Metal oxide composite material for preparing ethane through photocatalytic conversion of methane and preparation method of metal oxide composite material

Abstract

The invention relates to the field of catalytic materials, and provides a metal oxide composite material for preparing ethane through photocatalytic methane conversion and a preparation method of the metal oxide composite material. The material is a ternary composite functional material with a multi-stage structure and multi-scale characteristics, and comprises a micron-scale porous alumina carrier skeleton which is prepared by a freeze drying method and has directional through columnar pore channels, CeO2 nanorods which grow in the pore channels in situ, and an anatase type TiO2 shell layer which coats the surfaces of the CeO2 nanorods. The composite material forms a CeO2-TiO2 core-shell structure and a TiO2 thin layer on the inner wall of a pore channel, and has excellent photocatalytic activity and thermal shock resistance. The activity attenuation rate does not exceed 10% after irradiation of a light source with the wavelength of 365 nm and 20 times of thermal cycles at the temperature of 25-800 DEG C. The preparation method comprises the steps of freeze-drying forming and heat treatment of an aluminum oxide framework, in-situ hydrothermal synthesis of CeO2 nanorods and hydrolytic deposition and calcination of a TiO2 shell layer, the process is simple and convenient, the repeatability is high, and the method is suitable for efficient and stable photocatalysis application.

Description

Technical Field

[0001] The present invention relates to the field of catalyst materials, and particularly to a metal oxide composite material for photocatalytic methane conversion to ethane and a preparation method thereof. Background Art

[0002] With the extensive development of natural gas resources and the continuous optimization of the energy structure, methane, as the main component of natural gas, its high-value conversion and utilization has become an important research direction in the field of energy chemistry. Under this background, the green conversion path of photocatalytic methane coupling to prepare high-value hydrocarbons such as ethane has received extensive attention, especially suitable for application scenarios such as solar-driven distributed energy conversion systems and in-situ value-added utilization of remote oil and gas field resources. Such systems usually operate in an unstable temperature environment and pose higher requirements for reaction efficiency and material durability. Therefore, in such practical applications, the photocatalytic materials used need to simultaneously possess high photocatalytic activity and good thermal shock stability to ensure stable reaction efficiency under complex and frequent thermal cycling or long-term operation conditions. High photocatalytic activity is directly related to the excitation, activation, and selective coupling efficiency of methane molecules and is the basis for achieving low-energy consumption and high-yield conversion; while thermal shock stability determines the structural integrity and performance persistence of the material under repeated thermal stress, which is of great significance for improving the system operation life and reducing maintenance costs. Therefore, developing advanced composite materials with both high photocatalytic activity and thermal shock stability not only has the potential to significantly improve the efficiency and reliability of the methane photocatalytic conversion process, but also is of great significance for promoting the development of clean energy conversion technologies and the high-value utilization of natural gas resources.

[0003] Although the photocatalytic methane coupling conversion technology has made certain progress in recent years, the catalytic materials currently used still have obvious deficiencies in terms of high photocatalytic activity and thermal shock stability, and it is difficult to meet the application requirements under actual complex working conditions. For example, the Chinese patent with publication number CN117000238A discloses a spherical ferro-titanium ore-supported photocatalyst for methane oxidative coupling to ethane. Although it has certain catalytic activity, it is prone to structural degradation during frequent temperature changes, resulting in poor long-term stability, which limits its industrialization potential. The main reason for this problem is that the existing material structure design is single and lacks multi-scale synergy. It is difficult to balance mechanical stability and thermal stress buffering capacity while ensuring high specific surface area and active site exposure; at the same time, the interface bonding between active components is not strong, and separation or migration is prone to occur under thermal cycling or high temperature conditions, further weakening the sustainability of catalytic performance. In addition, although some materials have been optimized in terms of light response range, exciton separation efficiency, etc., their structural response behavior under thermal shock environment has not been systematically considered, resulting in rapid performance decline under actual operating conditions. Therefore, the development of composite photocatalytic materials with multi-level structural regulation capabilities, strong interface stability and the ability to maintain catalytic performance over a wide temperature range has become a key path to solving existing technical bottlenecks and promoting the sustainable development of photocatalytic methane conversion technology. Summary of the invention

[0004] (1) Technical issues solved The purpose of the present invention is to provide a metal oxide composite material for photocatalytic methane conversion to ethane and a preparation method thereof, so as to solve the current problem of insufficient performance in high photocatalytic activity and thermal shock stability.

[0005] (2) Technical solution In order to achieve the above object, the present invention provides the following technical solutions: A metal oxide composite material for photocatalytic methane conversion to ethane, wherein the composite catalytic material is a ternary composite functional material with a multi-level structure and multi-scale characteristics, and specifically includes the following structural units: a) a micrometer-sized porous alumina carrier skeleton, wherein the alumina carrier skeleton is prepared by freeze-drying process and has a directional and through-type columnar pore structure; b) CeO2 nanorod functional structure, wherein the CeO2 nanorod is constructed inside the columnar pores of the alumina support skeleton by an in-situ growth method; c) TiO2 shell photocatalytic active unit: The TiO2 shell is formed by an in-situ deposition growth process and coated on the surface of CeO2 nanorods and the inner wall of the columnar pores, thereby constructing a CeO2-TiO2 core-shell structure and a TiO2 thin layer on the inner wall of the pores.

[0006] Furthermore, the columnar pore structure of the alumina support framework is micron-scale pores that are parallel to each other, through and oriented, with a pore diameter of 2.5 - 8.5 μm and a porosity of 65 - 85%.

[0007] Furthermore, the CeO₂ nanorods have a diameter of 200 - 350 nm and a length of 2600 - 8600 nm, and the CeO₂ nanorods are uniformly distributed and densely grown on the inner wall of the columnar pores of the alumina support framework.

[0008] Furthermore, the TiO₂ shell layer has a thickness of 20 - 65 nm and continuously and uniformly coats the outer surface of the CeO₂ nanorods and the inner wall surface of the columnar pores, forming a dense interfacial bonding structure.

[0009] Furthermore, the crystal structure of the TiO₂ shell layer is anatase type.

[0010] Furthermore, the composite material has high catalytic activity for the photocatalytic coupling conversion of methane to ethane and high thermal shock stability, and after 20 thermal cycles at 25 - 800 °C under a light source with a wavelength of 365 nm, the decay rate of its photocatalytic activity does not exceed 10%.

[0011] The present invention adopts the design of ternary composite functional materials constructed by multi-level structures and multi-scale synergy, which is mainly used to enhance the catalytic activity and thermal shock stability performance in the process of photocatalytic methane conversion to ethane. By constructing a composite catalytic system composed of a micron-scale porous alumina support framework, CeO2 nanorod functional structures, and a TiO2 shell photocatalytic unit, the synergetic enhancement between the support structure and the catalytic components in terms of spatial configuration and functional mechanism is achieved. First, the alumina framework prepared by the freeze-drying process has a highly oriented and through-columnar pore network, which not only provides a structural basis for the uniform loading of subsequent functional components, but also helps to improve the mass transfer efficiency of reactant molecules in the pores and effectively buffer the change of thermal stress. Secondly, CeO2 nanorods are in-situ grown inside the pores, and their excellent oxygen migration ability and surface activity provide efficient reaction sites for the excitation and coupling of methane molecules, and their oriented growth morphology helps to improve the electron transfer rate and stability. In addition, the TiO2 shell is formed by in-situ deposition and uniformly coats the CeO2 nanorods and the inner wall of the pores, which not only constructs a stable CeO2-TiO2 core-shell structure, but also forms a dense bonding layer at the interface, thereby enhancing the separation efficiency of photogenerated carriers and suppressing the recombination of electrons and holes. The crystal form of this shell is anatase type, which has good light response performance and structural stability, enabling the overall composite material to maintain stable catalytic activity under high-temperature light irradiation and multiple thermal cycling conditions. Through the reasonable construction and organic integration of the above three structural units, the present invention not only takes into account the active sites, electron migration channels and light response ability required for efficient photocatalytic reactions, but also significantly improves the structural stability and service life of the material in a complex thermal environment, showing excellent functional performance, and providing a material solution with both high efficiency and stability for methane photocatalytic conversion.

[0012] The present invention also discloses a preparation method of a metal oxide composite material for photocatalytic methane conversion to ethane, which includes the following steps carried out in sequence: S1. Prepare a micron-scale porous alumina support framework with an oriented arrangement and a through-columnar pore structure by the freeze-drying process; S2. Prepare CeO2 nanorod functional structures in the columnar pores of the alumina support framework by an in-situ growth method; S3. Form a TiO2 shell on the surface of the CeO2 nanorods and the inner wall surface of the pores by an in-situ deposition growth process to construct a CeO2-TiO2 core-shell structure and a TiO2 nano-thin layer structure.

[0013] Further, the specific steps of step S1 include: dissolving 8.0 - 15.0 wt% aluminum nitrate and 1.5 - 3.0 wt% polyethylene glycol PEG - 6000 in deionized water to form an alumina precursor sol, injecting it into a mold, and then freeze - forming under a vacuum of 2 - 5 Pa. The freezing temperature is maintained at - 60 to - 40 °C for 12 - 24 h. Subsequently, vacuum freeze - drying is carried out under a vacuum of ≤50 Pa, and the drying temperature is - 30 to - 10 °C for 18 - 30 h. After obtaining a porous preform, it is placed in a muffle furnace and heated to 800 - 1000 °C at a heating rate of 5 - 8 °C / min for heat treatment for 3 - 4 h, and then cooled to room temperature with the furnace to obtain a porous alumina carrier skeleton.

[0014] Further, the specific steps of step S2 include: dissolving 1.2 - 2.5 wt% cerium nitrate, 0.3 - 0.8 wt% urea, and 0.1 - 0.3 wt% citric acid in deionized water to prepare a cerium oxide precursor solution. Immerse the porous alumina carrier skeleton obtained in S1 in the cerium oxide precursor solution. When immersing, control the liquid - solid ratio to be 5:1 - 8:1, and perform ultrasonic treatment at a power of 200 - 400 W and stir at 300 - 400 rpm for 20 - 40 min. Subsequently, add the mixed solution to a polytetrafluoroethylene - lined autoclave and set a safety relief valve. Carry out hydrothermal reaction at 160 - 180 °C for 12 - 24 h. After filtration, wash with deionized water until the pH value of the filtrate is 6.8 - 7.2. Retain the carrier loaded with CeO2 nanorods and dry it at 80 - 120 °C for 2 - 12 h to obtain a porous alumina carrier skeleton loaded with CeO2.

[0015] Further, the specific steps of step S3 include: dissolving 5.0 - 10.0 wt% tetrabutyl titanate in absolute ethanol to form a titanium source solution, adding deionized water according to 4.0 - 8.0 wt% of the total mass of the titanium source solution, and adding 0.2 - 0.6 wt% polyvinylpyrrolidone as a dispersant at the same time. Adjust the pH of the system to 3.5 - 4.5 with 25 - 28 wt% ammonia water. Immerse the porous alumina carrier skeleton loaded with CeO2 in the solution at a solid - liquid ratio of 1:10 - 1:15, and carry out hydrolysis - deposition reaction at a stirring rate of 500 - 700 rpm and a temperature of 80 - 100 °C for 8 - 16 h. After the reaction is completed, wash alternately with absolute ethanol and acetone 3 times to remove unreacted titanate and organic by - products. Retain the carrier coated with TiO2 and calcine it in an air atmosphere at a heating rate of 3 - 5 °C / min to 450 - 550 °C for 2 - 4 h, and finally obtain a metal oxide composite material for photocatalytic conversion of methane to ethane.

[0016] The composite material preparation method of the present invention, which takes freeze-drying, in-situ growth, and in-situ deposition as the core processes, is mainly used to enhance the catalytic activity and thermal shock stability performance in the photocatalytic conversion of methane to ethane. Through the rational design of three-step continuous and interconnected technological processes, the precise construction of multi-scale structures and the organic synergy of multi-functional components are realized. First, the micron-scale porous alumina support framework prepared by the freeze-drying process has a columnar pore structure with directional penetration, which not only provides a high specific surface area and good gas channels, helps the efficient diffusion of reactants and the timely desorption of products, but also lays a structural foundation for the precise positioning and uniform loading of subsequent functional components. Secondly, by dissolving cerium nitrate, urea, and citric acid together to form a cerium oxide precursor solution, and promoting its full penetration into the pores under the action of ultrasonic and stirring, and then inducing the in-situ directional growth of CeO2 nanorods through hydrothermal reaction, making them stably and densely attached to the inner wall of the pores, a highly active catalytic functional area is formed. Such nanorods have good structural order and uniform spatial distribution, which is beneficial to enhancing the electron transfer efficiency and improving the accessibility of reaction sites. Finally, by controlling the hydrolysis and deposition behavior of tetrabutyl titanate under specific pH value and temperature conditions, the TiO2 shell is uniformly coated on the CeO2 nanorods and the inner wall of the pores, forming a stable CeO2-TiO2 core-shell structure and a continuous and dense TiO2 thin layer, which not only effectively improves the interfacial bonding strength, enhances the mechanical stability and thermal stress resistance of the overall structure, but also further optimizes the separation process of photo-generated electrons and holes, enhancing the photocatalytic efficiency. Each technological process in the whole preparation process is closely linked, which not only ensures the spatial synergy and structural matching between functional components, but also realizes the complementarity and amplification of their respective performance advantages, making the composite material show excellent catalytic activity and thermal shock stability in the photocatalytic reaction, providing a solid material basis and technological support for the efficient, safe and sustainable photocatalytic conversion of methane.

[0017] (3) Beneficial technical effects 1. By constructing a synergistic system of porous alumina framework, CeO2 nanorods and TiO2 shell, the present invention realizes the dual improvement of high-efficiency methane conversion and thermal shock stability, solves the problems of low catalytic efficiency and thermal cycle deactivation in the prior art, has excellent structural stability and photocatalytic activity, is applicable to the field of clean energy conversion under high-temperature fluctuation environments, and shows significant application prospects and promotion values.

[0018] 2. By synergistically constructing a multi-scale ternary composite structure through freeze-drying, in-situ growth and deposition processes, the present invention realizes the efficient synergy among the alumina framework, CeO2 nanorods and TiO2 shell, significantly improves the photocatalytic efficiency and thermal shock stability, solves the problem that existing materials are easily deactivated during high-temperature cycling, and has broad application prospects in the high-value conversion of methane. Description of the drawings

[0019] Figure 1 SEM morphology photograph of the porous alumina support framework loaded with CeO₂ prepared in Example 1 of the present invention.

[0020] Figure 2 SEM morphology photograph of the metal oxide composite material for photocatalytic methane conversion to ethane prepared in Example 1 of the present invention.

[0021] Figure 3 Transmission electron microscopy morphology diagram of the CeO₂-TiO₂ core-shell structure prepared in Example 1 of the present invention.

[0022] Figure 4 XRD phase analysis diagram of the porous alumina support framework prepared in Example 1 of the present invention.

[0023] Figure 5 XRD phase analysis diagram of the porous alumina support framework loaded with CeO₂ prepared in Example 1 of the present invention.

[0024] Figure 6 XRD phase analysis diagram of the metal oxide composite material for photocatalytic methane conversion to ethane prepared in Example 1 of the present invention. Detailed implementation manners

[0025] To make the objectives, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0026] Example 1 A metal oxide composite material for photocatalytic methane conversion to ethane, the composite catalytic material is a ternary composite functional material with a multi-level structure and multi-scale characteristics, and specifically includes the following structural units: a) A porous alumina support framework at the micron scale, the alumina support framework is prepared by a freeze-drying process and has an oriented and through-columnar pore structure; b) A CeO₂ nanorod functional structure, the CeO₂ nanorods are constructed inside the columnar pores of the above alumina support framework by an in-situ growth method; c) A TiO₂ shell photocatalytic active unit: the TiO₂ shell is formed by an in-situ deposition growth process and covers the surface of the CeO₂ nanorods and the inner wall of the columnar pores, thereby constructing a CeO₂-TiO₂ core-shell structure and a TiO₂ thin layer on the inner wall of the pores.

[0027] The columnar pore structure of the alumina support framework in this example is micron-scale pores that are parallel, through, and oriented, with a pore diameter of 2.5 μm and a porosity of 65%.

[0028] The CeO2 nanorods of this embodiment have a diameter of 200 nm and a length of 2600 nm, and the CeO2 nanorods are uniformly distributed and densely grown on the inner wall of the columnar pores of the alumina carrier framework.

[0029] The TiO2 shell layer of this embodiment has a thickness of 20 nm and continuously and uniformly coats the outer surface of the CeO2 nanorods and the inner wall surface of the columnar pores, forming a dense interfacial bonding structure.

[0030] The crystal structure of the TiO2 shell layer of this embodiment is anatase type.

[0031] The composite material of this embodiment has high catalytic activity for the photocatalytic coupling conversion of methane to ethane and high thermal shock stability. And under a light source with a wavelength of 365 nm, after 20 thermal cycles at 25 °C, the photocatalytic activity decay rate does not exceed 10%.

[0032] A preparation method of a metal oxide composite material for photocatalytic methane conversion to ethane according to this embodiment includes the following steps in sequence: S1. Prepare a micron-scale porous alumina carrier framework with an oriented arrangement and through-columnar pore structure by freeze-drying process; specifically including: dissolving 10.1 wt% aluminum nitrate and 1.9 wt% polyethylene glycol PEG-6000 in deionized water to form an alumina precursor sol, injecting it into a mold and freeze-forming under a vacuum of 3 Pa, maintaining the freezing temperature at -54 °C for 16 h, then performing vacuum freeze-drying treatment under a vacuum of ≤50 Pa, with the drying temperature at -24 °C for 22 h, obtaining a porous preform and placing it in a muffle furnace, heating it to 860 °C at a heating rate of 6 °C / min for heat treatment for 3.3 h, and cooling it to room temperature with the furnace to obtain a porous alumina carrier framework.

[0033] S2. Prepare a CeO2 nanorod functional structure in the columnar pores of the alumina carrier framework by in-situ growth method; specifically including: dissolving 1.6 wt% cerium nitrate, 0.5 wt% urea, and 0.2 wt% citric acid in deionized water to prepare a cerium oxide precursor solution, impregnating the porous alumina carrier framework obtained in S1 in the cerium oxide precursor solution, controlling the liquid-solid ratio at 6:1 during impregnation, performing ultrasonic treatment at a power of 260 W and stirring at 330 rpm for 26 min, then adding the mixed solution to a polytetrafluoroethylene-lined autoclave and setting a safety relief valve, performing hydrothermal reaction at 166 °C for 16 h, filtering and washing with deionized water until the pH value of the filtrate is 6.9, retaining the carrier loaded with CeO2 nanorods and drying it at 92 °C for 5 h to obtain a porous alumina carrier framework loaded with CeO2.

[0034] S3. A TiO₂ shell layer is formed on the surface of the CeO₂ nanorods and the inner wall surface of the pores through an in-situ deposition growth process to construct a CeO₂-TiO₂ core-shell structure and a TiO₂ nanothin layer structure. Specifically, it includes: dissolving 6.5 wt% tetrabutyl titanate in absolute ethanol to form a titanium source solution, adding deionized water according to 5.2 wt% of the total mass of the titanium source solution, adding 0.3 wt% polyvinylpyrrolidone as a dispersant at the same time, adjusting the pH of the system to 3.8 with 26 wt% ammonia water, immersing the porous alumina carrier framework loaded with CeO₂ in the solution at a solid-liquid ratio of 1:12, and carrying out a hydrolysis-deposition reaction for 10 h under the conditions of a stirring rate of 560 rpm and a temperature of 86 °C. After the reaction is completed, it is washed alternately with absolute ethanol and acetone three times to remove unreacted titanates and organic by-products, and the carrier coated with TiO₂ is retained and calcined at 480 °C in an air atmosphere at a heating rate of 3.6 °C / min for 2.6 h to finally obtain a metal oxide composite for photocatalytic conversion of methane to ethane.

[0035] It can be seen from Figure 1 and Figure 2 that the porous alumina carrier framework prepared in Example 1 of the present invention has a highly ordered columnar pore structure, and the CeO₂ nanorods are uniformly loaded in the pores. The overall structure is complete, and the pore connectivity is good, which is beneficial to the diffusion of reactant gases and the exposure of active sites; Figure 2 It is further shown that a uniformly dense TiO₂ shell layer is successfully coated on the surface of the CeO₂ nanorods, forming a typical core-shell structure, indicating that the in-situ deposition process realizes good interfacial contact and structural integration. Figure 3 The transmission electron microscope images shown intuitively display the morphological characteristics of the CeO₂-TiO₂ core-shell structure at the nanoscale. The thickness of the TiO₂ shell layer is uniform and tightly coated on the outer surface of CeO₂, verifying the feasibility and controllability of the structural design. Figure 4 and Figure 5 The XRD patterns of Figure 6 show that the alumina framework prepared in the present invention is of the Al₂O₃ phase and has good thermal stability. After loading CeO₂, it still maintains the characteristics of the Al₂O₃ phase. At the same time, the characteristic diffraction peaks of TiO₂ are clearly observed in Figure 6 , indicating that TiO₂ is uniformly distributed in the composite material and has a good crystal phase. Based on the above results, it can be proved that the present invention successfully constructs a CeO₂-TiO₂ core-shell type metal oxide composite with regular structure, tight interface and stable crystal phase through controlling process parameters, providing a good structural basis and phase guarantee for subsequent efficient photocatalytic conversion of methane.

[0036] Example 2 A metal oxide composite material for photocatalytic methane conversion to ethane, the composite catalytic material being a ternary composite functional material with a hierarchical structure and multi-scale characteristics, specifically including the following structural units: a) A porous alumina support skeleton at the micron scale, the alumina support skeleton being prepared by a freeze-drying process and having a columnar pore structure with oriented and through-connected pores; b) A CeO2 nanorod functional structure, the CeO2 nanorods being constructed inside the columnar pores of the above-mentioned alumina support skeleton by an in-situ growth method; c) A TiO2 shell photocatalytic active unit: The TiO2 shell is formed by an in-situ deposition growth process and coats the surface of the CeO2 nanorods and the inner wall of the columnar pores, thereby constructing a CeO2-TiO2 core-shell structure and a TiO2 thin layer on the inner wall of the pores.

[0037] The columnar pore structure of the alumina support skeleton in this example is a micron-scale pore structure with parallel, through-connected, and oriented pores, the pore diameter is 4.3 μm, and the porosity is 71%.

[0038] The CeO2 nanorods in this example have a diameter of 245 nm and a length of 4400 nm, and the CeO2 nanorods are evenly distributed and densely grown on the inner wall of the columnar pores of the alumina support skeleton.

[0039] The TiO2 shell in this example has a thickness of 34 nm, continuously and evenly coats the outer surface of the CeO2 nanorods and the inner wall surface of the columnar pores, forming a dense interfacial bonding structure.

[0040] The crystal structure of the TiO2 shell in this example is anatase type.

[0041] The composite material in this example has high catalytic activity for photocatalytic methane coupling conversion to ethane and high thermal shock stability, and when irradiated by a light source with a wavelength of 365 nm and after 20 thermal cycles at 257 °C, the photocatalytic activity decay rate does not exceed 10%.

[0042] A preparation method of the metal oxide composite material for photocatalytic methane conversion to ethane in this example includes the following steps carried out in sequence: S1. Prepare a micron-scale porous alumina support framework with a directionally arranged and through-columnar pore structure by freeze-drying process; specifically including: Dissolve 8.0 wt% aluminum nitrate and 1.5 wt% polyethylene glycol PEG-6000 in deionized water to form an alumina precursor sol, inject it into a mold and freeze it under a vacuum of 2 Pa, maintain the freezing temperature at -60 °C for 12 h, then carry out vacuum freeze-drying treatment under a vacuum of ≤50 Pa, the drying temperature is -30 °C for 18 h, after obtaining a porous preform, place it in a muffle furnace and heat-treat it at a heating rate of 5 °C / min to 800 °C for 3 h, and cool it to room temperature with the furnace to obtain a porous alumina support framework.

[0043] S2. Prepare a CeO2 nanorod functional structure in the columnar pores of the alumina support framework by in-situ growth method; specifically including: Dissolve 1.2 wt% cerium nitrate, 0.3 wt% urea and 0.1 wt% citric acid in deionized water to prepare a cerium oxide precursor solution, immerse the porous alumina support framework obtained in S1 in the cerium oxide precursor solution, control the liquid-solid ratio at 5:1 during immersion, perform ultrasonic treatment at a power of 200 W and stir at 300 rpm for 20 min, then add the mixed solution to a polytetrafluoroethylene-lined autoclave, set a safety relief valve, carry out a hydrothermal reaction at 160 °C for 12 h, filter and wash with deionized water until the pH value of the filtrate is 6.8, retain the support loaded with CeO2 nanorods and dry it at 80 °C for 2 h to obtain a porous alumina support framework loaded with CeO2.

[0044] S3. Form a TiO2 shell layer on the surface of the CeO2 nanorods and the inner wall surface of the pores by in-situ deposition growth process to construct a CeO2-TiO2 core-shell structure and a TiO2 nanothin layer structure. Specifically including: Dissolve 5.0 wt% tetrabutyl titanate in absolute ethanol to form a titanium source solution, add 4.0 wt% of deionized water based on the total mass of the titanium source solution, and at the same time add 0.2 wt% polyvinylpyrrolidone as a dispersant, adjust the pH of the system to 3.5 with 25 wt% ammonia water, immerse the porous alumina support framework loaded with CeO2 in the solution at a solid-liquid ratio of 1:10, carry out a hydrolysis-deposition reaction at a stirring rate of 500 rpm and a temperature of 80 °C for 8 h, after the reaction is completed, wash it alternately with absolute ethanol and acetone 3 times to remove unreacted titanate and organic by-products, retain the carrier coated with TiO2 and calcine it in an air atmosphere at a heating rate of 3 °C / min to 450 °C for 2 h, and finally obtain a metal oxide composite material for photocatalytic conversion of methane to ethane.

[0045] Example 3 A metal oxide composite material for photocatalytic conversion of methane to ethane, the composite catalytic material being a ternary composite functional material with a hierarchical structure and multi-scale characteristics, specifically including the following structural units: a) A porous alumina support framework at the micron scale, the alumina support framework being prepared by a freeze-drying process and having a columnar pore structure with oriented and through-going pores; b) A CeO2 nanorod functional structure, the CeO2 nanorods being constructed inside the columnar pores of the above-mentioned alumina support framework by an in-situ growth method; c) A TiO2 shell photocatalytic active unit: The TiO2 shell is formed by an in-situ deposition growth process and coats the surface of the CeO2 nanorods and the inner wall of the columnar pores, thereby constructing a CeO2-TiO2 core-shell structure and a TiO2 thin layer on the inner wall of the pores.

[0046] The columnar pore structure of the alumina support framework in this embodiment is a micron-scale pore with parallel, through-going and oriented arrangement, the pore diameter is 8.5 μm, and the porosity is 85%.

[0047] The CeO2 nanorods in this embodiment have a diameter of 350 nm and a length of 8600 nm, and the CeO2 nanorods are evenly distributed and densely grown on the inner wall of the columnar pores of the alumina support framework.

[0048] The TiO2 shell in this embodiment has a thickness of 65 nm, continuously and evenly coats the outer surface of the CeO2 nanorods and the inner wall surface of the columnar pores, forming a dense interfacial bonding structure.

[0049] The crystal structure of the TiO2 shell in this embodiment is anatase type.

[0050] The composite material in this embodiment has high catalytic activity for photocatalytic coupling conversion of methane to ethane and high thermal shock stability, and when irradiated by a light source with a wavelength of 365 nm and after 20 thermal cycles at 800 °C, the photocatalytic activity decay rate does not exceed 10%.

[0051] A preparation method of the metal oxide composite material for photocatalytic conversion of methane to ethane in this embodiment includes the following steps carried out in sequence: S1. Prepare a micron-scale porous alumina carrier framework with an oriented and continuous columnar pore structure by freeze-drying process; specifically including: Dissolve 15.0 wt% aluminum nitrate and 3.0 wt% polyethylene glycol PEG-6000 in deionized water to form an alumina precursor sol, inject it into a mold and freeze it under a vacuum of 5 Pa, maintain the freezing temperature at -40 °C for 24 h, then perform vacuum freeze-drying treatment under a vacuum of ≤50 Pa, keep the drying temperature at -10 °C for 30 h, place the porous preform in a muffle furnace and heat it to 1000 °C at a heating rate of 8 °C / min for 4 h, and cool it to room temperature with the furnace to obtain the porous alumina carrier framework.

[0052] S2. Prepare a CeO2 nanorod functional structure in the columnar pores of the alumina carrier framework by in-situ growth method; specifically including: Dissolve 2.5 wt% cerium nitrate, 0.8 wt% urea and 0.3 wt% citric acid in deionized water to prepare a cerium oxide precursor solution, immerse the porous alumina carrier framework obtained in S1 in the cerium oxide precursor solution, control the liquid-solid ratio at 8:1 during immersion, perform ultrasonic treatment at a power of 400 W and stir at 400 rpm for 40 min, then add the mixed solution to a polytetrafluoroethylene-lined autoclave, set a safety relief valve, perform hydrothermal reaction at 180 °C for 24 h, filter and wash with deionized water until the pH value of the filtrate is 7.2, retain the carrier loaded with CeO2 nanorods and dry it at 120 °C for 12 h to obtain a porous alumina carrier framework loaded with CeO2.

[0053] S3. Form a TiO2 shell layer on the surface of the CeO2 nanorods and the inner wall surface of the pores by in-situ deposition growth process to construct a CeO2-TiO2 core-shell structure and a TiO2 nanothin layer structure. Specifically including: Dissolve 10.0 wt% tetrabutyl titanate in absolute ethanol to form a titanium source solution, add 8.0 wt% of deionized water based on the total mass of the titanium source solution, add 0.6 wt% polyvinylpyrrolidone as a dispersant at the same time, adjust the pH of the system to 4.5 with 28 wt% ammonia water, immerse the porous alumina carrier framework loaded with CeO2 in the solution at a solid-liquid ratio of 1:15, perform a hydrolysis-deposition reaction at a stirring rate of 700 rpm and a temperature of 100 °C for 16 h, after the reaction is completed, wash it alternately with absolute ethanol and acetone 3 times to remove unreacted titanates and organic by-products, retain the carrier coated with TiO2 and calcine it in an air atmosphere at a heating rate of 5 °C / min to 550 °C for 4 h, and finally obtain a metal oxide composite material for photocatalytic conversion of methane to ethane.

[0054] Example 4 A metal oxide composite material for photocatalytic conversion of methane to ethane. The composite catalytic material is a ternary composite functional material with a hierarchical structure and multi-scale characteristics, and specifically includes the following structural units: a) A porous alumina support framework at the micron scale. The alumina support framework is prepared by a freeze-drying process and has a columnar pore structure with oriented and through-hole arrangements; b) A CeO2 nanorod functional structure. The CeO2 nanorods are constructed inside the columnar pores of the above alumina support framework by an in-situ growth method; c) A TiO2 shell photocatalytic active unit: The TiO2 shell is formed by an in-situ deposition growth process and coats the surface of the CeO2 nanorods and the inner wall of the columnar pores, thereby constructing a CeO2-TiO2 core-shell structure and a TiO2 thin layer on the inner wall of the pores.

[0055] The columnar pore structure of the alumina support framework in this embodiment is a micron-scale pore with parallel, through-hole and oriented arrangements. The pore diameter is 6.1 μm and the porosity is 77%.

[0056] The CeO2 nanorods in this embodiment have a diameter of 290 nm and a length of 6200 nm, and the CeO2 nanorods are evenly distributed and densely grown on the inner wall of the columnar pores of the alumina support framework.

[0057] The TiO2 shell in this embodiment has a thickness of 47 nm and continuously and evenly coats the outer surface of the CeO2 nanorods and the inner wall surface of the columnar pores, forming a dense interfacial bonding structure.

[0058] The crystal structure of the TiO2 shell in this embodiment is anatase type.

[0059] The composite material in this embodiment has high catalytic activity for photocatalytic coupling conversion of methane to ethane and high thermal shock stability. And under a light source with a wavelength of 365 nm, after 20 thermal cycles at 490 °C, the photocatalytic activity decay rate does not exceed 10%.

[0060] A preparation method of the metal oxide composite material for photocatalytic conversion of methane to ethane in this embodiment includes the following steps carried out in sequence: S1. Prepare a micron-scale porous alumina carrier framework with a directionally arranged and through-columnar pore structure by freeze-drying process; specifically including: dissolving 12.2 wt% aluminum nitrate and 2.4 wt% polyethylene glycol PEG-6000 in deionized water to form an alumina precursor sol, injecting it into a mold and freeze-forming under a vacuum of 4 Pa, maintaining the freezing temperature at -48 °C for 20 h, then performing vacuum freeze-drying treatment under a vacuum of ≤50 Pa, with the drying temperature at -18 °C for 25 h. After obtaining the porous preform, place it in a muffle furnace and heat-treat it at a heating rate of 7 °C / min to 920 °C for 3.6 h, and cool it to room temperature with the furnace to obtain the porous alumina carrier framework.

[0061] S2. Prepare a CeO2 nanorod functional structure in the columnar pores of the alumina carrier framework by in-situ growth method; specifically including: dissolving 2.0 wt% cerium nitrate, 0.6 wt% urea, and 0.2 wt% citric acid in deionized water to prepare a cerium oxide precursor solution, impregnating the porous alumina carrier framework obtained in S1 in the cerium oxide precursor solution, controlling the liquid-solid ratio at 7:1 during impregnation, performing ultrasonic treatment at a power of 320 W and stirring at 360 rpm for 32 min, then adding the mixed solution to a polytetrafluoroethylene-lined autoclave, setting a safety relief valve, performing a hydrothermal reaction at 172 °C for 19 h, filtering and washing with deionized water until the pH value of the filtrate is 7.0, retaining the carrier loaded with CeO2 nanorods and drying it at 104 °C for 8 h to obtain a porous alumina carrier framework loaded with CeO2.

[0062] S3. Form a TiO2 shell layer on the surface of the CeO2 nanorods and the inner wall surface of the pores by in-situ deposition growth process to construct a CeO2-TiO2 core-shell structure and a TiO2 nanothin layer structure. Specifically including: dissolving 8.0 wt% tetrabutyl titanate in absolute ethanol to form a titanium source solution, adding 6.4 wt% deionized water based on the total mass of the titanium source solution, adding 0.4 wt% polyvinylpyrrolidone as a dispersant at the same time, adjusting the pH of the system to 4.1 with 27 wt% ammonia water, immersing the porous alumina carrier framework loaded with CeO2 in the solution at a solid-liquid ratio of 1:13, performing a hydrolysis-deposition reaction at a stirring rate of 620 rpm and a temperature of 92 °C for 13 h. After the reaction is completed, wash it alternately with absolute ethanol and acetone 3 times to remove the unreacted titanate and organic by-products, retain the carrier coated with TiO2 and calcine it in an air atmosphere at a heating rate of 4.2 °C / min to 510 °C for 3.2 h, and finally obtain a metal oxide composite material for photocatalytic conversion of methane to ethane.

[0063] Comparative Example 1 Basically the same as Example 1, except that the freezing temperature before freeze-drying of the alumina support skeleton was adjusted to -20°C, and no oriented columnar pore structure was formed.

[0064] Comparative Example 2 Basically the same as Example 1, except that the hydrothermal reaction temperature of the CeO2 nanorods was 140°C, resulting in insufficient growth of the nanorods and failure to fully fill the pore structure.

[0065] Comparative Example 3 Basically the same as Example 1, except that the freeze-drying temperature was 0°C and the freeze-drying time was only 6 h, and the porosity of the obtained alumina skeleton structure was lower than 60%.

[0066] Comparative Example 4 Basically the same as Example 1, except that the thickness of the TiO2 shell was less than 10 nm, and the coating was discontinuous, unable to form a dense core-shell structure.

[0067] Comparative Example 5 Basically the same as Example 1, except that the diameter of the CeO2 nanorods exceeded 450 nm, resulting in pore blockage and uneven core-shell structure.

[0068] Comparative Example 6 Basically the same as Example 1, except that the concentration of the titanium source used was 2.5 wt%, lower than 5.0 wt%, and the deposition of the TiO2 shell was insufficient.

[0069] Comparative Example 7 Basically the same as Example 1, except that the pH value of the system during in-situ deposition of the TiO2 shell was 6.0, resulting in the formation of a non-anatase TiO2 crystal form.

[0070] Comparative Example 8 Basically the same as Example 1, except that the freeze-drying process was not carried out under vacuum conditions, and the pore structure was loose and disordered.

[0071] Comparative Example 9 Basically the same as Example 1, except that the urea content in the CeO2 precursor solution was 1.2 wt%, higher than the recommended range, resulting in agglomeration of the CeO2 particles.

[0072] Comparative Example 10 Basically the same as Example 1, except that the TiO2 deposition reaction time was shortened to 4 h, and the thickness of the TiO2 coating layer was insufficient and unevenly distributed.

[0073] Comparative Example 11 Basically the same as Example 1, except that the dosage of PEG-6000 was 0.5 wt%, lower than the recommended range, resulting in insufficient pore-forming agent effect on the skeleton structure.

[0074] Comparative Example 12 It is basically the same as Example 1, except that no PVP dispersant is added to the tetrabutyl titanate hydrolysis system, and the TiO2 shell layer is uneven, with agglomeration phenomena occurring.

[0075] Comparative Example 13 It is basically the same as Example 1, except that the heat treatment temperature exceeds 1100 °C, the alumina skeleton undergoes sintering, and the pore structure collapses.

[0076] Comparative Example 14 It is basically the same as Example 1, except that the hydrothermal reaction time of the CeO2 nanorods is only 6 h, and the length is insufficient, so a complete core-shell structure cannot be formed with TiO2.

[0077] Comparative Example 15 It is basically the same as Example 1, except that the calcination temperature of TiO2 is 300 °C, and a complete crystal phase is not formed, resulting in a significant decrease in photocatalytic activity.

[0078] Performance Test: Photocatalytic Methane Coupling Activity Test: To evaluate the efficiency of the prepared metal oxide composite material in catalyzing the conversion of methane to ethane under light irradiation, a photocatalytic methane coupling activity test was carried out. In the experiment, a certain amount (100 mg) of the catalytic material was loaded into a quartz reactor, and irradiated with a 365 nm ultraviolet LED light source (light intensity of about 50 mW / cm²). At the same time, high-purity methane gas (99.99%) was introduced to ensure that the reaction temperature was stably controlled at about 400 °C, and the reaction time was set to 1 - 3 hours. After the reaction was completed, the reaction-generated gas was analyzed using gas chromatography (GC), and the generation rates and selectivities of ethane, ethylene, and other by-products were measured mainly to systematically evaluate the photocatalytic methane coupling activity of the material.

[0079] Photocatalytic Stability and Thermal Cycling Test: To further verify the stability and reusability of the composite catalytic material under high-temperature fluctuation conditions, a photocatalytic stability and thermal cycling test was carried out. Under the above photocatalytic reaction conditions, 20 consecutive thermal cycling tests were performed on the same batch of catalyst samples. Each cycle included a complete process of heating from room temperature to 800 °C, maintaining a constant temperature for 1 hour, and then naturally cooling to room temperature. After each thermal cycle was completed, the methane photocatalytic reaction was repeated and the generation rate of ethane was recorded. By comparing the changes in catalytic activity at different cycle numbers, the photocatalytic performance decay rate of the material was calculated to comprehensively evaluate its thermal stability and structural reproducibility.

[0080] Gas adsorption selectivity test (CH4 adsorption isotherm): To analyze the methane adsorption capacity of the material and provide theoretical support for the subsequent methane activation process, the methane adsorption isotherm test was carried out. The experiment was conducted at a constant temperature of 25 °C. A static gas sorption analyzer was used to measure the CH4 adsorption isotherm of the sample. During the test, the partial pressure of methane gas was gradually increased, and the adsorption amount on the surface of the composite material was recorded. By comparing the methane adsorption capacities of different materials or samples with different structures under the same conditions, the influence of their structures on the CH4 surface enrichment behavior can be revealed, thus providing a data basis for designing efficient photocatalysts.

[0081] The properties of the materials in Examples 1-4 and Comparative Examples 1-15 are summarized in Table 1.

[0082] Table 1 Summary of the properties of the materials in Examples 1-4 and Comparative Examples 1-15 As can be seen from Table 1, the factors affecting the performance of this material mainly include the pore structure of the alumina skeleton, the size and distribution of CeO2 nanorods, the thickness and crystal form of the TiO2 shell, the process parameters of each stage (such as freezing temperature, drying conditions, hydrothermal reaction temperature and time, deposition reaction conditions, calcination temperature) and the use of auxiliary components. These factors have a synergistic effect on the photocatalytic activity, thermal stability and gas adsorption performance. The orderliness and permeability of the pore structure are key factors affecting the mass transfer efficiency of reactants and the degree of exposure of reaction sites. If the freezing molding temperature is too high (such as -20°C) or it is not dried under vacuum, it will lead to disordered pore arrangement or loose structure, thereby reducing the diffusion efficiency of reactants and the support strength of the carrier, which is manifested as a decrease in photocatalytic activity and poor stability. Insufficient porosity (such as less than 60%) will also limit the gas from entering the reaction area, resulting in a decrease in adsorption capacity and catalytic reaction rate. The size and growth quality of CeO2 nanorods directly determine the number of surface active sites and the integrity of the core-shell structure. If the hydrothermal temperature is too low or the reaction time is insufficient, CeO2 nanorods will be short and sparsely distributed, resulting in insufficient active sites and poor electron transport paths, which will in turn affect the separation efficiency of photogenerated electron-hole pairs. If CeO2 particles agglomerate due to abnormal precursor ratio (such as excessive urea), the pores will be blocked, the specific surface area will be reduced, and the adsorption and catalytic performance will be further weakened. The thickness, uniformity, and crystal structure of the TiO2 shell are the core factors affecting the photocatalytic performance. If the TiO2 shell is too thin (<10 nm) or the deposition time is insufficient, it will not form a complete coating, which will lead to aggravated electron-hole pair recombination and a significant decrease in catalytic efficiency; while the shell thickness is moderate and the uniform coating can achieve effective interface coupling and charge separation. If the deposition pH is not appropriate or the calcination temperature is too low (such as 300°C), TiO2 will form a non-anatase or amorphous phase, which will seriously weaken its light absorption and catalytic activity. Auxiliary components such as pore formers (PEG-6000) and dispersants (PVP) have an important influence on the skeleton structure and shell quality. Insufficient PEG dosage will lead to insufficient pore formation of the skeleton, narrow or blocked pores; the lack of PVP will easily lead to TiO2 agglomeration, uneven coating, and reduced photocatalytic efficiency. In addition, too high a heat treatment temperature (such as >1100°C) will cause skeleton sintering and pore collapse, resulting in complete structural destruction and almost loss of performance. In summary, the influence of various factors on performance is reflected in the following: the pore structure affects gas mass transfer and adsorption, the CeO2 structure affects the number of active sites and electron transport, the TiO2 shell affects the photoresponse and interfacial charge separation, and the structural stability determines the service life and cycle performance of the material. There is a coupling relationship between the various process parameters, which need to be coordinated and regulated to optimize the overall catalytic performance.

[0083] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to the above embodiments, those of ordinary skill in the art should understand that any equivalent structural transformation made under the concept of the present invention by using the content of the specification and drawings of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A metal oxide composite material for photocatalytic methane conversion to ethane and a preparation method thereof, characterized in that: The composite catalytic material is a ternary composite functional material with multi-level structure and multi-scale characteristics, and specifically includes the following structural units: a) a micrometer-sized porous alumina carrier skeleton, wherein the alumina carrier skeleton is prepared by freeze-drying process and has a directional and through-type columnar pore structure; b) CeO2 nanorod functional structure, wherein the CeO2 nanorod is constructed inside the columnar pores of the alumina support skeleton by an in-situ growth method; c) TiO2 shell photocatalytic active unit: The TiO2 shell is formed by an in-situ deposition growth process and coated on the surface of CeO2 nanorods and the inner wall of the columnar pores, thereby constructing a CeO2-TiO2 core-shell structure and a TiO2 thin layer on the inner wall of the pores.

2. The metal oxide composite material for photocatalytic methane conversion to ethane according to claim 1, characterized in that: The columnar pore structure of the alumina carrier skeleton is micrometer-scale pores that are parallel, interpenetrating and directionally arranged, with a pore diameter of 2.5 to 8.5 μm and a porosity of 65 to 85%.

3. The metal oxide composite material for photocatalytic methane conversion to ethane according to claim 1, characterized in that: The CeO2 nanorods have a diameter of 200-350nm and a length of 2600-8600nm, and the CeO2 nanorods are evenly distributed and densely grown on the inner wall of the columnar pores of the alumina carrier skeleton.

4. The metal oxide composite material for photocatalytic methane conversion to ethane according to claim 1, characterized in that The TiO2 shell layer has a thickness of 20 to 65 nm and is continuously and evenly coated on the outer surface of the CeO2 nanorod and the inner wall surface of the columnar pores to form a dense interface bonding structure.

5. The metal oxide composite material for photocatalytic methane conversion to ethane according to claim 1, characterized in that: The crystal structure of the TiO2 shell layer is anatase type.

6. The metal oxide composite material for photocatalytic methane conversion to ethane according to claim 5, characterized in that: The composite material has high-efficiency catalytic activity for photocatalytic methane coupling conversion into ethane and high thermal shock stability, and the photocatalytic activity decay rate does not exceed 10% after 20 thermal cycles at 25-800° C. under a light source with a wavelength of 365 nm.

7. The method for preparing a metal oxide composite material for photocatalytic methane conversion to ethane according to claim 1, characterized in that: The process includes performing the following steps in sequence: S1. A micrometer-sized porous alumina carrier skeleton having a directional arrangement and a through-columnar pore structure was prepared by freeze-drying process; S2. CeO2 nanorod functional structure is prepared by in situ growth method in the columnar pores of the alumina support skeleton; S3. A TiO2 shell is formed on the surface of the CeO2 nanorods and the inner wall surface of the pores by an in-situ deposition growth process to construct a CeO2-TiO2 core-shell structure and a TiO2 nano-thin layer structure.

8. The method for preparing a metal oxide composite material for photocatalytic methane conversion to ethane as claimed in claim 7, wherein step S1 specifically comprises: 8.0~15.0 wt% aluminum nitrate and 1.5~3.0 wt% polyethylene glycol PEG-6000 are dissolved in deionized water to form an alumina precursor sol, which is injected into a mold and freeze-formed under a vacuum degree of 2~5 Pa, and the freezing temperature is -60~-40°C for 12~24 h. It is then vacuum freeze-dried under a vacuum degree of ≤50 Pa, and the drying temperature is -30~-10°C for 18~30 h. The porous preform is then placed in a muffle furnace and heated to 800~1000°C at a heating rate of 5~8°C / min for heat treatment for 3~4 h, and then cooled to room temperature to obtain a porous alumina carrier skeleton.

9. The method for preparing a metal oxide composite material for photocatalytic methane conversion to ethane according to claim 7, wherein step S2 specifically comprises: 1.2~2.5 wt% cerium nitrate, 0.3~0.8 wt% urea and 0.1~0.3 wt% citric acid are dissolved in deionized water to prepare a cerium oxide precursor solution. The porous alumina carrier skeleton obtained by S1 is immersed in the cerium oxide precursor solution. The liquid-solid ratio is controlled to be 5:1~8:1 during impregnation. The solution is stirred at an ultrasonic power of 200~400 W and 300~400 rpm for 20~40 min. Subsequently, the mixed solution is added to a polytetrafluoroethylene-lined high-pressure reactor, and a safety pressure relief valve is set. The mixture is hydrothermally reacted at 160~180°C for 12~24 h. After filtering, it is washed with deionized water until the pH value of the filtrate is 6.8~7.

2. The carrier loaded with CeO2 nanorods is retained and dried at 80~120°C for 2~12 h to obtain a porous alumina carrier skeleton loaded with CeO2.

10. The method for preparing a metal oxide composite material for photocatalytic methane conversion to ethane according to claim 7, characterized in that: The step S3 specifically comprises: dissolving 5.0-10.0 wt% tetrabutyl titanate in anhydrous ethanol to form a titanium source solution, adding deionized water at 4.0-8.0 wt% of the total mass of the titanium source solution, and adding 0.2-0.6 wt% polyvinyl pyrrolidone as a dispersant, using 25-28 wt% ammonia water to adjust the pH of the system to 3.5-4.5, immersing the porous alumina carrier skeleton loaded with CeO2 in the solution at a solid-liquid ratio of 1:10-1:15, and performing a hydrolysis-deposition reaction for 8-16 h at a stirring rate of 500-700 rpm and a temperature of 80-100°C. After the reaction is completed, washing with anhydrous ethanol and acetone alternately for 3 times to remove unreacted titanate and organic by-products, retaining the carrier coated with TiO2 and heating it to 450-550°C at a heating rate of 3-5°C / min in an air atmosphere and calcining it for 2-4 h, and finally a metal oxide composite material for photocatalytic methane conversion to ethane was obtained.

Images