxygen vacancies promote the separation and transport of photogenerated carriers.

[0124] PL analysis: PL spectra show that the recombination rate of photogenerated carriers in the A-OVs sample is significantly lower than that in the S-OVs sample, indicating that asymmetric oxygen vacancies help improve the separation efficiency of photogenerated carriers.

[0125] In situ IR analysis: In situ IR spectra showed that the characteristic peaks of the A-OVs sample were more obvious after CO2 adsorption, indicating that the asymmetric oxygen vacancies enhanced the adsorption and activation ability of CO2.

[0126] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the invention can be embodied in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as illustrative and non-restrictive, and the scope of the invention is defined by the appended claims, not the foregoing description, and all variations within the meaning and range of equivalents of the claims are intended to be encompassed therein. Any reference in a claim should not be construed as limiting the claim to which it relates.

[0127] In addition, it should be understood that although this specification is described in terms of implementation methods, not every implementation method contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation methods that can be understood by those skilled in the art.

Claims

1. A α-Fe2O3 / CdS / MXene heterojunction photoreduction catalyst based on surface oxygen vacancies regulation, characterized by Contains: α-Fe2O3 nanosheets with tetrameric oxygen vacancy clusters, OVs concentration 1.2~2.5×10 15 spins / g; CdS nanosheets forming an S-type heterojunction with α-Fe2O3; Mo2CT forming a Schottky junction with CdS x MXene nanosheets.

2. The α-Fe2O3 / CdS / MXene heterojunction photoreduction catalyst based on surface oxygen vacancy regulation according to claim 1, characterized in that: The tetramer oxygen vacancy is located on the α-Fe2O3 (001) crystal plane, and g=2.003±0.002 according to EPR spectrum.

3. The α-Fe2O3 / CdS / MXene heterojunction photoreduction catalyst based on surface oxygen vacancy regulation according to claim 1, characterized in that: The α-Fe2O3, CdS, Mo2CT x The thickness ratio is 1:(1.2~1.6):(0.2~0.4), and the mismatch between the CdS(002) crystal plane and the α-Fe2O3(001) crystal plane is < 3%.

4. A method for preparing the α-Fe2O3 / CdS / MXene heterojunction photoreduction catalyst based on surface oxygen vacancy regulation according to any one of claims 1 to 3, comprising: (1) FeCl3·6H2O and NaBH4 (15 ± 5 g / L) were dissolved in an ethanol / water mixed solvent and hydrothermally synthesized OVs cluster-modified α-Fe2O3 at 180°C; (2) In situ growth of CdS on the surface of OVs-α-Fe2O3, with L-cysteine ​​as the interface coupling agent; (3) Single-layer Mo2CT x Compound with the product of step (2) at pH = 9 and 500W ultrasound for 24 h.

5. The method according to claim 4, wherein: In step (1), the NaBH4 concentration was 15 g / L and the hydrothermal temperature deviation was ±5°C.

6. The method according to claim 4, wherein: In step (3), Mo2CTx is prepared by etching MAX phase Mo2Ga2C with 9M HCl+LiF, and the monolayer rate is > 90%.

7. A method for preparing a carbon dioxide photoreduction catalyst based on α-Fe2O3 / CdS / MXene heterojunction regulated by surface oxygen vacancies, characterized in that The catalyst according to claims 1-3 is used to reduce CO2 to methanol under visible light (λ≥420 nm), and the reaction system contains a 0.1M Na2S / 0.04M Na2SO3 vacancy sacrificial agent.

8. The method according to claim 7, wherein The reaction was carried out in a continuous flow photothermal reactor with a flow channel width of 500 ± 50 μm, a temperature of 80 ± 5°C, and a light intensity ≥100 mW / cm².

9. An α-Fe2O3 / CdS / MXene heterojunction photoreduction catalyst system based on surface oxygen vacancies regulation, characterized by: Integrate the catalyst described in claims 1-3, a 3D printed microfluidic reactor and a photothermal coordinated temperature control module.