Iron-aluminum-based catalyst for methane cracking reaction and application thereof

Abstract

The application relates to an iron-aluminum-based catalyst for a methane cracking reaction and application thereof, which is based on diiron trioxide, aluminum elements are replaced into the iron oxide crystal lattice to form a solid solution, and the chemical composition is Fe 2‑x Al x O3, wherein 0 2‑x Al x O3 solid solution crystal phase; the catalyst is prepared through twice plasma-assisted ball milling modification. According to the solid solution and twice plasma-assisted ball milling modification, the specific surface area of the catalyst is effectively increased, surface oxygen vacancies are increased, and catalytic active sites are enriched; the aluminum oxide framework can anchor active metal particles, inhibit high-temperature sintering agglomeration, and improve catalytic activity and long-term stability. The catalyst can generate a composite active phase in situ in a methane cracking atmosphere, does not need additional pre-reduction, simplifies a process, can guide directional growth of carbon, and can generate carbon nanotubes with few defects and regular morphology, can efficiently catalyze methane cracking, and can co-produce hydrogen and high-quality carbon nanotubes.

Description

Technical Field

[0001] This invention relates to the field of natural gas / methane cracking technology, and more specifically to an iron-aluminum based catalyst for methane cracking reactions and its applications. Background Technology

[0002] Methane is a major component of natural gas. The catalytic cracking (CDM) process can directly decompose methane to produce high-purity hydrogen and solid carbon materials without the participation of water vapor. The reaction process does not generate greenhouse gases such as CO and CO2. Compared with the traditional steam reforming (SMR) process for producing hydrogen from methane, it has the advantages of shorter process flow, resource utilization of carbon products, and significant carbon emission reduction. It has important application value in the field of low-carbon hydrogen production and co-production of high-value-added carbon materials.

[0003] Currently, the catalysts commonly used in CDM industrialization are mainly oxide supports such as alumina (Al2O3), magnesium oxide (MgO), zirconium oxide (ZrO2), and cerium oxide (CeO2) supported by transition metals such as nickel (Ni), cobalt (Co), and iron (Fe). Among them, nickel-based and cobalt-based catalysts are prone to sintering deactivation at high temperatures and have poor tolerance to sulfur and halogen impurities in the feed gas, making it difficult to meet the requirements of long-term continuous production of the plant. Iron-based catalysts have low raw material costs, are environmentally friendly, and have excellent selectivity for carbon nanotube products, making them the mainstream of research and development in the industry. At present, most existing iron-aluminum oxide (Fe-Al-O) catalysts adopt the configuration of supported Fe / Al2O3 or high-temperature calcined composite oxides. The existing preparation routes mostly rely on red mud acid leaching, co-precipitation combined with high-temperature roasting process. Before these catalysts are put into the cracking reaction, they must be pre-reduced and activated by external hydrogen gas, which adds a pretreatment process and supporting equipment, increases production costs, and lengthens the process.

[0004] While existing technologies include research and patent reports on the preparation of oxides through aerogel-modified supports and plasma-assisted mechanochemical processes, they do not integrate plasma mechanochemical modification processes with the research and development needs of iron-aluminum based methane cracking catalysts for pre-reduction-free and controllable crystal phase reconstruction. Existing conventional Fe-Al-O catalysts generally suffer from four industry shortcomings: first, catalyst production requires external hydrogen pre-reduction, making it impossible to achieve in-situ generation of the active phase using methane within the reaction system; second, the complex phase of the active metal and composite support is difficult to construct precisely and controllably, resulting in insufficient phase stability during operation; third, the active components are prone to sintering during high-temperature, long-term reactions, limiting catalyst lifespan; and fourth, it is difficult to directionally control the microstructure of cracking byproduct carbon.

[0005] Another type of methane cracking composite catalyst uses a multi-transition metal composite oxide system, including cobalt, nickel, chromium, and manganese. Catalyst regeneration is achieved through the reversible dissolution of metal ions in oxidizing and reducing atmospheres, enabling the preparation of various carbon materials. However, this approach has significant drawbacks: first, the large variety of metal components leads to high raw material costs; second, the system design is geared towards online regeneration, emphasizing fluidized bed applications and is not suitable for continuous rotary kiln operation; and third, the diverse range of carbon products makes it difficult to directionally prepare single, low-defect carbon nanotubes. Summary of the Invention

[0006] To address the problems of existing iron-aluminum based catalysts for methane cracking, such as limited active sites, easy sintering and deactivation of active metals at high temperatures, poor catalytic stability, and numerous defects and insufficient morphological regularity in carbon nanotube products, this invention aims to provide an iron-aluminum based catalyst for methane cracking and its application.

[0007] The iron-aluminum based catalyst for methane cracking according to the present invention uses ferric oxide as a matrix, with aluminum element replacing iron oxide in the ferric oxide lattice to form a solid solution, and has a chemical composition of Fe. 2-x Al x O3, where 0 < x < 2, and the main crystalline phase is Fe. 2- x Al x The catalyst is prepared by two plasma-assisted ball milling modifications, using an O3 solid solution crystalline phase. This invention employs an iron-aluminum composite solid solution and combines it with two plasma-assisted ball milling modifications to prepare an iron-aluminum based catalyst with excellent overall performance. This differs from conventional iron-aluminum oxide catalysts and lays the structural and technological foundation for improving the catalytic performance of methane cracking.

[0008] In a preferred embodiment, Fe 2-x Al x In O3, 0.5 ≤ x ≤ 1.5. This invention limits the range of aluminum element ratio in the solid solution, which can optimize the doping ratio of iron and aluminum elements, making the solid solution crystal structure more stable and further improving the intrinsic catalytic activity of the catalyst.

[0009] In a preferred embodiment, the catalyst possesses a large specific surface area and abundant surface oxygen vacancies, with a reduced valence state of iron on the catalyst surface and a decreased lattice oxygen content. This invention utilizes a second plasma-assisted ball milling process to regulate the catalyst's microstructure and surface state, significantly increasing the number of catalytically active sites and enhancing the catalyst's reactivity.

[0010] In a preferred embodiment, the catalyst precipitates elemental iron nanoparticles in a methane cracking atmosphere, which then form a Fe-FeAl2O4 composite active phase with iron and aluminum oxides. This invention utilizes the inherent properties of the catalyst to achieve in-situ construction of the composite active phase in the reaction atmosphere, eliminating the need for additional pre-activation with reducing gas. This simplifies the production process, ensures uniform particle size and good dispersion, and effectively improves catalytic reaction efficiency.

[0011] In a preferred embodiment, the aluminum oxide framework in the catalyst anchors the elemental iron nanoparticles, inhibiting the sintering of active particles. This invention, through the physical anchoring effect of the aluminum oxide framework, can hinder the migration and agglomeration of active metal particles during high-temperature reactions, significantly extending the catalyst's lifespan and improving long-term operational stability.

[0012] According to the application of the above-mentioned iron-aluminum based catalyst of the present invention, the catalyst is used to catalyze the methane cracking reaction, co-producing hydrogen and carbon nanotubes. The present invention applies the modified iron-aluminum based catalyst to the methane cracking system, enabling simultaneous and efficient methane conversion, hydrogen production, and carbon nanotube synthesis, expanding the practical application scenarios of the catalyst and achieving comprehensive resource utilization.

[0013] In a preferred embodiment, the methane cracking reaction temperature is 700~900℃, and the methane volume hourly space velocity is 1000~5000 mL·h. -1 ·gcat -1 The reaction is carried out under normal pressure. This invention defines the reaction operating range suitable for this catalyst, which can balance methane conversion efficiency and reaction capacity, ensuring stable and efficient reaction.

[0014] In a preferred embodiment, under a reaction time of 20 hours, 5-30 grams of carbon nanotubes (calculated as carbon) are generated per gram of catalyst. This invention leverages the high activity of the catalyst to achieve high-yield preparation of carbon nanotubes, thereby improving overall production efficiency.

[0015] In a preferred embodiment, the carbon nanotubes obtained by the reaction have a diameter of 10~200 nm, and the Raman spectrum of the carbon nanotubes is I. D / I G The value is 0.1~0.5. This invention utilizes this catalyst to prepare carbon nanotubes with a reasonable diameter distribution, few defects, and a high degree of graphitization, thereby improving product quality and performance.

[0016] In a preferred embodiment, an aluminum oxide framework serves as the growth substrate, guiding the directional growth of carbon to form carbon nanotubes. This invention utilizes the substrate-guiding effect of the carrier to avoid the generation of impurities such as amorphous carbon, ensuring that the carbon product grows into a regular tubular structure, and further improving product uniformity.

[0017] This invention employs a solid solution structure formed by iron and aluminum doping, and modifies the catalyst through two plasma-assisted ball milling processes. This effectively increases the catalyst's specific surface area and the number of surface oxygen vacancies, enriching the catalytic active sites. Simultaneously, the oxide framework provides a good anchoring effect on the active metal particles, inhibiting the sintering and agglomeration of active metals under high-temperature conditions, significantly improving the catalyst's catalytic activity and long-term operational stability. This catalyst can form an in-situ Fe-FeAl2O4 composite active phase in a methane cracking atmosphere without additional pre-reduction treatment, simplifying the operation process. It can also guide the directional growth of carbon components, producing carbon nanotube products with fewer defects and regular morphology. This effectively overcomes the shortcomings of existing iron-aluminum based catalysts, such as the need for exogenous hydrogen pre-reduction, poor operational stability, and numerous carbon product defects. It achieves efficient methane conversion and co-production of hydrogen and carbon nanotubes while simultaneously considering product quality and production efficiency. Attached Figure Description

[0018] Figure 1 The X-ray diffraction pattern is shown for the PM-FeAlO3 catalyst modified by a second plasma ball milling process prepared in Example 1 of this invention.

[0019] Figure 2 This is a high-resolution transmission electron microscope image of the PM-FeAlO3 catalyst prepared in Example 1 of this invention.

[0020] Figure 3 The X-ray diffraction pattern of the FeAlO3 catalyst prepared in Example 2 of this invention without undergoing a second plasma ball milling modification is shown.

[0021] Figure 4 The BET test N2 adsorption-desorption isotherms are for the PM-FeAlO3 catalyst prepared in Example 1 and the FeAlO3 catalyst prepared in Example 2 of this invention.

[0022] Figure 5 The image shows the deconvolution and integration peak fitting diagram of the Fe2p peak in the X-ray photoelectron spectroscopy data of the PM-FeAlO3 catalyst prepared in Example 1 and the FeAlO3 catalyst prepared in Example 2 of this invention.

[0023] Figure 6 The image shows the deconvolution and integration peak fitting diagram of the O1s peak of the X-ray photoelectron spectroscopy data of the PM-FeAlO3 catalyst prepared in Example 1 and the FeAlO3 catalyst prepared in Example 2 of this invention.

[0024] Figure 7 This is a scanning electron microscope (SEM) image of carbon nanotubes generated after the methane cracking reaction in Example 3.

[0025] Figure 8The graph shows the methane conversion rate versus reaction time changes in Example 3 (PM-FeAlO3 catalyst) and Comparative Example 5 (ordinary FeAlO3 catalyst) during the methane cracking reaction.

[0026] Figure 9 The Raman spectra of the carbon nanotubes produced in Example 3 and the carbon nanotubes produced in Comparative Example 5 are compared. Detailed Implementation

[0027] The technical solution of the present invention will be described in detail below with reference to specific embodiments and comparative examples. The following embodiments are only used to explain the present invention and are not intended to limit the scope of protection of the present invention.

[0028] This invention discloses a catalyst with ferric oxide (Fe₂O₃) as the matrix, in which aluminum (Al) element enters the iron oxide lattice in a substitutional manner to form a solid solution; the overall chemical composition of the material is Fe. 2-x Al x O3, wherein 0 < x < 2, preferably 0.5 ≤ x ≤ 1.5, and the main crystalline phase is Fe. 2-x Al x O3 solid solution crystal phase.

[0029] The present invention also discloses a method for preparing the above-mentioned catalyst, which includes three core processes: step S1: first plasma-assisted ball milling, step S2: calcination treatment, and step S3: second plasma-assisted ball milling.

[0030] In step S1, iron and aluminum raw materials are mixed and subjected to a first plasma-assisted ball milling (PM) to obtain precursor powder. Plasma-assisted ball milling technology introduces cold-field discharge plasma into mechanical vibration ball milling. Utilizing the synergistic effect of high-energy non-equilibrium plasma formed by near-atmospheric pressure gas in the milling jar and mechanical ball milling, it promotes powder refinement, activation, chemical reactions, and accelerates in-situ gas-solid phase reactions, significantly improving milling efficiency, significantly reducing milling contamination, and forming a unique structure that significantly improves material performance. In a preferred embodiment, iron oxide is selected as the iron source material. In a preferred embodiment, nano-iron oxide is selected as the iron source material. In a preferred embodiment, alumina is selected as the aluminum source material. In a preferred embodiment, nano-alumina is selected as the aluminum source material. In a preferred embodiment, the stoichiometric ratio of iron to aluminum ranges from 1:5 to 5:1. In a preferred embodiment, the stoichiometric ratio of iron to aluminum is 1:1. In a preferred embodiment, 0-8 wt% stearic acid is added to the mixture of iron oxide and alumina. Stearic acid improves the dispersion performance of the powder. During the calcination stage, stearic acid is completely decomposed by heat and does not interfere with the phase formation of the catalyst. In a preferred embodiment, 5 wt% stearic acid is added to the mixture. In a preferred embodiment, the ball milling atmosphere is an inert atmosphere or a vacuum atmosphere. In a preferred embodiment, the ball-to-material ratio of the first plasma-assisted ball milling ranges from 10:1 to 30:1. In a preferred embodiment, the ball-to-material ratio of the first plasma-assisted ball milling is preferably 25:1. In a preferred embodiment, the plasma discharge current ranges from 50 to 70 mA. In a preferred embodiment, the plasma discharge current is preferably 60 mA. In a preferred embodiment, the speed of the vibratory ball mill motor ranges from 1000 to 1350 rpm. In a preferred embodiment, the speed of the vibratory ball mill motor is preferably 1300 rpm. In a preferred embodiment, the total ball milling time ranges from 1 to 10 hours. In a preferred embodiment, the total ball milling time is preferably 5 hours.

[0031] In step S2, the precursor powder obtained in step S1 is calcined to obtain Fe. 2-x Al x O3 catalyst matrix. Fe / Al oxide feedstock forms the desired phase structure under high temperature, forming Fe... 2-x Al x The main crystalline phase of O3. In a preferred embodiment, the calcination temperature of the precursor powder is in the range of 1000~1500℃. In a preferred embodiment, the calcination temperature is preferably 1350℃. In a preferred embodiment, the heating rate is in the range of 1~10℃ / min. In a preferred embodiment, the heating rate is preferably 2℃ / min. In a preferred embodiment, the calcination time is in the range of 3~10h. In a preferred embodiment, the calcination time is preferably 5h.

[0032] In step S3, the Fe obtained in step S2 is... 2-x Al x The O3 catalyst matrix underwent a second plasma-assisted ball milling treatment to obtain plasma-modified PM-Fe. 2-x Al x O3 catalyst. Through the mechanical impact, plasma activation, and thermal effect of the second plasma-assisted ball milling, the uniform dispersion of iron and aluminum components can be promoted, increasing the number of defect sites and active centers on the catalyst surface and improving its surface chemical state, thereby enhancing the catalytic activity and stability of the catalyst in the natural gas / methane cracking reaction. The catalyst activity can be improved by the activation effect of the second plasma ball milling on the catalyst that has already formed its phase in step S2. Compared with the catalyst that has not undergone secondary ball milling modification, the catalyst modified by the second plasma ball milling has a larger specific surface area and more surface defects; specifically, the valence state of Fe on the catalyst surface is reduced, the lattice oxygen content is reduced, and the oxygen vacancy content is increased. In a preferred embodiment, 0-8 wt% stearic acid is added to the FeAlO3 catalyst matrix. The role of stearic acid is to improve the powder dispersion performance without interfering with the phase formation of the catalyst. In a preferred embodiment, 3 wt% stearic acid is added to the catalyst matrix. In a preferred embodiment, the ball milling atmosphere is an inert atmosphere or a vacuum atmosphere. In a preferred embodiment, the ball-to-material ratio in the second plasma-assisted ball milling ranges from 10:1 to 30:1. In another preferred embodiment, the ball-to-material ratio in the second plasma-assisted ball milling is preferably 25:1. In a preferred embodiment, the plasma discharge current ranges from 50 to 70 mA. In another preferred embodiment, the plasma discharge current is preferably 60 mA. In a preferred embodiment, the speed of the vibratory ball mill motor ranges from 1000 to 1350 rpm. In another preferred embodiment, the speed of the vibratory ball mill motor is preferably 1300 rpm. In a preferred embodiment, the total ball milling time ranges from 1 to 10 hours. In another preferred embodiment, the total ball milling time is preferably 5 hours.

[0033] This invention further discloses the application of the aforementioned catalyst in a natural gas / methane cracking reaction, co-producing hydrogen and carbon nanotubes. This catalyst can precipitate elemental iron nanoparticles in situ in a methane cracking atmosphere and construct a Fe-FeAl2O4 composite active phase with an iron-aluminum oxide matrix, without the need for additional hydrogen pre-reduction activation. Its catalytic activity and carbon product yield are significantly superior to the unmodified catalyst. Specifically, during the natural gas / methane cracking reaction, with the action of reducing methane and hydrogen, Fe... 2-x Al x Some of the Fe elements in the O3 phase are dissolved in situ and precipitated on the surface. 0Metal nanoparticles, together with the Fe / Al oxide matrix (FeAl2O4), form a Fe-FeAl2O4 composite phase structure. This composite phase catalyst further catalyzes the cracking of natural gas / methane, producing carbon and hydrogen. Fe, as the active metal, ensures the basic activity of the catalyst, while the in-situ desoluble Fe nanoparticles ensure the grain size, dispersion, and specific surface area of ​​the active metal. The support (i.e., the aluminum oxide framework) provides a stable anchoring effect on the elemental iron nanoparticles, effectively inhibiting the migration and sintering agglomeration of active particles during the high-temperature reaction, ensuring catalyst durability. Furthermore, the difference in carbon solubility between the support and the active metal provides a directional growth substrate for supersaturated carbon. The aluminum oxide framework can serve as a carbon growth substrate, guiding the directional growth of carbon components generated from cracking to form well-structured carbon nanotubes, inhibiting the formation of amorphous carbon, and ensuring that carbon grows in the form of carbon nanotubes. In a preferred embodiment, the methane cracking reaction temperature range is 700~900℃. In a preferred embodiment, the reaction temperature is preferably 815℃. In a preferred embodiment, the methane space velocity ranges from 1000 to 5000 mL·h. -1 ·g cat. -1 (That is, for every 1 gram of catalyst, 1000-5000 mL of methane gas is introduced per hour). In a preferred embodiment, the methane space velocity is preferably 3000 mL·h. -1 ·gcat -1 (i.e., 3L·h) -1 ·gcat -1 In a preferred embodiment, the reaction is carried out under normal pressure. In a preferred embodiment, the yield of carbon nanotubes after the reaction ranges from 5 to 30 g. C ·g cat. -1 / 20h (i.e., in a 20-hour reaction, 1 gram of catalyst can generate 5-30 grams of carbon nanotubes by mass of carbon). In a preferred embodiment, the carbon nanotube diameter ranges from 10 to 200 nm. In a preferred embodiment, the carbon nanotube Raman spectrum I D / I G The value ranges from 0.1 to 0.5. In preferred embodiments, the methane conversion rate ranges from 30% to 95%. In one preferred embodiment, the methane conversion rate is 92%.

[0034] Example 1: Preparation of PM-FeAlO3 catalyst (Fe:Al stoichiometric ratio 1:1, x=1)

[0035] Weigh the raw materials: 23.955g Fe2O3 and 15.294g Al2O3. Add stearic acid (5wt% of the total mass of raw materials) to the mixed powder, mix thoroughly, and then load into a plasma ball mill jar.

[0036] First plasma-assisted ball milling: ball-to-material ratio 25:1, discharge current 60mA, motor speed 1300rpm, ball milling time 5h, to obtain precursor powder;

[0037] High-temperature calcination treatment: The precursor powder was placed in a muffle furnace, and the heating rate was 2℃·min. -1 The temperature was raised to 1350℃ and calcined at a constant temperature for 5 hours. After calcination, FeAlO3 catalyst matrix was obtained, which was then naturally cooled and simply ground and crushed.

[0038] Second plasma-assisted ball milling: 3wt% stearic acid of the total matrix mass was added to the FeAlO3 matrix and mixed. The same ball milling parameters (ball-to-material ratio 25:1, 60mA, 1300rpm, 5h) were used for plasma ball milling again to finally obtain the modified catalyst PM-FeAlO3.

[0039] like Figure 1 As shown, the horizontal axis represents the diffraction angle 2θ (unit: °), and the vertical axis represents the diffraction signal intensity. The spectrum was compared and calibrated with reference to the standard PDF card #30-0024FeAlO3 to characterize the phase composition of the modified catalyst PM-FeAlO3, proving that the main crystalline phase of the prepared catalyst is the FeAlO3 phase.

[0040] like Figure 2 As shown, the low-magnification TEM image on the left (scale bar: 100 nm) reveals the aggregation state of the catalyst particles: the sample exhibits a loose structure formed by irregular blocky catalyst agglomerations, with agglomerate sizes ranging from hundreds of nanometers. The primary particles constituting the agglomerates are approximately tens of nanometers in size, and no obvious large-sized sintered particles are observed, preliminarily indicating good catalyst dispersion. The nanoscale primary particle size provides a structural basis for the subsequent formation of highly active and highly dispersed Fe active sites, echoing the anchoring and anti-sintering effect of the support. The high-resolution HRTEM image on the right (scale bar: 10 nm) clearly shows periodically arranged lattice fringes, confirming the crystallinity of the target phase (such as FeAlO3 or the subsequently reduced Fe-FeAl2O4). The nanoscale grain size and porous structure provide sufficient active sites and mass transfer channels for the methane cracking reaction, highly matching the design goals of this catalyst system.

[0041] Comparative Example 1: Preparation of PM-Fe4Al2O9 catalyst (Fe excess ratio)

[0042] Weigh 31.94g of Fe2O3 and 10.196g of Al2O3, and add 5wt% stearic acid to the total raw material mixture.

[0043] First plasma ball milling: 25:1 ball-to-material ratio, 60mA, 1300rpm, 5h, to obtain the precursor;

[0044] Calcination in a muffle furnace: heating to 1350℃ at 2℃ / min and holding for 5h to obtain Fe4Al2O9 matrix;

[0045] PM-Fe4Al2O9 was prepared by mixing 3wt% stearic acid into the matrix and then performing a second plasma ball milling under the same ball milling conditions (ball-to-material ratio 25:1, 60mA, 1300rpm, 5h).

[0046] Comparative Example 2: Preparation of PM-Fe2Al4O9 catalyst (Al excess ratio)

[0047] Weigh 15.97g of Fe2O3 and 20.392g of Al2O3, add 5wt% stearic acid and mix well;

[0048] Fe2Al4O9 matrix was obtained by plasma ball milling (25:1, 60mA, 1300rpm, 5h) followed by calcination at 1350℃ for 5h (heating rate 2℃ / min).

[0049] PM-Fe2Al4O9 was prepared by second plasma ball milling after adding 3wt% stearic acid to the matrix.

[0050] Example 2: Preparation of FeAlO3 catalyst without secondary ball milling

[0051] The raw material ratio, primary ball milling parameters, and calcination process were completely consistent with Example 1: Fe2O3 23.955g + Al2O3 15.294g + 5wt% stearic acid → primary plasma ball milling (25:1, 60mA, 1300rpm, 5h) → 2℃·min -1 The product was calcined at 1350℃ for 5 hours; the calcined product was then routinely ground, omitting the second plasma ball milling process, to obtain a common FeAlO3 catalyst.

[0052] like Figure 3 As shown, the horizontal axis represents the diffraction angle 2θ (unit: °), and the vertical axis represents the diffraction signal intensity. Phase calibration was performed according to the standard PDF card #30-0024FeAlO3. Figure 1 This provides a comparison, demonstrating that pure-phase FeAlO3 matrix can be synthesized through a single ball milling and calcination process, regardless of whether a secondary plasma ball milling is performed.

[0053] like Figure 4 The figure shows the N2 adsorption-desorption isotherms of the catalysts prepared in Examples 1 and 2. When the relative pressure P / P0 is close to 1.0, the adsorption capacity increases significantly without a clear plateau, indicating the presence of a mesoporous structure in the material of Example 1. Calculated using the BET model, its specific surface area is 4.64 m². 2 ·g -1 Analysis of the adsorption branch data using the BJH model yielded an average pore size of approximately 28.4 nm and a total pore volume of approximately 0.033 cm³. 3 ·g-1 The pore size distribution is mainly concentrated in the range of 20-30 nm. Compared with Example 2, the PM-FeAlO3 catalyst treated by plasma ball milling has a larger specific surface area and pore size distribution.

[0054] like Figure 5 The figure shows the deconvolution and integration peak fitting diagrams of the Fe2p peak in the X-ray photoelectron spectroscopy data of the catalysts prepared in Examples 1 and 2. The PM-FeAlO3 in Example 1 has a higher Fe content than the FeAlO3 in Example 2. 2+ The content has increased slightly.

[0055] like Figure 6 The figure shows the deconvolution and integration peak fitting diagrams of the O1s peak in the X-ray photoelectron spectroscopy data of the catalysts prepared in Examples 1 and 2. The PM-FeAlO3 in Example 1 has a greater number of vacant oxygen O compared to the FeAlO3 in Example 2. vac Adsorbed oxygen O ads The content increased significantly, lattice oxygen O lat The content has decreased.

[0056] Comparative Example 3: Preparation of Fe4Al2O9 catalyst without secondary ball milling

[0057] The raw materials, primary ball milling, and calcination parameters are the same as those in Comparative Example 1, eliminating the need for secondary plasma ball milling. The calcined product is directly ground and discharged to obtain Fe4Al2O9.

[0058] Comparative Example 4: Preparation of Fe2Al4O9 catalyst without secondary ball milling

[0059] The raw materials, primary ball milling, and calcination parameters are the same as those in Comparative Example 2, but the secondary plasma ball milling is omitted, and the calcined product is directly ground to obtain Fe2Al4O9.

[0060] Example 3: Preparation of PM as in Example 1 - FeAlO3 catalyst for methane cracking to produce hydrogen and co-produce carbon nanotubes

[0061] Catalytic evaluation experiments were conducted using the PM-FeAlO3 obtained in Example 1. The procedures were as follows:

[0062] Loading: Take 300mg of catalyst and load it into a quartz reaction tube with an inner diameter of 25mm, and fix it horizontally in the tube furnace;

[0063] Ventilation and heating: Introduce methane gas at a flow rate of 15 mL / min. -1 According to 5℃·min -1 The temperature was increased to the preferred reaction temperature of 815℃ at a rising rate, and the reaction was carried out continuously at this temperature for 20 hours; the entire reaction was conducted at atmospheric pressure and a space velocity of 3 L / h. -1 ·g cat. -1 ;

[0064] Online detection: The reactor tail gas is connected to a gas chromatograph to detect tail gas components in real time and to calculate hydrogen yield and methane conversion rate.

[0065] Cooling and discharging: After the reaction is complete, stop heating, switch to high-purity nitrogen to purge the pipeline, and remove the solid product when the furnace temperature drops below 100℃;

[0066] Product characterization: Solid carbon products were collected and subjected to SEM morphology and Raman spectroscopy.

[0067] like Figure 7 As shown, the carbon nanotubes prepared by methane cracking are one-dimensional continuous long tubular structures with a randomly distributed network morphology and no obvious agglomeration. The outer diameter of the carbon nanotubes is mainly distributed between 30 and 72 nm, such as 30.8 nm, 32.8 nm, 33.4 nm, 37.5 nm, 38.7 nm, 40.4 nm, 40.6 nm, 43.0 nm, 62.3 nm, and 71.6 nm, with most concentrated in the 30-45 nm range. The diameter distribution is relatively narrow and the uniformity is good. The surface of the carbon nanotubes exhibits typical textures of catalytically grown carbon nanotubes, without obvious amorphous carbon impurities, indicating good growth integrity. These results demonstrate that the catalyst of this invention can efficiently grow multi-walled carbon nanotubes with regular morphology and uniform diameter, resulting in excellent product quality.

[0068] Comparative Example 5: Catalytic Performance Test of Ordinary FeAlO3 Catalyst in Example 2

[0069] The catalyst used was FeAlO3 from Example 2 that had not undergone secondary plasma ball milling. The entire process of material preparation, aeration, heating, heat preservation, and post-treatment was completely consistent with that of Example 3.

[0070] like Figure 8 As shown, the horizontal axis represents the reaction duration in minutes (min), and the vertical axis represents the methane conversion rate, presented as a percentage. In Example 3, the methane conversion rate of the catalyst rapidly increased to approximately 85% within 100 min and remained stable between 100 and 500 min; at 1200 min, the conversion rate still remained at approximately 37%. In contrast, the conversion rate of the catalyst in Comparative Example 5 rapidly declined after reaching a peak of approximately 80%, dropping below 10% after 1000 min, indicating near-deactivation. These results demonstrate that the PM-FeAlO3 catalyst prepared in this invention exhibits higher initial activity and superior long-term stability, effectively solving the technical problems of easy sintering and deactivation of existing FeAlO3 catalysts.

[0071] like Figure 9 As shown, the horizontal axis represents the Raman shift (wavenumber), and the vertical axis represents the Raman signal intensity, with units of arbitrary units (au). The carbon nanotubes prepared in Example 3 and Comparative Example 5 both have a Raman shift of ~1350 cm⁻¹. -1 (D peak), ~1580cm-1 (G peak) and ~2700cm -1 The typical characteristic peak of carbon nanotubes appears at (G' peak). The I peak of the carbon nanotubes prepared in Example 3... D / I G The ratio of 0.34 is slightly lower than that of Comparative Example 5 (0.354), indicating that the carbon nanotubes in Example 3 have a higher degree of graphitization, lower defect density, and more regular structure. This result demonstrates that the catalyst of this invention can grow carbon nanotubes of higher quality with fewer defects, resulting in superior product performance.

[0072] When methane was used as a model reactant for evaluation, the PM-FeAlO3 catalyst of this invention exhibited a maximum methane conversion rate of 92% at 815 °C; at 3 L·h -1 ·g cat. -1 At space velocity, the maximum hydrogen yield is 3.65 mmol·min⁻¹. -1 ·g cat. -1 After 20 hours of reaction, the yield of carbon nanotubes reached 25.21 g. C ·g cat. -1 .

[0073] The catalyst according to the present invention exhibits good catalytic activity, high methane conversion and carbon / hydrogen yield, and good catalytic durability. The preparation method is simple, suitable for large-scale industrial application, and the synthesized catalyst has a pure phase, fine grains, and stable phase structure. It can produce carbon nanotubes and hydrogen with satisfactory yield and product quality. The catalyst designed in this invention features a simple phase structure, low elemental cost, simple operating conditions, and high catalytic efficiency and durability. The catalyst preparation method of this invention completes the mixing of raw materials and activation of the phase-forming products under the combined action of an coupled external plasma field and mechanical force. This provides a simple, efficient, and high-quality Fe / Al-based catalyst preparation method, which can effectively promote the engineering application of natural gas / methane cracking reactions.

[0074] Compared to existing technologies that use composite oxide systems of multiple transition metals such as cobalt, nickel, chromium, and manganese, this invention uses a pure iron-aluminum binary solid solution, abandoning the design of multiple metal doping such as cobalt, chromium, and manganese, resulting in lower raw material costs. Through two plasma-assisted ball milling modifications, the microstructure of the catalyst is controlled, enriching the active sites. Relying on the aluminum oxide framework to anchor active iron particles, the core design focuses on anti-sintering and long-term operation, which is different from the ion reversible regeneration route of existing multi-metal catalysts. The catalyst can form a composite active phase in situ in a methane atmosphere without the need for pre-reduction by external hydrogen. At the same time, it precisely guides the directional growth of carbon into carbon nanotubes, avoiding the problem of multiple product mixing, and significantly improving product uniformity and quality.

[0075] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of the invention. Various variations can be made to the above embodiments of the present invention. That is, all simple and equivalent changes and modifications made based on the claims and description of this invention fall within the protection scope of the claims. All aspects not described in detail in this invention are conventional technical content.

Claims

1. An iron-aluminum based catalyst for methane cracking reaction, characterized in that, The catalyst uses ferric oxide as a matrix, with aluminum element replacing the ferric oxide lattice to form a solid solution, and its chemical composition is Fe. 2-x Al x O3, where 0 < x < 2, and the main crystalline phase is Fe. 2-x Al x O3 solid solution crystalline phase; the catalyst was prepared by two plasma-assisted ball milling modifications.

2. The iron-aluminum based catalyst according to claim 1, characterized in that, Fe 2-x Al x In O3, 0.5≤x≤1.

5.

3. The iron-aluminum based catalyst according to claim 1, characterized in that, The catalyst has a large specific surface area and abundant surface oxygen vacancies. The valence state of iron on the catalyst surface is reduced, and the lattice oxygen content is decreased.

4. The iron-aluminum based catalyst according to claim 1, characterized in that, The catalyst precipitates elemental iron nanoparticles in a methane cracking atmosphere and forms a Fe-FeAl2O4 composite active phase with iron and aluminum oxides.

5. The iron-aluminum based catalyst according to claim 1, characterized in that, The aluminum oxide framework in the catalyst anchors the elemental iron nanoparticles, inhibiting the sintering of active particles.

6. The application of the iron-aluminum based catalyst according to any one of claims 1 to 5, characterized in that, The catalyst was used to catalyze the cracking reaction of methane, co-producing hydrogen and carbon nanotubes.

7. The application according to claim 6, characterized in that, The methane cracking reaction temperature is 700~900℃, and the methane volume hourly space velocity is 1000~5000 mL·h. -1 ·g cat. -1 The reaction takes place under normal pressure.

8. The application according to claim 6, characterized in that, Under a reaction time of 20 hours, each gram of catalyst produces 5 to 30 grams of carbon nanotubes based on carbon content.

9. The application according to claim 6, characterized in that, The carbon nanotubes obtained by the reaction have a diameter of 10~200 nm. The Raman spectrum of the carbon nanotubes is shown in Figure I. D / I G The value is 0.1 to 0.

5.

10. The application according to claim 6, characterized in that, An aluminum oxide framework serves as a growth substrate, guiding the directional growth of carbon to form carbon nanotubes.

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