Mixed crystal form alumina supported palladium catalyst as well as preparation method and application thereof

Abstract

The invention discloses a preparation method of a mixed crystal form alumina supported palladium catalyst. The preparation method comprises the following steps: (1) tabletting aluminum precursor powder to prepare a ceramic wafer; (2) putting the ceramic wafer into a Joule heat heating device, and heating and roasting at 1000-1300 DEG C under the conditions that the Joule heat heating output voltage is 5-30V and the output current is 50-200A to prepare mixed crystal aluminum oxide; and (3) dipping the mixed crystal form alumina carrier in a precursor solution of palladium, and roasting to obtain the palladium catalyst. The mixed crystal aluminum oxide of the theta crystal form and the alpha crystal form is prepared in a Joule heating mode, the mixed crystal aluminum oxide serves as a carrier to prepare the palladium catalyst through an impregnation method, palladium active sites with high dispersion and stability are obtained on the surface of the aluminum oxide carrier, and the activity and stability of the catalyst are improved. The catalyst is applied to a methane catalytic oxidation reaction, the methane activation temperature is low, and the catalyst stability is high.

Description

Technical Field

[0001] The invention belongs to the technical field of precious metal catalysts and relates to a palladium-based catalyst, in particular to a method for preparing a mixed crystal alumina-supported palladium catalyst, and a catalyst prepared by the method and application thereof. Background Art

[0002] Methane (CH4) is the second largest greenhouse gas causing global warming, contributing 25% to global warming, second only to carbon dioxide (CO2). Compared with CO2, methane has a stronger ability to absorb heat, with a global warming potential (GWP) of 84 times that of CO2 within 20 years and 28 times that of CO2 within 100 years. Methane emissions mainly come from agriculture, energy, industry and waste treatment industries, of which the energy industry contributes about 40% of methane emissions. Methane emissions in the energy industry are mainly composed of oil and gas systems, coal mining, biomass combustion and fossil fuel combustion, of which methane emissions caused by coal mining account for as much as 21%. Low-negative pressure extraction gas with a methane concentration of less than 5% and ventilation gas with a methane concentration of less than 0.75% in coal mining have the characteristics of low concentration and large air volume. However, this type of gas cannot be used for power generation due to its low methane concentration and is directly emitted. The control and comprehensive utilization of low-concentration methane have significant economic and social value.

[0003] Methane, as a greenhouse gas, can completely oxidize CH4 to H2O and CO2 under the action of catalysts. Catalytic oxidation technology is a common method for methane treatment, among which palladium-based catalysts are considered to be the most active for methane catalytic oxidation reactions. Studies have shown that crystalline palladium oxide (PdO) or a combination of Pd and PdO is more active than metallic Pd. The amount of PdO in the supported Pd catalyst, the Pd dispersion, and the Pd-support interaction play an important role in determining the catalytic activity. Alumina, especially γ-Al2O3, is widely used as a carrier for methane combustion catalysts due to its high surface area, moderate chemical activity, and low cost.

[0004] Nano-alumina has many types of crystals, such as α, β, γ, δ, η, and θ. Different preparation methods and process conditions can obtain nano-alumina with different structures: β and γ alumina are characterized by porosity, high dispersion, high activity, large specific surface area, high porosity, and are active alumina, which are widely used as catalysts and catalyst carriers; α-Al2O3, generally called inert alumina, is the most common and most stable alumina crystal form, with a low specific surface area and high temperature resistance, but it is not active alumina and has almost no catalytic activity; θ-alumina is an intermediate crystal form between active alumina and α-Al2O3. Various types of alumina will eventually generate the thermodynamically stable phase α-Al2O3 after high-temperature treatment.

[0005] At present, most palladium-based catalysts use activated alumina γ-Al2O3, or mainly γ-Al2O3 as a carrier, and θ-Al2O3 or α-Al2O3 are rarely used in catalyst carriers. CN202110313336A discloses a methane catalytic oxidation palladium-based catalyst, which uses precious metal palladium as an active component and θ-crystal Al2O3 as a carrier. The θ-crystal alumina is prepared by calcining pseudo-boehmite at 1000℃~1100℃ for 1~3h. Replacing the γ-crystal with the uncommon θ-crystal alumina carrier, the θ-crystal alumina carrier has a stronger interaction with the active substance, which makes up for the inherent defects of the small pore volume and specific surface area of ​​the alumina carrier. CN103203233A discloses a methane catalytic combustion palladium-based catalyst, which uses precious metal palladium as an active component and pseudo-boehmite as an aluminum source to prepare a mesoporous alumina as a carrier. The synthesis of mesoporous alumina uses industrial raw material pseudo-boehmite as aluminum source, and is obtained by calcination with the addition of surfactant. The palladium-based catalyst is prepared by an equal volume impregnation method.

[0006] The key to methane catalytic oxidation technology lies in the development of catalysts. However, at present, the ignition activity of catalysts still has room for improvement. Unlike γ-Al2O3, θ-Al2O3 and α-Al2O3 have no catalytic activity and are less studied as carriers of Pd catalysts. Commercial θ-Al2O3 is not common, and the catalytic ability of mixed alumina of θ and α crystals is still unclear. Summary of the invention

[0007] The purpose of the present invention is to provide a method for preparing a mixed crystalline alumina-supported palladium catalyst, wherein a mixed crystalline alumina of θ crystal and α crystal is prepared by Joule heat heating, and the mixed crystalline alumina is used as a carrier to reduce metal-carrier interaction, overcome the problem of easy agglomeration of metal nanoparticles, obtain highly dispersed and stable palladium active sites on the surface of the mixed crystalline alumina carrier, and improve the activity of the catalyst.

[0008] Another object of the present invention is to provide a catalyst prepared by the method and its application in the catalytic oxidation reaction of methane, which has the advantages of low methane activation temperature and high catalyst stability.

[0009] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical solutions: A method for preparing a mixed crystalline alumina-supported palladium catalyst of the present invention comprises the following steps: (1) Pressing aluminum precursor powder into a ceramic sheet; (2) placing the ceramic sheet obtained in step (1) in a Joule heat heating device, wherein the Joule heat heating output voltage is 5-30 V, the output current is 50-200 A, and heating and calcining is performed at 1000-1300° C. to prepare mixed crystalline alumina, wherein the θ crystal accounts for 10-90%wt and the α crystal accounts for 90%-10%wt; (3) Dispersing the mixed crystal alumina carrier obtained in step (2) into a precursor solution of precious metal palladium, separating the solid and liquid after impregnation, washing, drying, and calcining to obtain the mixed crystal alumina-supported palladium catalyst.

[0010] In the step (1), the aluminum precursor is one or more of gibbsite, boehmite, ammonium aluminum sulfate, ammonium aluminum carbonate and γ-Al2O3, preferably γ-Al2O3.

[0011] In the step (2), the conductive material of the Joule heat heating device is one of graphite felt, carbon cloth, graphite plate and conductive sponge, preferably graphite felt.

[0012] The Joule heat heating device in step (2) preferably has an output voltage of 10-20 V and an output current of 100-200 A.

[0013] The Joule heat heating and roasting time in step (2) is 5 to 30 minutes, and the heating temperature is preferably 1100 to 1200°C.

[0014] The mixed crystalline alumina obtained in step (2) is a θ and α mixed phase. Preferably, the θ crystal of the alumina accounts for 30-70%wt, and the α crystal accounts for 70%-30%wt, more preferably, the θ crystal accounts for 60-70% and the α crystal accounts for 40-30%. The specific surface area of ​​the mixed crystalline alumina is 10-100m 2 g - .

[0015] The palladium precursor in step (3) includes, but is not limited to, one or more of palladium nitrate, palladium acetate, and palladium chloride, preferably palladium nitrate.

[0016] The aluminum oxide in step (3) is dispersed in the palladium precursor, immersed for 30 to 180 minutes, filtered, washed and dried.

[0017] In step (3), the calcination temperature is 300-600°C for 30-120 min. Preferably, the calcination temperature is 400-450°C for 100-120 min.

[0018] The palladium aluminum catalyst obtained by the above preparation method preferably has a palladium content of 0.01wt%~1.5wt%.

[0019] The invention also relates to a mixed crystal alumina-supported palladium catalyst prepared by the method.

[0020] The palladium content of the mixed crystal alumina-supported palladium catalyst is preferably 0.01wt% to 1.5wt%.

[0021] In the mixed crystal alumina supported palladium catalyst, the θ crystal of the carrier mixed crystal alumina accounts for 30-70%wt, and the α crystal accounts for 70%-30%wt, preferably the θ crystal accounts for 60-70% and the α crystal accounts for 40-30%.

[0022] The present invention also provides application of the mixed crystal alumina-supported palladium catalyst prepared by the method in a methane catalytic oxidation reaction.

[0023] Furthermore, the methane catalytic oxidation reaction is carried out in a fixed bed reactor at a space velocity of 20,000 to 10,000 h -1 The catalytic oxidation reaction temperature is 200℃~900℃.

[0024] Beneficial effects: The present invention adopts Joule heat heating to prepare mixed crystal alumina, and uses mixed crystal alumina of θ crystal and α crystal as a carrier, and prepares a palladium catalyst by an impregnation method. Mixed crystal alumina as a carrier can reduce the metal-carrier interaction, overcome the problem of easy agglomeration of metal nanoparticles, obtain highly dispersed and stable palladium active sites on the surface of the mixed crystal alumina carrier, and improve the activity and stability of the catalyst. The mixed crystal alumina-supported palladium catalyst prepared by the present invention is applied to the catalytic oxidation reaction of methane, and the methane activation temperature is low and the catalyst stability is high. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 XRD analysis diagram of the mixed crystal alumina prepared in Example 1-5; Figure 2 Stability test of the mixed crystalline alumina-supported palladium catalyst prepared in Example 1. DETAILED DESCRIPTION

[0026] The specific embodiments of the present invention are described in detail below, but it should be pointed out that the protection scope of the present invention is not limited by these embodiments but is determined by the claims.

[0027] Example 1 A method for preparing a mixed crystalline alumina-supported palladium catalyst comprises the following steps: (1) Take 1 g of γ-Al2O3 powder and use a tablet press to make alumina flakes with a diameter of 10 mm and a thickness of 2 mm under a pressure of 10 MPa (pressure indication number).

[0028] (2) Place the ceramic sheet of step (1) in a Joule heating device with an output voltage of 16 V, an output current of 200 A, a sintering time of 15 min, and a sintering temperature of 1100°C. Disconnect the circuit, cool to 25°C, and take out the alumina and grind it into powder.

[0029] (3) Repeat steps (1) and (2) until a sufficient amount of alumina is produced.

[0030] (4) Take 1 mL of 10 g / L palladium nitrate solution and add it dropwise to 39 mL of water. Take 2 g of the alumina sample prepared in step (3) and add it thereto. Soak at room temperature for 60 min. Heat to 80 °C and stir for 2 h. Stop heating, centrifuge, wash, and dry in an oven at 110 °C for 3 h.

[0031] (5) The sample obtained in step (4) was placed in a tube furnace, air was introduced (flow rate was 50 ml / min), and calcined at 400°C for 2 h (heating rate was 5°C / min). After the tube furnace was cooled to about 25°C, the sample was taken out and stored in a dry environment to obtain a mixed crystalline alumina-supported palladium catalyst with a palladium content of 0.5 wt%, recorded as Pd / Al-1.

[0032] The XRD spectrum of the obtained mixed crystal phase alumina is as follows: Figure 1 As shown. Figure 1 It can be seen that the spectrum has obvious θ-Al2O3 diffraction peaks at 31.35, 32.89, 36.95 and 67.41, and obvious α-Al2O3 diffraction peaks at 43.36, 35.15, 57.5 and 37.78, indicating that after Joule heat heating and roasting (temperature is 1100℃), alumina has a mixed crystal phase of θ and α. The results of jade semi-quantitative analysis are shown in Table 1. CO pulse analysis was performed, and the sample was pretreated in 10% H2 / N2, dehydrated and reduced for 2 hours, cooled to room temperature and then switched to He gas, degassed, and cooled to 30°C. CO / He was injected into the sample every 2 minutes, and CO and Pd were adsorbed and combined until saturated. Assuming that the CO / Pd stoichiometric ratio is 1:1, the dispersion of Pd was calculated, and the results are shown in Table 1. From the results of the CO pulse in Table 1, the dispersion of Pd is higher when the mixed crystal phase alumina is used as a carrier.

[0033] Example 2 A method for preparing a mixed crystalline alumina-supported palladium catalyst comprises the following steps: (1) Take 1 g of γ-Al2O3 powder and use a tablet press to make alumina flakes with a diameter of 10 mm and a thickness of 2 mm under a pressure of 10 MPa (pressure indication number).

[0034] (2) Place the ceramic sheet of step (1) in a Joule heating device with an output voltage of 10 V, an output current of 100 A, a calcination time of 5 min, and a calcination temperature of 1000°C. Disconnect the circuit, cool to 25°C, and take out the alumina and grind it into powder.

[0035] (3) Repeat steps (1) and (2) until a sufficient amount of alumina is produced.

[0036] (4) Take 1 mL of 10 g / L palladium nitrate solution and add it dropwise to 39 mL of water. Take 2 g of the alumina sample prepared in step (3) and add it thereto. Soak at room temperature for 60 min. Heat to 80 °C and stir for 2 h. Stop heating, centrifuge, wash, and dry in an oven at 110 °C for 3 h.

[0037] (5) The sample obtained in step (4) was placed in a tube furnace, air was introduced (flow rate was 50 ml / min), and calcined at 400°C for 2 h (heating rate was 5°C / min). After the tube furnace was cooled to about 25°C, the sample was taken out and stored in a dry environment to obtain a mixed crystalline alumina-supported palladium catalyst, which was recorded as Pd / Al-2.

[0038] The XRD spectrum of the obtained mixed crystal phase alumina is as follows: Figure 1 The dispersion of Pd is shown in Table 1.

[0039] Example 3 A method for preparing a mixed crystalline alumina-supported palladium catalyst comprises the following steps: (1) Take 1 g of γ-Al2O3 powder and use a tablet press to make alumina flakes with a diameter of 10 mm and a thickness of 2 mm under a pressure of 10 MPa (pressure indication number).

[0040] (2) Place the ceramic sheet of step (1) in a Joule heating device with an output voltage of 20 V, an output current of 200 A, a calcination time of 20 min, and a calcination temperature of 1200°C. Disconnect the circuit, cool to 25°C, take out the alumina and grind it into powder.

[0041] (3) Repeat steps (1) and (2) until a sufficient amount of alumina is produced.

[0042] (4) Take 1 mL of 10 g / L palladium nitrate solution and add it dropwise to 39 mL of water. Take 2 g of the alumina sample prepared in step (3) and add it thereto. Soak at room temperature for 60 min. Heat to 80 °C and stir for 2 h. Stop heating, centrifuge, wash, and dry in an oven at 110 °C for 3 h.

[0043] (5) The sample obtained in step (4) was placed in a tube furnace, air was introduced (flow rate was 50 ml / min), and calcined at 400°C for 2 h (heating rate was 5°C / min). After the tube furnace was cooled to about 25°C, the sample was taken out and stored in a dry environment to obtain a mixed crystalline alumina-supported palladium catalyst, which was recorded as Pd / Al-3.

[0044] The XRD spectrum of the obtained mixed crystal phase alumina is as follows: Figure 1 The dispersion of Pd is shown in Table 1.

[0045] Example 4 A method for preparing a mixed crystalline alumina-supported palladium catalyst comprises the following steps: (1) Take 1 g of γ-Al2O3 powder and use a tablet press to make alumina flakes with a diameter of 10 mm and a thickness of 2 mm under a pressure of 10 MPa (pressure indication number).

[0046] (2) Place the ceramic sheet of step (1) in a Joule heating device with an output voltage of 20 V, an output current of 200 A, a calcination time of 30 min, and a calcination temperature of 1200°C. Disconnect the circuit, cool to 25°C, take out the alumina and grind it into powder.

[0047] (3) Repeat steps (1) and (2) until a sufficient amount of alumina is produced.

[0048] (4) Take 1 mL of 10 g / L palladium nitrate solution and add it dropwise to 39 mL of water. Take 2 g of the alumina sample prepared in step (3) and add it thereto. Soak at room temperature for 60 min. Heat to 80 °C and stir for 2 h. Stop heating, centrifuge, wash, and dry in an oven at 110 °C for 3 h.

[0049] (5) The sample obtained in step (4) was placed in a tube furnace, air was introduced (flow rate was 50 ml / min), and calcined at 400°C for 2 h (heating rate was 5°C / min). After the tube furnace was cooled to about 25°C, the sample was taken out and stored in a dry environment to obtain a mixed crystalline alumina-supported palladium catalyst, which was recorded as Pd / Al-4.

[0050] The XRD spectrum of the obtained mixed crystal phase alumina is as follows: Figure 1 The dispersion of Pd is shown in Table 1.

[0051] Example 5 A method for preparing a mixed crystalline alumina-supported palladium catalyst comprises the following steps: (1) 1 g of boehmite powder was used to press a tablet at a pressure of 10 MPa (pressure indication number) to form a tablet with a diameter of 10 mm and a thickness of 2 mm.

[0052] (2) Place the ceramic sheet of step (1) in a Joule heating device with an output voltage of 20 V, an output current of 200 A, a calcination time of 15 min, and a calcination temperature of 1100°C. Disconnect the circuit, cool to 25°C, take out the sheet and grind it into powder.

[0053] (3) Repeat steps (1) and (2) until a sufficient amount of alumina is produced.

[0054] (4) Take 1 mL of 10 g / L palladium nitrate solution and add it dropwise to 39 mL of water. Take 2 g of the alumina sample prepared in step (3) and add it thereto. Soak at room temperature for 60 min. Heat to 80 °C and stir for 2 h. Stop heating, centrifuge, wash, and dry in an oven at 110 °C for 3 h.

[0055] (5) The sample obtained in step (4) was placed in a tube furnace, air was introduced (flow rate was 50 ml / min), and calcined at 400°C for 2 h (heating rate was 5°C / min). After the tube furnace was cooled to about 25°C, the sample was taken out and stored in a dry environment to obtain a mixed crystalline alumina-supported palladium catalyst, which was recorded as Pd / Al-5.

[0056] The XRD spectrum of the obtained mixed crystal phase alumina is as follows: Figure 1 The dispersion of Pd is shown in Table 1.

[0057] Example 6 (1) Take 1 g of γ-Al2O3 powder and use a tablet press to make alumina flakes with a diameter of 10 mm and a thickness of 2 mm under a pressure of 10 MPa (pressure indication number).

[0058] (2) Place the ceramic sheet of step (1) in a Joule heating device with an output voltage of 16 V, an output current of 200 A, a sintering time of 15 min, and a sintering temperature of 1100°C. Disconnect the circuit, cool to 25°C, and take out the alumina and grind it into powder.

[0059] (3) Repeat steps (1) and (2) until a sufficient amount of alumina is produced.

[0060] (4) Take 3 mL of 10 g / L palladium nitrate solution and add it dropwise to 39 mL of water. Take 2 g of the alumina sample prepared in step (3) and add it thereto. Soak at room temperature for 60 min. Heat to 80 °C and stir for 2 h. Stop heating, centrifuge, wash, and dry in an oven at 110 °C for 3 h.

[0061] (5) The sample obtained in step (4) was placed in a tube furnace, air was introduced (flow rate was 50 ml / min), and calcined at 400°C for 2 h (heating rate was 5°C / min). After the tube furnace was cooled to about 25°C, the sample was taken out and stored in a dry environment to obtain a mixed crystalline alumina-supported palladium catalyst with a palladium content of 1.5 wt%, recorded as Pd / Al-6.

[0062] Comparative Example 1 (1) Place 2 g of alumina in a tube furnace, introduce air (flow rate 50 ml / min, heating rate 5 °C / min), calcine at 1100 °C for 15 min, and take it out after the tube furnace cools down to about 25 °C; (2) Take 1 mL of 1 g / L palladium nitrate solution and add it dropwise to 39 mL of water, take 2 g of the sample in step (1) and add it thereto, soak at room temperature for 60 min, heat to 80 °C and stir for 2 h, stop heating, centrifuge, wash, and dry in an oven at 110 °C for 3 h; (3) The sample obtained in step (2) was placed in a tube furnace, air was introduced (flow rate was 50 ml / min), and calcined at 400°C for 2 h (heating rate was 5°C / min). After the tube furnace was cooled to about 25°C, the sample was taken out and stored in a dry environment to obtain a γ-alumina supported palladium catalyst with a palladium content of 0.5 wt%, recorded as Pd / Al-1.

[0063] Comparative Example 2 (1) Pseudo-boehmite is calcined at 1050°C for 2 hours, and the obtained alumina is mixed with water to prepare an alumina dispersion; (2) Pd(NO3)2 and alumina dispersion are mixed and boiled at 70°C to obtain a solid; (3) The solid was dried at 100°C and then calcined at 600°C for 2 h to obtain a palladium-based catalyst. The heating rate during the calcination process was 10°C / min. A catalyst with a palladium content of 0.5% was obtained.

[0064] Catalyst performance evaluation Method: The evaluation of methane catalytic oxidation activity was carried out in a fixed bed catalytic reaction system. The reaction gas consisted of 5000ppmCH4, 21%O2, and N2 supplementary gas, with a total flow rate of 300ml / min. First, the catalyst sample was ground, pressed and sieved to 40-60 mesh. 72mg of catalyst was mixed with 1.8g of 40-60 mesh quartz sand to form a catalyst bed. The catalyst bed volume was about 0.9mL, and the reaction space velocity was 20000h -1, both ends of the reaction tube are filled with quartz wool. Heat the reactor in a N2 atmosphere, raise the temperature to 300℃ and cut to the reaction path. The test temperature range is 300~550℃, with a temperature interval of 40℃. Each temperature point is maintained for 1h, and the time to heat to the next temperature point is 4 minutes. The concentration of residual CH4 in the product is tested by gas chromatography and FID detector. The temperature at which the methane conversion rate is 90% is recorded as T90. The results are shown in Table 1: Table 1 Catalyst activity and dispersion of Examples 1 to 6 and Comparative Examples 1 to 2 .

[0065] The catalyst stability experiment was carried out by mixing 72 mg of Pd-Al-1 sample with 1.8 g of 40-60 mesh quartz sand to form a catalyst bed. The catalyst bed volume was about 0.9 mL and the reaction space velocity was 20000 h -1 The two ends of the reaction tube were filled with quartz wool. The reaction furnace was heated to 600°C under N2 atmosphere for 120 hours to test the thermal stability activity. The residual CH4 concentration of the test product was tested by gas chromatography and FID detector. The results are shown in Figure 2 .

[0066] From Table 1 and the XRD graph, it can be seen that under the same conditions, the mixed crystal alumina prepared by Joule heating with 16 voltage, 200A current, and maintaining for 15 minutes can achieve a CH4 conversion rate of 90% at 371°C, and can maintain a catalyst activity of nearly 100% for 120 hours at 600°C without deactivation. This mixed crystal has the highest Pd dispersion, and XRD shows that the mixed crystal is higher in θ crystal than α crystal alumina.

[0067] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should know that any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed in the present invention fall within the protection scope of the present invention.

Claims

1. A method for preparing a mixed crystalline alumina-supported palladium catalyst, characterized in that: The following steps are involved: (1) Pressing aluminum precursor powder into a ceramic sheet; (2) placing the ceramic sheet obtained in step (1) in a Joule heat heating device, wherein the Joule heat heating output voltage is 5-30 V, the output current is 50-200 A, and heating and calcining is performed at 1000-1300° C. to obtain mixed crystalline alumina, wherein the θ crystal accounts for 10-90%wt and the α crystal accounts for 90%-10%wt; (3) Dispersing the mixed crystal alumina carrier obtained in step (2) into a precursor solution of precious metal palladium, separating the solid and liquid after impregnation, washing, drying, and calcining to obtain the mixed crystal alumina-supported palladium catalyst.

2. The method for preparing a mixed crystal alumina-supported palladium catalyst according to claim 1, wherein: The aluminum precursor described in step (1) is one or more of gibbsite, boehmite, ammonium aluminum sulfate, ammonium aluminum carbonate and γ-Al2O3.

3. The method for preparing a mixed crystal alumina-supported palladium catalyst according to claim 1, characterized in that: The conductive material of the Joule heat heating device in step (2) is one of graphite felt, carbon cloth, graphite plate and conductive sponge.

4. The method for preparing a mixed crystal alumina-supported palladium catalyst according to claim 1, characterized in that: In step (2), the output voltage of the Joule heat heating is 10-30V, and the output current is 100-200A.

5. The method for preparing a mixed crystal alumina-supported palladium catalyst according to claim 1, characterized in that: In step (2), the heating temperature is 1100-1200° C., and the heating and calcining time is 5-30 min.

6. The method for preparing a mixed crystal alumina-supported palladium catalyst according to claim 1, characterized in that: The palladium precursor in step (3) is one or more of palladium nitrate, palladium acetate, and palladium chloride.

7. The method for preparing a mixed crystal alumina-supported palladium catalyst according to claim 1, characterized in that: In the mixed crystalline alumina obtained in step (2), the θ crystal accounts for 30-70%wt, the α crystal accounts for 70%-30%wt, and the specific surface area is 10-100m 2 g -1 .

8. A mixed crystal alumina-supported palladium catalyst prepared according to the method for preparing a mixed crystal alumina-supported palladium catalyst according to any one of claims 1 to 7.

9. The mixed crystal alumina-supported palladium catalyst according to claim 8, characterized in that: The palladium content is 0.01wt%~1.5wt%, the θ crystal form of the carrier mixed crystalline alumina accounts for 30~70%wt, and the α crystal form accounts for 70~30%wt.

10. Use of the mixed crystal alumina supported palladium catalyst as claimed in claim 9 in catalytic oxidation of methane.

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