ZnO / BTO composite catalyst and preparation and application methods thereof

Abstract

The invention relates to the technical field of catalyst preparation, in particular to a ZnO / BTO composite catalyst and a preparation and application method thereof.The preparation method comprises the steps that Bi4Ti3O12 (BTO) powder and ZIF-8 powder are subjected to dry grinding and mixing according to the mass ratio of (5-20): 1, then a polar solvent is added for wet grinding, obtained slurry is aged for 8-48 h and then dried, and a precursor is obtained; placing the precursor in an inert atmosphere, raising the temperature to 350-400 DEG C, keeping the temperature for 20-60 minutes, pyrolyzing and converting ZIF-8 into a porous zinc oxide precursor, then switching to an atmosphere of 1-10% hydrogen, raising the temperature to 500-600 DEG C, and keeping the temperature for 60-120 minutes to form a heterojunction structure; the oxygen vacancy concentration of the material is measured after cooling, secondary reduction treatment is selected according to the measurement result, direct rapid cooling or mild oxidation treatment is performed, and the problems of weak interface bonding and uncontrollable defects are solved.

Description

Technical Field

[0001] This invention relates to the field of catalyst preparation technology, and specifically to a ZnO / BTO composite catalyst and its preparation and application methods. Background Technology

[0002] As a major greenhouse gas, carbon dioxide capture and resource utilization are key to reducing carbon content. Among them, reducing CO2 to high-value-added fuels or chemicals (such as methanol) is considered a highly promising green conversion pathway, because methanol is not only a basic chemical raw material, but can also be used as a clean fuel.

[0003] Currently, the most widely used method for producing methanol from CO2 in industry and research is thermocatalytic hydrogenation. This method is typically carried out under high temperature and high pressure, using copper-based catalysts (such as Cu / ZnO / Al2O3) or noble metal catalysts (such as Pd and Pt-based catalysts). The technology is relatively mature and has been implemented industrially; it features a fast reaction rate, high space-time yield (STY), and enables large-scale continuous production.

[0004] However, precious metal co-catalysts significantly increase material costs, deviating from the original intention of the technology's application. Copper-based catalysts require a large amount of energy to maintain high-temperature and high-pressure reaction conditions. More importantly, the hydrogen required for the reaction currently mainly relies on the steam reforming or coal gasification of fossil fuels (such as natural gas). These processes themselves generate a large amount of CO2, which greatly reduces the overall carbon emission reduction effect of the process and may even lead to carbon leakage. Furthermore, the high-temperature and high-pressure reaction environment, especially the water generated in the reaction, can easily cause the active components of copper-based catalysts to sinter, be lost, or be oxidized, leading to rapid deactivation and requiring frequent regeneration or replacement.

[0005] To overcome the aforementioned shortcomings of thermocatalytic hydrogenation, solar-driven photocatalytic CO2 reduction technology has emerged. This technology can directly convert CO2 and water vapor into products such as methanol under normal temperature and pressure conditions, using only water and sunlight as energy. Theoretically, it is a near-ideal negative carbon or zero carbon conversion pathway.

[0006] However, the core bottleneck hindering the practical application of photocatalysis technology lies in the lack of efficient, stable, and highly selective catalysts. An ideal catalyst needs to simultaneously meet the following requirements: good absorption of visible light; efficient separation of photogenerated electrons and holes; and provision of specific adsorption and activation sites for CO2 molecules to selectively generate the target product, methanol.

[0007] Although researchers have developed numerous photocatalytic materials, including TiO2, CdS, g-C3N4, metal-organic frameworks (MOFs) and their derivatives, they generally suffer from one or more of the following problems: wide-bandgap semiconductors such as TiO2 can only utilize ultraviolet light, resulting in extremely low solar energy utilization efficiency; narrow-bandgap semiconductors such as CdS and some organic polymers, while exhibiting good visible light response, suffer from severe recombination of photogenerated carriers and poor photochemical stability, making them susceptible to photocorrosion. Furthermore, due to the complex CO2 reduction reaction pathway, the products are typically mixtures of CO, CH4, formic acid, methanol, etc., making it difficult to obtain a single high-value product with high selectivity and high yield. In particular, precisely guiding the reaction to produce methanol rather than the simpler CO or CH4 is a significant challenge. Therefore, developing a novel catalyst that can photocatalytically reduce CO2 to methanol with high selectivity under mild conditions without the need for precious metals is an urgent technical problem to be solved in this field.

[0008] The information disclosed in this background section is intended only to enhance the understanding of the overall background of the present invention and should not be construed as an admission or in any way implying that the information constitutes prior art known to those skilled in the art. Summary of the Invention

[0009] The first objective of this invention is to provide a method for preparing a ZnO / BTO composite catalyst. By combining stepwise mechanochemical activation, programmed atmosphere calcination, and defect state regulation, a gradient distribution of oxygen vacancies can be precisely and directionally introduced into the heterojunction interface, solving the problems of weak interfacial bonding and uncontrollable defects in traditional methods.

[0010] The above-mentioned technical objective of the present invention is achieved through the following technical solution: A method for preparing a ZnO / BTO composite catalyst includes the following steps: S1 will use Bi4Ti3O 12 (BTO) powder and ZIF-8 powder were dry-milled and mixed at a mass ratio of 5~20:1, and then 0.05~0.3mL / g of polar solvent was added for wet milling. The resulting slurry was aged for 8~48h and then dried at 40~80℃ to obtain the precursor. Preferably, in step S1, the dry grinding conditions are as follows: using zirconia grinding balls with diameters of 10 mm and 5 mm respectively, with a mass ratio of 1:2, grinding at a speed of 200-500 rpm for 15-60 min in a planetary ball mill, with a ball-to-material ratio of 10-20:1; and the wet grinding conditions are as follows: using the same grinding media, grinding at a speed of 300-600 rpm for 30-120 min in a planetary ball mill; the polar solvent is at least one of anhydrous ethanol, methanol, isopropanol, and deionized water. This step is carried out near room temperature, avoiding premature decomposition of ZIF-8 or structural changes of BTO at high temperatures, thus preserving the activity of the precursor; at the same time, the ball mill used is conventional, and the process is easy to scale up.

[0011] Specifically, under dry grinding and mixing, mechanical force breaks the old chemical bonds on the particle surface, generating a large number of new surfaces with high surface energy and unsaturated dangling bonds. Then, solvent-assisted wet grinding is used, where polar solvent molecules adsorb onto the surface of the new material. Their dipole moments can interact with the surface dangling bonds. Under continuous mechanical force, the solvent molecules dissociate or polarize, generating ions or active groups (such as H+). + ,OH - ,RO - The active groups on the surfaces of the two phases are attacked and chemically modified. Through solvent bridging or direct contact, the active groups on the surfaces of the two phases undergo condensation or coordination reactions under the promotion of mechanical force, and the initial formation of chemical bonds such as Zn-O-Ti or Zn-O-Bi is achieved.

[0012] Preferably, the wet-milled slurry, along with the grinding media, is poured into a beaker containing an appropriate amount of the same solvent. The ball mill jar is rinsed multiple times to ensure quantitative transfer of the material, resulting in a uniform suspension that guarantees complete material recovery and constant composition. Then, the slurry is sealed and aged at room temperature. During aging, the container is manually shaken or sonicated for 1-2 minutes every 6-12 hours to redisperse any particles that may have settled and promote a uniform reaction.

[0013] Preferably, the suspension can be subjected to short-term low-frequency ultrasonic treatment before drying to break up temporary soft agglomerates, so that the particles reach the optimal dispersion state in the solvent, and then dried to obtain a looser and more uniform precursor powder.

[0014] In this invention, dry grinding initially mixes particles to reduce their size and increase their specific surface area. Adding solvent during wet grinding lowers the grinding temperature, preventing localized overheating that could lead to ZIF-8 decomposition, and allows ZIF-8 particles to disperse more uniformly and adhere to the BTO surface. The wet-milled slurry is in a high-energy, metastable state. After aging for 8–48 hours, the slurry is dried at 40–80°C to obtain the precursor. During aging, mechanical stress gradually relaxes, and the solvent further promotes the compact arrangement of particles under capillary action. Simultaneously, some interfacial chemical reactions initiated during wet grinding have sufficient time to continue under static conditions and reach a more stable state. After aging, the solvent is slowly removed at a lower temperature of 40–80°C to avoid structural collapse or particle re-aggregation caused by rapid solvent evaporation, preserving the microstructure formed after aging. This yields a dry, uniform precursor composite powder rich in pre-coupled interfaces, preparing it for the next calcination step.

[0015] S2 The precursor is placed in an inert atmosphere and heated to 350-400℃ at a heating rate of 1-5℃ / min and held for 20-60min to pyrolyze ZIF-8 into a porous zinc oxide precursor. Then, the temperature is switched to an atmosphere of 1-10% hydrogen and heated to 500-600℃ and held for 60-120min to convert the zinc oxide precursor into porous ZnO and form a heterojunction structure with BTO. The heating rate of 2-3℃ / min is further preferred.

[0016] Preferably, the inert atmosphere in step S2 is selected from at least one of argon and nitrogen.

[0017] Preferably, in step S2, after being kept at a constant temperature in an inert atmosphere, the porous zinc oxide precursor formed by ZIF-8 conversion is a loose initial porous network composed of zinc species and residual carbon. The subsequent heating and holding in a hydrogen-containing atmosphere reduces and crystallizes the porous zinc oxide precursor, transforming it into porous ZnO. This ZnO forms a heterojunction structure with BTO powder at the interface, while oxygen vacancies are introduced into the interface region to enhance the activity and selectivity of the photocatalyst in reducing carbon dioxide to methanol.

[0018] Specifically, in the first stage, the precursor is placed in an inert atmosphere. The organic ligands in ZIF-8 undergo controlled pyrolysis and volatilization in an oxygen-deficient environment, exposing zinc nodes and forming highly dispersed, amorphous or crystalline zinc-based active species. This process occurs in situ within the tight enclosure of BTO particles. The newly formed highly active zinc species interact with adjacent BTO surfaces to form initial chemical bonds. This temperature window removes most of the organic ligands without oxidizing BTO or disrupting mixing homogeneity, forming an intermediate with initial porosity and extremely high reactivity. This avoids conditions where excessively low temperatures lead to incomplete decomposition and residual organic matter interfering with heterojunction formation, or excessively high temperatures cause premature crystallization and particle agglomeration of zinc species, thus maintaining extremely high specific surface area and surface activity. A slow heating rate promotes uniform heat and mass transfer, preventing the ZIF-8 framework from collapsing due to rapid decomposition.

[0019] In the second stage, under a weak reducing atmosphere and higher temperatures, the removal of residual carbon opens up the pores, exposing more active zinc surfaces. This completely reduces the amorphous precursor from the first stage and crystallizes it into porous ZnO. Under these conditions, the residual carbon in the ZIF-8 precursor is effectively hydrogenated or transformed into a highly graphitized stable carbon structure. This stable graphitic carbon acts as a hard template, preventing excessive growth of ZnO grains and supporting the formation of a porous structure. Furthermore, it is extremely inert under room-temperature photocatalytic conditions, making it difficult to be oxidized by photoactive species (such as holes or ·OH) to produce methanol. ZnO and BTO undergo solid-state ion diffusion and reaction at the contact interface, forming a chemically tightly bonded heterojunction structure with a high lattice matching degree, thereby significantly reducing the interfacial charge transport resistance. Reducible hydrogen preferentially reacts with oxygen atoms at the interface, which have a larger strain due to lattice mismatch, to generate water and leave oxygen vacancies. These oxygen vacancies are enriched at the interface and diffuse from the high-concentration interface to the low-concentration bulk phase. Due to the limited holding time, the diffusion is incomplete, thus naturally forming an oxygen vacancy concentration gradient from the interface to the bulk phase. The interface becomes a highly efficient catalytic active site and a promoting center for charge separation.

[0020] Low-concentration hydrogen can achieve mild and controllable reduction. Too low a concentration will result in insufficient defect introduction, while too high a concentration may lead to over-reduction of BTO or deep reduction of ZnO. A temperature of 500-600℃ can drive the solid-state interface reaction to form a strong heterojunction. At the same time, it is a suitable window for hydrogen to effectively reduce oxides and generate oxygen vacancies. It is also much lower than the phase transition or decomposition temperature of BTO. Holding at this temperature for 60-120 minutes ensures that the heterojunction is fully bonded and crystallized, while controlling the amount and gradient distribution of oxygen vacancies.

[0021] Preferably, after the first stage of the heat preservation process at 350~400℃ is completed, before the system temperature begins to rise to 500~600℃, the atmosphere is switched to an atmosphere containing 1~10% hydrogen.

[0022] Preferably, in a hydrogen-containing atmosphere, the temperature is first raised to 400 - 450 °C at a rate of 2 - 8 °C / min and held for 30 min to promote the initial formation and homogenization of interfacial oxygen vacancies, and then the temperature is raised to the final temperature at the same rate for sufficient crystallization. 400 - 450 °C is the temperature at which hydrogen begins to effectively reduce metal oxides, but the reaction rate is relatively mild. Holding at this temperature allows oxygen vacancies to nucleate controllably and evenly, avoiding excessive concentration of defects in local areas or the formation of large-sized defect clusters due to violent reactions when rapidly rising to high temperatures.

[0023] Preferably, the dried precursor powder is loosely and evenly spread on an open high-temperature resistant ceramic boat or quartz boat, and the powder stacking thickness does not exceed 5 mm. This ensures that heat can be quickly and evenly transferred to each powder particle, while the reaction atmosphere and product gas can smoothly diffuse in and out, eliminating the temperature gradient and atmosphere concentration gradient caused by too thick bed layers.

[0024] S3 After cooling, measure the oxygen vacancy concentration of the material in step S2, and select to perform secondary reduction treatment, direct rapid cooling, or mild oxidation treatment according to the measurement results to obtain the ZnO / BTO composite catalyst.

[0025] Due to factors such as precursor differences and minor fluctuations in the temperature zones of the equipment, even if steps S1 and S2 are strictly followed, the oxygen vacancy concentration of the produced material will fluctuate around the ideal value. In this step, real-time adjustment is made according to the results of the previous two stages. When the oxygen vacancies are insufficient, secondary reduction is carried out; when the oxygen vacancies are excessive, mild oxidation is carried out; when the oxygen vacancies are moderate, direct rapid cooling is carried out. The purpose is to finely adjust the interfacial electron structure, optimize the carrier separation efficiency, and ensure that the performance of each batch of final products is stable in the optimal range.

[0026] Preferably, the material after being treated in step S2 is cooled to room temperature under the protection of an inert atmosphere to avoid interference from high-temperature oxidation, and then a representative sample is taken from it for oxygen vacancy concentration measurement. For example, the sample is obtained by sampling using the quartering method to avoid misjudgment caused by uneven sampling.

[0027] Preferably, in step S3, adjustment is made according to the oxygen vacancy concentration in the material. The oxygen vacancy concentration is characterized by the signal intensity I at g≈2.002 in the electron paramagnetic resonance (EPR) spectrum; compare I with the reference signal intensity I0 measured from the control sample. When I < k1I0, perform secondary reduction treatment. The secondary reduction treatment is carried out in a reducing atmosphere containing 0.5 - 5% hydrogen at 500 - 600 °C for 5 - 30 min; supplement oxygen vacancies to the material to make it reach the optimal range. When I>k2I0, mild oxidation treatment is performed. Mild oxidation treatment is carried out in air or a mixed gas containing 1~21% oxygen at 500~600℃ for 2~20min. This selectively fills some of the excess or unstable oxygen vacancies, while retaining the shallow oxygen vacancies at the interface that are beneficial to catalysis, thereby reducing deep defects. When k1I0≤I≤k2I0, direct rapid cooling is performed; the rate of direct rapid cooling is not less than 20℃ / min, and it is carried out under an inert gas to prevent oxygen vacancies from annihilating, migrating, or structurally relaxing during the slow cooling process.

[0028] Where k1 = 1.2~1.5, meaning the lower threshold of the optimal range is 1.2~1.5 times the signal intensity of the control sample, and k2 = 2.5~3.5, meaning the upper threshold of the optimal range is 2.5~3.5 times the signal intensity of the control sample.

[0029] Preferably, the control sample is obtained by mechanically mixing equal masses of BTO powder and ZIF-8 powder for 30 min, followed by calcination at 550°C for 2 h in air. The signal intensity I0 of the control sample is tested by obtaining representative test samples from the control sample powder using a standard quartering method. These representative test samples are placed in standardized quartz EPR sample tubes and tested at room temperature using an X-band electron paramagnetic resonance (EPR) spectrometer. The test parameters are fixed as follows: microwave frequency approximately 9.8 GHz, modulation amplitude 0.5 mT, modulation frequency 100 kHz, and scan width 10 mT. The EPR spectrum is recorded and integrated, and the area of ​​the resonance peaks with g-factor in the range of 2.001 to 2.003 (either single-integrated or double-integrated) is calculated. This area value is recorded as the reference signal intensity I0. To ensure accuracy, at least three parallel tests should be performed on the same control sample, and the average value is taken as the final I0 value. The method for determining the signal intensity I of the sample is exactly the same as that for determining the I0 of the control sample.

[0030] The second objective of this invention is to provide a ZnO / BTO composite catalyst that solves the problems of insufficient charge separation efficiency, scarce active sites, and low selectivity in existing photocatalytic materials.

[0031] The above-mentioned technical objective of the present invention is achieved through the following technical solution: A ZnO / BTO composite catalyst, prepared by the above method, uses ZIF-8 powder and BTO powder as starting materials. After treatment, ZnO nanoparticles derived from the thermal conversion of ZIF-8 are uniformly distributed on the BTO surface, and chemically bonded Zn-O-Ti or Zn-O-Bi bonds exist at the interface. The heterojunction interface of the composite catalyst has a gradient distribution of oxygen vacancies, with the oxygen vacancy concentration in the interface region being 1.5 to 5 times that in the bulk region. The oxygen vacancy enrichment at the interface strongly adsorbs CO2 molecules and weakens the C=O bond by injecting electrons into the π antibonding orbitals of CO2, making it easier for protonation reduction. This catalyst, without relying on any noble metal co-catalyst, possesses both extremely high photogenerated carrier separation efficiency and abundant CO2 adsorption activation sites, ensuring the catalyst can selectively reduce CO2 to the target product methanol while exhibiting good structural stability.

[0032] The mass ratio of BTO powder to ZIF-8 powder is 5~20:1, ensuring that BTO is the main light absorption and charge generation phase, while ZnO serves as a modifying phase to optimize the interface and provide additional active sites. Too much ZnO will cover the active surface of BTO; too little ZnO will result in insufficient interface effect.

[0033] Preferably, the BTO powder is bismuth-rich Bi4Ti3O. 12 The molar ratio of Bi to Ti is 4.1~4.5:3. The bismuth-rich BTO surface has more active bismuth sites and oxygen vacancies, and exhibits higher interfacial reactivity during mechanochemical activation with ZIF-8 and subsequent calcination, making it easier to form strongly chemically bonded heterojunctions.

[0034] As a preferred method, the preparation of BTO powder is as follows: Bi₂O₃ and TiO₂ powders are accurately weighed according to a Bi:Ti molar ratio of 4.1~4.5:3, and wet-milled to obtain an ultrafine and uniform mixed precursor. The precursor is then pressed into tablets and placed in a covered corundum crucible, and calcined at 600~700℃ for 30~90 min in an air atmosphere. After calcination, the tablets are cooled, ground, washed, and dried to obtain bismuth-rich Bi₄Ti₃O₃. 12 Powder. Prolonged wet milling achieves nanoscale uniform mixing of reactants, greatly shortening the solid-phase diffusion distance and enabling the reaction to be completed rapidly at lower temperatures. This minimizes bismuth volatilization. By controlling the excess Bi2O3 and using a local sealing design, and utilizing low-temperature and rapid heat treatment, the excess bismuth occupies normal bismuth sites or interstitial sites in the crystal lattice during the reaction. To maintain electroneutrality, intrinsic oxygen vacancies or bismuth vacancies of corresponding concentrations are generated in the crystal, achieving the controllable preparation of bismuth-rich BTO.

[0035] As a preferred method, the preparation of ZIF-8 powder is as follows: Weigh Zn(NO3)2·6H2O and dissolve it in methanol to obtain solution A; weigh 2-methylimidazole and dissolve it in methanol to obtain solution B; under vigorous stirring, quickly pour solution B into solution A; Zn... 2+ The molar ratio of ZIF-8 to 2-methylimidazole was 1:8. The reaction produced a white precipitate. The mixture was stirred at room temperature for 24 hours. The suspension was filtered, and the precipitate was washed three times with methanol. The filter cake was dried under vacuum at 60°C for 12 hours to obtain white ZIF-8 powder.

[0036] A third objective of this invention is to provide a method for applying the aforementioned ZnO / BTO composite catalyst to the photocatalytic reduction of carbon dioxide to methanol. This application can achieve highly efficient and selective conversion of CO2 to methanol under mild conditions of ambient temperature and pressure, using only water and sunlight as energy and proton sources.

[0037] The above-mentioned technical objective of the present invention is achieved through the following technical solution: A method for photocatalytic reduction of carbon dioxide to methanol, using the above-mentioned ZnO / BTO composite catalyst, includes the following steps: A1 places the ZnO / BTO composite catalyst in the light-illuminated area of ​​the photocatalytic reactor and maintains good dispersion by uniform spreading or immobilization to ensure the maximum light-harvesting area and the contact interface with the reactants. A2 introduces carbon dioxide gas and water vapor as a proton source into the reactor, and exhausts the air to create a carbon dioxide atmosphere inside the reactor. Preferably, in step A2, the carbon dioxide-containing gas is pure carbon dioxide gas, or a mixture of carbon dioxide and an inert gas, wherein the volume fraction of carbon dioxide is not less than 20%. This invention allows the direct use of a mixed gas with a CO2 concentration of not less than 20%. The high concentration of gradient oxygen vacancies at the catalyst interface has a strong chemical affinity and capture ability for CO2, enabling effective enrichment of CO2 locally on the catalyst surface, thereby ensuring a considerable surface reaction rate is maintained even at a low gas phase partial pressure.

[0038] Preferably, the catalyst dosage is 0.1-5 g per liter of reactor volume, and the carbon dioxide space velocity is 100-2000 mL·g. -1 ·h -1 This range balances light capture efficiency with reactor processing capacity, achieving high methanol space-time yield while maintaining high selectivity.

[0039] Preferably, water vapor is introduced by bubbling a carrier gas into a constant-temperature water bath, with the water bath temperature controlled at 10~60℃; thus achieving precise and stable control of the proton source supply rate in the reaction system.

[0040] Preferably, the volume ratio of carbon dioxide-containing gas to water vapor is 5 to 20:1. This ensures preferential adsorption and activation of CO2 and provides a sufficient but not excessive proton source.

[0041] A3 uses light intensity of 50~500mW / cm². 2 When a light source irradiates a catalyst, carbon dioxide reacts with water vapor to produce methanol-containing products under the catalytic action of the catalyst.

[0042] Preferably, in step A3, the light source is a xenon lamp that simulates sunlight, or an LED light source with an emission wavelength range of 300~500nm.

[0043] Preferably, the photocatalytic reaction is carried out at room temperature to 150°C and atmospheric pressure to 2 MPa, and the selectivity of methanol in the photocatalytic reaction is not less than 70%.

[0044] During the process, CO2 molecules are preferentially chemisorbed and activated by the gradient enrichment of oxygen vacancies at the catalyst heterojunction interface; water vapor is oxidized by photogenerated holes on the catalyst surface, providing the protons (H+) required for the reduction reaction. + The oxygen-free environment avoids the competitive consumption of photogenerated electrons by O2, ensuring that electrons are used efficiently for CO2 reduction. Under illumination, the ferroelectric polarization field of BTO and the built-in electric field of the heterojunction synergistically drive the photogenerated electrons (e... - Electrons migrate directionally to the ZnO side, and are injected into the activated CO2 through the oxygen vacancy at the interface. The CO2 is then coupled with protons for multi-step hydrogenation reduction, ultimately producing methanol.

[0045] Compared with the prior art, the present invention has the following beneficial effects: This invention prepares a ZnO / BTO composite catalyst using ZIF-8 powder and BTO powder through stepwise mechanochemical activation and atmosphere-controlled calcination. By performing multi-step mechanochemical treatment and precisely controlling the calcination atmosphere, partial chemical reactions between precursors are induced during the mechanical mixing stage. During the calcination stage, the interfacial defect state of the composite material is finely controlled by atmosphere control, thereby creating a gradient distribution of active oxygen vacancies at the heterojunction interface. These vacancies, in synergy with the ferroelectric polarization field of BTO, drive charge separation and greatly accelerate the transfer of electrons from BTO to ZnO.

[0046] The ZnO / BTO composite catalyst prepared in this invention precisely optimizes the adsorption and activation pathway of CO2 in the photocatalytic CO2 reduction reaction through a gradient distribution of interfacial oxygen vacancies, guiding the reaction towards methanol production and suppressing the formation of byproducts such as CO and CH4. It achieves high selectivity for the target product methanol while significantly improving methanol yield. Compared to the control group that underwent simple mechanical mixing followed by calcination, the methanol yield can be increased by 2 times or even more. Attached Figure Description

[0047] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0048] Figure 1 This is a flowchart of the preparation method for ZnO / BTO composite catalysts; Figure 2 Here is a SEM image of the BTO powder in Example 1; Figure 3 The image shows a SEM image of the ZnO / BTO composite catalyst prepared in Example 1. Figure 4 This is a flowchart of the defect feedback control decision-making process; Figure 5 The figures show a comparison of the photocatalytic performance of the examples and comparative examples. Detailed Implementation

[0049] To further illustrate the technical means and effects adopted by the present invention to achieve its intended purpose, a ZnO / BTO composite catalyst and its preparation and application methods according to the present invention are described in detail below, including their specific implementation methods, features, and effects. It should be understood that the specific embodiments described herein are only for explaining the present invention and are not intended to limit the present invention.

[0050] Example 1: like Figures 1-5 As shown, a method for preparing a ZnO / BTO composite catalyst includes the following steps: S1. Preparation of BTO powder: 5.236 g of Bi₂O₃ and 0.799 g of TiO₂ powder were accurately weighed according to the molar ratio of Bi:Ti = 4.2:3. The powders were wet-milled to obtain an ultrafine and uniform mixed precursor. The precursor was pressed into tablets and placed in a covered corundum crucible. It was calcined at 650 °C for 60 min in air. After calcination, the tablets were cooled, ground, washed, and dried to obtain bismuth-rich Bi₄Ti₃O₃. 12 powder; S2. Preparation of ZIF-8 powder: Weigh 2.97g Zn(NO3)2·6H2O and dissolve it in 100mL methanol to obtain solution A. Weigh 6.57g 2-methylimidazole and dissolve it in 100mL methanol to obtain solution B. Under vigorous stirring, quickly pour solution B into solution A. The reaction produces a white precipitate. Continue stirring at room temperature for 24h. Filter the suspension and wash the precipitate three times with methanol. Dry the filter cake under vacuum at 60℃ for 12h to obtain white ZIF-8 powder. S3. 1.8g of BTO powder and 0.2g of ZIF-8 powder, along with zirconia grinding balls (5mm and 10mm in diameter, ball-to-powder ratio 15:1), were placed in a zirconia ball mill jar. The mixture was dry-milled at 400rpm for 30min on a planetary ball mill, then 0.2mL / g of polar solvent was added and wet-milled at 500rpm for 90min. The resulting slurry was transferred to a stoppered glass bottle, sealed and aged at room temperature for 24h, and then dried at 60℃ to obtain the precursor. S4. Loosely spread the precursor in a quartz boat to a thickness of about 3 mm, place it in a tube furnace, and introduce argon gas at a flow rate of 50 mL / min. Increase the temperature to 300 °C at a rate of 2 °C / min and hold for 30 min to allow ZIF-8 to pyrolyze and transform into a porous zinc oxide precursor. Then switch to a 5% H2 / Ar mixed gas at a flow rate of 50 mL / min, increase the temperature to 550 °C at a rate of 5 °C / min, and hold for 90 min to transform the zinc oxide precursor into porous ZnO and react with Bi4Ti3O4. 12 Formation of heterojunction structure; S5. After cooling, the oxygen vacancy concentration of the material from step S4 is measured and compared with the EPR signal intensity I0 of the control sample. In this example, I / I0 = 2.8, which is between the thresholds k1 = 1.5 and k2 = 3.0. Based on the determination result, the sample is rapidly quenched to room temperature in an argon gas flow at a rate >100℃ / min to obtain the final catalyst, denoted as C-1.

[0051] A method for photocatalytic reduction of carbon dioxide to methanol, using a ZnO / BTO composite catalyst prepared by the above method, includes the following steps: A1. A 300mL cylindrical quartz photocatalytic reactor is used, equipped with a quartz window, gas inlet and outlet, and sampling port at the top. The light source is a 300W xenon lamp with an AM1.5G filter. The light intensity at the reactor window is controlled to 100mW / cm by adjusting the distance. 2 ; A2. Weigh 0.15g of the prepared C-1 catalyst and spread it evenly on the circular quartz plate at the bottom of the reactor. The diameter is 5cm and the catalyst loading is about 1g / L of the reactor volume. A3. High-purity CO2 (99.999%) is introduced into the reactor at a flow rate of 50 mL / min through a water bath bubbler at a constant temperature of 30°C, carrying water vapor. The volume ratio of CO2 to H2O is controlled by the temperature of the bubbler and the CO2 flow rate. In this embodiment, it is about 10:1. The aeration is continued for 30 minutes to completely replace the air in the reactor. A4. Turn on the xenon lamp and start timing. The reaction is carried out at room temperature and normal pressure. During the reaction, the gaseous products are analyzed by online gas chromatography every 1 hour through the reactor circulation loop. After 6 hours of reaction, turn off the light source and collect the condensed liquid products by passing the gas in the reactor through a -10℃ cold trap.

[0052] The liquid product was filtered through a 0.45 μm filter membrane and qualitatively analyzed using gas chromatography-mass spectrometry (GC-MS). The methanol content was quantitatively analyzed using GC with FID (Fluid Identification), and quantified using the external standard method. The methanol yield and selectivity were calculated, yielding a methanol yield of 185 μmol·g for the C-1 catalyst. -1 ·h -1 The methanol selectivity was 82%.

[0053] Example 2: Using catalyst C-1 prepared in Example 1, the carbon dioxide composition in step A3 of the photocatalytic reduction of carbon dioxide to methanol was changed. Specifically, a mixture of 40% CO2 and 60% N2 was used as the carbon dioxide-containing gas. While keeping the total flow rate, water vapor partial pressure, and illumination conditions constant, the methanol yield was measured to be 138 μmol·g. -1 ·h -1 The methanol selectivity was 76%, demonstrating that the bismuth-rich BTO composite with porous ZnO and abundant interfacial oxygen vacancies exhibit high selective adsorption and activation capabilities for CO2 molecules. Even in the presence of abundant N2, the affinity of its active sites for CO2 is much higher than that for N2, thus effectively resisting N2 interference.

[0054] Example 3: The difference between this embodiment and Example 1 is that stoichiometric BTO was used. Following steps S2-S5 of Example 1, a composite catalyst, denoted as C-2, was prepared. The photocatalytic reduction of carbon dioxide to methanol was performed under identical conditions to test the catalytic performance of C-2. The methanol yield of C-2 was 142 μmol·g. -1 ·h -1 With a selectivity of 68% and an EPR signal strength ratio I / I0 of 1.5, it is demonstrated that the bismuth-rich design can improve performance.

[0055] Example 4: Using catalyst C-1 prepared in Example 1, the reaction conditions of the photocatalytic reduction of carbon dioxide to methanol were modified, wherein the light source was an LED array with an emission wavelength of 385 nm, and the light intensity was adjusted to 200 mW / cm². 2 The reaction system was sealed, and the back pressure valve was adjusted to allow the reaction to proceed under a mild pressure of 0.5 MPa. The water bath temperature was increased to 50°C, and the volume ratio of CO2 to H2O was adjusted to 15:1. Under these conditions, the methanol yield reached 235 μmol·g. -1 ·h -1 The selectivity was 85%, demonstrating that the catalyst performed excellently within the optimal parameter range.

[0056] Comparative Example 1: The BTO powder obtained in steps S1 and S2 of Example 1 was simply mixed with ZIF-8 powder at a mass ratio of 9:1 for 30 min, and then calcined in a muffle furnace at 550°C for 2 h in air atmosphere to obtain the comparative catalyst C-0, which had an EPR signal intensity of I0. Photocatalysis was performed under identical conditions, and the methanol yield of the C-0 catalyst was 65 μmol·g⁻¹. -1 ·h -1 The methanol selectivity was 58%.

[0057] Comparative Example 2: In this comparative example, the precursor preparation was the same as step S3 in Example 1, except that the calcination process only involved the first step: after holding at 300°C for 30 min in argon, instead of switching to H2 / Ar, the temperature was raised to 550°C and held for 90 min in pure argon, followed by direct cooling. The resulting catalyst was designated C-3, and its methanol yield was 45 μmol·g. -1 ·h -1 The selectivity was less than 50%, proving that the programmed introduction of a reducing atmosphere is the key to forming highly active interface oxygen vacancies.

[0058] Comparative Example 3: The preparation process was the same as steps S1-S4 in Example 1. In step S5, EPR measurement and threshold judgment were not performed; instead, direct rapid cooling was used as the default post-treatment method. The three batches of catalysts obtained were designated C-4a, C-4b, and C-4c. Performance tests were performed on C-4a, C-4b, and C-4c sequentially, and their batch reproducibility was calculated. Their methanol yields were 155 μmol·g⁻¹, respectively. -1 ·h -1 192 μmol·g -1 ·h -1 205 μmol·g -1 ·h -1 The results showed significant batch fluctuations, proving that the feedback control in step S5 is necessary to ensure product performance consistency and a high pass rate.

[0059] Those skilled in the art should understand that this invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to this invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a ZnO / BTO composite catalyst, characterized in that, It includes the following steps: S1 will use Bi4Ti3O 12 The powder and ZIF-8 powder were dry-milled and mixed at a mass ratio of 5~20:1, and then 0.05~0.3mL / g of polar solvent was added for wet milling. The resulting slurry was aged for 8~48h and then dried to obtain the precursor. S2. The precursor is placed in an inert atmosphere and heated to 350-400℃ at a heating rate of 1-5℃ / min, and held for 20-60 min to pyrolyze the ZIF-8 into a porous zinc oxide precursor. Then, the temperature is switched to an atmosphere of 1-10% hydrogen and heated to 500-600℃, held for 60-120 min to convert the zinc oxide precursor into porous ZnO, which then reacts with Bi4Ti3O. 12 Formation of heterojunction structure; After cooling in step S3, measure the oxygen vacancy concentration of the material in step S2, and select secondary reduction treatment, direct rapid cooling or mild oxidation treatment according to the measurement results.

2. The preparation method of the ZnO / BTO composite catalyst according to claim 1, characterized in that, In step S2, after holding at a temperature in an inert atmosphere, the porous zinc oxide precursor formed by the ZIF-8 transformation is a loose initial porous network composed of zinc species and residual carbon. The subsequent heating and holding at a temperature in a hydrogen-containing atmosphere reduces and crystallizes the porous zinc oxide precursor, transforming it into porous ZnO, which then reacts with Bi4Ti3O. 12 The powder forms a heterojunction structure at the interface, while oxygen vacancies are introduced into the interface region.

3. The method for preparing the ZnO / BTO composite catalyst according to claim 1, characterized in that, The oxygen vacancy concentration described in step S3 is characterized by the signal intensity I at g≈2.002 in the EPR spectrum; secondary reduction treatment is carried out when I<k1I0, mild oxidation treatment is carried out when I>k2I0, and direct rapid cooling is carried out when k1I0≤I≤k2I0; where k1 = 1.2 - 1.5, k2 = 2.5 - 3.5, and I0 is the EPR signal intensity of the control sample.

4. The preparation method of the ZnO / BTO composite catalyst according to claim 3, characterized in that, The control sample was composed of the same mass of Bi4Ti3O 12 The sample was obtained by mechanically mixing the powder with ZIF-8 powder for only 30 min, followed by calcination at 550 °C for 2 h in air atmosphere.

5. The method for preparing the ZnO / BTO composite catalyst according to claim 1, characterized in that, The secondary reduction treatment is carried out by heating to 500 - 600°C in an atmosphere containing 0.5 - 5% hydrogen and holding for 5 - 30 min; the mild oxidation treatment is carried out by heating to 500 - 600°C in air or a mixed gas containing 1 - 21% oxygen and holding for 2 - 20 min; the rate of the direct rapid cooling is not less than 20°C / min.

6. The method for preparing the ZnO / BTO composite catalyst according to claim 1, characterized in that, The conditions of the dry grinding in step S1 are grinding at a rotation speed of 200 - 500 rpm for 15 - 60 min; the conditions of the wet grinding are grinding at a rotation speed of 300 - 600 rpm for 30 - 120 min; the polar solvent is at least one of anhydrous ethanol, methanol, isopropanol, and deionized water.

7. A ZnO / BTO composite catalyst, prepared by the method according to any one of claims 1 to 6, characterized in that, After treatment, ZnO nanoparticles derived from ZIF-8 thermal conversion are uniformly distributed in Bi4Ti3O. 12 On the surface, the heterojunction interface of the composite catalyst has a gradient distribution of oxygen vacancies, and the oxygen vacancy concentration in the interface region is 1.5 to 5 times that in the bulk region.

8. The ZnO / BTO composite catalyst according to claim 7, characterized in that, The Bi4Ti3O 12 The powder is bismuth-rich Bi4Ti3O 12 The molar ratio of Bi to Ti is 4.1 to 4.5:

3.

9. A method for preparing methanol by photocatalytic reduction of carbon dioxide, characterized in that, Using the ZnO / BTO composite catalyst according to claim 7 or 8, it includes the following steps: A1 Place the ZnO / BTO composite catalyst in a photocatalytic reactor. A2 Introduce a gas containing carbon dioxide and water vapor as a proton source into the reactor, and discharge air to make the inside of the reactor in a carbon dioxide atmosphere. A3 uses light intensity of 50~500mW / cm². 2 When the catalyst is irradiated by a light source, carbon dioxide reacts with water vapor to produce a product containing methanol under the catalytic action of the catalyst.

10. The method for preparing methanol by photocatalytic reduction of carbon dioxide according to claim 9, characterized in that, In step A2, the gas containing carbon dioxide is pure carbon dioxide gas or a mixture of carbon dioxide and an inert gas, where the volume fraction of carbon dioxide is not less than 20%.

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