Aluminum gel coated mullite fiber carrier supported palladium catalyst for catalytic oxidation of methane

Abstract

The invention discloses an aluminum gel coated mullite fiber carrier supported palladium catalyst Pd / AlOOH-M. The catalyst takes aluminum gel coated mullite fiber as a carrier, the carrier has the characteristics of good thermal stability and good formability, palladium is supported by adopting an impregnation method, and the Pd / AlOOH-M catalyst is obtained by roasting. The catalyst has the advantages of high space velocity resistance, frequent heating and cooling start, long service life and the like. Therefore, the catalyst has important application potential in the fields of replacement of traditional noble metal catalysts and industrial methane emission reduction.

Description

Technical Field

[0001] The present invention relates to a preparation method of a palladium-loaded catalyst and its application in methane catalytic oxidation, belonging to the technical field of environmental protection. Background Art

[0002] Methane, the core component of natural gas, has been widely used in industrial, power generation and civil fields. However, methane leakage problems always exist in the processes of extraction, storage, transportation and end-use. Engineering monitoring data shows that under non-steady operating conditions such as equipment start-stop and load fluctuations, about 12%-18% of methane forms waste gas emissions due to incomplete combustion. Since the global warming potential of methane on a hundred-year time scale is as high as 28-36 times that of carbon dioxide, such gases are intensifying the global greenhouse effect, and their purification has become a major technical challenge in the process of energy low-carbonization. Among current methane treatment technologies, the physical adsorption and recovery method is limited by economy and difficult to treat low-concentration waste gas, while the traditional combustion method cannot be directly applied because the methane concentration is often lower than the lower combustion limit of 4%. In contrast, the catalytic oxidation technology can reduce the reaction activation energy through a catalyst and achieve efficient decomposition under low-concentration methane conditions, becoming the most potential solution to solve low-concentration methane emissions. Among them, the research and development of high-performance catalytic materials is the core breakthrough direction for the engineering application of this technology.

[0003] In the field of natural gas vehicle exhaust purification, the application of palladium-based catalysts is a classic case. Since the exhaust gas from engine incomplete combustion often contains high concentrations of methane and carbon monoxide, these residual substances not only cause a decrease in fuel utilization rate but also trigger dual environmental problems. However, it is found in practical applications that Pd catalysts exhibit obvious defects during the reuse process: when the reaction system is in a high-temperature working condition and there is water vapor, the metal active components are prone to sintering to form larger particles, thus causing significant attenuation of catalyst activity and deterioration of long-term stability.

[0004] In the design of palladium-based catalyst systems, the selection of the support has a decisive influence on the efficiency of active sites and thermodynamic stability. The support systems mainly studied by Wang et al. (Science China Chemistry, 66.4 (2023): 1032-1051) include alumina, silica, metal oxide systems such as cerium zirconium titanium, etc. It is worth noting that Chen et al. (ACS Catalysis, 11.9 (2021): 5666-5677) found that the cerium dioxide support, with its dynamic oxygen storage characteristics and abundant surface defect structures, can effectively promote the dissociative adsorption of methane molecules. However, the inherent defects of this material in terms of thermodynamic stability lead to lattice reconstruction at high-temperature conditions. Similarly, zirconia, also a transition metal oxide, although having specific acidic characteristics, its tendency to undergo phase transformation at high temperatures also restricts its application expansion in the deep oxidation system of methane. The research by Li et al. (Catalysis Communications 6.12 (2005): 796-801) shows that although palladium supported on a basic support has the highest degree of oxidation, the over-stability of PdO under the action of electrophoretic cations results in low catalytic activity. At the same time, the research by Wills et al. (ACS Catal., 2017, 7: 7810-7821) also proves that when using MgO (basic) as the support material, the low activity of the Pd / MgO catalyst is because the basic MgO enhances the stability of the electron-rich PdO phase, leading to an increase in the energy required to form oxygen vacancies and a decrease in the methane reaction rate. In addition, MgO easily adsorbs CO2 to form magnesium carbonate, which may affect the Pd particle surface or the metal-support interface, thus covering the active sites. Through literature research, it is found that existing palladium catalysts supported on supports are prone to deactivation under complex conditions and cannot meet the actual requirements, and it is necessary to develop new noble metal palladium catalysts supported on supports. Summary of the Invention

[0005] To achieve the above objectives, the technical solution of the present invention is as follows: The present invention creatively uses mullite fibers coated with aluminum gel as the support, and prepares a palladium catalyst supported on mullite fibers coated with aluminum gel by the impregnation method, abbreviated as Pd / AlOOH-M catalyst. This catalyst uses mullite fibers coated with aluminum gel as the support, and Pd 2+ as the active component, rich in a high concentration of PdO, and having characteristics of high surface oxygen and high oxygen vacancy content. In the catalytic oxidation reaction of methane, it exhibits excellent properties such as resistance to high space velocity, frequent heating and cooling startup, and long life.

[0006] Using methane and air as raw materials, the air is provided by an air generator and enters a fixed-bed reactor according to a certain ratio and flow rate. The volume concentration of methane is 500 - 20,000 ppm, the total flow rate is 10 - 200 mL / min, and air is used as the balance gas. 0.02 - 0.2 g of Pd / AlOOH-M catalyst is filled in the fixed-bed reactor, and the overall mass space velocity is 3,000 mL / g cat / h - 600,000 mL / g cat / h. The catalytic oxidation performance of methane is tested by a programmed temperature rise method. The backend is directly connected to a gas chromatograph for on-line detection and analysis. The gas chromatograph is equipped with an FID detector, and the programmed temperature rise range is 200 - 600 °C.

[0007] The palladium catalyst Pd / AlOOH-M supported on mullite fiber coated with aluminum gel of the present invention includes the following preparation steps: (1) Preparation of mullite fiber support coated with aluminum gel: Dissolve an aluminum source and urea in a mixed solution of deionized water and acetic acid, stir evenly in a high-temperature water bath, slowly add a concentrated nitric acid solution to adjust the pH to about 2, and continue stirring. After the reaction is completed, the resulting mixture is in a gel state. Coat it evenly on the mullite fiber, and prepare the AlOOH-M support after high-temperature drying and calcination; (2) Preparation of Pd catalyst supported on mullite fiber support coated with aluminum gel: Add the AlOOH-M support prepared in step (1) to a certain mass of palladium salt solution for impregnation, and then obtain the catalyst after drying and calcination.

[0008] In the above technical solution, further, the aluminum source in step (1) is one of pseudoboehmite, γ-Al2O3 or α-Al2O3.

[0009] In the above technical solution, further, the volume ratio of deionized water to acetic acid in step (1) is: 9:1, 8:2, 7:3, 1:1 and 1:9.

[0010] In the above technical solution, further, the high-temperature water bath temperature in step (1) is 60 - 100 °C; the high-temperature drying time is 6 - 12 h.

[0011] In the above technical solution, further, the calcination temperature in step (1) is 400 - 800 °C; the calcination time is 3 - 6 h.

[0012] In the above technical solution, further, the palladium salt in step (2) is one of palladium nitrate, palladium acetate or palladium chloride, and the loading amount is 0.1 wt% - 2 wt%.

[0013] In the above technical solution, further, the calcination temperature in step (2) is 400-800 °C; the calcination time is 1-4 h.

[0014] The present invention creatively designs for the first time mullite fiber coated with aluminum gel as a carrier. This carrier has the advantages of good thermal stability and formability. The mullite fiber carrier coated with aluminum gel is prepared by the sol-gel method. The catalyst preparation method is simple. After loading precious metal palladium on the carrier by the impregnation method and then calcining, a palladium-loaded catalyst Pd / AlOOH-M supported on a mullite fiber carrier coated with aluminum gel is obtained. This catalyst has excellent catalytic activity.

[0015] Such as Figure 1 and Figure 2 The results show that for the catalyst of Example 1, under the reaction conditions of a methane volume concentration of 10,000 ppm and a mass space velocity ≤ 120,000 mL / g cat / h, the temperature required to catalyze 50% of methane is 342 °C, and the temperature required to catalyze 90% of methane is 400 °C. Even when the mass space velocity reaches 150,000 mL / g cat / h, the temperature required to catalyze 50% of methane is only 350 °C, and the temperature required to catalyze 90% of methane is 410 °C. The increase in temperature is very small, indicating that the catalyst still has excellent catalytic activity and high resistance to high space velocity under extreme conditions; Figure 3 As shown, there is no obvious downward trend in the conversion rate of the first activity test and the eighth test of the catalyst of Example 1, which indicates that the catalyst in Example 1 has good cycle stability and excellent catalytic stability ability. Even under the loading conditions, it can still maintain high catalytic activity, while Figure 4 in Comparative Example 1, the catalytic activity decreased significantly after four heating and cooling cycle tests. Figure 5 The results show that the methane conversion rate of Example 1 still remained at about 60% without change after a 50-hour test at 350 °C, proving that the catalyst has excellent stability.

[0016] Figure 6XRD characterization results of the catalysts of Example 1 and Comparative Examples 1, 2, and 3. The PdO / Pd0 values of the catalysts decreased in the following order: Example 1 (1.56) > Comparative Example 1 (1.45) > Comparative Example 2 (1.07) > Comparative Example 3 (0.92). This result confirmed that there were more active component PdO on the surface of the catalyst of Example 1, which was very consistent with the change trend of catalytic activity. The adsorbed oxygen / lattice oxygen values of the catalysts decreased in the following order: Example 1 (2.9) > Comparative Example 1 (2.62) > Comparative Example 2 (2.37) > Comparative Example 3 (1.77). This result confirmed that there were more oxygen vacancies on the surface of Example 1, which was also consistent with the change trend of catalytic activity, indicating that the catalyst had high catalytic activity for the complete oxidation of methane when the surface of the catalyst had high PdO and adsorbed oxygen concentrations.

[0017] The object of the present invention is to provide a Pd catalyst supported on mullite fibers coated with aluminum gel for the complete oxidation of methane. The prepared mullite fiber support coated with aluminum gel is an anchoring support for the active metal component PdO. The surface of this catalyst has the advantages of high PdO concentration and many oxygen vacancies. This catalyst has excellent catalytic performance such as resistance to high space velocity, frequent heating and cooling startup, and long service life. The prepared catalyst has good catalytic activity and cyclic stability, providing a new idea for improving the cyclic use ability of the catalyst in the complete oxidation of methane. Description of the Drawings

[0018] Figure 1 It is a test chart of the catalytic activity of Examples 1-7 and Comparative Examples 1-3 for the complete oxidation of methane.

[0019] Figure 2 It is a test chart of Example 1 for the complete oxidation of methane at different mass space velocities.

[0020] Figure 3 It is a test chart of the cyclic cooling of Example 1 for the complete oxidation of methane.

[0021] Figure 4 It is a test chart of the cyclic cooling of Comparative Example 1 for the complete oxidation of methane.

[0022] Figure 5 It is a stability curve chart of Example 1 for the complete oxidation of methane.

[0023] Figure 6 It is an XPS Pd 3d and O1s chart of Example 1 and Comparative Examples 1, 2, and 3. Detailed Description of the Invention

[0024] The present invention will be further described in detail below in combination with examples and comparative examples. It should also be understood that the following examples and comparative examples are only used to further illustrate the present invention and should not be construed as limiting the protection scope of the present invention. The specific quality, reaction time and temperature, process parameters, etc. in the examples are also only examples within a suitable range. Some non-essential improvements and adjustments made by those skilled in the art based on the above content of the present invention fall within the protection scope of the present invention. For those not specified in the examples in terms of specific technology or conditions, they are carried out according to the technology or conditions described in the literature in the field or according to the product specification. For the reagents or instruments not specified in terms of the manufacturer, they are all conventional products that can be purchased on the market.

[0025] The embodiments of the present invention will be described in detail below. The following described embodiments are exemplary and are only used to explain the present invention and should not be construed as limiting the present invention. Example

[0026] The steps for preparing a palladium catalyst supported on mullite fibers coated with alumina gel are as follows: (1) Preparation of a mullite fiber support coated with alumina gel: Add a mixed solution of deionized water and acetic acid with a volume ratio of 9:1 to a beaker. Then, weigh 0.7 g of pseudoboehmite and 1.0 g of urea and dissolve them in the above mixed solution. Place the beaker on a water bath at 60 - 100 °C and stir evenly. Use a pipette to slowly drop the acid solution into the beaker. After adjusting the pH to ~2, stir until the solution becomes gel-like. After the stirring is completed, evenly coat it on the mullite fibers. Transfer it to an oven for drying, and then transfer it to a muffle furnace for calcination. After maintaining at 400 - 800 °C for 3 - 6 h, a support is obtained, denoted as AlOOH-M-0.1.

[0027] (2) Preparation of a palladium catalyst supported on mullite fibers coated with alumina gel: Take the mullite fiber support prepared in step (1), prepare an impregnation solution with palladium nitrate, impregnate the support, and after drying, calcine the obtained sample at 400 - 800 °C for 1 - 4 h to obtain the catalyst. The obtained catalyst is denoted as Pd / AlOOH-M-0.1-N.

[0028] The method for catalytic oxidation of methane is as follows: (1) The composition of the reaction gas is as follows: The total flow rate is 50 mL / min; among them, the methane concentration is 10,000 ppm, and the reaction is carried out with a volume ratio of methane to oxygen of 1:16.8, and nitrogen is used as the balance gas. The mass space velocity is 30,000 mL / g cat / h. The catalyst dosage was 0.1 g, which was filled into a quartz tube for performance evaluation. The inner diameter of the quartz tube loaded with the catalyst was about 8 mm, and the length was about 380 mm. The thermocouple was placed directly above the catalyst, and the constant temperature range was 4 - 6 cm, ensuring the accuracy of temperature measurement. When analyzing the test results with a GC - 2014C chromatograph, the heating rate was 10 °C / min, continuously heating from 200 °C to 600 °C, maintaining for 30 min at every 50 °C increase. At each temperature point, the CH4 concentration in the fixed - bed tail gas was detected twice using a gas chromatograph equipped with a flame ionization detector (FID).

[0029] (II) Tests at different space velocities: Adjust the mass space velocity to 30,000, 60,000, 120,000, 150,000 mL / g cat / h, and other test conditions were the same as in (I).

[0030] (III) Heating - cooling cycle test: The fixed - bed temperature was continuously heated from 200 °C to 600 °C, maintaining for 30 min at every 50 °C increase. At each temperature point, the CH4 concentration in the fixed - bed tail gas was detected twice using a gas chromatograph equipped with a flame ionization detector (FID). After heating to 600 °C, it was cooled naturally. The CH4 concentration was detected at every 50 °C decrease, and other test conditions were the same as in (I).

[0031] (IV) Stability test: Directly heat from 200 °C to 350 °C and maintain for 50 h. At each temperature point, the CH4 concentration in the fixed - bed tail gas was detected every 30 min using a gas chromatograph equipped with a flame ionization detector (FID), and other test conditions were the same as in (I). Example

[0032] The steps for preparing a palladium catalyst supported on mullite fibers coated with alumina gel are as follows: (1) Preparation of a mullite fiber support coated with alumina gel: Change the volume ratio of deionized water to acetic acid to 8:2 for mixing and addition. The addition amounts of other reagents and the experimental steps were the same as in (1) of Example 1 to obtain the support, denoted as AlOOH - M - 0.2.

[0033] (2) Preparation of a palladium catalyst supported on mullite fibers coated with alumina gel: Take the support prepared in step (1). The addition amounts of other reagents and the experimental steps were the same as in (2) of Example 1 to obtain the catalyst, denoted as Pd / AlOOH - M - 0.2 - N.

[0034] The method for catalytic oxidation of methane was: the same as the method for catalytic oxidation of methane (I) in Example 1. Example

[0035] The steps for preparing a palladium catalyst supported on mullite fibers coated with alumina gel are as follows: (1) Preparation of mullite fiber support coated with alumina gel: Change the volume ratio of deionized water to acetic acid to 7:3 and add them together. The dosage of other reagents and the experimental steps are the same as those in (1) of Example 1 to obtain the support, denoted as AlOOH-M-0.3.

[0036] (2) Preparation of palladium catalyst supported on mullite fiber coated with alumina gel: Take the support prepared in step (1). The dosage of other reagents and the experimental steps are the same as those in (2) of Example 1 to obtain the catalyst, denoted as Pd / AlOOH-M-0.3-N.

[0037] The method for catalytic oxidation of methane is the same as that in (1) of Example 1. Example

[0038] The steps for preparing a palladium catalyst supported on mullite fiber coated with alumina gel are as follows: (1) Preparation of mullite fiber support coated with alumina gel: Change the volume ratio of deionized water to acetic acid to 1:1 and add them together. The dosage of other reagents and the experimental steps are the same as those in (1) of Example 1 to obtain the support, denoted as AlOOH-M-0.5.

[0039] (2) Preparation of palladium catalyst supported on mullite fiber coated with alumina gel: Take the support prepared in step (1). The dosage of other reagents and the experimental steps are the same as those in (2) of Example 1 to obtain the catalyst, denoted as Pd / AlOOH-M-0.5-N.

[0040] The method for catalytic oxidation of methane is the same as that in (1) of Example 1. Example

[0041] The steps for preparing a palladium catalyst supported on mullite fiber coated with alumina gel are as follows: (1) Preparation of mullite fiber support coated with alumina gel: Change the volume ratio of deionized water to acetic acid to 1:9 and add them together. The dosage of other reagents and the experimental steps are the same as those in (1) of Example 1 to obtain the support, denoted as AlOOH-M-0.9.

[0042] (2) Preparation of palladium catalyst supported on mullite fiber coated with alumina gel: Take the support prepared in step (1). The dosage of other reagents and the experimental steps are the same as those in (2) of Example 1 to obtain the catalyst, denoted as Pd / AlOOH-M-0.9-N.

[0043] The method for catalytic oxidation of methane is the same as that in (1) of Example 1. Example

[0044] The steps for preparing a palladium catalyst supported on mullite fibers coated with alumina gel are as follows: (1) Preparation of a mullite fiber support coated with alumina gel: The support was prepared as in (1) of Example 1, denoted as AlOOH-M-0.1.

[0045] (2) Preparation of a palladium catalyst supported on mullite fibers coated with alumina gel: The support obtained in step (1) was taken and impregnated with a solution prepared from palladium acetate. The dosages of other reagents and the experimental procedures were the same as in (2) of Example 1 to obtain the catalyst, denoted as Pd / AlOOH-M-0.1-C.

[0046] The method for catalytic oxidation of methane was the same as that in Example 1 (Method (I)) for catalytic oxidation of methane. Example

[0047] The steps for preparing a palladium catalyst supported on mullite fibers coated with alumina gel are as follows: (1) Preparation of a mullite fiber support coated with alumina gel: The support was prepared as in (1) of Example 1, denoted as AlOOH-M-0.1.

[0048] (2) Preparation of a palladium catalyst supported on mullite fibers coated with alumina gel: The support obtained in step (1) was taken and impregnated with a solution prepared from palladium chloride. The dosages of other reagents and the experimental procedures were the same as in (2) of Example 1 to obtain the catalyst, denoted as Pd / AlOOH-M-0.1-Cl.

[0049] The method for catalytic oxidation of methane was the same as that in Example 1 (Method (I)) for catalytic oxidation of methane.

[0050] The comparative examples of the present invention are described in detail below. The following nasal drip examples are illustrative and are only used to explain the present invention and should not be construed as limiting the present invention.

[0051] Comparative Example 1 The steps for preparing a palladium catalyst supported on mullite fibers coated with Al2O3 are as follows: (1) Preparation of a mullite fiber support coated with Al2O3: Samples of Al2O3 and polyvinyl alcohol powder with a mass ratio of 1:10 were dissolved in 2 mL of deionized water. After stirring at a constant temperature in a water bath at 60 - 100 °C until the solution was homogeneous, mullite fibers were added to the solution and impregnated evenly, then placed in an oven to dry. Subsequently, it was transferred to a muffle furnace, and the heating rate was 5 °C / min. It was maintained at 400 - 800 °C for 3 - 6 h during calcination.

[0052] (2) Preparation of Pd catalyst supported on Al2O3-coated mullite fibers: Take the Al2O3-coated mullite fiber support prepared in step (1), prepare an impregnation solution with palladium nitrate, impregnate the support, and after drying, calcine the obtained sample at 400-800 °C for 1-4 h to obtain the catalyst, and denote the obtained catalyst as Pd / Al2O3-M.

[0053] The method for catalytic oxidation of methane is as follows: (I) The same as the method for catalytic oxidation of methane (I) in Example 1.

[0054] (II) Heating and cooling cycle test: The same as the method for catalytic oxidation of methane (III) in Example 1.

[0055] Comparative Example 2 The steps for preparing Pd catalyst supported on Al2O3 are as follows: (1) Select commercial Al2O3 as the support.

[0056] (2) Preparation of Pd catalyst supported on Al2O3: Take the Al2O3 support selected in step (1), prepare an impregnation solution with palladium nitrate, impregnate the support and dry it, and then calcine the obtained sample at 400-800 °C for 1-4 h to obtain the catalyst, and denote the obtained catalyst as Pd / Al2O3.

[0057] The method for catalytic oxidation of methane is as follows: The same as the method for catalytic oxidation of methane (I) in Example 1.

[0058] Comparative Example 3 The steps for preparing Pd catalyst supported on mullite fibers are as follows: (1) Select mullite fibers as the support: (2) Preparation of Pd catalyst supported on mullite fibers: Take the mullite fibers in step (1) as the support, prepare an impregnation solution with palladium nitrate, impregnate the support and dry it, and then calcine the obtained sample at 400-800 °C for 1-4 h to obtain the catalyst, and denote the obtained catalyst as Pd / M.

[0059] The method for catalytic oxidation of methane is as follows: (I) The same as the method for catalytic oxidation of methane (I) in Example 1.

[0060] Figure 1 For the cases where the catalysts of Examples 1-7 and Comparative Examples 1-3 are used in the complete oxidation reaction of methane. The experimental results show that the catalyst of Example 1 has the best activity, the temperature required for 50% conversion of methane is 342 °C, and the temperature required for 90% conversion of methane is 400 °C.

[0061] Figure 2 The results show that at up to 150,000 mL / g catUnder the mass space velocity condition of / h, the catalyst in Example 1 can still maintain excellent activity equivalent to that under the low mass space velocity condition of 30,000 mL / g cat / h, which fully indicates that the catalyst has significantly enhanced stability.

[0062] Figure 3 There is no obvious downward trend in the conversion rate of the first activity test and the eighth test of the catalyst in Example 1 shown, which indicates that the catalyst in Example 1 has good cyclic stability and excellent resistance to thermal stress damage. The crystal phase stability of the carrier material and the anchoring effect of the active component jointly inhibit the phenomenon of high-temperature sintering.

[0063] Figure 4 The activity of Comparative Example 1 decreased significantly after four heating and cooling cycle tests.

[0064] Figure 5 The results show that the methane conversion rate of Example 1 remained unchanged at about 60% under the test for 50 h, proving that the catalyst has excellent stability.

[0065] Figure 6 XRD characterization results of the catalysts of Example 1 and Comparative Examples 1, 2, and 3.

[0066] XPS characterization was carried out on the samples of Example 1 and Comparative Examples 1, 2, and 3, and O1s energy spectrum and Pd 3d energy spectrum were obtained through characterization analysis. The two peaks in the O1s energy spectrum respectively correspond to lattice oxygen and adsorbed oxygen. Usually, the ratio of adsorbed oxygen / lattice oxygen can be used to compare the content of oxygen vacancies in the catalyst. The two peaks in the Pd 3d energy spectrum respectively correspond to PdO and Pd 0 。

[0067] According to Figure 6 The PdO / Pd 0 values of the catalyst decrease in the following order: Example 1 (1.56) > Comparative Example 1 (1.45) > Comparative Example 2 (1.07) > Comparative Example 3 (0.92). This result confirms that there are more active component PdO on the surface of the catalyst in Example 1, which is very consistent with the change trend of catalytic activity.

[0068] According to Figure 6 The adsorbed oxygen / lattice oxygen values of the catalyst can be analyzed to decrease in the following order: Example 1 (2.9) > Comparative Example 1 (2.62) > Comparative Example 2 (2.37) > Comparative Example 3 (1.77). This result confirms that there are more oxygen vacancies on the surface of Example 1, which is very consistent with the change trend of catalytic activity.

[0069] The above content is a further detailed description of the present invention in combination with specific embodiments. It cannot be considered that the specific implementation of the present invention is only limited to these descriptions. For those of ordinary skill in the technical field to which the present invention pertains, without departing from the concept of the present invention, simple modifications and substitutions made should be regarded as belonging to the protection scope of the present invention.

Claims

1. A palladium catalyst Pd / AlOOH-M supported on mullite fiber carrier coated with aluminum gel for methane catalytic oxidation, and its methane catalytic oxidation characteristics are as follows: (1) The dosage of Pd / AlOOH-M catalyst is 0.02 - 0.2 g; (2) The volume concentration of methane is 500 - 20,000 ppm; (3) The total flow rate is 10 - 200 mL / min, and the balance gas is air; (4) The mass space velocity is 3,000 mL / g cat / h - 600,000 mL / g cat / h; (5) The reaction temperature is 200 - 600 °C.

2. The catalyst according to claim 1, and its preparation method is as follows: (1) Preparation of mullite fiber carrier coated with aluminum gel: Dissolve the aluminum source and urea in a mixed solution of deionized water and acetic acid, stir evenly in a high-temperature water bath, slowly add concentrated nitric acid solution to adjust the pH to ~2, continue stirring, and the resulting mixture is in a gel state after the reaction. Coat it evenly on the mullite fiber, and prepare the AlOOH-M carrier after high-temperature drying and calcination; (2) Preparation of Pd catalyst supported on mullite fiber carrier coated with aluminum gel: Immerse the AlOOH-M carrier prepared in step (1) into a certain mass of palladium salt solution, and then obtain the catalyst after drying and calcination.

3. The preparation method of the Pd catalyst supported on the mullite fiber carrier coated with aluminum gel according to claim 2, characterized in that The aluminum source described in step (1) is one or a mixture of boehmite, γ-Al2O3 or α-Al2O3.

4. The preparation method of the alumina gel-coated Pd catalyst supported on mullite fibers according to claim 2, characterized in that The volume ratio of deionized water to acetic acid described in step (1) is: 9:1, 8:2, 7:3, 1:1 and 1:

9.

5. The preparation method of the aluminum gel-coated Pd catalyst supported on mullite fibers according to claim 2, wherein The high-temperature water bath temperature described in step (1) is 60 - 100 °C; the high-temperature drying time is 6 - 12 h.

6. The preparation method of the mullite fiber-supported Pd catalyst coated with an aluminum gel according to claim 2, characterized in that The calcination temperature described in step (1) is 400 - 800 °C; the calcination time is 3 - 6 h.

7. The preparation method of the palladium catalyst Pd / AlOOH-M supported on mullite fibers coated with aluminum gel according to claim 2, characterized in that The palladium salt described in step (2) is one or a mixture of palladium nitrate, palladium acetate or palladium chloride, and the loading amount is 0.1 wt% - 5 wt%.

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