Preparation method and application of cerium tin pyrochlore-based composite catalyst for catalytic denitration

Abstract

The invention relates to a preparation method and application of a cerium tin pyrochlore-based composite catalyst for catalytic denitration. A hydrothermal-physical mixing method is adopted to prepare the catalyst, Ce2Sn2O7 pyrochlore powder is obtained through preparation of cerium salt and tin salt precursor solutions, mixing, precipitation with a precipitator, hydrothermal aging, pumping washing, grinding and roasting, and finally the Ce2Sn2O7 pyrochlore powder is mixed with a Beta molecular sieve to obtain the catalyst. The denitration performance test of the catalyst shows that the NOx conversion rate of the Ce2Sn2O7: Beta = 2: 3 catalyst in the temperature interval of 250-450 DEG C is higher than 90%, the N2 selectivity is also higher than 95%, meanwhile, the good water-resistant and sulfur-resistant stability is kept, and the number of acid sites of the catalyst is increased; the preparation cost is low, the preparation method is simple, the process is non-toxic and harmless, the requirement for expensive instruments is extremely low, and the risk of secondary pollution to the environment is avoided.

Description

Technical Field

[0001] The present invention belongs to the technical field of tail gas purification catalysis. It relates to a preparation method and application of a cerium tin pyrochlore-based composite catalyst Background Art

[0002] Although clean power generation technologies (nuclear energy, hydropower, etc.) are being vigorously developed nowadays, traditional coal-fired power generation is difficult to be replaced in a short time. A large amount of nitrogen oxides (NO x ) are generated during the coal-fired power generation process, which not only cause environmental pollution problems such as acid rain, haze, and photochemical smog, but also endanger human health. At the same time, with the increasingly stringent emission standards, it is particularly crucial to improve the purification efficiency of nitrogen oxides in the tail gas

[0003] The selective catalytic reduction of nitrogen oxides technology (NO x -SCR) is widely used in industrial sites such as steel plants, coal-fired power plants, and industrial boilers due to its high denitrification efficiency and relatively low reaction temperature. The core of the SCR technology is the selection of the catalyst. The commercially available V 2 O 5 -WO 3 / TiO 2 catalyst is the mainstream choice for coal-fired power plants, but it has problems such as a narrow denitrification window and the biological toxicity of V 2 O 5 . The noble metal catalyst is expensive and its application is limited. Therefore, it is imperative to design and develop a catalyst with high efficiency, low cost, environmental friendliness, and high hydrothermal and sulfur resistance stability

[0004] CeO 2 has a cubic fluorite structure. Its unique Ce 3+ / Ce 4+ redox cycle characteristics and excellent oxygen storage and release performance have been applied in many fields. Rutile-type SnO 2 is rich in Lewis acid centers and surface oxygen vacancies, which promotes its wide application in the catalytic field. Generally, the catalytic performance of the CeO 2 -SnO 2 mixture or the Ce-Sn solid solution type composite oxide is significantly improved compared with that of the single oxide. In addition, the formation of the A 2 B 2 O 7 pyrochlore-type composite oxide structure endows it with good thermal stability and intrinsic oxygen vacancies, which helps to promote the adsorption and activation of reactant molecules, thereby improving the reaction performance Summary of the Invention

[0005] The present invention provides a preparation method and application of a catalytic denitrification cerium tin pyrochlore-based composite catalyst. Based on the design of dual active sites, through Ce 2Sn 2 O 7 The pyrochlore composite oxide provides abundant oxygen vacancies and introduces an acidic substance (Beta zeolite) to regulate the redox ability of the catalyst and improve the distribution of acidic sites on the catalyst. The synergistic effect between the two enhances the adsorption and activation of NH 3 and the reduction ability of NO x to achieve efficient denitrification, while showing excellent stability against water and sulfur resistance.

[0006] A catalytic denitrification cerium-tin pyrochlore-based composite catalyst described in the present invention is named Ce 2 Sn 2 O 7 :Beta = a:b (a:b is the mass ratio of the two components), where the crystal phase structure of Ce 2 Sn 2 O 7 is a disordered pyrochlore structure.

[0007] A preparation method of a catalytic denitrification cerium-tin pyrochlore-based composite catalyst described in the present invention adopts a hydrothermal-physical mixing method, which includes the following steps.

[0008] (1) Prepare a precursor salt solution: Weigh cerium and tin precursor salts according to an equimolar ratio, dissolve them separately in deionized water, and ultrasonically assist the dissolution until the solution is clear.

[0009] (2) Solution mixing: Slowly add the clear solution containing the cerium precursor to the tin solution, heat and stir the mixture in a constant temperature water bath at 40 - 50 °C for 20 - 30 min to form a mixed metal ion solution.

[0010] (3) Under low-speed stirring, slowly add a precipitant to adjust the pH of the mixed metal solution to 10 - 14, and continue to maintain magnetic stirring at 300 - 400 rpm for 3 - 4 h.

[0011] (4) High-temperature and high-pressure hydrothermal aging: Add the suspension in step (3) to a 150 mL polytetrafluoroethylene-sealed autoclave, and hydrothermally age it in a forced-air drying oven at 160 - 200 °C for 12 - 36 hours.

[0012] (5) After the hydrothermal aging is completed, the product is separated by vacuum filtration and washed alternately with deionized water and absolute ethanol until the sodium ion concentration in the filtrate is lower than 20 ppm.

[0013] (6) The obtained filter cake is dried overnight in a vacuum drying oven at 90 - 110 °C, ground into fine powder, then placed in a muffle furnace, and heated at a heating rate of 2 °C·min -1 , and calcined at 550 °C for 4 hours. The obtained light green powder is Ce 2 Sn 2 O7 Pyrochlore

[0014] (7)Weigh pyrochlore and Beta zeolite in different mass ratios and place them in an agate mortar, and grind them thoroughly for 30 - 40 min. 2 Sn 2 O 7 Put the ground solid powder into a muffle furnace and calcine it at a heating rate of 2 °C / min under anoxic conditions at 550 °C for 4 h to obtain the catalyst of the present invention.

[0015] Further, the tin precursor in step (1) includes but is not limited to compounds such as sodium stannate trihydrate, anhydrous sodium stannate, stannic chloride pentahydrate, or stannous chloride dihydrate.

[0016] Preferably, the tin precursor is preferably sodium stannate trihydrate. The structural stability of the Ce

[0017] Sn 2 Sn 2 O 7 pyrochlore powder obtained using sodium stannate trihydrate as the tin source is significantly improved, and XRD characterization shows that its crystallinity is optimal. At the same time, it exhibits the highest NO 3 -SCR denitration reaction x conversion rate and N 2 selectivity.

[0018] To coprecipitate the cerium salt and tin salt in the mixed metal ion solution to form a cerium-tin composite oxide and inhibit the formation of a single oxide phase (SnO 2 or CeO 2 ), it is necessary to precisely control the pH and the precipitation process.

[0019] Further, the precipitant in step (3) includes at least one or two selected from urea, ammonia water, sodium hydroxide, potassium hydroxide, ammonium bicarbonate, and ensure that no other side precipitates are produced.

[0020] Preferably, under other unchanged conditions, the cerium-tin composite oxide prepared with ammonia water with a mass percentage of 28 - 30% as the precipitant has a more uniform particle size distribution (11.6 ± 0.5 nm) and better denitration performance.

[0021] The pH in step (3) needs to be precisely controlled within the range of 10 - 14. Under strong alkaline conditions, it will promote the complex precipitation of Ce 3+ (or Ce 4+ ) and Sn 4+ ions to form Ce(OH) 3 / Sn(OH) 4Intermediates such as further form a stable cerium-tin composite oxide. Preferably, Ce synthesized under the condition of pH = 12 2 Sn 2 O 7 The composite oxide has better NH 3 -SCR catalytic denitrification performance and better stability compared with samples prepared at other pH values.

[0022] Furthermore, in the hydrothermal aging described in step (4), the hydrothermal temperature and hydrothermal time are reasonably optimized to promote the formation of Ce-Sn-O crystal nuclei and inhibit the formation of SnO 2 phase, and adjust the specific surface area of the Ce 2 Sn 2 O 7 composite oxide. Preferably, hydrothermal treatment is carried out at 180 °C for 18 - 24 hours, and the obtained sample has better NH 3 -SCR denitrification performance.

[0023] Furthermore, in step (7), pyrochlore and Beta zeolite are mixed with each other in different mass ratios of Ce 2 Sn 2 O 7 The mass ratio is 4:1 - 1:4. The test results show that the catalyst with Ce 2 Sn 2 O 7 :Beta = 2:3 is the optimal ratio, and its NO x conversion rate reaches 97% at 250 °C, showing excellent reaction activity and N 2 selectivity, and also has excellent water and sulfur resistance stability.

[0024] The application of the catalytic denitrification cerium-tin pyrochlore-based composite catalyst described in the present invention in denitrifying motor vehicle exhaust or industrial waste gas includes the following steps.

[0025] (1) Weigh 50 mg of the catalyst and place it in a quartz fixed-bed reactor with an inner diameter of 6 mm. Quartz wool is filled at both the upper and lower ends as a support layer and gently compacted to form a uniform catalyst bed layer with a height of about 5 mm.

[0026] (2) Introduce a mixed atmosphere of 500 ppm NO + 500 ppm NH -1 + 5% O 3 and N 2 balanced at a flow rate of 50 mL·min 2 . The total mass hourly space velocity (WHSV) is 6×10 4 mL g -1 ·h -1 .

[0027] (3) Starting from room temperature, it is heated to 50 °C, and then the temperature is programmed to increase with a temperature gradient of 50 °C (heating rate 5 °C·min -1 ). After reaching the target temperature of 100 - 500 °C, it is stabilized for 20 min to obtain stable kinetic data.

[0028] (4) The motor vehicle exhaust gas or industrial waste gas after the reaction is introduced into the FGA10 type flue gas analyzer, which can efficiently and accurately detect the concentration changes of NO, NO 2 , N 2 O and NH 3 in the exhaust gas or industrial waste gas.

[0029] Compared with the prior art, the catalyst prepared by the present invention has the following advantages.

[0030] (1) The catalyst described in the present invention has low preparation cost, simple preparation method, non-toxic and harmless process, minimal requirement for expensive instruments, and no risk of secondary environmental pollution.

[0031] (2) The cerium tin pyrochlore-based composite catalyst described in the present invention has high denitrification efficiency, a relatively wide temperature window, and excellent water and sulfur resistance stability, and can be applied to the denitrification of motor vehicle exhaust gas or industrial waste gas.

[0032] (3) By introducing Beta zeolite into the cerium tin pyrochlore-based composite catalyst described in the present invention, the number of acid centers of the catalyst is increased. And the oxygen vacancies of Ce 2 Sn 2 O 7 and the Brønsted acid sites of Beta zeolite act synergistically to promote the oxidation of NO molecules and the adsorption and activation of NH 3 molecules respectively, thereby achieving high-efficiency denitrification in a wide temperature range. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 is the X-ray diffraction (XRD) comparison pattern of the Ce 2 Sn 2 O 7 and composite catalysts prepared in Examples 1 - 6.

[0034] Figure 2 is the NH 2 Sn 2 O 7 and composite catalysts prepared in Examples 1 - 6 for NH 3 -SCR reaction performance test.

[0035] Figure 3 is the Ce 2 Sn 2 O 7NH of the composite catalyst 3 -SCR reaction anti-water and anti-sulfur stability test.

[0036] Figure 4 For the Ce prepared in Example 1 and Examples 4 - 6 2 Sn 2 O 7 and the NH of the composite catalyst 3 Temperature-programmed desorption (NH 3 -TPD) experiment. Specific implementation mode

[0037] To more clearly illustrate the content of the present invention, the following examples are listed, but they have no limitation on the scope of the present invention. The reagents or instruments used can all be obtained through commercial purchase. Example 1

[0038] CeSnO pyrochlore was prepared by the hydrothermal method. 2 Sn 2 O 7 pyrochlore.

[0039] Accurately weigh 4.3400 g of Ce(NO 3 ) 3 •6H 2 O (AR) and 2.6770 g of Na 2 SnO 3 •3H 2 O (AR), and ensure that the Ce:Sn molar ratio is controlled at 1:1. Dissolve the weighed precursor salts in 20 mL of deionized water respectively, and ultrasonically assist in dissolving for 15 min. Then mix the clarified solutions of the two in a 150 mL beaker, heat and stir in a 50 °C constant temperature water bath for 20 min to ensure that Ce 3+ and Sn 4+ are fully mixed. Then transfer to normal temperature stirring, and slowly drop 28 - 30% of NH 3 •H 2 O solution until pH = 12. During this process, a grayish-brown precipitate is formed and the solution becomes a suspension. Then continue to stir for 3 - 4 hours, load it into a 150 mL polytetrafluoroethylene sealed autoclave, and place it in a forced air drying oven for aging at 180 °C for 24 hours. After aging, wash the precipitate alternately with deionized water and absolute ethanol until the sodium ion concentration in the last wash water is lower than 20 ppm. The obtained filter cake is dried in a 100 °C vacuum drying oven for 10 - 12 h, ground into fine powder, and then placed in a muffle furnace for calcination at 550 °C (heating rate is 2 °C·min -1 ) for 4 hours. The obtained sample is Ce 2 Sn 2 O 7 pyrochlore powder.

[0040] The specific surface area of the catalyst in Example 1 measured by nitrogen adsorption - desorption characterization was 82 m 2 / g. Example 2

[0041] Ce 2 Sn 2 O 7 pyrochlore was prepared by the hydrothermal method.

[0042] Accurately weigh 4.3400 g of Ce(NO 3 ) 3 •6H 2 O (AR) and 2.6770 g of Na 2 SnO 3 •3H 2 O (AR), and ensure that the Ce:Sn molar ratio is controlled at 1:1. Dissolve the weighed precursor salts separately in 20 mL of deionized water and ultrasonically assist the dissolution for 15 min. Then mix the clear solutions of the two in a 150 mL beaker, heat and stir in a 50 °C constant - temperature water bath for 20 min to ensure that Ce 3+ and Sn 4+ are fully mixed. Then transfer to room - temperature stirring, slowly add NaOH solution until pH = 14, and a gray - brown precipitate is formed during this process, and the solution becomes a suspension. Then continue stirring for 3 - 4 hours, load it into a 150 mL polytetrafluoroethylene - sealed autoclave, and place it in a forced - air drying oven for aging at 200 °C for 30 hours. After aging, wash the precipitate alternately with deionized water and absolute ethanol until the sodium ion concentration in the last wash water is lower than 20 ppm. The obtained filter cake is dried in a vacuum drying oven at 110 °C for 10 - 12 h, ground into fine powder, and then placed in a muffle furnace for calcination at 550 °C (heating rate is 2 °C·min -1 ) for 4 hours, and the obtained sample is Ce 2 Sn 2 O 7 pyrochlore powder.

[0043] The specific surface area of the catalyst in Example 2 measured by nitrogen adsorption - desorption characterization was 69 m 2 / g. Example 3

[0044] Ce 2 Sn 2 O 7 pyrochlore was prepared by the hydrothermal method.

[0045] Accurately weigh 4.3400 g of Ce(NO 3 ) 3 •6H 2O (AR) and 2.6770 g Na 2 SnO 3 •3H 2 O (AR), and ensure that the Ce:Sn molar ratio is controlled at 1:1. Weigh the precursor salts and dissolve them in 20 mL of deionized water respectively, and ultrasonically assist the dissolution for 15 min. Then mix the clarified solutions of the two in a 150 mL beaker, heat and stir in a 50 °C constant temperature water bath for 20 min to ensure that Ce 3+ and Sn 4+ are fully mixed. Then transfer it to room temperature stirring, and slowly add 28 - 30% NH 3 •H 2 O solution until pH = 10. A grayish-brown precipitate is formed during this process, and the solution becomes a suspension. Then continue to stir for 3 - 4 hours, put it into a 150 mL polytetrafluoroethylene-sealed autoclave, and place it in a forced-air drying oven for aging at 160 °C for 18 hours. After aging, wash the precipitate alternately with deionized water and absolute ethanol until the sodium ion concentration in the last wash water is lower than 20 ppm. The obtained filter cake is dried in a 90 °C vacuum drying oven for 10 - 12 h, ground into fine powder, and then put into a muffle furnace and calcined at 550 °C (heating rate is 2 °C·min -1 ) for 4 hours. The obtained sample is the Ce 2 Sn 2 O 7 pyrochlore powder.

[0046] The specific surface area of the catalyst in Example 3 measured by nitrogen adsorption-desorption characterization is 74 m 2 / g. Example 4

[0047] Weigh 1.2 g of the sample obtained in Example 1 and 0.8 g of Beta zeolite (commercial, Si / Al = 21) in an agate mortar and grind them thoroughly for 30 min. Then, transfer the powder to a crucible, put it into a muffle furnace and heat it to 550 °C at a heating rate of 2 °C·min -1 , and keep it calcined for 4 h. The obtained catalyst is named Ce 2 Sn 2 O 7 :Beta = 3:2 composite catalyst.

[0048] The specific surface area of the catalyst in Example 4 measured by nitrogen adsorption-desorption characterization is 219 m 2 / g. Example 5

[0049] Weigh 0.8 g of the sample obtained in Example 1 and 1.2 g of Beta zeolite (commercial, Si / Al = 21) in an agate mortar and grind them thoroughly for 40 min. Then, transfer the powder to a crucible and place it in a muffle furnace, heating it at a rate of 2 °C·min -1 until the temperature reaches 550 °C, and keep it calcined for 4 h. The obtained catalyst is named Ce 2 Sn 2 O 7 :Beta = 2:3 composite catalyst.

[0050] The specific surface area of the catalyst in Example 5 measured by nitrogen adsorption-desorption characterization is 311 m 2 / g. Example 6

[0051] Weigh 0.4 g of the sample obtained in Example 1 and 1.6 g of Beta zeolite (commercial, Si / Al = 21) in an agate mortar and grind them thoroughly for 35 min. Then, transfer the powder to a crucible and place it in a muffle furnace, heating it at a rate of 2 °C·min -1 until the temperature reaches 550 °C, and keep it calcined for 4 h. The obtained catalyst is named Ce 2 Sn 2 O 7 :Beta = 1:4 composite catalyst.

[0052] The specific surface area of the catalyst in Example 6 measured by nitrogen adsorption-desorption characterization is 337 m 2 / g.

[0053] Figure 1 In the XRD patterns of Examples 1 to 3, the characteristic diffraction peaks at 2 θ = 29.26°, 33.78°, 48.96° and 58.02° can be attributed to the (222), (400), (440) and (622) crystal planes of the cubic pyrochlore structure (PDF#48-0640) with the space group of Fd -3m. In Examples 4 to 6, in addition to observing the above pyrochlore characteristic peaks, there is a characteristic diffraction peak unique to Beta zeolite at 2 θ = 22.26°, and the peak intensity increases with the increase in the amount of Beta zeolite.

[0054] The catalyst dosage for the activity test is 0.05 g, and the reaction atmosphere is 500 ppm NO + 500 ppm NH 3 + 5%O 2 + N 2 balanced mixed atmosphere, and the total flow rate is 50 mL·min -1, equivalent to a weight hourly space velocity (WHSV) of 60000 mL·g -1 ·h -1 , water vapor is introduced by bubbling, and the bubbling amount is 5 - 10%, and the SO 2 concentration is 200 ppm.

[0055] Figure 2 shows the performance of the catalysts of six examples for the selective catalytic reduction of nitrogen oxides by ammonia. Table 1 lists the denitrification conversion rates of the six catalysts of Examples 1 - 6 at different temperatures.

[0056] Table 1 Activity of NH 3 -SCR of catalysts of different examples at different temperatures

[0057] It can be seen from the results that the introduction of Beta zeolite significantly improves the performance of the catalyst, and in Example 5, the Ce 2 Sn 2 O 7 :Beta = 2:3 catalyst has a NO x conversion rate higher than 90% and an N 2 selectivity higher than 95% in the temperature range of 250 - 450 °C.

[0058] Figure 3 The water and sulfur resistance stability of the catalysts of Example 4 and Example 5 was tested at 300 °C. It can be seen that although the introduction of H 2 O or SO 2 will reduce the NH 3 -SCR performance of the catalyst, it will not damage the structural stability of the catalyst, indicating that water vapor or SO 2 will form a competitive adsorption relationship with the reactant molecules, reducing the adsorption and activation ability of the catalyst for NH 3 or NO x , and thus reducing the reaction performance. In addition, the water and sulfur resistance performance of the catalyst of Example 5 is better than that of Example 4.

[0059] Figure 4 shows the NH 3 temperature-programmed desorption spectra of the catalysts of Example 1 and Examples 4 - 6. Compared with Example 1, the introduction of Beta zeolite on the catalysts of Examples 4 - 6 significantly increases the number of acid centers of the catalyst, which is beneficial to promoting the adsorption and activation of the catalyst for NH 3 molecules.

Claims

1. A method for preparing a cerium-tin pyrochlore-based composite catalyst for catalytic denitration, characterized in that The following steps are involved: (1) Weighing cerium and tin precursor salts in equal molar ratios, dissolving them in deionized water, and dissolving them with ultrasound assistance until the solution is clear; (2) slowly adding the clarified solution containing the cerium precursor into the tin solution, heating and stirring the mixture in a constant temperature water bath at 40 to 50 °C for 20 to 30 min to form a mixed metal ion solution; (3) While stirring at a low speed, slowly add the precipitant dropwise to adjust the pH of the mixed metal solution to 10-14, and continue to maintain magnetic stirring at 300-400 rpm for 3-4 h to obtain a suspension; (4) adding the suspension in step (3) into a polytetrafluoroethylene sealed autoclave, and hydrothermally aging at 160-200° C. in a forced air drying oven for 12-36 hours; (5) After the hydrothermal aging is completed, the product is separated by vacuum filtration and washed alternately with deionized water and anhydrous ethanol until the sodium ion concentration in the filtrate is less than 20 ppm; (6) The obtained filter cake was dried in a vacuum oven at 90-110 °C overnight, ground into fine powder, and placed in a muffle furnace at a heating rate of 2 °C min. -1 , calcined at 550 °C for 4 hours, the obtained Ce2Sn2O7 pyrochlore; (7) Weigh Ce2Sn2O7 pyrochlore and Beta molecular sieve in a mass ratio of 4:1 to 1:4, place in an agate mortar, and grind thoroughly for 30 to 40 min; (8) The ground powder was placed in a muffle furnace and calcined at 550 °C in an oxygen-deficient condition at a heating rate of 2 °C / min for 4 h to obtain the catalyst of the present invention.

2. The method for preparing a cerium-tin pyrochlore-based composite catalyst for catalytic denitration according to claim 1, characterized in that The tin precursor in step (1) includes sodium stannate trihydrate, anhydrous sodium stannate, tin tetrachloride pentahydrate or stannous chloride dihydrate.

3. The method for preparing a cerium-tin pyrochlore-based composite catalyst for catalytic denitration according to claim 2, wherein the tin precursor is sodium stannate trihydrate.

4. The method for preparing a cerium-tin pyrochlore-based composite catalyst for catalytic denitration according to claim 1, characterized in that The precipitant in step (3) includes one or two of urea, ammonia water, sodium hydroxide, potassium hydroxide, and ammonium bicarbonate.

5. The method for preparing a catalytic denitrification cerium-tin pyrochlore-based composite catalyst according to claim 4, characterized in that the precipitant is ammonia water with a mass percentage of 28-30%.

6. The method for preparing a cerium-tin pyrochlore-based composite catalyst for catalytic denitration according to claim 1, characterized in that The pH in step (3) is 12.

7. The method for preparing a cerium-tin pyrochlore-based composite catalyst for catalytic denitration according to claim 1, characterized in that In the hydrothermal aging described in step (4), the sample is hydrothermally aged at 180° C. for 18 to 24 hours.

8. The method for preparing a cerium-tin pyrochlore-based composite catalyst for catalytic denitration according to claim 1, characterized in that In step (7), the mass ratio of Ce2Sn2O7:Beta is 2:

3.

9. Use of the cerium-tin pyrochlore-based composite catalyst for catalytic denitration according to any one of claims 1 to 8 in denitration of motor vehicle exhaust or industrial waste gas.

10. The use of the cerium-tin pyrochlore-based composite catalyst for catalytic denitration according to claim 9, characterized in that The following steps are involved: (1) Weigh 50 mg of the catalyst and place it in a quartz fixed bed reactor with an inner diameter of 6 mm. Fill the upper and lower ends with quartz wool as a support layer and gently compact it to form a uniform catalyst bed with a height of about 5 mm. (2) The flow rate is 50 mL min -1 The mixed atmosphere is 500 ppm NO + 500 ppm NH3 + 5%O2 and N2 balance, with a total mass space velocity WHSV of 6×10 4 mL g -1 ·h -1 ; (3) Start from room temperature and heat up to 50 °C, then increase the temperature by 50 °C at a rate of 5 °C min -1 , and stabilize for 20 min after reaching the target temperature of 100~500 ℃.

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