A method and system for improving selectivity of photocatalytic reduction of carbon dioxide products

Abstract

The application belongs to the field of photocatalysis, and relates to a method and system for improving product selectivity of photocatalytic reduction of carbon dioxide. The method places a magnetic composite photocatalyst with a negative magnetic resistance effect in a reaction container, applies an external magnetic field to the reaction system under light conditions, uses the negative magnetic resistance effect of the magnetic photocatalyst under the action of the external magnetic field to promote the separation of photo-generated carriers, and improves the efficiency and product selectivity of photocatalytic reduction of carbon dioxide. The magnetic composite photocatalyst is preferably a MOF-derived carbon layer-wrapped nickel-iron composite oxide with a chemical composition of NiFe2O4 / Fe2O3@C, and has oxygen vacancies and heterojunctions. The application also discloses a system for realizing the above method, which comprises a reaction container, a light source, a magnetic composite photocatalyst and a magnetic field generating device. The application controls the behavior of photo-generated carriers through a magnetic field, effectively suppresses electron-hole recombination, and significantly improves the performance of photocatalytic reduction of CO2.

Description

Technical Field

[0001] This invention belongs to the field of photocatalysis technology, specifically relating to a method and system for improving the selectivity of photocatalytic reduction of carbon dioxide products. Background Technology

[0002] Over the past few decades, the large-scale combustion of fossil fuels has led to a 30% increase in atmospheric carbon dioxide (CO2) concentrations, resulting in a series of severe environmental problems such as global warming and sea-level rise. These issues pose a serious threat to the ecological environment and human survival and development. Therefore, developing effective CO2 emission reduction and conversion technologies to address the environmental hazards caused by excessive CO2 emissions has become an urgent technical challenge in this field.

[0003] Solar energy, as a clean and renewable energy source, has been widely researched and applied in the field of CO2 emission reduction. Among these technologies, photocatalytic CO2 reduction is considered one of the most promising CO2 treatment technologies because it can directly utilize solar energy to convert CO2 into high-value-added products. However, the practical application of photocatalytic CO2 reduction technology is currently severely limited, with the core bottleneck being the low efficiency of the photocatalytic reaction. This problem mainly stems from the inherent defects of traditional photocatalysts, such as wide bandgap and low photogenerated charge separation efficiency. Therefore, developing high-performance photocatalysts is key to improving the efficiency of photocatalytic CO2 reduction and promoting the industrial application of this technology.

[0004] Among numerous photocatalysts, ferric oxide (Fe2O3) has attracted widespread attention in the field of photocatalysis due to its advantages such as wide availability, environmental friendliness, and low cost. However, the relatively wide band gap of Fe2O3 limits its absorption range of sunlight, thus restricting its effective utilization in photocatalytic reactions. Nickel tetroxide (NiFe2O4), as a composite oxide, possesses superior conductivity and high chemical stability, and has gradually become a research hotspot in the field of photocatalysis in recent years. However, its photocatalytic efficiency has still not met the requirements for practical applications, mainly because the number of its surface active sites is limited, failing to provide sufficient reaction sites for photocatalytic reactions.

[0005] Furthermore, due to the inherent properties of photocatalysts, the photogenerated electron-hole pairs generated during the photocatalytic reaction are prone to recombination on the catalyst surface, resulting in a significant reduction in the utilization rate of photogenerated charge carriers, which further restricts the improvement of photocatalytic reaction efficiency. Therefore, how to effectively suppress the recombination of photogenerated charge carriers has become a core technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0006] The purpose of this invention is to provide a method and system for improving the selectivity of products from photocatalytic reduction of carbon dioxide, thereby solving the problem of reduced utilization of photogenerated carriers in existing photocatalytic reduction of carbon dioxide reactions.

[0007] This invention is achieved through the following technical solution: This invention discloses a method for improving the selectivity of photocatalytic reduction of carbon dioxide products, comprising the following steps: A magnetic composite photocatalyst with a negative magnetoresistance effect is placed in a reaction vessel. Under illumination, an external magnetic field is applied to the reaction system. The negative magnetoresistance effect of the magnetic photocatalyst under the action of the external magnetic field promotes the separation of photogenerated charge carriers and improves the efficiency and product selectivity of photocatalytic reduction of carbon dioxide.

[0008] Furthermore, the magnetic composite photocatalyst with negative magnetoresistance effect is a nickel-iron composite oxide encapsulated by a carbon layer derived from MOF, with a chemical composition of NiFe2O4 / Fe2O3@C and oxygen vacancies; NiFe2O4 / Fe2O3 has a heterojunction.

[0009] Furthermore, the strength of the external magnetic field is 200 GS ~ 4000 GS.

[0010] Furthermore, the photocatalyst is obtained by calcining MIL-101 (Fe) with nickel doping under an inert atmosphere.

[0011] Furthermore, the preparation process of the magnetic composite photocatalyst with negative magnetoresistance effect includes the following steps: (1) The iron salt and organic ligand were subjected to a hydrothermal reaction in a solvent to obtain a MIL-101 (Fe) precursor solution; (2) Add nickel salt to the MIL-101(Fe) precursor solution, mix and photo-assisted to obtain a MIL-101(Fe) solution containing nickel; (3) The MIL-101 (Fe) solution containing nickel was ultrasonically treated and then subjected to hydrothermal reaction to obtain a mixed solution; (4) After cooling the mixture to room temperature, centrifuge and wash to obtain nickel-doped MOF products, and then vacuum dry them; (5) The nickel-doped MOF product after vacuum drying was subjected to a first stage calcination under an inert atmosphere to obtain an intermediate product; (6) The intermediate product is calcined in air to obtain carbon-coated NiFe2O4 / Fe2O3, which is a magnetic composite photocatalyst.

[0012] Furthermore, in step (2), the amount of nickel ions added is 2% to 8% of the molar amount of iron ions.

[0013] Furthermore, in step (3), the hydrothermal reaction temperature is 105~125°C and the time is 20~25 h; In step (4), vacuum drying is performed at 60~80°C for 12 h.

[0014] Furthermore, in step (5), the calcination temperature in the first stage is 400~600°C, the heating rate is 3~5 °C / min, and the holding time is 4~6 h; In step (6), the calcination temperature in the second stage is 400~600°C, the heating rate is 3~5 °C / min, and the holding time is 4~6 h.

[0015] The present invention also discloses a system for photocatalytic reduction of carbon dioxide, comprising: The reaction vessel has a light-transmitting window at the top; A light source, positioned above or outside the light-transmitting window, is used to provide the illumination required for the photocatalytic reaction. A magnetic composite photocatalyst is disposed inside the reaction vessel; A magnetic field generating device is installed inside or outside the reaction vessel to apply an external magnetic field to the reaction system.

[0016] Furthermore, the magnetic field generating device is a permanent magnet or an electromagnet.

[0017] Compared with the prior art, the present invention has the following beneficial technical effects: This invention discloses a method for improving the selectivity of photocatalytic carbon dioxide reduction products by introducing a negative magnetoresistance effect into the photocatalytic carbon dioxide reduction system. The core mechanism lies in the fact that, under the influence of an external magnetic field, the resistance of a magnetic photocatalyst with a negative magnetoresistance effect decreases, allowing photogenerated charge carriers (electron-hole pairs) to migrate more efficiently to the catalyst surface to participate in the reaction, rather than recombinating in the bulk phase or at the interface. This mechanism solves the core problem of high carrier recombination rate in photocatalysis from a spintronics perspective.

[0018] Furthermore, this invention specifies that the magnetic composite photocatalyst with negative magnetoresistance effect is a nickel-iron composite oxide encapsulated by a MOF-derived carbon layer, with a chemical composition of NiFe2O4 / Fe2O3@C and oxygen vacancies; NiFe2O4 / Fe2O3 has a heterojunction. The catalyst structure of this invention is the structural basis for realizing the negative magnetoresistance effect. Specifically: Oxygen vacancies: XPS confirmed that the oxygen vacancy content in NFO / FO@C-4 is as high as 36.12%. Oxygen vacancies enhance the spin polarization of the material, making it more ferromagnetic and providing an electron source for spin polarization for the negative magnetoresistance effect.

[0019] NiFe2O4 / Fe2O3 heterojunction: Strong electronic interactions at the interface (confirmed by the binding energy shifts of Fe 2p and Ni 2p in XPS) promote interfacial charge transfer, enabling efficient injection of spin-polarized electrons.

[0020] Carbon layer encapsulation: On the one hand, the carbon layer protects the oxide nanoparticles, and on the other hand, it improves conductivity and promotes the migration of photogenerated carriers.

[0021] Furthermore, the strength of the external magnetic field is limited to 800 GS ~ 4000 GS, and the magnetic field strength is a key parameter for controlling the degree of negative magnetoresistance effect. The negative magnetoresistance effect rapidly increases at lower field strengths and tends to saturate above 20 kOe (approximately 2 million GS). The 800~4000 GS range defined in this invention covers the entire process of the negative magnetoresistance effect from rapid response to saturation. Figure 7 Further analysis confirmed that as the magnetic field strength increased from 3000 GS to 4000 GS, the CO yield increased from 124.5 µmol g / g. -1 h -1 Increased to 230.6 µmol g -1 h -1 They show a positive correlation.

[0022] Furthermore, this invention specifies the detailed preparation steps of the magnetic composite photocatalyst, including photo-assisted doping, two-step calcination (inert atmosphere + air atmosphere), and specific parameters such as nickel doping amount (2%~8%) and calcination temperature (400~600°C). The specific effects are as follows: Photo-assisted doping: Xenon lamp irradiation promotes uniform doping of nickel ions, which is conducive to the formation of oxygen vacancies and enhances spin polarization.

[0023] Two-step calcination: The first step, inert atmosphere calcination, achieves carbonization and heterojunction formation; the second step, air calcination, moderately stabilizes the structure and regulates the oxygen vacancy concentration. XPS data shows that the oxygen vacancy content is highest (36.12%) when the nickel doping amount is 4%, corresponding to the best photocatalytic performance.

[0024] Nickel doping concentration of 2% to 8%: Within this range, the photocatalytic performance is superior to that of undoped and air-calcined control samples, demonstrating the correlation between the doping ratio and oxygen vacancy content, as well as the relationship between magnetism and catalytic performance.

[0025] This invention also discloses a system for photocatalytic reduction of carbon dioxide, comprising a reaction vessel (with a light-transmitting window), a light source, a magnetic composite photocatalyst, and a magnetic field generating device. This system integrates the "photocatalytic reaction vessel + magnetic photocatalyst + external magnetic field" into a synergistic whole. Its mechanism is as follows: the light source provides light energy to excite the catalyst to generate electron-hole pairs; the magnetic field generating device provides an external magnetic field, which, in conjunction with the negative magnetoresistance effect of the catalyst, reduces resistance and promotes carrier separation; the reaction vessel ensures the stable coupling of multiple fields—light, magnetism, gas, and liquid. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the experimental apparatus of the present invention; Figure 2a Hysteresis (MH) rings of NiFe2O4, Fe2O3, and composite material NFO / FO@C-4 were measured at room temperature. Figure 2b Magnetoresistive (MR) effects of NiFe2O4, Fe2O3 and composite material NFO / FO@C-4 were measured at room temperature. Figure 2c A schematic diagram of a spin-state electron under conditions without a magnetic field; Figure 2d A schematic diagram of a spin-state electron under an applied magnetic field; Figure 3a Full-spectrum XPS spectra of different catalysts; Figure 3b The XPS spectrum of carbon is shown in Figure 1 (1s). Figure 3c XPS spectrum of oxygen at 1s; Figure 3d XPS spectrum of iron 2p; Figure 3e XPS spectrum of nickel 2p; Figure 4a Photocurrent response spectra of different catalysts; Figure 4b Photocurrent response spectra of NFO / FO@C-4 under different magnetic fields; Figure 4c Time-resolved fluorescence decay spectra of different catalysts; Figure 4d Time-resolved fluorescence emission spectra of NFO / FO@C-4 under different magnetic fields; Figure 5a In-situ infrared spectra of NFO / FO@C-4 at different reaction times; Figure 5b The TDOS diagram of Fe2O3; Figure 5cTDOS diagram for NFO / FO@CX; Figure 5d A diagram illustrating the suppression of carrier recombination through electronic spin polarization; Figure 6 Figure 1 shows the yield of CO / CH4 under different photocatalysts under no magnetic field and applied magnetic field conditions; where Figure (a) represents the no magnetic field condition and Figure (b) represents the applied magnetic field condition. Figure 7 The photocatalytic reduction CO2 activity of NFO / FO@C-4 catalyst under different magnetic field conditions; Figure 8 The photocatalytic reduction activity of NFO / FO@C-4 catalyst is used to assess the cycle stability. Detailed Implementation

[0027] To make the objectives, technical solutions, and advantages of the present invention clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention; that is, the described embodiments are only a part of the embodiments of the present invention, and not all of them.

[0028] The detailed description of the embodiments of the present invention provided in the following figures is not intended to limit the scope of the claimed invention, but merely to illustrate one selected embodiment of the invention. All other embodiments obtained by those skilled in the art based on the figures and embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0029] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0030] Example 1 like Figure 1 As shown, this invention discloses a system for photocatalytic reduction of carbon dioxide, comprising: The reaction vessel has a light-transmitting window at the top; A light source, positioned above or outside the light-transmitting window, is used to provide the illumination required for the photocatalytic reaction. A magnetic composite photocatalyst is disposed inside the reaction vessel; A magnetic field generating device is installed inside or outside the reaction vessel to apply an external magnetic field to the reaction system.

[0031] The magnetic field generating device is a permanent magnet or an electromagnet. The magnet, air inlet, air outlet, catalyst material, and reaction gas are all placed in the same reaction vessel to avoid experimental errors during the reaction process.

[0032] This embodiment discloses a method for preparing a magnetic composite photocatalyst, including the following steps: 1) Preparation of MIL-101 (Fe): 2.45 mmol FeCl3·6H2O and 1.24 mmol terephthalic acid were dissolved in 15 mL DMF, sonicated until transparent, and then transferred to a hydrothermal reactor and heated at 110°C for 20 hours to obtain a mixed solution. After the reaction was cooled to room temperature, the mixture A, namely the MIL-101 (Fe) precursor solution, was obtained by centrifuging three times alternately with 20 mL DMF and 20 mL methanol.

[0033] 2) Preparation of Ni-doped MIL-101 (Fe): Ni(NO3)2·6H2O was added to the above mixture A, and the mixture was stirred continuously at 300 rpm with a magnetic stirrer and irradiated with a xenon lamp for 30 min to obtain mixture B. The amount of nickel ions added was 4% of the molar amount of iron ions.

[0034] 3) After ultrasonic treatment of the above mixture B until it becomes transparent, it is transferred to a hydrothermal reactor and heated at 110°C for 20 h to obtain mixture C.

[0035] 4) After the reaction was cooled to room temperature, the nickel-doped MOF product D was obtained by centrifugation three times with alternating amounts of 20 mL DMF and 20 mL methanol. The product was recovered by centrifugation and further dried in vacuum at 60°C for 12 h.

[0036] 5) Purge the muffle furnace with argon for 30 min, and then place the vacuum-dried nickel-doped MOF product D in the muffle furnace for heating.

[0037] The heating conditions are as follows: starting at 25°C, and increasing by 3 °C for 3 minutes. -1 The temperature steadily increased until it reached 400°C, and was maintained at this temperature for 4 hours to obtain product E.

[0038] 6) Product E was heated in air from 25°C to 400°C at the same heating rate and held at that temperature for 4 h to obtain a magnetic composite photocatalyst, which is an oxide NiFe2O4 / Fe2O3@C-4 encapsulated by a carbon layer derived from the magnetic product MOF, denoted as NFO / FO@C-4.

[0039] The photocatalytic reduction of carbon dioxide was carried out in a custom-made stainless steel reaction vessel (approximately 450 mL in volume), which was equipped with a quartz window at the top for light illumination. The specific operating procedures are as follows: First, a 3000 GS cylindrical neodymium magnet (3 cm in diameter) was placed in the reaction vessel, and 20 mg of the pre-prepared magnetic composite photocatalyst was weighed and added to the vessel. Next, 100 µL of high-purity water (H2O) was injected into the vessel, and air was purged by introducing 99.999% high-purity argon (Ar). Subsequently, CO2 was injected into the vessel, and an LED light source (wavelength 420 nm, POLIFIC, Beijing) was activated to drive the photocatalytic reaction. During the reaction, the temperature of the reaction system was controlled by an external heating element. After the reaction, the generated gases were qualitatively and quantitatively analyzed using an Agilent GC-7890B gas chromatograph equipped with two thermal conductivity detectors (TCDs), with one detection line using argon as the carrier gas for hydrogen detection. Before conducting the photocatalytic reaction test, it was ensured that all air was removed from the reaction vessel to improve the reaction's effectiveness. The power of the light source was measured using a PLS-LED100B photoradiometer (POLIFIC, Beijing) to ensure the accuracy of the reaction conditions. The test results are shown in Table 1.

[0040] Example 2 A cylindrical neodymium magnet (3 cm in diameter) of 3500 GS was placed in the reaction vessel. The rest was the same as in Example 1.

[0041] Example 3 A cylindrical neodymium magnet (3 cm in diameter) of 4000 GS was placed in the reaction vessel. The rest was the same as in Example 1.

[0042] Example 4 A 200 GS rectangular ordinary magnet (1 cm in diameter) was placed in the reaction vessel. The rest was the same as in Example 1.

[0043] Example 5 A 300 GS rectangular ordinary magnet (2 cm in diameter) was placed in the reaction vessel. The rest was the same as in Example 1.

[0044] Example 6 A 400 GS rectangular ordinary magnet (3 cm in diameter) was placed in the reaction vessel. The rest was the same as in Example 1.

[0045] Example 7 Based on Example 1, in step 2) of the preparation method of the magnetic composite photocatalyst, nickel ions are added to the mixture A at an amount equal to 2% of the molar amount of iron ions, and the final product is denoted as NFO / FO@C-2.

[0046] Example 8 Based on Example 1, in step 2) of the preparation method of the magnetic composite photocatalyst, Ni(NO3)2·6H2O with nickel ions added at 6% of the molar amount of iron ions is added to the mixture A, and the final product is denoted as NFO / FO@C-6.

[0047] Example 9 Based on Example 1, in step 2) of the preparation method of the magnetic composite photocatalyst, Ni(NO3)2·6H2O with nickel ions added at 8% of the molar amount of iron ions is added to the mixture A, and the final product is denoted as NFO / FO@C-8.

[0048] Example 10 Based on Example 1, the preparation method of the magnetic composite photocatalyst is as follows: 1) Preparation of MIL-101 (Fe) precursor solution: 2.45 mmol FeCl3·6H2O and 1.24 mmol terephthalic acid were dissolved in 15 mL DMF, sonicated until transparent, and then transferred to a hydrothermal reactor and heated at 105°C for 25 h. After the reaction was cooled to room temperature, mixture A was obtained by centrifuging three times alternately with 20 mL DMF and 20 mL methanol. 2) Preparation of Ni-doped MIL-101 (Fe) solution: Add Ni(NO3)2·6H2O, which is 4% of the molar amount of iron ions, to the above mixture A. Stir continuously with a magnetic stirrer at 300 rpm and irradiate with a xenon lamp for 30 min to obtain mixture B.

[0049] 3) After ultrasonic treatment of the above mixture B until it becomes transparent, it is transferred to a hydrothermal reactor and heated at 105°C for 25 hours to obtain mixture C.

[0050] 4) After the reaction was cooled to room temperature, the nickel-containing MOF product D was obtained by centrifugation three times alternately with 20 mL DMF and 20 mL methanol; the product was recovered by centrifugation and dried in vacuum at 70°C for 12 h.

[0051] 5) Purge the muffle furnace with argon for 30 min, and then place the vacuum-dried nickel-doped MOF product D in the muffle furnace for heating.

[0052] The heating conditions are as follows: starting at 25°C, and increasing by 4°C per minute. -1 The temperature steadily increased until it reached 600°C, and was maintained at this temperature for 4 hours to obtain product E.

[0053] 6) Product E was heated in air from 25°C to 600°C at the same heating rate and held at that temperature for 4 h to obtain a magnetic composite photocatalyst, which is an oxide encapsulated by a carbon layer derived from a product MOF containing oxygen vacancies.

[0054] Example 11 Based on Example 1, the preparation method of the magnetic composite photocatalyst is as follows: 1) Preparation of MIL-101 (Fe) precursor solution: 2.45 mmol FeCl3·6H2O and 1.24 mmol terephthalic acid were dissolved in 15 mL DMF, sonicated until transparent, and then transferred to a hydrothermal reactor and heated at 125°C for 20 h. After the reaction was cooled to room temperature, mixture A was obtained by centrifuging three times alternately with 20 mL DMF and 20 mL methanol. 2) Preparation of Ni-doped MIL-101 (Fe) solution: Add Ni(NO3)2·6H2O, which is 4% of the molar amount of iron ions, to the above mixture A. Stir continuously with a magnetic stirrer at 300 rpm and irradiate with a xenon lamp for 30 min to obtain mixture B.

[0055] 3) After ultrasonic treatment of the above mixture B until it becomes transparent, it is transferred to a hydrothermal reactor and heated at 125°C for 20 hours to obtain mixture C.

[0056] 4) After the reaction was cooled to room temperature, the nickel-containing MOF product D was obtained by alternating centrifugation with 20 mL DMF and 20 mL methanol three times; the product was recovered by centrifugation and dried in vacuum at 80°C for 12 h.

[0057] 5) Purge the muffle furnace with argon for 30 min, and then place the vacuum-dried nickel-doped MOF product D in the muffle furnace for heating.

[0058] The heating conditions are as follows: starting at 25°C, and increasing by 5°C per minute. -1 The temperature steadily increases until it reaches 500℃, and is maintained at this temperature for 4 hours to obtain product E.

[0059] 6) Product E was heated in air from 25°C to 500°C at the same heating rate and held at that temperature for 4 h to obtain a magnetic composite photocatalyst, which is an oxide encapsulated by a carbon layer derived from a product MOF containing oxygen vacancies.

[0060] Comparative Example 1 (Undoped Ni) Unlike the method for preparing magnetic composite photocatalysts in Example 1, step 2 was omitted, and the resulting product was Fe-MIL, a weakly magnetic material.

[0061] Comparative Example 2 (Ni-doped but not calcined in an inert atmosphere) Unlike the method for preparing magnetic composite photocatalysts in Example 1, step 5 was omitted, and the resulting product was Ni / Fe-MIL, which is a weakly magnetic material.

[0062] Table 1. Yields of photocatalyst products under external magnet conditions

[0063] The materials prepared in Examples 1-6 were the same. The products prepared in Examples 1-9 and Comparative Examples 1-2 were used as photocatalysts to carry out the photocatalytic reduction of CO2 without the application of an external magnetic field. The test results are shown in Table 2. Table 2. Yields of time-catalyzed products without external magnets

[0064] Tables 1 and 2 show that the magnetic field strength has a significant impact on the photocatalytic performance of the catalysts. The applied magnetic field significantly improved the CO production efficiency of all catalysts, indicating that the CO2 reduction rate was enhanced under this external force. Under the influence of the applied magnetic field, the CO production of the NFO / FO@C-4 photocatalyst increased from 38.6 μmol g / L under no magnetic field conditions. -1 h -1 Increased to 230.6 μmol g -1 h -1 The high yield of NFO / FO@C-4 is consistent with the EPR results, which is attributed to its highest O2 content. V Content. Furthermore, the presence of a magnetic field significantly affects the selectivity of the CO product. Without an external magnetic field, the CO selectivity in the NFO / FO@C-4 product is 60.2%. With the introduction of an external magnetic field, the CO production selectivity of all catalysts is significantly improved, reaching 90.1%. This improvement is attributed to the spin-polarized electrons generated by the NFO / FO@C-4 composite material, which promote CO2 adsorption and enhance its selectivity. The synergistic effect of the electron spin state and the external magnetic field leads to a negative MR effect, enhancing the electron transfer capability between heterojunctions, thereby increasing the product yield.

[0065] like Figure 2aAs shown, the hysteresis (MH) loops of NiFe2O4, Fe2O3, and the composite material NFO / FO@C-4 were measured at room temperature. The results show that the ferromagnetism of the composite material NFO / FO@C-4 is significantly superior to that of NiFe2O4 and Fe2O3 alone. This enhanced ferromagnetism is mainly attributed to the O content in NFO / FO@C-4. V The increase in content was supported by XPS test results. V The increase in these vacancies promotes spin polarization within the catalyst, thereby enhancing its ferromagnetism. These vacancies promote spin polarization of electrons in NFO / FO@C-4, which is beneficial for reducing recombination of photogenerated carriers during catalysis. Furthermore, the close interaction between NiFe2O4 and Fe2O3 alters the physical and chemical properties of the composite material, leading to an increase in magnetic susceptibility. This enhancement further improves catalytic performance and increases the material's recyclability.

[0066] Figure 2b The magnetoresistance (MR) effect of NiFe2O4, Fe2O3, and the composite material NFO / FO@C-4 at room temperature was demonstrated. The results show that the NFO / FO@C-4 composite material exhibits a significant negative MR effect, with a maximum value of approximately -2.5% at an external magnetic field strength of 20 kOe. Figure 2b It can be observed that the negative MR of the composite material decreases rapidly at lower electric fields, while tending to saturate at higher electric fields, especially above 20 kOe. This trend is consistent with... Figure 2a The magnetic saturation behavior described in the MH ring is consistent, with the material's magnetization almost reaching saturation at 20 kOe. The negative MR effect of the NFO / FO@C-4 composite material is due to the ordered arrangement of magnetic moments near this magnetic field strength. Beyond this threshold, further increases in the magnetic field do not significantly enhance the negative MR effect.

[0067] like Figure 2c As shown, without an external magnetic field, the magnetic moments align in one direction within a specific region, but the overall arrangement appears somewhat chaotic. When an external magnetic field is applied, as... Figure 2d As shown, all magnetic moments tend to align with the direction of the magnetic field, thereby promoting charge transfer. This ordered alignment of magnetic moments significantly improves the conductivity of the material and enhances the negative MR effect.

[0068] Figures 3a-3e Based on XPS measurements of different catalysts, the elemental composition and chemical valence state of Fe2O3, NiFe2O4, NFO / FO@C-2, NFO / FO@C-4, and NFO / FO@C-6 were determined.

[0069] Complete XPS spectrum as Figure 3aAs shown, apart from the characteristic peaks corresponding to elements C, Ni, Fe and O, no other irrelevant element peaks were detected.

[0070] like Figure 3b As shown, the C 1s spectra of the five materials were decomposed into three peaks: C=O at 288.7 eV, CO at 286.3 eV, and CC at 284.8 eV.

[0071] exist Figure 3c In the O 1s phase, three distinct characteristic peaks are observed, among which the characteristic peak at 529.6 eV is associated with the metal-oxygen bond (O). L The characteristic peaks at 531.7 eV and 533.2 eV were determined to be O, respectively. V and surface hydroxyl groups (O C Furthermore, in NFO / FO@C-4, O V The percentage of O (36.12%) was higher than that of NFO / FO@C-2 (35.11%) and NFO / FO@C-6 (33.99%), indicating that NFO / FO@C-4 contains more O. V O in Fe2O3 L The proportion was 43.02%, higher than the 38.34% in NFO / FO@C-4. In contrast, the O content in Fe2O3 was... V The percentage was 28.40%, lower than the 36.12% in the NFO / FO@C-4 catalyst. However, the O content in both catalysts... C The content was almost the same. The results showed that O V The formation may originate from NFO / FO@C-4 octahedron (O h ) lattice structure of the site.

[0072] like Figure 3d As shown, Fe 2p exhibits two peaks at 710.8 eV and 712.8 eV, corresponding to Fe in Fe2O3, respectively. 3+ The peak values ​​at 711.8 eV and 725.0 eV for NFO / FO@C-2, NFO / FO@C-4, and NFO / FO@C-6 are attributed to Fe. 2+ The presence of Fe was also observed at 709.8 eV and 723.2 eV. 3+ The relevant peaks show that, compared to NFO / FO@C-4, Fe 2p in Fe2O3 shifts towards lower binding energies, while O 1s shifts towards higher binding energies. This shift indicates electron transfer from NiFe2O4 to Fe2O3, demonstrating a strong interfacial electronic interaction between the two compounds.

[0073] like Figure 3eAs shown, the characteristic peaks at 860.8 eV and 878.7 eV correspond to the Ni 2p of NiFe2O4, respectively. 3 / 2 and Ni2p 1 / 2 The characteristic peaks at 854.7 eV and 872.3 eV are attributed to satellite peaks. In NFO / FO@C-2, NFO / FO@C-4, and NFO / FO@C-6, two distinct peaks were observed in the ranges of 850–858 eV and 870–878 eV, respectively, corresponding to Ni₂P. 3 / 2 and Ni 2P 1 / 2 The characteristics are attributed to the high-spin state of Ni. 2+ Ions. This shift compared to single-phase NiFe2O4 is due to strong electronic interactions at the interface of the heterojunction complex NFO / FO@CX.

[0074] Figure 4a The transient photocurrent response spectra of different catalysts prepared in Examples 1-4 of this invention under photoexcitation are shown. When the samples are exposed to light, the photocurrent increases rapidly, indicating the effective generation of photogenerated carriers. Conversely, when the light source is removed, the photocurrent drops rapidly to zero, indicating rapid recombination of electron-hole pairs. Notably, the NFO / FO@C-4 photocatalyst exhibits a significantly higher transient photocurrent than other catalysts, demonstrating its superiority in enhancing charge separation, which is attributed to its carefully designed heterojunction structure. Photocurrent analysis shows that, under conditions without a magnetic field, the maximum photocurrent density of NFO / FO@C-4 reaches 2.8 μA cm⁻¹. -2 .

[0075] like Figure 4b As shown, the photocurrent intensity increases accordingly with the increase of the applied magnetic field strength, from 22.5 μA cm⁻¹. -2 Increased to 38.5 μA cm -2 This enhancement is attributed to the behavior of the ferromagnetic catalyst under an external magnetic field, where the spin-polarized electrons align with the magnetic field direction. Due to the negative MR effect induced by the spin-polarized electrons and the applied magnetic field, the spin-polarized electrons can efficiently cross the interface of the composite catalyst. Considering that both Fe₂O₃ and NiFe₂O₄ possess magnetic moments, their arrangement at the interface significantly affects the transport dynamics of photogenerated carriers. Under the condition of an applied magnetic field, the magnetic moments of Fe₂O₃ and NiFe₂O₄ align with the magnetic field direction. Figure 2b Consistent with the above, this ordered arrangement improves the efficiency of injecting spin-polarized charge carriers from NiFe2O4 to Fe2O3. Therefore, more photogenerated charge carriers can be transported through the interface between Fe2O3 and NiFe2O4 per unit time, thereby increasing the current density of the composite material and reducing its resistivity.

[0076] like Figure 4c As shown, without the application of an external magnetic field, the maximum carrier lifetime of NFO / FO@C-4 is 17.63 ns, which is higher than that of other catalysts.

[0077] To further investigate the effect of magnetic fields on carrier recombination, a self-made time-resolved emission spectrometer with an external magnet was constructed, and lifetime measurements were performed, such as... Figure 4d As shown, the fluorescence lifetime of the NFO / FO@C-4 catalyst changed significantly under different magnetic field strengths. Under stronger magnetic field conditions, the maximum carrier lifetime of NFO / FO@C-4 increased to 20.00 ns. This increase in carrier lifetime is likely due to the negative MR effect, which effectively suppressed charge recombination in the NFO / FO@C-4 catalyst. These results indicate that a stronger magnetic field plays a crucial role in enhancing the separation of electron-hole pairs in the photocatalyst, further improving its overall photocatalytic efficiency.

[0078] Figure 5a In-situ DRIFTS testing was conducted using the NFO / FO@C-4 catalyst to verify the photocatalytic reaction pathway of CO2. First, after introducing CO2 and H2O at 200 °C, a reaction was observed at 3400 cm⁻¹. -1 A broad peak appears nearby, indicating the presence of free radicals (OH*) generated by the dissociation of H2O. The characteristic peaks of these hydroxyl radicals overlap with the adsorbed CO2 peak, indicating an interaction between H2O and CO2 on the NFO / FO@C-4 surface. As the reaction proceeds, a peak appears at 1656 cm⁻¹. -1 The peak at 1432 cm⁻¹ indicates that H₂O molecules are adsorbed on the NFO / FO@C₄ surface. Furthermore, the peak at 1432 cm⁻¹... -1 and 1222 cm -1 Bidentate bicarbonate (b-HCO3) was detected at the site. 2- The peak at 1396 cm⁻¹ further confirms that CO₂ molecules are activated on the NFO / FO@C₄ surface. -1 and 1417 cm -1 The peaks appearing at [location] represent *CH3 and *CH2O species, respectively. During the reaction, [value] 2918 cm⁻¹ -1 2848 cm -1 and 1260 cm -1 The appearance of the new peak signifies the formation of *COOH, a necessary intermediate in the conversion of CO2 to CO. These *COOH species then, under light conditions, react with H+ produced from H2O molecules. +The reaction promotes the conversion of *COOH to *CO. The gradual hydrogenation of *CO further produces *CH2O and *CH3, ultimately generating CH4. Based on the results of in-situ DRIFTS, the following possible CO2 reduction pathways are proposed. First, CO2 and H2O are adsorbed on the catalyst surface. Under light irradiation, CO2 molecules are activated and react with water molecules to form the *COOH intermediate species. Subsequently, *COOH further reacts with protons and electrons to generate CO and CH4 as the final products. Notably, *COOH is the key species for CO2 to CO conversion. Overall, in-situ DRIFTS provides detailed mechanistic insights into the complex pathway of CO2 reduction in the NFO / FO@C-4 catalyst, highlighting the importance of various intermediate species and their interactions for efficient CO2 reduction. The density of states (DOS) of Fe2O3 and NFO / FO@CX was calculated using DFT. Fe2O3 exhibits a symmetric DOS, indicating its limited electronic activity (see [link to DFT]). Figure 5b ).

[0079] Figure 5b This indicates that Fe₂O₃ exhibits a completely symmetrical DOS, suggesting limited electronic activity. In contrast, such as Figure 5c As shown, in NFO / FO@CX, the spin-up and spin-down electrons exhibit an asymmetric DOS. This asymmetry in the DOS indicates the presence of spin-polarized electrons in NFO / FO@CX. Furthermore, spin-polarized electrons can be excited to E CB In E VB This leaves holes with similar spin orientation characteristics. However, due to spin-orbit interactions, spin-polarized electrons may undergo spin flipping and lose their original spin orientation. More importantly, in E... VB The lack of spin-polarized holes that match the equivalent spin orientation severely hinders carrier recombination (see...). Figure 5d Compared to pristine Fe2O3, the extended carrier lifetime in NFO / FO@CX resulted in a lower carrier recombination rate and higher photocatalytic activity. These theoretical insights are consistent with the experimental results of NFO / FO@CX, highlighting the importance of altered electronic properties in improving the photocatalytic efficiency of CO2 reduction.

[0080] Figure 6 Figure a shows the CO2 photoreduction reaction activity tests using different catalysts under no magnetic field conditions. The results indicate that the main products in the catalytic reaction are CO and CH4. The CO yield of NFO / FO@C-4 is 5.0 times that of Fe-MIL and 2.1 times that of Ni / Fe-MIL. Furthermore, the highest CO yield of NFO / FO@C-4 is 38.6 μmol g. -1 h-1 It was significantly higher than that of NFO / FO@C-2 (28.5 μmol g). -1 h -1 ), NFO / FO@C-6 (32.3 μmol g -1 h -1 ) and NFO / FO@C-8 (30.7 μmol g -1 h -1 The high yield of NFO / FO@C-4 is consistent with the XPS results, which is attributed to its highest O2 content. V Content. Furthermore, the NFO / FO@CX catalyst exhibits the ability to generate eight-electron CH4. In the catalyst activity test, increased nickel content was associated with higher methane yield. The maximum methane yield of NFO / FO@C-4 was 25.6 μmol g. -1 h -1 However, further increasing the nickel content leads to a decrease in photocatalytic performance. This decrease may be due to the higher proportion of NiFe2O4, which reduces the number of active sites, thus adversely affecting the overall efficiency of the catalyst system. The reasons for the reduced catalytic activity include: the higher proportion of NiFe2O4 reduces the number of active sites available for photocatalytic reactions; O V The effects of factors such as changes in light absorption characteristics, poisoning of active sites, and surface covering effects all negatively impact the overall efficiency of the catalyst system. For example... Figure 6 As shown in Figure b, the magnetic field has a significant impact on the photocatalytic performance of the catalyst. The applied magnetic field significantly improved the CO production efficiency of all catalysts, indicating that the CO2 reduction rate was enhanced under this external force. Under the action of the applied magnetic field, the CO production of the NFO / FO@C-4 photocatalyst increased from 38.6 μmol g / L under the condition without a magnetic field. -1 h -1 Increased to 230.6 μmol g -1 h -1 Furthermore, the presence of a magnetic field significantly affects the selectivity of the CO product. Without an external magnetic field, the CO selectivity of NFO / FO@C-4 is 60.2%. However, with the introduction of an external magnetic field, the selectivity of NFO / FO@C-4 increases from 60.2% to 90.1%. This improvement is attributed to the spin-polarized electrons generated by the NFO / FO@C-4 composite material, which promote CO2 adsorption and enhance its selectivity. The synergistic effect of the electron spin state and the external magnetic field leads to a negative MR effect, enhancing the electron transfer ability between Fe2O3 and NiFe2O4, thereby increasing the product yield. Due to the superior photocatalytic performance of the NFO / FO@C-4 catalyst, it will be used as a representative material for further research.

[0081] Figure 7 The performance of the NFO / FO@C-4 catalyst under different magnetic field strengths was investigated. The strengths of the three magnets were measured using a Gaussian measuring instrument at 3000 GS, 3500 GS, and 4000 GS. When NFO / FO@C-4 was exposed to a strong magnetic field of 4000 GS, its CO formation rate reached 230.6 μmol g⁻¹. -1 h -1 In contrast, under low and medium magnetic field conditions, the CO yields of NFO / FO@C-4 were 124.5 and 160.1 μmol g, respectively. -1 h -1 This indicates that there is a significant correlation between magnetic intensity and CO generation rate during the photocatalytic CO2 reduction process using the NFO / FO@C-4 catalyst.

[0082] Figure 8 Cyclic stability experiments of the NFO / FO@C-4 catalyst were demonstrated, with each cycle lasting 5 h for a total of 4 cycles. The results show that NFO / FO@C-4 exhibits excellent cyclic stability, a key factor in ensuring continuous photocatalytic reaction. Even after 20 h of cycling, the photocatalytic performance of the catalyst remained unchanged, demonstrating its superior cyclic stability.

[0083] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for improving the selectivity of photocatalytic reduction of carbon dioxide products, characterized in that, Includes the following processes: A magnetic composite photocatalyst with a negative magnetoresistance effect is placed in a reaction vessel. Under illumination, an external magnetic field is applied to the reaction system. The negative magnetoresistance effect of the magnetic photocatalyst under the action of the external magnetic field promotes the separation of photogenerated carriers and improves the efficiency and product selectivity of photocatalytic reduction of carbon dioxide.

2. The method for improving the selectivity of photocatalytic reduction of carbon dioxide products according to claim 1, characterized in that, The magnetic composite photocatalyst with negative magnetoresistance effect is a nickel-iron composite oxide encapsulated by a carbon layer derived from MOF, with a chemical composition of NiFe2O4 / Fe2O3@C and oxygen vacancies; NiFe2O4 / Fe2O3 has a heterojunction.

3. The method for improving the selectivity of photocatalytic reduction of carbon dioxide products according to claim 1, characterized in that, The strength of the external magnetic field is 200 GS ~ 4000 GS.

4. The method for improving the selectivity of photocatalytic reduction of carbon dioxide products according to claim 1, characterized in that, The photocatalyst was obtained by calcining MIL-101 (Fe) with nickel doping under an inert atmosphere.

5. The method for improving the selectivity of photocatalytic reduction of carbon dioxide products according to claim 1, characterized in that, The preparation process of the magnetic composite photocatalyst with negative magnetoresistance effect includes the following steps: (1) The iron salt and organic ligand were subjected to a hydrothermal reaction in a solvent to obtain a MIL-101 (Fe) precursor solution; (2) Add nickel salt to the MIL-101(Fe) precursor solution, mix and photo-assisted to obtain a MIL-101(Fe) solution containing nickel; (3) The MIL-101 (Fe) solution containing nickel was ultrasonically treated and then subjected to hydrothermal reaction to obtain a mixed solution; (4) After cooling the mixture to room temperature, centrifuge and wash to obtain nickel-doped MOF products, and then vacuum dry them; (5) The nickel-doped MOF product after vacuum drying was subjected to a first stage calcination under an inert atmosphere to obtain an intermediate product; (6) The intermediate product is calcined in air to obtain carbon-coated NiFe2O4 / Fe2O3, which is a magnetic composite photocatalyst.

6. The method for improving the selectivity of photocatalytic reduction of carbon dioxide products according to claim 5, characterized in that, In step (2), the amount of nickel ions added is 2% to 8% of the molar amount of iron ions.

7. The method for improving the selectivity of photocatalytic reduction of carbon dioxide products according to claim 5, characterized in that, In step (3), the hydrothermal reaction temperature is 105~125°C and the time is 20~25 h; In step (4), vacuum drying is performed at 60~80°C for 12 h.

8. The method for improving the selectivity of photocatalytic reduction of carbon dioxide products according to claim 5, characterized in that, In step (5), the temperature of the first stage of calcination is 400~600°C, the heating rate is 3~5 °C / min, and the holding time is 4~6 h; In step (6), the calcination temperature in the second stage is 400~600°C, the heating rate is 3~5 °C / min, and the holding time is 4~6 h.

9. A system for implementing the photocatalytic reduction of carbon dioxide according to any one of claims 1-8, characterized in that, include: The reaction vessel has a light-transmitting window at the top; A light source, positioned above or outside the light-transmitting window, is used to provide the illumination required for the photocatalytic reaction. A magnetic composite photocatalyst is disposed inside the reaction vessel; A magnetic field generating device is installed inside or outside the reaction vessel to apply an external magnetic field to the reaction system.

10. The system according to claim 9, characterized in that, The magnetic field generating device is a permanent magnet or an electromagnet.

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