Preparation method and application of silicon-doped iron oxide-based ammoxidation catalyst with high n2 selectivity

Abstract

The application discloses a preparation method and application of a silicon-doped iron oxide-based ammoxidation catalyst with high N2 selectivity, and belongs to the field of ammoxidation catalysts. The catalyst is prepared by an ethanol solvent-assisted ammonia water-coprecipitation method, which can significantly improve the N2 selectivity while maintaining the ammoxidation activity of the iron oxide catalyst. The silicon doping of the iron oxide can be realized through a simple catalytic synthesis method, and an amorphous iron-silicon composite oxide catalyst is obtained. The novel iron-silicon composite oxide catalyst has the characteristics of large specific surface area and many acid sites, and exhibits good activity and superior N2 selectivity in the ammoxidation reaction. The catalyst can solve the ammonia escape problem of an ammonia selective catalytic reduction (NH3-SCR) module in a diesel vehicle exhaust aftertreatment system, realize efficient oxidation of ammonia, and reduce the generation of nitrogen oxides, and has a wide industrial application prospect.

Description

Technical Field

[0001] This invention belongs to the field of ammonia oxidation catalysts in diesel vehicle exhaust aftertreatment systems, specifically relating to a method for preparing a silicon-doped iron oxide-based ammonia oxidation catalyst with high N2 selectivity, and the application of the prepared silicon-doped iron oxide catalyst in the selective catalytic oxidation of ammonia. Background Technology

[0002] Ammonia (NH3) is a commonly used chemical raw material, frequently used to produce nitrogen fertilizers, nitric acid, and biomass. It is also a common reducing agent in selective catalytic reduction (SCR) reactions. Ammonia (NH3) emissions are enormous, reaching up to 220,000 tons annually. Ammonia escape is a widespread problem in production and daily life, especially in NH3-SCR reaction systems. Once released into the atmosphere, ammonia causes serious environmental damage (e.g., forming secondary pollutants such as aerosols, and causing eutrophication in water). When atmospheric NH3 levels exceed 300 ppm, human health is also threatened. Therefore, controlling ammonia escape is of great importance.

[0003] Selective catalytic oxidation (NH3-SCO) can convert NH3 into environmentally friendly N2 and H2O, providing a highly efficient and green method for solving ammonia slip. While NH3-SCR systems in motor vehicles (especially diesel vehicles) can efficiently catalyze the removal of NO from exhaust gases... x However, there is a problem of ammonia escape.

[0004] Iron oxide (Fe₂O₃)-based catalysts are widely used in catalyzing various redox reactions due to their excellent redox performance, environmental friendliness, and low cost. Fe₂O₃ catalysts also exhibit superior catalytic activity in ammonia oxidation, but poor N₂ selectivity limits their practical application. Doping Fe₂O₃ may be an effective method to improve its catalytic performance. Silica (SiO₂) has a large specific surface area, good stability, and can provide a large amount of... Due to its characteristics such as acid sites participating in the reaction, Fe₂O₃ is often used as a catalyst support or for modifying catalysts. Meanwhile, it has been reported that enhanced surface acidity is beneficial for improving the N₂ selectivity of Fe-based catalysts in the ammonia oxidation reaction. Therefore, increasing the specific surface area of ​​Fe₂O₃ and enhancing its surface acidity through Si doping is an effective method to improve its N₂ selectivity while maintaining the high NH₃ oxidation activity of Fe₂O₃ catalysts. Summary of the Invention

[0005] In view of the above problems of the prior art, the present application aims to provide a preparation method of a silicon-doped iron oxide-based ammonia oxidation catalyst with high N2 selectivity, which can significantly improve the N2 selectivity of the iron-based catalyst while maintaining the high NH3 oxidation activity of the Fe2O3 catalyst, reduce the generation of by-products such as NO and N2O in the NH3 oxidation reaction, and increase the environmental and economic benefits. x

[0006] To solve the above problems, the technical scheme adopted by the present application is as follows:

[0007] The present application provides a preparation method of a silicon-doped iron oxide-based ammonia oxidation catalyst with high N2 selectivity, which comprises the following steps: dissolving iron nitrate nonahydrate and tetraethyl orthosilicate in ethanol, uniformly mixing them, and then adding excess ammonia water drop by drop until the precipitation is complete; and then aging, washing, drying, and calcining the precipitate to obtain the silicon-doped iron oxide-based ammonia oxidation catalyst with high N2 selectivity.

[0008] Further, the molar ratio of the iron nitrate nonahydrate to the tetraethyl orthosilicate is 8:1-1:1.

[0009] Further, the concentration of the iron nitrate nonahydrate ethanol solution is 0.003 mol / L-0.03 mol / L.

[0010] Further, the addition of the ammonia water is stopped when the pH of the solution reaches 9, and the concentration of the ammonia water is 13.2 mol / L.

[0011] Further, the aging time of the precipitate is 24 h.

[0012] Further, the drying temperature is 80-150°C, and the drying time is 12-24 h.

[0013] Further, the calcination temperature is 500°C, and the calcination time is 4 h.

[0014] Further, the steps of the above preparation method are as follows:

[0015] 1) Dissolve iron nitrate nonahydrate in anhydrous ethanol to obtain an iron nitrate ethanol solution, and then add tetraethyl orthosilicate drop by drop through a dropper at a molar ratio of Fe:Si=8:1-1:1 under stirring at room temperature to obtain a mixed solution;

[0016] 2) Add 13.2 mol / L ammonia water drop by drop to the mixed solution obtained in step 1) while stirring until the pH of the solution reaches about 9, stop stirring, and then let it stand for 24 h to obtain a precipitate;

[0017] ​3) The precipitate obtained in step 2) is separated by centrifugation, washed with deionized water for 3 times, dried at 100℃ for 12h, ground and calcined in a muffle furnace at 500℃ for 4h with a heating rate of 5℃ / min to obtain the Si-doped Fe2O3 catalytic material.

[0018] Further, the high N2-selectivity silicon-doped iron oxide-based ammoxidation catalyst prepared by the preparation method of the catalyst.

[0019] Further, the application of the high N2-selectivity silicon-doped iron oxide-based ammoxidation catalyst in the catalytic oxidation reaction of NH3.

[0020] Compared with the prior art, the application has the following advantages:

[0021] 1) The application increases the specific surface area of Fe2O3 and enhances the surface acidity of Fe2O3 by Si doping, thereby significantly improving the N2 selectivity while maintaining the ammoxidation activity.

[0022] 2) The raw materials required by the preparation method of the application are cheap and easy to obtain, and the resources are abundant.

[0023] 3) The application uses the principle that both iron nitrate nonahydrate (Fe(NO3)3·9H2O) and tetraethyl orthosilicate (TEOS) can be dissolved in ethanol, and both the iron ions and TEOS can be quickly hydrolyzed and precipitated after adding concentrated ammonia water into the solution, to prepare the Fe2O3-SiO2 composite oxide catalyst by using the ammonia water co-precipitation method, so that the preparation method is simple and suitable for large-scale industrial production.

[0024] 4) The application develops an efficient NH3-SCO catalyst, which has superior catalytic activity and solves the problems of low energy consumption and high economic benefits caused by ammonia escape.

[0025] 5) The application improves the N2 selectivity while maintaining the ammoxidation activity by using the new type of catalytic material, thereby reducing the generation of by-products such as NO and N2O in the NH3 oxidation reaction, and is environmentally friendly. x and N2O in the NH3 oxidation reaction, and is environmentally friendly. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 is the XRD result graph of Fe2O3 and FexSiy;

[0027] Figure 2 is the H2-TPR graph of Fe2O3 and FexSiy;

[0028] Figure 3 is the NH3-TPD graph of Fe2O3 and FexSiy;

[0029] Figure 4 is the NH3-SCO reaction conversion rate graph of Fe2O3 and FexSiy;

[0030] Figure 5 NH3-SCO reaction N2 selectivity diagram for Fe2O3 and FexSiy; DETAILED DESCRIPTION

[0031] In order to make the above objectives, features and advantages of the present application more obvious and easy to understand, the specific embodiments of the present application will be described in detail below.

[0032] Principle of the present application: since both iron nitrate nonahydrate (Fe(NO3)3·9H2O) and tetraethyl orthosilicate (TEOS) can be dissolved in ethanol, and both iron ions and TEOS can be quickly hydrolyzed and precipitated after adding concentrated ammonia water dropwise into the solution, therefore, by using ammonia water co-precipitation method, Fe(NO3)3·9H2O and TEOS are dissolved and mixed uniformly in ethanol, and then excess ammonia water is added dropwise to make the precipitation complete, and then after aging, washing, drying and calcination, the Fe2O3-SiO2 composite oxide catalyst can be prepared.

[0033] Example 1

[0034] Preparation of Fe2O3 catalyst

[0035] 20.2 g of iron nitrate nonahydrate was weighed and dissolved in 200 mL of anhydrous ethanol, stirred at room temperature for 30 min to form a uniform solution system, and then 1.4 mL of tetraethyl orthosilicate (TEOS) was added dropwise through a dropper, and the solution was mixed uniformly after stirring for 30 min. While stirring, 13.2 mol / L ammonia water was added dropwise until the pH was about 9, stirring was stopped, and the system was aged for 24 h. The obtained precipitate was centrifuged and washed with water for 3 times, dried at 100℃ for 12 h, and then ground uniformly and placed in a muffle furnace for calcination at 500℃ for 4 h, with a heating rate of 5℃ / min. The obtained product was Fe8Si1 catalyst.

[0036] Example 2

[0037] Preparation of Fe8Si1 catalyst

[0038] 20.2 g of iron nitrate nonahydrate was weighed and dissolved in 200 mL of anhydrous ethanol, stirred at room temperature for 30 min to form a uniform solution system, and then 1.4 mL of tetraethyl orthosilicate (TEOS) was added dropwise through a dropper, and the solution was mixed uniformly after stirring for 30 min. While stirring, 13.2 mol / L ammonia water was added dropwise until the pH was about 9, stirring was stopped, and the system was aged for 24 h. The obtained precipitate was centrifuged and washed with water for 3 times, dried at 100℃ for 12 h, and then ground uniformly and placed in a muffle furnace for calcination at 500℃ for 4 h, with a heating rate of 5℃ / min. The obtained product was Fe8Si1 catalyst.

[0039] Example 3

[0040] Preparation of Fe4Si1 catalyst

[0041] Take 20.2 g of nine hydrated ferric nitrate and dissolve it in 200 mL of anhydrous ethanol, stir at room temperature for 30 min to form a uniform solution system, then add 2.8 mL of tetraethyl orthosilicate (TEOS) drop by drop through a dropper, stir for 30 min to mix the solution uniformly. Add 13.2 mol / L of ammonia water drop by drop while stirring until pH≈9, stop stirring, and let it stand for 24 h. Centrifuge and wash the obtained precipitate with water 3 times, dry at 100℃ for 12 h, grind uniformly, and then put it into a muffle furnace for calcination at 500℃ for 4 h, with a heating rate of 5℃ / min. The obtained product is the Fe4Si1 catalyst.

[0042] Example 4

[0043] Preparation of Fe2Si1 catalyst

[0044] Take 20.2 g of nine hydrated ferric nitrate and dissolve it in 200 mL of anhydrous ethanol, stir at room temperature for 30 min to form a uniform solution system, then add 5.6 mL of tetraethyl orthosilicate (TEOS) drop by drop through a dropper, stir for 30 min to mix the solution uniformly. Add 13.2 mol / L of ammonia water drop by drop while stirring until pH≈9, stop stirring, and let it stand for 24 h. Centrifuge and wash the obtained precipitate with water 3 times, dry at 100℃ for 12 h, grind uniformly, and then put it into a muffle furnace for calcination at 500℃ for 4 h, with a heating rate of 5℃ / min. The obtained product is the Fe2Si1 catalyst.

[0045] Example 5

[0046] Preparation of Fe1Si1 catalyst

[0047] Take 20.2 g of nine hydrated ferric nitrate and dissolve it in 200 mL of anhydrous ethanol, stir at room temperature for 30 min to form a uniform solution system, then add 11.2 mL of tetraethyl orthosilicate (TEOS) drop by drop through a dropper, stir for 30 min to mix the solution uniformly. Add 13.2 mol / L of ammonia water drop by drop while stirring until pH≈9, stop stirring, and let it stand for 24 h. Centrifuge and wash the obtained precipitate with water 3 times, dry at 100℃ for 12 h, grind uniformly, and then put it into a muffle furnace for calcination at 500℃ for 4 h, with a heating rate of 5℃ / min. The obtained product is the Fe1Si1 catalyst.

[0048] The Fe2O3-SiO2 composite oxide catalysts prepared in Examples 1-5 of the present application were respectively evaluated for structure and catalytic performance by means of N2 adsorption-desorption (N2-physisorption), X-ray diffraction (XRD), H2 temperature programmed reduction (H2-TPR), NH3 temperature programmed desorption (NH3-TPD), and NH3 selective catalytic oxidation (NH3-SCO) performance tests. The test results are shown in Table 1 and the accompanying figures. Figures 1-5The N2-physisorption and XRD results of Table 1 show that after Si doping, the specific surface area of Fe2O3 is greatly improved, and the crystallinity is obviously reduced, basically showing an amorphous state, indicating that Si doping can effectively improve the specific surface area of Fe2O3 and reduce the crystallinity of Fe2O3; the TPR results of Table 1 show that the low-temperature redox capacity of Fe2O3 is basically unchanged after Si doping; the NH3-TPD results of Table 1 show that Si doping can effectively increase the number of surface acid sites of Fe2O3, which is more conducive to the adsorption of NH3; after NH3-SCO reaction activity test, it is found that the N2 selectivity of FexSiy catalyst is greatly improved on the premise of maintaining the ammonia oxidation activity basically the same as Fe2O3. Figure 1 The N2-physisorption and XRD results of Table 1 show that after Si doping, the specific surface area of Fe2O3 is greatly improved, and the crystallinity is obviously reduced, basically showing an amorphous state, indicating that Si doping can effectively improve the specific surface area of Fe2O3 and reduce the crystallinity of Fe2O3; the TPR results of Table 1 show that the low-temperature redox capacity of Fe2O3 is basically unchanged after Si doping; the NH3-TPD results of Table 1 show that Si doping can effectively increase the number of surface acid sites of Fe2O3, which is more conducive to the adsorption of NH3; after NH3-SCO reaction activity test, it is found that the N2 selectivity of FexSiy catalyst is greatly improved on the premise of maintaining the ammonia oxidation activity basically the same as Fe2O3. Figure 2 The N2-physisorption and XRD results of Table 1 show that after Si doping, the specific surface area of Fe2O3 is greatly improved, and the crystallinity is obviously reduced, basically showing an amorphous state, indicating that Si doping can effectively improve the specific surface area of Fe2O3 and reduce the crystallinity of Fe2O3; the TPR results of Table 1 show that the low-temperature redox capacity of Fe2O3 is basically unchanged after Si doping; the NH3-TPD results of Table 1 show that Si doping can effectively increase the number of surface acid sites of Fe2O3, which is more conducive to the adsorption of NH3; after NH3-SCO reaction activity test, it is found that the N2 selectivity of FexSiy catalyst is greatly improved on the premise of maintaining the ammonia oxidation activity basically the same as Fe2O3. Figure 3 The N2-physisorption and XRD results of Table 1 show that after Si doping, the specific surface area of Fe2O3 is greatly improved, and the crystallinity is obviously reduced, basically showing an amorphous state, indicating that Si doping can effectively improve the specific surface area of Fe2O3 and reduce the crystallinity of Fe2O3; the TPR results of Table 1 show that the low-temperature redox capacity of Fe2O3 is basically unchanged after Si doping; the NH3-TPD results of Table 1 show that Si doping can effectively increase the number of surface acid sites of Fe2O3, which is more conducive to the adsorption of NH3; after NH3-SCO reaction activity test, it is found that the N2 selectivity of FexSiy catalyst is greatly improved on the premise of maintaining the ammonia oxidation activity basically the same as Fe2O3.

[0049] Table 1: Comparison of specific surface area results of Fe2O3 and FexSiy tested by N2-adsorption and desorption method

[0050] Samples S BET (m 2 / g)]]> Fe2O3 17.4 Fe8Si1 206.1 Fe4Si1 215.4 Fe2Si1 271.6 Fe1Si1 274.6

[0051] Example 6

[0052] Application of FexSiy catalyst in NH3 oxidation reaction

[0053] The prepared Fe2O3 and FexSiy catalysts were applied to NH3 oxidation reaction, and the FexSiy catalyst showed NH3 oxidation activity comparable to that of Fe2O3 catalyst, while the N2 selectivity was greatly improved, among which Fe2Si1 had the highest N2 selectivity, which could reach 93% at 325°C, and the results are shown in Table 2 and Figure 1. Figure 4 Figure 5 .

[0054] The specific reaction conditions are as follows: the catalytic reaction test was carried out in a fixed bed continuous flow quartz reactor, the catalyst particle size was 40-60 mesh, the amount used was 100 mg, the reaction gas composition was: 500 ppm NH3, 4% O2, Ar as balance gas. The mass space velocity tested was 60000 mL·g -1 ·h -1 -1·h in . The catalytic reaction was carried out at 150-450°C, and the catalytic performance data at each test temperature point was collected after the reaction reached equilibrium. The products were detected and analyzed by Thermofisher IS10 FTIR spectrometer, and the NH3 conversion was calculated by the following formula:

[0055] NH3 conversion(%)={([NH3] in -[NH3] out ) / [NH3] in}×100%

[0056] N2 selectivity (%) = {1- ([NO2] + 2[N2O] ) / ([NH3] - [NH3] )} x 100% out + 2[N2O] out out ) / ([NH3] in - [NH3] out )} x 100%

[0057] The preferred embodiments of the present application only are described above and not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.​

Claims

1. Use of a silicon-doped iron oxide-based ammoxidation catalyst having high N2 selectivity in the catalytic oxidation of NH3, characterized in that, The preparation method of the silicon-doped iron oxide base ammoxidation catalyst is as follows: dissolving iron nitrate nonahydrate and tetraethyl orthosilicate in ethanol, adding excess ammonia water drop by drop after uniform mixing, allowing the precipitation to be complete, then aging, washing, drying, and calcining to obtain the silicon-doped iron oxide base catalyst with high N2 selectivity for ammoxidation; the specific preparation steps are as follows: 1) dissolving iron nitrate nonahydrate in anhydrous ethanol to obtain an iron nitrate ethanol solution, adding tetraethyl orthosilicate drop by drop through a dropper at a molar ratio of Fe:Si=8:1-1:1 under stirring at room temperature to obtain a mixed solution; 2) adding 13.2 mol / L ammonia water drop by drop to the mixed solution obtained in step 1) under stirring until the solution pH=9, stopping stirring, and allowing to stand for 24 h to obtain a precipitate; 3) centrifuging the precipitate obtained in step 2), washing with deionized water for 3 times, drying at 100℃ for 12 h, grinding, and then placing in a muffle furnace at 500℃ for calcination for 4 h at a temperature rising rate of 5℃ / min to obtain a Si-doped Fe2O3 catalyst material.

2. Use according to claim 1, characterized in that, The concentration of the iron nitrate nonahydrate ethanol solution is 0.003 mol / L-0.03 mol / L.

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