Catalyst for low-temperature direct catalytic decomposition denitration as well as preparation method and application thereof

Abstract

The invention discloses a catalyst for low-temperature direct catalytic decomposition denitration as well as a preparation method and application thereof, and belongs to the technical field of direct catalytic decomposition denitration. The preparation method comprises the following steps: by taking potassium ferricyanide, manganese nitrate and polyvinylpyrrolidone as raw materials, preparing a ferromanganese oxide carrier with a hollow structure by adopting a coprecipitation method and high-temperature annealing, and then loading noble metal rhodium on the ferromanganese oxide carrier by adopting an impregnation method, thereby obtaining the denitration catalyst for low-temperature direct catalytic decomposition. When the low-temperature direct catalytic decomposition denitration catalyst is prepared, the reduction-state nano hollow ferromanganese oxide carrier with the hollow structure is adopted, oxidation inactivation of rhodium can be effectively inhibited, and the low-temperature direct denitration activity of the catalyst is greatly improved. The operation process is simple, a reducing agent is not needed, and secondary pollution is avoided; meanwhile, the reaction temperature is low, the denitration efficiency is high, and the direct emission standard of nitric oxide can be met.

Description

Technical Field

[0001] The invention belongs to the technical field of direct catalytic decomposition and denitration, and in particular relates to a catalyst for low-temperature direct catalytic decomposition and denitration, and a preparation method and application thereof. Background Art

[0002] Nitrogen oxides (NOx) are considered to be one of the main sources of air pollution. They can cause serious harm to the ecological environment and human health and must be properly handled. In the past few decades, various denitrification technologies have been used to remove nitrogen oxides. At present, the most widely used NO selective catalytic reduction technology (SCR) using NH3 as a reductant still has some disadvantages, such as the catalyst (V2O5-WO3 / TiO2) is not only toxic and harmful, but also has a high operating temperature and a narrow range (300-400℃), and is prone to escape and cause secondary pollution. In contrast, direct catalytic decomposition of NO is considered to be the most ideal technology because it does not require a reductant and does not form harmful products. However, its widespread application has been hindered by the low activity of the catalyst under actual conditions. Therefore, the key to direct catalytic denitrification technology lies in the development of highly active direct decomposition catalysts.

[0003] A large number of studies have shown that three-way catalysts with Pt, Pd and Rh as active components have high NO direct decomposition activity. However, precious metal catalysts have poor resistance to oxygen, and the high reaction temperature (>600°C) hinders their practical application. In contrast, although transition metal oxides and zeolite catalysts are cheaper than precious metal catalysts and can show the activity of directly decomposing NO at medium and high temperatures (400-650°C), their activity is far from meeting the requirements. For example, the Chinese invention patent application with publication number CN113457717A discloses "a catalyst for low-temperature and low-oxygen flue gas denitrification, preparation method and application thereof", with transition metal manganese Mn and iron Fe as active components loaded on SBA-15 molecular sieve, and finally doped with urea; this process does not load precious metals, resulting in the final catalyst denitrification efficiency still not meeting the requirements.

[0004] In summary, if the problem of noble metal oxygen poisoning can be solved and the reaction temperature of the catalyst can be reduced by combining with transition metals, it is possible to find a composite metal catalyst with good activity at low temperatures. Therefore, the development of efficient catalysts for direct decomposition of NO at low temperatures will open up a direction for the direct removal of nitrogen oxides. Summary of the invention

[0005] In order to overcome the shortcomings of the above-mentioned prior art, the purpose of the present invention is to provide a catalyst for low-temperature direct catalytic decomposition and denitrification, and a preparation method and application thereof, so as to solve the technical problem that the denitrification catalyst prepared by the existing method has low activity and cannot achieve low-temperature direct denitrification treatment efficiency.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions: The invention discloses a method for preparing a catalyst for low-temperature direct catalytic decomposition and denitration, comprising: using potassium ferrocyanide, manganese nitrate and polyvinyl pyrrolidone as raw materials, adopting a coprecipitation method and high-temperature annealing to prepare a manganese iron oxide carrier with a hollow structure, adopting an impregnation method to load precious metal rhodium on the manganese iron oxide carrier, and then subjecting the carrier to hydrogen reduction treatment to obtain the catalyst for low-temperature direct catalytic decomposition and denitration.

[0007] Furthermore, the above-mentioned method for preparing the low-temperature direct catalytic decomposition denitrification catalyst comprises the following steps: 1) dissolving polyvinyl pyrrolidone in a mixed solution of ethanol and water to obtain solution A; dissolving potassium ferrocyanide in water to obtain solution B; mixing solution A and solution B and stirring them uniformly to obtain a mixed solution A; then adding manganese nitrate solution to the mixed solution A and stirring them uniformly at room temperature to obtain a mixed solution B; 2) The mixed solution B is allowed to stand for treatment and then centrifuged and filtered, the precipitate is collected, dried and then ground, and then subjected to high temperature annealing treatment to obtain a manganese iron oxide carrier with a hollow structure; 3) ultrasonically mixing the rhodium chloride solution and the manganese iron oxide carrier, and then standing to obtain a mixture, and then drying the mixture and calcining it at high temperature to obtain a rhodium-based catalyst precursor; 4) The rhodium-based catalyst precursor is reduced in a hydrogen gas flow, and then cooled to room temperature in an argon gas flow to obtain a denitrification catalyst for low-temperature direct catalytic decomposition, namely, a rhodium-based catalyst supported by reduced nanocage manganese iron oxide.

[0008] More preferably, in step 1), the volume ratio of ethanol to water in the mixed solution of ethanol and water is 1:1.5-1:3; the mass ratio of potassium ferrocyanide to manganese nitrate is 1:2-3:4; and the amount ratio of polyvinyl pyrrolidone to the mixed solution of ethanol and water is (3-6) g: 100 mL.

[0009] More preferably, in step 2), the drying temperature is 25-50°C, and the powder is ground into 60-80 mesh powder; the temperature of the high temperature annealing treatment is 500-550°C, the heating rate during the high temperature annealing treatment is 1-1.5°C / min, and the holding time is 2-3 h.

[0010] More preferably, in step 3), the amount of doped rhodium is 0.1%-0.4% of the total mass of the low-temperature direct catalytic decomposition denitrification catalyst; the high-temperature calcination temperature is 500-550°C, the heating rate during the high-temperature calcination process is 8-10°C / min, and the insulation time is 2-3 h.

[0011] More preferably, in step 4), the temperature of the reduction treatment in the hydrogen gas flow is 500-550°C, the heating rate is 7-10°C / min, and the holding time is 2-3 h.

[0012] More preferably, in step 4), the flow rate of hydrogen gas is controlled to be 40-60 mL / min, and the flow rate of argon gas is controlled to be 40-60 mL / min. After reduction under hydrogen atmosphere, the cooling process is protected by an inert atmosphere (argon) to effectively prevent the product from being oxidized again.

[0013] The invention also discloses a low-temperature direct catalytic decomposition denitration catalyst prepared by the above-mentioned preparation method for the low-temperature direct catalytic decomposition denitration catalyst. The low-temperature direct catalytic decomposition denitration catalyst is a rhodium-based catalyst supported by reduced nano cage manganese iron oxide.

[0014] Preferably, the catalyst for low-temperature direct catalytic decomposition and denitration is a polyhedral hollow structure with an average size of 140-160 nm.

[0015] The present invention also discloses the use of the above-mentioned catalyst for low-temperature direct catalytic decomposition and denitration in a direct denitration reaction at low temperature. The low temperature range for the direct denitration reaction is 25-250°C.

[0016] Compared with the prior art, the present invention has the following beneficial effects: The invention discloses a method for preparing a low-temperature direct catalytic decomposition denitration catalyst, which uses potassium ferrocyanide, manganese nitrate and polyvinyl pyrrolidone as raw materials, adopts a coprecipitation method and a high-temperature annealing technology to prepare a manganese iron oxide carrier with a hollow structure, and then adopts an impregnation method to load precious metal rhodium (Rh), and then adopts hydrogen reduction to prepare a rhodium-based direct denitration catalyst, that is, a low-temperature direct catalytic decomposition denitration catalyst. Among them, a small amount of precious metal rhodium loaded as an active center can effectively decompose NO at low temperature, greatly reducing the reaction temperature, and the reduced manganese iron oxide carrier treated with hydrogen can transfer the oxygen generated by the decomposition of NO on Rh, effectively inhibiting the oxidation deactivation of Rh, thereby solving the problem of noble metal oxygen poisoning. At the same time, the hollow structure of the low-temperature direct catalytic decomposition denitration catalyst can concentrate the active components on the surface, reduce the mass transfer time, and strengthen the interface reaction. The preparation method of the invention has a simple operation process, does not need to add an additional ammonia reducing agent, and can avoid secondary pollution of ammonia. At the same time, the reaction temperature of the entire process is low, the denitration efficiency is high, and the direct emission standard of nitrogen oxides can be met.

[0017] The catalyst for low-temperature direct catalytic decomposition and denitration prepared by the present invention has a main active component of manganese iron oxide and a small amount (0.2 wt%-0.4 wt%) of precious metal Rh active component, which greatly reduces the use of precious metals and effectively reduces economic costs. The reduced nano hollow manganese iron oxide carrier (manganese iron oxide carrier with a hollow structure) formed after hydrogen reduction can effectively inhibit the oxidation deactivation of Rh, greatly reduce the reaction temperature, and extend the service life. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 It is a schematic diagram of the structure of the fixed bed denitration reactor used in the present invention; In the figure, 1 is an argon gas cylinder; 2 is a nitric oxide gas cylinder; 3 is a tubular furnace; 4 is a flow meter; 5 is a flue gas analyzer; 6 is a mass spectrometer; 7 is a catalyst; Figure 2 It is a comparison chart of the results of direct catalytic decomposition of NO by the denitration catalysts prepared in Example 2 of the present invention and various comparative examples at 25-250°C; Figure 3 is a comparison chart of the results of direct catalytic decomposition of NO by rhodium-based direct denitration catalysts (Rh / NC-MnFe) with different Rh loadings prepared in Examples 1 to 4; Figure 4 These are SEM and TEM images of the rhodium-based direct denitration catalyst (Rh(0.2) / NC-MnFe) prepared in Example 2 of the present invention; wherein a is a SEM image (200 nm); b and c are TEM images at different magnifications, with b being 500 nm and c being 20 nm. DETAILED DESCRIPTION

[0018] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0019] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the data used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions, for example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units that are clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0020] The present invention is further described in detail below in conjunction with the accompanying drawings: Comparative Example 1 A method for preparing a nano cage manganese iron oxide direct denitration catalyst, the specific steps are: Step 1: Dissolve 3 g of polyvinyl pyrrolidone in a mixed solution of 100 mL of ethanol and deionized water (the volume ratio of ethanol to water is 1:1), and dissolve 0.75 g of potassium ferrocyanide in 200 mL of deionized water. Mix the two solutions and stir them evenly to obtain a mixed solution. Then, weigh 2 g of a 50 wt% manganese nitrate solution and drop it into 100 mL of deionized water for dilution. Then, add the diluted manganese nitrate solution dropwise into the mixed solution and stir at room temperature for 6 h. Step 2: The mixed solution obtained by the treatment in step 1 was allowed to stand at room temperature overnight, centrifuged and filtered, and dried at room temperature for 12 h to obtain a brown precipitate, which was ground into a 60-80 mesh powder. Then, the powder was placed in a tube furnace, heated from room temperature to 500 °C at a heating rate of 1 °C / min, and annealed at 500 °C for 2 h to obtain a nano cage manganese iron oxide direct denitrification catalyst (referred to as NC-MnFeOx).

[0021] Comparative Example 2 A method for preparing a reduced nano-cage manganese iron oxide direct denitration catalyst, which is different from the above comparative example 1 in that the obtained nano-cage manganese iron oxide catalyst is subjected to reduction treatment. The specific steps are: Step 1: Dissolve 3 g of polyvinyl pyrrolidone in a mixed solution of 100 mL of ethanol and deionized water (the volume ratio of ethanol to water is 1:1), and dissolve 0.75 g of potassium ferrocyanide in 200 mL of deionized water, mix the above two solutions and stir them evenly to obtain a mixed solution; then, drop 2 g of a 50wt% manganese nitrate solution into 100 mL of deionized water for dilution, and then add the diluted manganese nitrate solution dropwise into the above mixed solution, and stir at room temperature for 6 h; Step 2: The mixed solution obtained by the treatment in step 1 was allowed to stand at room temperature overnight, centrifuged and filtered, and dried at room temperature for 12 hours to obtain a brown precipitate, which was ground into a 60-80 mesh powder. Then, the powder was placed in a tube furnace, heated from room temperature to 500 °C at a heating rate of 1 °C / min, and annealed at 500 °C for 2 hours to obtain a catalyst precursor; Step 3: The catalyst precursor prepared in step 2 was reduced at 500 °C in a hydrogen gas flow at a flow rate of 60 mL / min for 2 h, and then cooled to 25 °C in an argon gas flow at a flow rate of 60 mL / min to obtain a reduced nanocage manganese iron oxide direct denitrification catalyst (denoted as NC-MnFe).

[0022] Comparative Example 3 A common method for preparing a commercial SCR denitration catalyst comprises the following steps: Step 1: Weigh 0.023 g of ammonium metavanadate and 0.135 g of ammonium metatungstate, i.e., the mass fraction m(V):m(W)=1:10, and 1 g of titanium dioxide powder and dissolve them in 100 mL of deionized water, stir at room temperature for 1 hour, and then stand for 24 hours; Step 2: Place the mixture obtained in step 1 in a forced air drying oven and completely dry it at 60°C. Place the dried powder in a tubular furnace and heat it from room temperature to 500°C at a heating rate of 10°C / min. Calcinate at 500°C for 2 h to obtain a commercial SCR denitrification catalyst, namely V2O5-WO3 / TiO2 catalyst (vanadium pentoxide and tungsten trioxide loaded on titanium dioxide).

[0023] Comparative Example 4 A method for preparing a rhodium-based direct denitration catalyst supported by nano-particle manganese iron oxide, comprising the following specific steps: Step 1: Weigh 2 g of 50 wt% manganese nitrate solution, 1.84 g of ferric nitrate nonahydrate and 4 g of urea and dissolve them in 50 mL of deionized water. Stir at room temperature for 1 h and then transfer them into a 100 mL hydrothermal reactor. React at 120 °C for 12 h and collect the precipitate. Step 2: Filter the precipitate obtained by the hydrothermal reaction in step 1, wash it with deionized water three times, and then dry it in a forced air drying oven at 60°C for 12 h. Place the dried powder in a tube furnace and calcine it at 500°C for 2 h at a heating rate of 10°C / min to obtain a manganese iron oxide carrier; Step 3: Weigh 1 g of the manganese iron oxide carrier obtained in step 2, use a pipette to transfer 400 μL of a rhodium chloride solution with a mass concentration of 5 g / L (the amount of Rh accounts for 0.2 wt% of the total mass of the rhodium-based catalyst supported by the nano-particle manganese iron oxide obtained), add it to 50 mL of deionized water, stir it at room temperature for 1 h, let the mixture stand for 24 h, and then completely dry it in a forced air drying oven at 60 ° C. The dried powder is placed in a tube furnace, and the temperature is raised from room temperature to 500 ° C at a heating rate of 10 ° C / min, and calcined at 500 ° C for 2 hours to obtain a catalyst precursor; Step 4: The catalyst precursor obtained in step 3 was reduced at 500 ° C in a hydrogen gas flow at a flow rate of 60 mL / min for 2 h, and then cooled to 25 ° C in an argon gas flow at a flow rate of 60 mL / min to obtain a nanoparticle manganese iron oxide supported rhodium-based catalyst (denoted as Rh(0.2) / MnFeOx).

[0024] Comparative Example 5 A method for preparing a rhodium-based direct denitration catalyst supported by an SBA-15 molecular sieve, comprising the following specific steps: Step 1: Weigh 1 g of SBA-15 molecular sieve, use a pipette to take 400 μL of a rhodium chloride solution with a mass concentration of 5 g / L (the amount of Rh accounts for 0.2 wt% of the total mass of the rhodium-based catalyst loaded on the final SBA-15 molecular sieve), add it to 50 mL of deionized water, stir at room temperature for 1 h, and let it stand for 24 hours to obtain a mixture; Step 2: The mixture obtained in step 1 is completely dried in a forced air drying oven at 60°C, the dried powder is placed in a tube furnace, the temperature is increased from room temperature to 500°C at a heating rate of 10°C / min, and calcined at 500°C for 2 h to obtain a catalyst precursor; Step 3: The catalyst precursor prepared in step 2 was reduced at 500 °C in a hydrogen gas flow at a flow rate of 60 mL / min for 2 h, and then cooled to 25 °C in an argon gas flow at a flow rate of 60 mL / min to obtain a rhodium-based direct denitrification catalyst supported on SBA-15 molecular sieve (denoted as Rh(0.2) / SBA-15).

[0025] Example 1 This embodiment provides a method for preparing a catalyst for low-temperature direct catalytic decomposition and denitration, which is different from the above comparative example 2 in that 0.1 wt% of rhodium is loaded in the catalyst. The specific steps are: Step 1: Dissolve 3 g of polyvinyl pyrrolidone in a mixed solution of 100 mL of ethanol and deionized water (the volume ratio of ethanol to water is 1:1) to form solution A; at the same time, dissolve 0.75 g of potassium ferrocyanide in 200 mL of deionized water to form solution B; mix solution A and solution B and stir evenly to obtain a mixed solution A; then, drop 2 g of a 50wt% manganese nitrate solution into 100 mL of deionized water for dilution, and then add the diluted manganese nitrate solution dropwise to the mixed solution A, and stir at room temperature for 6 h to obtain a mixed solution B; Step 2: The mixed solution B obtained by the treatment in step 1 is allowed to stand at room temperature overnight, centrifuged and filtered, and dried at room temperature for 12 hours to obtain a brown precipitate, which is then ground into a 60-80 mesh powder. Then, the powder is placed in a tube furnace, heated from room temperature to 500°C at a heating rate of 1°C / min, and annealed at 500°C for 2 hours to obtain a manganese iron oxide carrier with a hollow structure; Step 3: Weigh 1 g of the manganese iron oxide carrier with a hollow structure obtained in step 2, then use a pipette to transfer 400 μL of a rhodium chloride solution with a mass concentration of 5 g / L (the amount of Rh accounts for 0.1wt% of the total mass of the denitration catalyst for low-temperature direct catalytic decomposition), add it to 50 mL of deionized water, treat it under ultrasonic conditions of 40 kHz at room temperature for 30 min, let the mixture stand for 24 h, then completely dry the mixture at 60 ° C, place the dried powder in a tube furnace, heat it from room temperature to 500 ° C at a heating rate of 10 ° C / min, and calcine it at 500 ° C for 2 h to obtain a rhodium-based catalyst precursor; Step 4: The rhodium-based catalyst precursor prepared in step 3 is reduced in a hydrogen gas flow at a flow rate of 60 mL / min at 500°C for 2 h, and then cooled to 25°C in an argon gas flow at a flow rate of 60 mL / min to obtain a rhodium-based direct denitrification catalyst supported on a reduced nanocage manganese iron oxide, i.e., a low-temperature direct catalytic decomposition denitrification catalyst, denoted as Rh(0.1) / NC-MnFe.

[0026] Example 2 This embodiment provides a method for preparing a catalyst for low-temperature direct catalytic decomposition and denitration, which is different from that of Example 1 in that 0.2 wt% of rhodium is loaded in the catalyst. The specific steps are: Step 1: Dissolve 3 g of polyvinyl pyrrolidone in a mixed solution of 100 mL of ethanol and deionized water (the volume ratio of ethanol to water is 1:1) to form solution A; at the same time, dissolve 0.75 g of potassium ferrocyanide in 200 mL of deionized water to form solution B; mix solution A and solution B and stir evenly to obtain a mixed solution A; then, drop 2 g of a 50 wt% manganese nitrate solution into 100 mL of deionized water for dilution, and add the diluted manganese nitrate solution dropwise to the mixed solution A, and stir at room temperature for 6 h to obtain a mixed solution B; Step 2: The mixed solution B obtained by the treatment in step 1 is allowed to stand at room temperature overnight, centrifuged and filtered, and dried at room temperature for 12 hours to obtain a brown precipitate, which is then ground into a 60-80 mesh powder. Then, the powder is placed in a tube furnace, heated from room temperature to 500°C at a heating rate of 1°C / min, and annealed at 500°C for 2 hours to obtain a manganese iron oxide carrier with a hollow structure; Step 3: Weigh 1 g of the manganese iron oxide carrier with a hollow structure obtained in step 2, then use a pipette to transfer 400 μL of a rhodium chloride solution with a mass concentration of 5 g / L (the amount of Rh accounts for 0.2 wt% of the total mass of the denitrification catalyst for low-temperature direct catalytic decomposition), add it to 50 mL of deionized water, ultrasonically treat for 30 min, let the mixture stand for 24 h, then completely dry the mixture at 60 ° C, place the dried powder in a tube furnace, and heat it from room temperature to 500 ° C at a heating rate of 10 ° C / min under still air, and calcine it at 500 ° C for 2 h to obtain a rhodium-based catalyst precursor; Step 4: The rhodium-based catalyst precursor prepared in step 3 is reduced at 550 ° C in a hydrogen gas flow with a flow rate of 50 mL / min for 3 h, and then cooled to 25 ° C in an argon gas flow with a flow rate of 50 mL / min to obtain a rhodium-based direct denitrification catalyst supported on a reduced nanocage manganese iron oxide, that is, a low-temperature direct catalytic decomposition denitrification catalyst, recorded as Rh(0.2) / NC-MnFe.

[0027] See also Figure 4 , Figure 4 The SEM and TEM images of Rh(0.2) / NC-MnFe were taken with the help of scanning and transmission electron microscopes. It can be seen from the figures that Rh(0.2) / NC-MnFe is a polyhedral hollow structure with an average size of about 150 nm.

[0028] Example 3 This embodiment provides a method for preparing a catalyst for low-temperature direct catalytic decomposition and denitration, which is different from that of Example 1 in that 0.3 wt% of rhodium is loaded in the catalyst. The specific steps are: Step 1: Dissolve 3 g of polyvinyl pyrrolidone in a mixed solution of 100 mL of ethanol and deionized water (the volume ratio of ethanol to water is 1:1) to form solution A; at the same time, dissolve 0.75 g of potassium ferrocyanide in 200 mL of deionized water to form solution B; mix solution A and solution B and stir evenly to obtain a mixed solution A; then, drop 2 g of a 50 wt% manganese nitrate solution into 100 mL of deionized water for dilution, and add the diluted manganese nitrate solution dropwise to the mixed solution A, and stir at room temperature for 6 h to obtain a mixed solution B; Step 2: The mixed solution B obtained by the treatment in step 1 is allowed to stand at room temperature overnight, centrifuged and filtered, and dried at room temperature for 12 hours to obtain a brown precipitate, which is then ground into a 60-80 mesh powder. Then, the powder is placed in a tube furnace, heated from room temperature to 500°C at a heating rate of 1°C / min, and annealed at 500°C for 2 hours to obtain a manganese iron oxide carrier with a hollow structure; Step 3: Weigh 1 g of the manganese iron oxide carrier with a hollow structure obtained in step 2, then use a pipette to transfer 1200 μL of a rhodium chloride solution with a mass concentration of 5 g / L (the amount of Rh accounts for 0.3wt% of the total mass of the denitrification catalyst for low-temperature direct catalytic decomposition), add it to 50 mL of deionized water, ultrasonically treat for 30 min, let the mixture stand for 24 h, then completely dry the mixture at 60 ° C, place the dried powder in a tube furnace, and heat it from room temperature to 500 ° C at a heating rate of 10 ° C / min under still air, and calcine it at 500 ° C for 2 h to obtain a rhodium-based catalyst precursor; Step 4: The rhodium-based catalyst precursor prepared in step 3 is reduced in a hydrogen gas flow at a flow rate of 60 mL / min at 500°C for 2 h, and then cooled to 25°C in an argon gas flow at a flow rate of 60 mL / min to obtain a rhodium-based direct denitrification catalyst supported on a reduced nanocage manganese iron oxide, i.e., a low-temperature direct catalytic decomposition denitrification catalyst, denoted as Rh(0.3) / NC-MnFe.

[0029] Example 4 This embodiment provides a method for preparing a catalyst for low-temperature direct catalytic decomposition and denitration, which is different from Example 1 in that 0.4 wt% of rhodium is loaded in the catalyst. The specific steps are: Step 1: Dissolve 3 g of polyvinyl pyrrolidone in a mixed solution of 100 mL of ethanol and deionized water (the volume ratio of ethanol to water is 1:1) to form solution A; at the same time, dissolve 0.75 g of potassium ferrocyanide in 200 mL of deionized water to form solution B; mix solution A and solution B and stir evenly to obtain a mixed solution A; then, drop 2 g of a 50 wt% manganese nitrate solution into 100 mL of deionized water for dilution, and then drop the diluted manganese nitrate solution into the mixed solution A, and stir at room temperature for 6 h to obtain a mixed solution B; Step 2: The mixed solution B obtained by the treatment in step 1 is allowed to stand at room temperature overnight, centrifuged and filtered, and dried at room temperature for 12 hours to obtain a brown precipitate, which is then ground into a 60-80 mesh powder. Then, the powder is placed in a tube furnace, heated from room temperature to 500°C at a heating rate of 1°C / min, and annealed at 500°C for 2 hours to obtain a manganese iron oxide carrier with a hollow structure; Step 3: Weigh 1 g of the manganese iron oxide carrier with a hollow structure obtained in step 2, then use a pipette to transfer 1600 μL of a rhodium chloride solution with a mass concentration of 5 g / L (the amount of Rh accounts for 0.4 wt% of the total mass of the denitrification catalyst for low-temperature direct catalytic decomposition), add it to 50 mL of deionized water, ultrasonically treat for 30 min, let the mixture stand for 24 h, then completely dry the mixture at 60 ° C, place the dried powder in a tube furnace, and heat it from room temperature to 500 ° C at a heating rate of 8 ° C / min under still air, and calcine it at 500 ° C for 2 h to obtain a rhodium-based catalyst precursor; Step 4: The rhodium-based catalyst precursor prepared in step 3 is reduced in a hydrogen gas flow at a flow rate of 40 mL / min at 550°C for 3 h, and then cooled to 25°C in an argon gas flow at a flow rate of 40 mL / min to obtain a rhodium-based direct denitrification catalyst supported on a reduced nanocage manganese iron oxide, i.e., a low-temperature direct catalytic decomposition denitrification catalyst, denoted as Rh(0.4) / NC-MnFe.

[0030] See also Figure 1 , a schematic diagram of the structure of the fixed bed denitration reactor used in the present invention, when conducting a denitration performance test experiment, the denitration catalysts prepared in the above embodiments and comparative examples are placed in a fixed bed quartz tube reactor for testing, and both ends of the catalyst 7 are fixed in a tube furnace 3 with quartz wool, and the reaction gas consists of Ar and NO, which are provided by an argon gas bottle 1 and a nitric oxide gas bottle 2, respectively, wherein the NO concentration is 300ppm, and Ar is used as a balance gas, and a flow meter 4 is provided to control the total flow rate to 80 mL / min, and the reaction space velocity is 30000 h -1The outlet components were tested by online mass spectrometry using a mass spectrometer 6, and the concentration of NO in the reaction tail gas was detected by an AFRISO M60x flue gas analyzer 5 with a detection accuracy of 0.1 ppm.

[0031] During the test, data was collected 30 minutes after the reaction reached a steady state. The temperature range of the catalyst reaction was 25-250 °C, and the conversion rate of NO was calculated using the following formula:

[0032] Among them, X NO represents the conversion rate of nitrogen oxides, [NO] in and [NO] out Respectively represent the inlet and outlet concentrations of nitrogen oxides, in ppm.

[0033] Table 1 Denitration activity of catalysts prepared in Examples and Comparative Examples

[0034] The test results are combined with the above Table 1 and Figure 2 , Figure 3 The analysis is as follows: The rhodium-based direct denitration catalysts prepared in the examples all have good low-temperature direct denitration activity, wherein the rhodium-based direct denitration catalysts prepared in Examples 1, 2, 3 and 4 have a removal rate of NO of more than 94% at 150-250°C, and in particular, the rhodium-based direct denitration catalyst Rh(0.2) / NC-MnFe with a rhodium content of 0.2wt% in Example 2 has the best activity. Comparative Examples 1 and 2 are respectively unreduced and hydrogen-reduced manganese iron oxide carriers (note: no rhodium group is added), and the results show that the reduced carrier has better activity. At the same time, the comparison of the results of Comparative Example 2 and Examples 1-4 shows that the presence of precious metal rhodium can greatly improve the low-temperature direct denitration activity of the catalyst.

[0035] Comparative Example 3 provides a common commercial SCR denitration catalyst (V2O5-WO3 / TiO2 catalyst). The results show that the direct denitration activity of the commercial SCR denitration catalyst in the entire reaction zone is less than 10%, indicating that it is not suitable for the direct denitration reaction process. Comparative Example 4 is a rhodium-based direct denitration catalyst supported by a nanoparticle manganese iron carrier prepared by a common hydrothermal method. The test results show that the low-temperature activity of the rhodium-based direct denitration catalyst supported by the nanoparticle manganese iron carrier is lower than that of the rhodium-based catalyst supported by the nanocage structure manganese iron oxide carrier. This is due to the hollow structure of the manganese iron oxide carrier with a nanocage structure, which can concentrate the active components on the surface, reduce the mass transfer time, and strengthen the interface reaction. Comparative Example 5 is a rhodium-based direct denitration catalyst supported by a molecular sieve SBA-15 with a high specific surface area, and its activity is much lower than that of Examples 1-4, which shows that even with the same amount of precious metal Rh loading, the reduced manganese iron oxide can avoid Rh oxygen poisoning and greatly improve the low-temperature activity of the catalyst.

[0036] In summary, the present invention is based on the principle of low-temperature direct catalytic decomposition and denitrification technology. Reduced nano-cage manganese iron oxide is used as a carrier (NC-MnFe), and a small amount of precious metal Rh is loaded on the carrier to obtain a rhodium-based direct denitrification catalyst (Rh / NC-MnFe) loaded with reduced nano-cage manganese iron oxide. The catalyst can directly decompose nitrogen oxides (NOx) in flue gas into non-toxic nitrogen and oxygen. The use of reduced nano hollow manganese iron oxide carriers can effectively inhibit the oxidative deactivation of Rh, thereby greatly improving the low-temperature direct denitrification activity of the catalyst.

[0037] The above contents are only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A method for preparing a catalyst for low-temperature direct catalytic decomposition and denitration, characterized in that: include: Potassium ferrocyanide, manganese nitrate and polyvinyl pyrrolidone are used as raw materials, a manganese iron oxide carrier with a hollow structure is prepared by a coprecipitation method and high-temperature annealing, noble metal rhodium is loaded on the manganese iron oxide carrier by an impregnation method, and then a low-temperature direct catalytic decomposition denitrification catalyst is prepared by hydrogen reduction treatment.

2. The method for preparing a low-temperature direct catalytic decomposition denitration catalyst according to claim 1, characterized in that: The following steps are involved: 1) dissolving polyvinyl pyrrolidone in a mixed solution of ethanol and water to obtain solution A; dissolving potassium ferrocyanide in water to obtain solution B; mixing solution A and solution B and stirring them uniformly to obtain a mixed solution A; then adding manganese nitrate solution to the mixed solution A and stirring them uniformly at room temperature to obtain a mixed solution B; 2) The mixed solution B is allowed to stand for treatment and then centrifuged and filtered, the precipitate is collected, dried and then ground, and then subjected to high temperature annealing treatment to obtain a manganese iron oxide carrier with a hollow structure; 3) ultrasonically mixing the rhodium chloride solution and the manganese iron oxide carrier, and then standing to obtain a mixture, and then drying the mixture and calcining it at high temperature to obtain a rhodium-based catalyst precursor; 4) The rhodium-based catalyst precursor is reduced in a hydrogen gas flow, and then cooled to room temperature in an argon gas flow to obtain a denitrification catalyst for low-temperature direct catalytic decomposition.

3. The method for preparing a low-temperature direct catalytic decomposition denitration catalyst according to claim 2, characterized in that: In step 1), the volume ratio of ethanol to water in the mixed solution of ethanol and water is 1:1.5-1:3; the mass ratio of potassium ferrocyanide to manganese nitrate is 1:2-3:4; and the amount ratio of polyvinyl pyrrolidone to the mixed solution of ethanol and water is (3-6) g: 100 mL.

4. The method for preparing a low-temperature direct catalytic decomposition denitration catalyst according to claim 2, characterized in that: In step 2), the drying temperature is 25-50°C, and the powder is ground into 60-80 mesh powder; the temperature of the high temperature annealing treatment is 500-550°C, the heating rate during the high temperature annealing treatment is 1-1.5°C / min, and the insulation time is 2-3h.

5. The method for preparing a low-temperature direct catalytic decomposition denitration catalyst according to claim 2, characterized in that: In step 3), the amount of doped rhodium is 0.1%-0.4% of the total mass of the low-temperature direct catalytic decomposition denitrification catalyst; the high-temperature calcination temperature is 500-550°C, the heating rate during the high-temperature calcination process is 8-10°C / min, and the insulation time is 2-3h.

6. The method for preparing a low-temperature direct catalytic decomposition denitration catalyst according to claim 2, characterized in that: In step 4), the temperature of the reduction treatment in the hydrogen gas flow is 500-550°C, the heating rate is 7-10°C / min, and the holding time is 2-3h.

7. The method for preparing a low-temperature direct catalytic decomposition denitration catalyst according to claim 2, characterized in that: In step 4), the flow rate of the hydrogen gas flow is controlled to be 40-60 mL / min, and the flow rate of the argon gas flow is controlled to be 40-60 mL / min.

8. A catalyst for low temperature direct catalytic decomposition of denitrification prepared by the method for preparing a catalyst for low temperature direct catalytic decomposition of denitrification according to any one of claims 1 to 7, characterized in that: The catalyst for low-temperature direct catalytic decomposition and denitration is a rhodium-based catalyst supported by reduced nano cage manganese iron oxide.

9. The catalyst for low-temperature direct catalytic decomposition and denitration as claimed in claim 8, characterized in that: The catalyst for low-temperature direct catalytic decomposition and denitrification is a polyhedral hollow structure with an average size of 140-160 nm.

10. Use of the catalyst for low-temperature direct catalytic decomposition and denitration according to claim 8 or 9 in a direct denitration reaction at low temperature, characterized in that: The low temperature range for direct denitrification reaction is 25-250 ℃.

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