Silver-based scr catalyst, its preparation method and application

Abstract

This invention provides a silver-based SCR catalyst, its preparation method, and its application, belonging to the field of air pollution control technology. The preparation method of the silver-based SCR catalyst provided by this invention includes the following steps: impregnating alumina hydroxyaluminate in a soluble silver salt aqueous solution to obtain a catalyst precursor; subjecting a portion of the catalyst precursor to a first calcination at 450–550°C to obtain a first calcined product; subjecting the remaining catalyst precursor to a second calcination at 850–950°C to obtain a second calcined product; and mixing the first and second calcined products to obtain the silver-based SCR catalyst. The preparation method provided by this invention is green and low-cost. When the obtained silver-based SCR catalyst is used for flue gas denitrification, it can maintain a high NO conversion rate from low to high temperatures.

Description

Technical Field

[0001] This invention belongs to the field of air pollution control technology, specifically relating to a silver-based SCR catalyst, its preparation method, and its application. Background Technology

[0002] With the development and progress of industry and transportation, a large amount of pollutant gases are emitted into the atmosphere, including nitrogen oxides (NOx). x Nitrogen oxides are among the most common air pollutants, specifically including nitrogen dioxide (NO2), nitric oxide (NO), and nitrous oxide (N2O). Nitrogen oxides not only harm the human respiratory system, causing alveolar shrinkage and pulmonary fluid leakage, but they can also further form nitrites that enter the bloodstream, leading to oxygen supply difficulties and damage to the nervous system. Furthermore, nitrogen oxides are a major contributor to photochemical pollution, smog, and acid rain. The nitrates formed from nitrogen oxides dissolve in water, causing significant harm to water quality and aquatic life.

[0003] Among existing flue gas denitrification technologies, those for NO x Selective catalytic reduction (SCR) is considered the most effective denitrification method currently available. Depending on the reducing agent, SCR technology is mainly divided into ammonia selective catalytic reduction (NH3-SCR) and hydrocarbon selective catalytic reduction (HC-SCR). Using hydrocarbons such as propylene (C3H6) instead of NH3 as the reducing agent in SCR can avoid the corrosion and leakage problems associated with ammonia during storage, transportation, and use in NH3-SCR technology. It can also reduce infrastructure investment and simplify vehicle exhaust purification systems.

[0004] The reported C3H6-SCR catalysts are mainly classified into three categories: noble metal catalysts, molecular sieve catalysts, and metal oxide catalysts. Among them, the metal oxide catalysts are composed of CuO and Fe. x O y Mn x O y While it is inexpensive, its reactivity needs improvement and it is difficult to meet the growing demand for denitrification. Summary of the Invention

[0005] The purpose of this invention is to provide a silver-based SCR catalyst, its preparation method, and its application. The silver-based SCR catalyst prepared by the method provided by this invention can maintain a high NO conversion rate from low temperature to high temperature.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] A method for preparing a silver-based SCR catalyst includes the following steps:

[0008] Alumina hydroxide was impregnated in an aqueous solution of soluble silver salt to obtain a catalyst precursor;

[0009] A portion of the catalyst precursor was subjected to a first calcination at 450–550 °C to obtain a first calcined product.

[0010] The remaining catalyst precursor was subjected to a second calcination at 850–950°C to obtain the second calcined product.

[0011] The first calcined product and the second calcined product were mixed to obtain a silver-based SCR catalyst.

[0012] Preferably, the soluble silver salt aqueous solution comprises soluble silver salt and water; the mass ratio of the hydroxyalumina to the soluble silver salt is 4-8:0.063-0.126.

[0013] Preferably, the impregnation temperature is 20-25°C and the time is 2-3 hours.

[0014] Preferably, the holding time for the first roasting is 2.5 to 3.5 hours; the temperature for the second roasting is 850 to 950°C, and the holding time is 2.5 to 3.5 hours.

[0015] Preferably, the mass ratio of the first roasted product to the second roasted product is 1:1.

[0016] This invention provides a silver-based SCR catalyst prepared by the preparation method described in the above technical solution.

[0017] Preferably, the mass fraction of silver in the silver-based SCR catalyst is 1-5%.

[0018] This invention provides the application of the silver-based SCR catalyst described in the above technical solution in flue gas denitrification treatment.

[0019] Preferably, the temperature of the denitrification treatment is 200–500°C.

[0020] Preferably, the NO in the flue gas x The concentration is 700-900 ppm; the reducing agent used in the denitrification treatment is propylene.

[0021] This invention provides a method for preparing a silver-based SCR catalyst, comprising the following steps: impregnating alumina hydroxyaluminate in an aqueous solution of a soluble silver salt to obtain a catalyst precursor; subjecting a portion of the catalyst precursor to a first calcination at 450–550°C to obtain a first calcined product; subjecting the remaining catalyst precursor to a second calcination at 850–950°C to obtain a second calcined product; and mixing the first calcined product and the second calcined product to obtain a silver-based SCR catalyst. This invention first obtains a catalyst precursor by loading silver salts onto alumina hydroxyalumina using an impregnation method. Then, the obtained catalyst precursor is divided into two parts for calcination: one part is calcined at 450–550°C, and the other part is calcined at 850–950°C. During the first and second calcinations, as the temperature increases, the number of hydroxyl groups on the alumina surface gradually increases. Since hydroxyl groups serve as anchoring sites for the active components, their content directly affects the distribution of the active components on the support. This invention calcines the catalyst precursor under different temperature conditions to obtain calcined products with active sites mainly composed of oxidized silver and calcined products with active sites mainly composed of silver clusters. Mixing these two products results in a silver-based SCR catalyst that simultaneously possesses a large number of oxidized silver active sites and silver cluster active sites. When this silver-based SCR catalyst is used for flue gas denitrification, it can maintain a high NO conversion rate from low to high temperatures. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 The graph shows the NO conversion rate of sample A at different temperatures.

[0024] Figure 2 The graph shows the NO conversion rate of sample B at different temperatures.

[0025] Figure 3 The graph shows the NO conversion rate of sample C at different temperatures. Detailed Implementation

[0026] This invention provides a method for preparing a silver-based SCR catalyst, comprising the following steps:

[0027] Alumina hydroxide was impregnated in an aqueous solution of soluble silver salt to obtain a catalyst precursor;

[0028] A portion of the catalyst precursor was subjected to a first calcination at 450–550 °C to obtain a first calcined product.

[0029] The remaining catalyst precursor was subjected to a second calcination at 850–950°C to obtain the second calcined product.

[0030] The first calcined product and the second calcined product were mixed to obtain a silver-based SCR catalyst.

[0031] Unless otherwise specified, all raw materials used in this invention are commercially available products well known to those skilled in the art.

[0032] This invention involves impregnating alumina hydroxyacid in an aqueous solution of a soluble silver salt to obtain a catalyst precursor. In this invention, the aqueous solution of the soluble silver salt preferably comprises soluble silver salt and water; the preferred ratio of soluble silver salt to water in the aqueous solution is 0.063–0.126 g: 100–200 mL, more preferably 0.063 g: 130–170 mL. The preferred mass ratio of alumina hydroxyacid to soluble silver salt is 4–8: 0.063–0.126, more preferably 4: 0.063. The preferred particle size of the alumina hydroxyacid is 10–100 nm, more preferably 10 nm. The preferred soluble silver salt comprises silver nitrate and / or silver sulfate, more preferably silver nitrate. In this invention, the impregnation temperature is preferably 20–25°C, more preferably 22–23°C; the impregnation time is preferably 2–3 h, specifically 2 h, 2.5 h, or 3 h. The present invention preferably performs impregnation under stirring conditions. The present invention does not specifically limit the stirring method and conditions; any stirring method well known to those skilled in the art can be used. In the present invention, the impregnation process preferably includes sequentially evaporating and drying the resulting liquid material. The evaporation temperature is preferably 60–90°C, more preferably 80–90°C; the evaporation time is preferably 0.2–0.4 h, more preferably 0.3 h. In an embodiment of the present invention, the evaporation is specifically rotary evaporation. The drying temperature is preferably 90–110°C, more preferably 100°C; the drying time is preferably 10–14 h, more preferably 12 h. The present invention, by impregnating the alumina hydroxya with loaded silver salts, ensures that the silver salts are uniformly distributed on the surface of the alumina hydroxya, which is beneficial to improving the activity of the catalyst.

[0033] After obtaining the catalyst precursor, the present invention performs a first calcination on a portion of the catalyst precursor at 450–550°C to obtain a first calcined product; the remaining catalyst precursor is then subjected to a second calcination at 850–950°C to obtain a second calcined product. In the present invention, the temperature of the first calcination is 450–550°C, preferably 500°C; the holding time of the first calcination is preferably 2.5–3.5 h, more preferably 3 h; the rate of heating to the temperature of the first calcination is preferably 3–7°C / min, more preferably 5°C / min. The first calcination of the present invention is preferably carried out in an air atmosphere. In embodiments of the present invention, the first calcination is specifically carried out in a muffle furnace. In the present invention, the first calcination is preferably followed by a first cooling, grinding, tableting, and sieving in sequence. The first cooling of the present invention preferably includes cooling the first solid material obtained from the first calcination to room temperature. The present invention does not have a particular limitation on the method of the first cooling; any cooling method well known to those skilled in the art can be used. This invention does not specifically limit the grinding, tableting, and sieving methods after the first cooling; any grinding, tableting, and sieving methods well-known to those skilled in the art can be used. In embodiments of this invention, the particle size of the first calcined product is 40-60 mesh. In this invention, the temperature of the second calcination is 850-950℃, preferably 900℃; the holding time of the second calcination is preferably 2.5-3.5h, more preferably 3h; the rate of heating to the second calcination temperature is preferably 3-7℃ / min, more preferably 5℃ / min. The second calcination of this invention is preferably carried out in an air atmosphere. In embodiments of this invention, the second calcination is specifically carried out in a muffle furnace. In this invention, the second calcination preferably includes a second cooling, grinding, tableting, and sieving performed sequentially. The second cooling of this invention preferably includes cooling the second solid material obtained from the second calcination to room temperature. This invention does not specifically limit the method of the second cooling; any cooling method well-known to those skilled in the art can be used. This invention does not specifically limit the grinding, tableting, and sieving methods after the second cooling; any grinding, tableting, and sieving methods well-known to those skilled in the art can be used. In an embodiment of the present invention, the particle size of the second calcined product is 40-60 mesh.This invention involves calcining the obtained catalyst precursor in two parts: one part is calcined at 450–550°C, and the other part is calcined at 850–950°C. During the first and second calcinations, as the temperature increases, the number of hydroxyl groups on the alumina surface gradually increases. Since hydroxyl groups act as anchoring sites for the active components, their content directly affects the distribution of the active components on the support. This invention calcines the catalyst precursor under different temperature conditions to obtain calcined products with active sites mainly composed of oxidized silver and calcined products with active sites mainly composed of silver clusters. Mixing these two products results in a silver-based SCR catalyst that simultaneously possesses a large number of oxidized silver active sites and silver cluster active sites. When this silver-based SCR catalyst is used for flue gas denitrification, it can maintain a high NO conversion rate from low to high temperatures. Furthermore, the preparation method provided by this invention is simple, easy to implement, and has the characteristics of being green and low-cost.

[0034] After obtaining the first calcined product and the second calcined product, the present invention mixes the first calcined product and the second calcined product to obtain a silver-based SCR catalyst. In the present invention, the preferred mass ratio of the first calcined product and the second calcined product is 1:1. The present invention does not have any particular limitation on the mixing method of the first calcined product and the second calcined product; any mixing method well known to those skilled in the art can be used. The present invention, by mixing the first calcined product and the second calcined product to obtain a silver-based SCR catalyst, can achieve a high NO conversion rate from low temperature to high temperature.

[0035] This invention also provides a silver-based SCR catalyst prepared by the preparation method described above. In this invention, the mass fraction of silver in the silver-based SCR catalyst is preferably 1-5%, more preferably 1%. The silver-based SCR catalyst provided by this invention can achieve high denitrification effect at a low silver loading and maintain high activity from low temperature to high temperature.

[0036] This invention also provides the application of the silver-based SCR catalyst described in the above-mentioned technical solution in flue gas denitrification treatment. Preferably, the flue gas is denitrified in the presence of the silver-based SCR catalyst. In this invention, the temperature of the denitrification treatment is preferably 200–500°C, more preferably 275–475°C. The flue gas containing NO... xThe concentration of the nitrogen is preferably 700-900 ppm, more preferably 800 ppm; the reducing agent used in the denitrification treatment is preferably propylene; the concentration of the propylene is preferably 1500-1700 ppm, more preferably 1600 ppm. In an embodiment of the present invention, the flue gas includes NO, H2 and O2; the denitrification treatment conditions are: propylene concentration of 1600 ppm, NO concentration of 800 ppm, H2 volume fraction of 1%, O2 volume fraction of 10%; the amount of silver-based SCR catalyst is 30 mg, and the space velocity is 120000 h⁻¹. -1 The temperature is 200–500℃. In this invention, the denitrification treatment is preferably carried out in a fixed-bed reactor. The denitrification treatment of this invention preferably includes online collection and analysis of inlet and outlet gases. This invention does not have specific limitations on the instruments used for online collection and analysis of inlet and outlet gases; instruments well-known to those skilled in the art can be used. In an embodiment of this invention, a Thermo Fisher Scientific infrared spectrometer is specifically used. This invention uses a silver-based SCR catalyst for denitrification treatment, which can remove NO... x It is converted into water and nitrogen gas at a space velocity of 120,000 h⁻¹. -1 At that time, the NO conversion rate is higher than 90%, and the silver-based SCR catalyst exhibits good activity from low temperature to high temperature within the reaction temperature range of 200-500℃, meeting the flue gas treatment requirements of modern industrial SCR denitrification systems. Furthermore, this invention uses propylene instead of ammonia as the reducing agent in the denitrification system, which not only effectively solves the problem of light hydrocarbon pollution in various industrial flue gases but also effectively addresses the high costs associated with ammonia storage and transportation in NH3-SCR systems, as well as the problems of ammonia escape, ammonia wastewater generation, and catalyst ammonia poisoning caused by large-scale ammonia injection and equipment leaks in NH3-SCR systems. In addition, the silver-based SCR catalyst of this invention produces no polluting gases or waste liquids during the denitrification process, making it environmentally friendly.

[0037] The technical solutions of this invention will be clearly and completely described below with reference to the embodiments thereof. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0038] All other reagents used in the examples were commercially available.

[0039] Example 1

[0040] 4g of aluminum hydroxide and 0.063g of silver nitrate were mixed with 100mL of water and impregnated at 22℃ with stirring for 2h; the impregnated liquid material was rotary evaporated at 80℃ for 0.5h and then dried at 100℃ for 12h to obtain the catalyst precursor.

[0041] A portion of the catalyst precursor was placed in a muffle furnace, and the temperature of the muffle furnace was increased to 500°C at a heating rate of 5°C / min. The resulting catalyst precursor was calcined at 500°C for 3 hours in an air atmosphere. The resulting solid material was cooled to room temperature, ground, pressed into tablets, and sieved to obtain a first calcined product with a particle size of 40-60 mesh. The remaining catalyst precursor was placed in a muffle furnace, and the temperature of the muffle furnace was increased to 900°C at a heating rate of 5°C / min. The resulting catalyst precursor was calcined at 900°C for 3 hours in an air atmosphere. The resulting solid material was cooled to room temperature, ground, pressed into tablets, and sieved to obtain a second calcined product with a particle size of 40-60 mesh.

[0042] The first calcination product and the second calcination product obtained in the above step are mixed evenly in a mass ratio of 1:1 to obtain a silver-based SCR catalyst. The mass fraction of silver in the silver-based SCR catalyst is 1% (denoted as sample A).

[0043] Comparative Example 1

[0044] 2g of aluminum hydroxide, 0.0315g of silver nitrate, and 100mL of water were mixed and impregnated at 22℃ with stirring for 2h; the impregnated liquid material was rotary evaporated at 80℃ for 0.5h, and then dried at 100℃ for 12h to obtain the catalyst precursor.

[0045] The catalyst precursor was placed in a muffle furnace, and the temperature of the muffle furnace was increased to 500°C at a heating rate of 5°C / min. The resulting catalyst precursor was calcined at 500°C for 3 hours in an air atmosphere. The resulting solid material was cooled to room temperature, ground, pressed into tablets, and sieved to obtain a silver-based SCR catalyst with a particle size of 40-60 mesh, denoted as sample B.

[0046] Comparative Example 2

[0047] 2g of aluminum hydroxide, 0.0315g of silver nitrate, and 100mL of water were mixed and impregnated at 22℃ with stirring for 2h; the impregnated liquid material was rotary evaporated at 80℃ for 0.5h, and then dried at 100℃ for 12h to obtain the catalyst precursor.

[0048] The catalyst precursor was placed in a muffle furnace, and the temperature of the muffle furnace was increased to 900°C at a heating rate of 5°C / min. The resulting catalyst precursor was calcined at 900°C for 3 hours in an air atmosphere. The resulting solid material was cooled to room temperature, ground, pressed into tablets, and sieved to obtain a silver-based SCR catalyst with a particle size of 40-60 mesh, denoted as sample C.

[0049] Evaluation of the denitrification performance of silver-based SCR catalysts:

[0050] In a fixed-bed reactor, flue gas was subjected to denitrification treatment in the presence of samples A, B, and C, respectively. The flue gas consisted of NO, H2, and O2. The denitrification conditions were: propylene concentration of 1600 ppm, NO concentration of 800 ppm, H2 volume fraction of 1%, and O2 volume fraction of 10%; the amount of sample A, B, or C was 30 mg, and the space velocity was 120,000 h⁻¹. -1 The temperature ranges from 200 to 500℃; the inlet and outlet gases were collected and analyzed online using a Thermo Fisher Scientific infrared spectrometer, and the specific results are as follows: Figure 1 , Figure 2 , Figure 3 And as shown in Table 1; where, Figure 1 The graph shows the NO conversion rate of sample A at different temperatures. Figure 2 The graph shows the NO conversion rate of sample B at different temperatures. Figure 3 Figure 1 shows the NO conversion rate of sample C at different temperatures; Table 1 shows the NO conversion rates of samples A, B, and C at different temperatures.

[0051] Table 1. NO conversion rates of samples A, B, and C at different temperatures.

[0052] Temperature (°C) Sample A Sample B Sample C 200 30％ 3％ 14％ 225 51％ 5％ 33％ 250 75％ 18％ 71％ 275 89％ 22％ 91％ 300 93％ 39％ 89％ 350 89％ 73％ 88％ 400 88％ 97％ 78％ 450 86％ 92％ 67％ 500 87％ 77％ 41％

[0053] Depend on Figures 1-3 As shown in Table 1, within the temperature range of 200–500℃, the NO conversion rates of samples A, B, and C all showed a significant upward trend with increasing temperature. Specifically, from Figure 1 As shown in Table 1, the NO conversion rate of sample A reached 51% at 225℃; at 300℃, the NO conversion rate of sample A reached 93%, and within the temperature range of 300–500℃, the NO conversion rate of sample A remained consistently above 85%. Figure 2 As shown in Table 1, at 300℃, the NO conversion rate of sample B was only 39%; at 400℃, the NO conversion rate of sample B reached 97%; however, as the temperature continued to rise, at 500℃, the NO conversion rate of sample B dropped to 77%. Figure 3As shown in Table 1, the NO conversion rate of sample C reached 91% at 275℃; however, as the temperature continued to rise, the NO conversion rate of sample C dropped to 41% at 500℃. This indicates that the silver-based SCR catalyst obtained by the present invention through first calcination to obtain a first calcination product, second calcination to obtain a second calcination product, and mixing the first and second calcination products can maintain a high NO conversion rate from low to high temperatures.

[0054] Although the above embodiments have provided a detailed description of the present invention, they are only some embodiments of the present invention, and not all embodiments. People can obtain other embodiments based on these embodiments without creative effort, and these embodiments all fall within the protection scope of the present invention.

Claims

1. The application of silver-based SCR catalyst in flue gas denitrification treatment, characterized in that, The denitrification treatment temperature is 200~500℃, and the preparation method of the silver-based SCR catalyst includes the following steps: Alumina hydroxide is impregnated in an aqueous solution of soluble silver salt to obtain a catalyst precursor; the aqueous solution of soluble silver salt includes soluble silver salt and water; the mass ratio of alumina hydroxide to soluble silver salt is 4~8:0.063; the impregnation temperature is 20~25℃ and the time is 2~3h. A portion of the catalyst precursor was subjected to a first calcination at 500°C to obtain a first calcination product; the holding time for the first calcination was 2.5~3.5 h. The remaining catalyst precursor was subjected to a second calcination at 900°C to obtain a second calcined product; the holding time for the second calcination was 2.5~3.5h. The first calcined product and the second calcined product are mixed to obtain a silver-based SCR catalyst; the mass ratio of the first calcined product and the second calcined product is 1:

1.

2. The application according to claim 1, characterized in that, The NOx concentration in the flue gas is 700~900ppm; The reducing agent used in the denitrification process is propylene.

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