A modified cobalt-based catalyst for low-temperature decomposition of nitrous oxide, and its preparation method and application

Abstract

The present invention relates to a modified cobalt-based catalyst for low-temperature decomposition of nitrous oxide and a preparation method and application thereof. Co3O4 nanoparticles are used as carriers, and Co3O4 is modified by cesium nitrate to obtain a modified cobalt-based catalyst whose active component is a Co metal site, wherein the mass proportion of the cesium element is about 2-4wt.%. Co3O4 nanoparticles are used as carrier materials, and a liquid phase impregnation method is used to prepare the modified cobalt-based catalyst. The present invention utilizes the electron donating effect of cesium to reduce the reaction activation energy, promote the regeneration of active sites, and thus improve the catalytic performance. Compared with the prior art, the present invention has the advantages of safe and simple process, excellent decomposition performance, and strong water vapor tolerance.

Description

Technical Field

[0001] The invention belongs to the technical field of environmental protection, and specifically relates to a modified cobalt-based catalyst for decomposing nitrous oxide at low temperature, and a preparation method and application thereof. Background Art

[0002] The concentration of N2O in the atmosphere is generally around 0.33 ppm, but its greenhouse effect potential is 298 times that of CO2 (CH4 is 25 times that of CO2), so N2O is an extremely important greenhouse gas. The concentration of N2O in the atmosphere is about 310 ppbv, and its half-life in the natural state is 114 years, which is difficult to decompose. N2O in the upper atmosphere will also generate NO, which reacts with O3 to destroy the ozone layer and form an ozone hole. In addition, N2O has certain hallucinogenicity and certain neurotoxicity. In the context of pollution reduction and carbon reduction, N2O has attracted the attention of more and more researchers due to its extremely strong greenhouse effect.

[0003] It is estimated that the global annual N2O emission is about 17 Tg N yr -1 , of which natural emissions account for 9.7 Tg N yr -1 , mainly from natural releases from soil and the ocean. Anthropogenic emissions are 6.3 Tg N yr -1 The largest source (60.3%) is agriculture, with 3.8 Tg N yr per year -1 , from agricultural activities such as nitrogen fertilizer application. Industrial sources are the second largest source, 1.0 Tg N yr -1 , accounting for 15.9%. my country’s N2O situation is similar to that of the world, with emissions of about 2150 Gg N2O yr -1 , of which agriculture accounts for 64% and energy production accounts for 17%. In terms of geographical distribution, it is concentrated in the eastern coastal areas, mainly because of the dense population and intensive industrial activities.

[0004] Industrial sources can be divided into stationary sources and mobile sources. Stationary sources mainly use NH3-SCR to remove NO from flue gas in coal-fired power plants. x Byproduct, about 168.8 Gg N2O yr per year -1 , accounting for 32.3%, with an emission concentration of about; followed by the manufacturing and construction industries, with an annual emission of about 110.0 Gg N2O yr -1 , accounting for 21.1%, with an emission concentration of; the second is the nitrification and denitrification process of the sewage treatment plant, about 107.6 Gg N2O yr per year -1, accounting for 20.6%, with N2O concentrations up to 600 ppm; in addition, when N2O is used as an anesthetic, the concentration can reach 400 ppm in rooms without ventilation systems. Mobile sources are mainly motor vehicle exhaust emissions, with 27.0 Gg N2O yr per year. -1 The main source of gasoline vehicle emissions is NO degradation by three-way catalysts (TWCs). x The concentration of NO is usually 1000 ppm, while that of diesel vehicles mainly comes from the selective catalytic reduction of NO by ammonia. x (NH3-SCR) side reaction. The United States has set requirements for the separate emission of N2O in the latest emission regulations, requiring that the emission of light vehicles should be below 10 mg / mile and that of heavy vehicles should be below 50 mg / mile. Therefore, the control and purification of N2O in mobile source emissions and industrial production processes is an important research topic. Catalytic decomposition is an efficient N2O control strategy. The development of N2O low-temperature decomposition catalysts is of great significance. It will provide theoretical and technical support for efficient control of N2O and promote the realization of my country's dual carbon goals.

[0005] The main N2O emission reduction technologies can be divided into high temperature decomposition method and direct catalytic method. The direct catalytic decomposition method refers to the direct catalytic decomposition of N2O into N2 and O2 that do not pollute the atmosphere under the action of a catalyst. Compared with the high temperature decomposition method, the direct catalytic decomposition method has become the most direct and effective N2O post-treatment method favored by researchers due to its many advantages such as simple process, economic and environmental protection, high N2O conversion rate, and no need for reducing agents. Since Co-based oxide catalysts generally have the advantages of higher N2O decomposition catalytic activity and resistance to impurity gases, they also have the disadvantage of being intolerant to impurity gases.

[0006] Patent application CN202211017382.X A rare earth element Sm modified Ni-based water-resistant catalyst and its preparation method and application discloses a Sm-modified Ni-based catalyst and its preparation method and use. The catalyst uses NiO as an active component and Sm2O3 as an auxiliary agent, and is prepared by coprecipitation. The catalyst is characterized in that Sm is highly dispersed on the surface of NiO nanoparticles in the form of Sm2O3, and the grain size of NiO is greatly reduced, and a certain number of edge dislocations and defects are generated on the catalyst surface, increasing the number of active sites. By adjusting the amount of additive Sm added, a Ni-based catalyst with excellent catalytic decomposition performance of N2O can be obtained, and it has satisfactory resistance to impurity gases (NO, O2 and H2O). Although this patented technology reduces the temperature window, its catalytic performance in the medium and low temperature section of 350 ℃ and below is not high, and the working space velocity is low, which is difficult to meet the actual application needs.

[0007] Patent application CN202211179840.X A transition metal oxide catalyst for low-temperature catalytic decomposition of N2O and its preparation method and application discloses a transition metal oxide catalyst for low-temperature catalytic decomposition of N2O and its preparation method and application, which utilizes the chemical reaction between acid and transition metal oxide to achieve the purpose of appropriately corroding the catalyst surface, destroying the surface lattice, and forming a large number of oxygen vacancies and lattice defects, thereby improving the performance of the catalyst in catalytic decomposition of N2O. Although the patented technology uses acid washing to increase the active sites of the catalyst, thereby improving the performance of decomposing N2O, the acid washing process is relatively complicated and has certain risks, and the decomposition performance in the medium and low temperature range is poor. Summary of the invention

[0008] The purpose of the present invention is to overcome the defects of the above-mentioned prior art and to provide a modified cobalt-based catalyst for decomposing nitrous oxide at low temperature and with high efficiency, as well as a preparation method and application thereof.

[0009] The purpose of the present invention can be achieved by the following technical scheme: a modified cobalt-based catalyst for decomposing nitrous oxide at low temperature and high efficiency, using Co3O4 nanoparticles as a carrier, modifying Co3O4 with cesium nitrate to obtain a modified cobalt-based catalyst whose active component is a Co metal site, wherein the mass proportion of cesium element is about 2-4 wt.%.

[0010] Furthermore, the carrier is cobalt oxide solid powder with a particle size of 60-80 mesh, which can well support the catalytic sites.

[0011] Furthermore, the cesium-containing nitrate is CsNO3, and the introduced nitrate ions will not have additional effects on the catalyst preparation. The present invention selects cesium nitrate with relatively mild properties and relatively safe preparation process, avoiding the potential preparation danger of directly using cesium oxide. Secondly, the cesium species introduced in the cesium nitrate used in the present invention has no effect on the chemical reaction. In addition, cesium nitrate will not produce surface corrosion to cobalt oxide.

[0012] Furthermore, the carrier Co3O4 nanoparticles are prepared by a hard template method, which specifically includes the following steps:

[0013] S1. Dissolve Co(NO3)2 and silica mesoporous molecular sieve in an appropriate amount of ethanol, stir vigorously for 3-5 h, slowly heat the obtained mixed liquid to 60 ℃ - 70 ℃, evaporate excess ethanol to obtain a paste mixture, and then place it at a constant temperature of 50-70 ℃ to dry to obtain a solid;

[0014] S2, raising the temperature of the solid obtained in step S1 to 150-200°C at a heating rate of 1-2°C / min, and continuously calcining in static air at 150-200°C for 4-6 hours to obtain an intermediate product;

[0015] S3, then repeat the above steps S1 and S2 for multiple times with the intermediate product, after the last Co(NO3)2 solution immersion and drying, increase the temperature to 150-200°C at a heating rate of 1-2°C / min, continue calcining for 4-6 hours, and then increase the temperature to 550-600°C at a heating rate of 1-2°C / min, continue calcining for 6-8 hours, to obtain the primary product;

[0016] S4. After washing away the silicon template on the surface of the initial product with 60-80 ℃ 2-3 M NaOH solution, wash with deionized water until the solution is neutral, and then place it at 50 ℃ - 70 ℃ constant temperature for drying to obtain Co3O4 nanoparticle powder.

[0017] The present invention also provides a method for preparing a modified cobalt-based catalyst for decomposing nitrous oxide at low temperature and high efficiency, wherein the modified cobalt-based catalyst is prepared by a liquid phase impregnation method using Co3O4 nanoparticle powder as a carrier material.

[0018] Furthermore, the specific steps of preparing the modified cobalt-based catalyst by liquid phase impregnation method are: adding Co3O4 nanoparticle powder to cesium nitrate aqueous solution, stirring vigorously, slowly heating the obtained mixed liquid to 60 ℃-70 ℃, evaporating excess water to obtain a paste mixture, and then placing it at 50-70 ℃ constant temperature for drying, and then heating the obtained solid to 550-600 ℃ at a heating rate of 1-2 ℃ / min, and continuously calcining in static air at 550-600 ℃ for 4 h-6 h to obtain the product. The present invention adopts the step drying method for drying because it prevents the temperature from rising too fast and can fully impregnate. The calcination heating rate of the present invention is 1-2 ℃ / min. The purpose of this is to prevent the collapse of the catalyst pores and maintain the specific surface area, thereby avoiding the decomposition of nitrates and causing a significant decrease in the specific surface area.

[0019] Furthermore, vigorous stirring refers to stirring on a magnetic stirrer at a speed of 400-500 rpm for 3-5 h.

[0020] Furthermore, constant temperature drying refers to drying in a constant temperature drying oven, and the drying time is 10-12 hours.

[0021] Furthermore, the heating rate of the reaction is 1 ℃ / min – 2 ℃ / min. A heating rate that is too fast will cause the structure of the catalyst to be destroyed and the specific surface area to decrease. A heating rate that is too slow will result in a too long reaction time. After comprehensive consideration, 1 ℃ / min – 2 ℃ / min is selected.

[0022] The present invention also provides an application of a modified cobalt-based catalyst for decomposing nitrous oxide at low temperature and high efficiency, wherein the modified cobalt-based catalyst is used as a catalyst in a N2O decomposition reaction.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] 1. The present invention develops a modified cobalt-based catalyst for decomposing nitrous oxide at low temperature and with high efficiency. The modified cobalt-based catalyst is prepared by nitrate impregnation. The conversion rate of N2O is significantly higher than that of commonly used industrial catalysts. Compared with the conversion rate of N2O by the catalyst described in patent application CN202211017382.X, the conversion rate of N2O by the catalyst of the present invention is higher, and the material has a more obvious advantage in water resistance.

[0025] 2. The modified cobalt-based catalyst developed by the present invention exhibits stronger resistance to water and oxygen than common industrial catalysts and other existing modified cobalt-based catalysts. This is because the present invention uses cobalt oxide with good performance as a carrier, and uses cesium nitrate to modify the cobalt oxide, which can increase the storage of active oxygen and improve the continuous oxygen mobility, construct alkaline sites to promote the redox characteristics of the catalyst, and greatly improve the catalyst's resistance to water and oxygen. In addition, the materials selected by the present invention are low-cost and environmentally friendly.

[0026] 3. The present invention adopts a liquid phase impregnation method to prepare a modified cobalt-based catalyst. By controlling the reaction process, cesium species can be introduced on the catalyst surface. The species can provide alkaline sites and regulate the redox properties, which can effectively promote the N2O decomposition reaction. DETAILED DESCRIPTION

[0027] The present invention is described in detail below with reference to specific embodiments. Example 1

[0028] (1) Preparation of cobalt-based carrier

[0029] S1. Dissolve 2.3286 g of cobalt nitrate hexahydrate (Co(NO3)2·6H2O) and 1 g of SBA-15 in 25 mL of ethanol solution (CH3CH2OH) and stir vigorously for 4 h. Slowly heat the resulting mixed liquid to 60 ℃ - 70 ℃, evaporate excess ethanol to obtain a paste-like mixture, and then dry it at 60 ℃ to obtain a solid;

[0030] S2, raising the temperature of the solid obtained in step S1 to 200°C at a heating rate of 1°C / min, and continuously calcining in static air at 200°C for 4 hours to obtain an intermediate product;

[0031] S3, then repeat the above steps S1 and S2 twice with the intermediate product, after the last Co(NO3)2 solution immersion and drying, increase the temperature to 200°C at a heating rate of 1°C / min, continue calcining for 4 hours, and then increase the temperature to 600°C at a heating rate of 1°C / min, continue calcining for 6 hours, to obtain the primary product;

[0032] S4. After washing away the silicon template on the surface of the initial product with 80°C 2 M NaOH solution, wash with deionized water until the solution is neutral, and then dry at a constant temperature of 50°C - 70°C to obtain Co3O4 nanoparticle powder.

[0033] (2) Preparation of modified cobalt-based catalysts:

[0034] S1. Dissolve 0.0586 g of CsNO3 solid in 10 ml of deionized water to prepare a cesium nitrate aqueous solution;

[0035] S2, gradually adding 1 g of Co3O4 nanoparticle powder to the cesium nitrate aqueous solution prepared in step S1, and vigorously stirring for 4 h;

[0036] S3, slowly heating the mixed liquid after completely mixing in step S2 to 60°C - 70°C, evaporating excess water to obtain a paste-like mixture;

[0037] S4, placing the paste mixture obtained in step S3 into a drying oven at a constant temperature of 60° C. to obtain a dry solid;

[0038] S5, placing the dried solid obtained in step S4 into a muffle furnace for calcination, raising the temperature to 600°C at a heating rate of 1°C / min, and calcining in static air at 600°C for 4 hours;

[0039] S6. Before the catalytic test, the modified cobalt catalyst was pressed into pellets. The mass proportion of cesium in the final modified cobalt catalyst was about 4 wt.%. Example 2

[0040] (1) Preparation of cobalt-based carrier

[0041] S1. Dissolve 2.3286 g of cobalt nitrate hexahydrate (Co(NO3)2·6H2O) and 1 g of SBA-15 in 25 mL of ethanol solution (CH3CH2OH) and stir vigorously for 6 h. Slowly heat the resulting mixed liquid to 60 ℃ - 70 ℃, evaporate excess ethanol to obtain a paste-like mixture, and then place it at 70 ℃ to dry to obtain a solid;

[0042] S2, raising the temperature of the solid obtained in step S1 to 150°C at a heating rate of 1°C / min, and continuously calcining in static air at 150°C for 6 hours to obtain an intermediate product;

[0043] S3, then repeat the above steps S1 and S2 twice with the intermediate product, after the last Co(NO3)2 solution immersion and drying, increase the temperature to 150°C at a heating rate of 1°C / min, continue calcining for 6 h, and then increase the temperature to 550°C at a heating rate of 1°C / min, continue calcining for 4 h, to obtain the primary product;

[0044] S4. After washing away the silicon template on the surface of the initial product with 80°C 2 M NaOH solution, wash with deionized water until the solution is neutral, and then dry at a constant temperature of 50°C - 70°C to obtain Co3O4 nanoparticle powder.

[0045] (2) Preparation of modified cobalt-based catalysts:

[0046] S1. Dissolve 0.0293 g of CsNO3 solid in 15 ml of deionized water to prepare a cesium nitrate aqueous solution;

[0047] S2, gradually adding 1 g of Co3O4 nanoparticle powder to the cesium nitrate aqueous solution prepared in step S1, and vigorously stirring for 5 h;

[0048] S3, slowly heating the mixed liquid after completely mixing in step S2 to 60°C - 70°C, evaporating excess water to obtain a paste-like mixture;

[0049] S4, placing the paste mixture obtained in step S3 into a drying oven at a constant temperature of 70° C. to obtain a dry solid;

[0050] S5, placing the dried solid obtained in step S4 into a muffle furnace for calcination, raising the temperature to 550°C at a heating rate of 1°C / min, and calcining in static air at 550°C for 6 hours;

[0051] S6. Before the catalytic test, the modified cobalt catalyst was pressed into pellets. The mass proportion of cesium in the final modified cobalt catalyst was about 2 wt.%.

[0052] Comparative Example 1

[0053] Preparation of Co3O4 catalyst:

[0054] S1. Dissolve 2.3286 g of cobalt nitrate hexahydrate (Co(NO3)2·6H2O) and 1 g of SBA-15 in 25 mL of ethanol solution (CH3CH2OH) and stir vigorously for more than 4 h.

[0055] S2, slowly heating the completely mixed liquid in step S1 to 60°C - 70°C to evaporate excess ethanol to obtain a paste-like mixture;

[0056] S3, placing the paste mixture obtained in step S2 into a drying oven at a constant temperature of 60° C. to obtain a dry solid;

[0057] S4, the dried solid obtained in step S3 is placed in a muffle furnace and calcined at 1 The heating rate was increased to 200 °C at 1.2 °C / min and continued in static air at 200 °C for 4 h to obtain the intermediate product.

[0058] S5. Repeat the above steps S1-S4 twice with the intermediate product. After the last Co(NO3)2 solution immersion and drying, increase the temperature to 200°C at a heating rate of 1°C / min, continue calcining for 4 h, and then increase the temperature to 600°C at a heating rate of 1°C / min, continue calcining for 6 h to obtain the primary product.

[0059] S6, washing the silicon template on the surface of the initial product with 80 ℃ 2 M NaOH solution, then washing with deionized water until the solution is neutral, and then drying at a constant temperature of 50 ℃ - 70 ℃ to obtain Co3O4 nanoparticle powder;

[0060] S7. Grind and sieve the catalyst obtained in step S6 for subsequent performance evaluation.

[0061] Comparative Example 2

[0062] The Sm-modified Ni-based catalyst was prepared by the method of Example 1 of patent application CN202211017382.X.

[0063] Comparative Example 3

[0064] The acid-treated transition metal oxide catalyst was prepared by the method of Example 1 of patent application CN202211179840.X.

[0065] The catalysts prepared in the above-mentioned embodiments and comparative examples were pressed into particles, sieved through a 60-80 mesh screen, and placed in a catalyst activity evaluation device. The activity evaluation was carried out in a fixed bed reactor.

[0066] The working conditions for performance testing are: N2O concentration 800 ppm - 1000 ppm. The working conditions are: simulated flue gas composition: N2O concentration 800 ppm - 1000 ppm, N2 as balance gas, total flow rate 50 mL / min, reaction space velocity 30000 mL·g -1 ·h -1 The test results are shown in Table 1 below:

[0067] Table 1 N2O removal efficiency of the catalysts obtained in the examples and comparative examples under different reaction conditions

[0068]

[0069] As can be seen from the table above, the N2O conversion rate of the modified cobalt-based catalyst after treatment is higher than 50% at 300°C and higher than 50% at 350°C. The conversion rate is as high as 91% at ℃, and the performance of the catalyst in the medium and low temperature sections is significantly better than that of pure Co3O4, patent application CN202211017382.X Example 1, and patent application CN202211179840.X Example 1. In addition, the present invention adopts a simple and safe impregnation method to prepare the catalyst, and the working conditions are more practical, which is conducive to industrial application. It can be seen from the data of Example 2 that when the mass proportion of cesium element is 2 wt.%, its performance is also significantly improved compared with pure Co3O4, and the conversion rate at 350 ℃ is increased from 21% to 55%, which proves that the scope of cesium promotion is wider.

[0070] In order to investigate the performance of the catalyst in water-containing flue gas, the above working conditions were changed to: N2O concentration 800 ppm-1000 ppm, 5% O2 or H2O, N2 as the balance gas, total flow rate of 50 mL / min, reaction space velocity of 30000 mL·g -1 · h -1 The test results are shown in Table 2 below:

[0071] Table 2 N2O removal efficiency of the catalysts obtained in the examples and comparative examples in water-containing flue gas

[0072]

[0073] It can be seen from the above table that the performance of the modified cobalt-based catalyst is still better than that of pure Co3O4 under water and oxygen conditions. This is because the present invention selects cobalt oxide with good performance as a carrier, and uses cesium nitrate to modify cobalt oxide, which can increase active oxygen storage, improve continuous oxygen mobility, and construct alkaline sites to promote the redox characteristics of the catalyst, thereby greatly improving the catalyst's resistance to water and oxygen.

[0074] The performance of the present invention in the medium and low temperature range below 350°C is significantly better than that of patent applications CN202211017382.X and CN202211179840.X. The conversion rate is as high as 91% at ℃, and it still has good catalytic performance under water and oxygen conditions, with a reaction space velocity of 30000 mL∙min -1 ∙g -1, which is more in line with actual working conditions. In addition, the present invention adopts a simple and safe impregnation method to prepare the catalyst, which is conducive to industrial application.

Claims

1. A modified cobalt-based catalyst for low-temperature decomposition of nitrous oxide, characterized in that: Using Co3O4 nanoparticles as carriers, Co3O4 is modified with cesium nitrate to obtain a modified cobalt-based catalyst with Co metal sites as active components, wherein the mass proportion of cesium element is 2-4wt.%; The carrier is prepared by a hard template method, which specifically includes the following steps: S1. Dissolve Co(NO3)2 and silica mesoporous molecular sieve in ethanol, stir vigorously for 3-5h, slowly heat the obtained mixed liquid to 60℃-70℃, evaporate excess ethanol to obtain a paste mixture, and then place it at 50℃-70℃ for constant temperature drying to obtain a solid product, wherein the silica mesoporous molecular sieve is SBA-15; S2, raising the temperature of the solid obtained in step S1 to 150-200° C. at a heating rate of 1-2° C. / min, and continuously calcining in static air at 150-200° C. for 4-6 hours to obtain an intermediate product; S3, then repeat the above steps S1 and S2 for multiple times with the intermediate product, after the last Co(NO3)2 solution immersion and drying, increase the temperature to 150-200°C at a heating rate of 1-2°C / min, continue calcining for 4-6h, and then increase the temperature to 550-600°C at a heating rate of 1-2°C / min, continue calcining for 6-8h, to obtain the primary product; S4. Wash the silicon template on the surface of the initial product with 60-80°C 2-3M NaOH solution, then wash with deionized water until the solution is neutral, and then dry at a constant temperature of 50°C-70°C to obtain Co3O4 nanoparticle powder.

2. The modified cobalt-based catalyst for low-temperature decomposition of nitrous oxide according to claim 1, characterized in that: The carrier is Co3O4 nanoparticle powder with a particle size of 60-80 meshes.

3. The modified cobalt-based catalyst for low-temperature decomposition of nitrous oxide according to claim 1, characterized in that: The cesium-containing nitrate is CsNO3.

4. A method for preparing a modified cobalt-based catalyst for low-temperature decomposition of nitrous oxide as claimed in claim 1, characterized in that: The modified cobalt-based catalyst was prepared by liquid phase impregnation method using Co3O4 nanoparticles as carrier material.

5. The method for preparing a modified cobalt-based catalyst for low-temperature decomposition of nitrous oxide according to claim 4, characterized in that: The specific steps of preparing the modified cobalt-based catalyst by liquid phase impregnation method are as follows: dissolving Co3O4 nanoparticles in cesium nitrate aqueous solution and stirring vigorously, slowly heating the obtained mixed liquid to 60°C-70°C, evaporating excess water to obtain a paste mixture, and then drying it at a constant temperature of 50-70°C. The obtained solid is then heated to 550-600°C and continuously calcined in static air at 550-600°C for 4-6 hours to obtain the product.

6. The method for preparing a modified cobalt-based catalyst for low-temperature decomposition of nitrous oxide according to claim 5, characterized in that: Vigorous stirring refers to stirring at a speed of 400-500 rpm on a magnetic stirrer for 3-5 hours.

7. The method for preparing a modified cobalt-based catalyst for low-temperature decomposition of nitrous oxide according to claim 5, characterized in that: The constant temperature drying refers to drying in a constant temperature drying oven, and the drying time is 10-12 hours.

8. The method for preparing a modified cobalt-based catalyst for low-temperature decomposition of nitrous oxide according to claim 5, characterized in that: The heating rate is 1°C / min-2°C / min.

9. Use of the modified cobalt-based catalyst for low-temperature decomposition of nitrous oxide as claimed in claim 1, characterized in that: The modified cobalt-based catalyst is used as a catalyst in a N2O decomposition reaction.

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