Urea hydrolysis ammonia preparation catalyst and preparation method thereof

Abstract

The invention belongs to the technical field of catalysts, and discloses a catalyst for preparing ammonia through urea hydrolysis and a preparation method thereof.The preparation method comprises the following steps that 1, after a carrier is calcined, the carrier is added into a mixed solution of a silane coupling agent and absolute ethyl alcohol to be subjected to ultrasonic treatment, the carrier is taken out and dried, and a modified carrier is obtained; (2) immersing the modified carrier into a transition metal salt solution for multiple times, after each time of immersion, firstly carrying out ultrasonic treatment, then taking out, drying and calcining, and repeating for multiple times to obtain a transition metal oxide-loaded modified carrier; and (3) adding a rare earth metal salt solution or an alkali metal salt solution into the modified carrier loaded with the transition metal oxide by several times, carrying out ultrasonic treatment and then stirring treatment after each time of addition, standing, taking out, drying and calcining after all the addition is finished, thereby obtaining the catalyst for preparing ammonia through urea hydrolysis. According to the invention, the catalytic activity, thermal stability, anti-clogging performance and anti-poisoning capability of the catalyst in the process of preparing ammonia by hydrolyzing urea are obviously improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalysts, and in particular relates to a urea hydrolysis catalyst for producing ammonia and a preparation method thereof. Background Art

[0002] The urea hydrolysis process plays a key role in denitrification systems in industries like steel. Its mechanism of action is that urea hydrolysis generates ammonia, which reacts with nitrogen oxides in flue gas to convert them into harmless ammonia and water, reducing their impact on the atmospheric environment. However, the slow reaction rate and long reaction time of urea hydrolysis hinder its rapid response to unit load changes, hindering its development. Catalysts can accelerate the reaction and offset the shortcomings of conventional hydrolysis.

[0003] However, existing urea hydrolysis catalysts are mostly based on transition metal oxides and their carrier materials. They still show insufficient catalytic performance under the high temperature, high dust and complex gaseous environment of the steel industry. The urea hydrolysis rate is slow, affecting the ammonia yield. In addition, sulfur (SO2), alkali metals (such as K, Na) and dust particles in the exhaust gas can easily cause catalyst poisoning or blockage, shortening the service life. Under long-term operation conditions, the catalyst is also prone to sintering, resulting in a decrease in specific surface area and activity attenuation. Summary of the Invention

[0004] The technical problem to be solved by the present invention is to address the deficiencies in the prior art and provide a urea hydrolysis catalyst for producing ammonia and a preparation method thereof. The catalyst uses a porous material as a carrier, a transition metal oxide as an active component, and a rare earth metal oxide or an alkali metal oxide as an auxiliary component, thereby significantly improving the catalytic activity, thermal stability, anti-clogging performance, and anti-poisoning ability during urea hydrolysis to produce ammonia.

[0005] In order to solve the technical problem raised by the present invention, the present invention provides a method for preparing a catalyst for producing ammonia by hydrolysis of urea, comprising the following steps:

[0006] 1) After calcining the support, adding a mixed solution of a silane coupling agent and anhydrous ethanol, performing ultrasonic treatment, taking out and drying to obtain a modified support;

[0007] 2) immersing the modified support in a transition metal salt solution multiple times, and after each immersion, first ultrasonically treating it, then removing it, drying it, and calcining it, and repeating this process multiple times to obtain a modified support loaded with a transition metal oxide;

[0008] 3) Adding a rare earth metal salt solution or an alkali metal salt solution to the modified support loaded with the transition metal oxide in portions, first ultrasonically treating and then stirring after each addition, and after all additions are completed, allowing the solution to stand, taking it out, drying it, and calcining it to obtain a urea hydrolysis catalyst for ammonia production.

[0009] In the above scheme, the carrier is γ-Al2O3 or SiO2, with a particle size of 0.1 to 2 mm and a specific surface area of ​​≥200 m 2 / g, the average pore diameter is 5-10nm, and the porosity is 0.4-0.6.

[0010] In the above scheme, in step 1), the calcination temperature of the carrier is 600-800° C. and the calcination time is 2-4 hours to remove impurities and optimize the pore structure.

[0011] In the above solution, the silane coupling agent is KH-550 or KH-570.

[0012] In the above scheme, the mixed solution of silane coupling agent and anhydrous ethanol is composed of silane coupling agent and anhydrous ethanol in a mass ratio of 1: (10-50).

[0013] In the above scheme, the mass ratio of the carrier to the silane coupling agent is (10-50):1.

[0014] In the above scheme, in step 1), the frequency of ultrasonic treatment is 40 to 60 kHz, the power is 80 to 120 W, and the ultrasonic time is 20 to 40 minutes, so that the silane molecules are grafted onto the surface of the carrier.

[0015] In the above scheme, in step 1), the drying temperature is 80-100° C., the drying time is 4-6 hours, and after drying, the mixture is cooled to room temperature and sealed for storage to avoid moisture absorption or contamination.

[0016] In the above scheme, the transition metal salt solution is one or more of titanium nitrate solution, zirconium nitrate solution, and cerium nitrate solution, with a concentration of 0.05 to 1.0 mol / L.

[0017] In the above scheme, the modified support is immersed in the transition metal salt solution 2 to 4 times, and the concentration of the transition metal salt solution is gradually increased with an increase gradient of 0.2 to 0.4 mol / L.

[0018] Preferably, the modified support is immersed in transition metal salt solution three times, the concentration of the first transition metal salt solution is 0.1-0.2 mol / L, the concentration of the second transition metal salt solution is 0.4-0.6 mol / L, and the concentration of the third transition metal salt solution is 0.8-1.0 mol / L.

[0019] Preferably, during each immersion, the ratio of the mass of the modified support to the volume of the transition metal salt solution is 1 g: (0.5-1) mL.

[0020] In the above scheme, in step 2), the frequency of ultrasonic treatment is 40-60 kHz, the power is 80-120 W, and the single ultrasonic treatment time is 5-15 minutes.

[0021] In the above scheme, in step 2), the drying temperature is 80-120° C., and the single drying time is 8-12 hours.

[0022] In the above scheme, in step 2), the calcination temperature is 300-600° C., and the single calcination time is 2-4 hours.

[0023] Preferably, in step 2), the calcination temperature is gradually increased.

[0024] In the above scheme, the rare earth metal salt solution is one of lanthanum nitrate solution and yttrium nitrate solution, and the alkali metal salt solution is one of potassium nitrate solution and sodium nitrate solution, and the concentration of both is 0.05-0.8 mol / L.

[0025] In the above scheme, the rare earth metal salt solution or alkali metal salt solution is added in 2 to 3 times, and the mass ratio of the total added volume to the modified support loaded with transition metal oxide is (1 to 2) mL:1 g.

[0026] In the above scheme, in step 3), the frequency of ultrasonic treatment is 40-60 kHz, the power is 80-120 W, and the single time is 5-15 minutes.

[0027] In the above scheme, in step 3), the stirring speed of the stirring treatment is 100 to 300 rpm, and the single stirring time is 20 to 40 minutes.

[0028] In the above scheme, in step 3), the standing time is 12 to 24 hours, so that the rare earth metal salt or alkali metal salt can penetrate into the surface and pores of the active layer.

[0029] In the above scheme, in step 3), the drying temperature is 80-120° C., and the drying time is 8-12 hours.

[0030] In the above scheme, in step 3), the calcination temperature is 400-600° C., and the calcination time is 2-4 hours.

[0031] Preferably, the calcination temperature in step 3) is not lower than the highest calcination temperature in step 2).

[0032] In the above scheme, the particle size of the urea hydrolysis ammonia production catalyst is 0.1-2 mm, and the specific surface area is 150-250 m 2 / g, the average pore diameter is 4-10nm, and the porosity is 0.35-0.55.

[0033] The technical concept of the present invention is:

[0034] The present invention uses a porous material with a high specific surface area as a carrier to provide sufficient active sites and improve high-temperature resistance and dust clogging resistance; uses transition metal oxides as active components to provide excellent urea hydrolysis catalytic activity; and introduces rare earth metal oxides or alkali metal oxides as auxiliary components to enhance electronic effects and resistance to SO2 and alkali metal poisoning. However, under complex operating conditions, the porous carrier is prone to pore clogging due to dust and moisture adsorption, reducing catalytic efficiency. When the traditional impregnation method is used to load the metal oxide, the loaded metal oxide is difficult to fully exert the composite effect, which will further limit the performance of the catalyst. The present invention solves these difficult problems by optimizing the carrier properties and loading process.

[0035] 1) Surface modification of porous carriers: To address the problem that porous materials are easily clogged by dust and moisture in high-temperature and high-humidity environments, the present invention introduces a silane coupling agent (KH-550 or KH-570) during carrier pretreatment for surface modification. The silane molecules form a hydrophobic layer through chemical grafting. On the one hand, the carrier's tendency to adsorb moisture is reduced, while the surface chemical stability is enhanced, reducing the adhesion of dust particles, thereby improving anti-clogging performance. On the other hand, the surface hydrophilicity is reduced, slowing the adsorption rate of the precursor, which is particularly beneficial for the deep penetration of subsequent low-concentration transition metal salt solutions, extending their diffusion time in the pores, while high-concentration transition metal salt solutions can still be quickly deposited on the surface.

[0036] 2) Gradient loading of active components: In order to address the defects of uneven distribution of metal oxides and insufficient loading in deep pores in traditional impregnation methods, the present invention adopts an ultrasound-assisted gradient impregnation method. Through the gradient distribution of active groups to balance the surface activity and internal patency of the catalyst, the modified carrier is immersed in a transition metal salt solution multiple times, and the concentration of the transition metal salt solution is gradually increased. Low concentration deep deposition, medium concentration middle deposition, and high concentration surface deposition are finally formed. A gradient with decreasing concentration from the surface to the center is formed. The gradient law not only increases the loading depth, but also optimizes the distribution of high-activity sites on the surface and the patency of internal pores, reducing the occurrence of blockage. Specifically, the low-concentration solution has a lower diffusion driving force. The precursor molecules preferentially penetrate into the deep pores of the carrier (50-80% of the particle radius) under the action of ultrasound to form a base layer; after the medium-concentration solution partially occupies the pore sites in the base layer, it is further deposited in the middle layer area, and ultrasound drives moderate penetration to form a transition layer; the catalytic activity is enhanced. The high-concentration solution preferentially deposits on the surface of the carrier and shallow pores due to the larger concentration gradient and adsorption rate, forming a high-concentration protective layer. The high surface concentration provides initial high activity, while the low internal concentration keeps the pores open, synergizing with the rare earth components to enhance electron transfer and reduce alkali metal poisoning.

[0037] 3) Uniform loading of auxiliary components: The main function of auxiliary components is to enhance electron transfer in the catalytic reaction and prevent the catalyst from poisoning. A single concentration can be added in batches to achieve uniform coverage of the active layer, without the need for a gradient design. Transition metal oxides (such as TiO2) provide active sites with high oxygen vacancies, while rare earth / alkali metal oxides (such as La2O3) enhance electron transfer and anti-poisoning capabilities. Rare earth metal salt solutions or alkali metal salt solutions are added in multiple batches to avoid surface overload caused by a single addition. This allows the precursor molecules to gradually penetrate deeper into the pores after each addition, achieving uniform loading.

[0038] Compared with the prior art, the present invention has the following beneficial effects:

[0039] The catalyst of the present invention uses a porous material as a carrier, a transition metal oxide as an active component, and a rare earth metal oxide or an alkali metal oxide as an auxiliary component. Through surface modification of the porous carrier, gradient loading of the active component, and uniform loading of the auxiliary component, the clogging problem of the porous material in a high-temperature and high-humidity environment and the defects of uneven distribution of metal oxides and insufficient deep-pore loading in the traditional impregnation method are solved. The catalytic activity, thermal stability, anti-clogging performance, and anti-poisoning ability of the catalyst in urea hydrolysis to produce ammonia are significantly improved.

[0040] The catalyst of the present invention has a urea hydrolysis efficiency of over 95% at 200-400°C, an activity retention rate of over 90% after high-temperature operation at 400-600°C for 2000-5000 hours, and an activity decrease rate of less than 7% and a pore blockage rate of less than 10% after operation at 200-400°C for 1000-2000 hours in a complex environment containing SO2, alkali metals and dust, significantly superior to conventional catalysts. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 This is the XRD pattern of the urea hydrolysis catalyst for ammonia production prepared in Example 1 of the present invention.

[0042] Figure 2 This is the XRD pattern of the urea hydrolysis catalyst for ammonia production prepared in Example 2 of the present invention. DETAILED DESCRIPTION

[0043] In order to better understand the present invention, the content of the present invention is further illustrated below in conjunction with the examples, but the content of the present invention is not limited to the following examples.

[0044] Example 1

[0045] The preparation method of the catalyst for producing ammonia by hydrolysis of urea in this embodiment comprises the following steps:

[0046] 1) Surface modification of carrier

[0047] Select particle size 0.5mm, specific surface area 220m 2 / g, an average pore size of 7nm, and a porosity of 0.49 as the support;

[0048] The γ-Al2O3 support was placed in a muffle furnace and heated to 600℃, and calcined at this temperature for 3h.

[0049] The calcined γ-Al2O3 support was added to a mixed solution of a silane coupling agent KH-550 and anhydrous ethanol in a mass ratio of 1:40 (the mass ratio of the support to the silane coupling agent was 20:1), and ultrasonically treated for 30 minutes (frequency 40 kHz, power 100 W). After removal, the solution was dried at 100°C for 6 hours to obtain a modified γ-Al2O3 support.

[0050] 2) Gradient loading of active components

[0051] The modified support was immersed in a 0.1 mol / L titanium nitrate solution at a volume ratio of 1 g to 0.5 mL of the transition metal salt solution. The support was ultrasonically treated for 10 min (frequency 40 kHz, power 100 W), dried at 100 °C for 12 h, and then calcined at 300 °C for 3 h.

[0052] The modified support was then immersed in a 0.5 mol / L titanium nitrate solution at a volume ratio of 1 g to 0.5 mL of the transition metal salt solution. The support was ultrasonically treated for 10 min (frequency 40 kHz, power 100 W). After removal, the support was dried at 100°C for 12 h and then calcined at 400°C for 3 h.

[0053] The modified γ-Al2O3 support was then immersed in a 0.8 mol / L titanium nitrate solution at a volume ratio of 1 g to the transition metal salt solution. The support was ultrasonically treated for 10 min (frequency 40 kHz, power 100 W). After removal, the support was dried at 100°C for 12 h and then calcined at 500°C for 3 h to obtain a TiO2-loaded modified γ-Al2O3 support.

[0054] 3) Uniform loading of auxiliary components

[0055] 0.3 mol / L lanthanum nitrate solution was added dropwise twice (equally) to the modified γ-Al2O3 carrier loaded with TiO2, with the mass ratio of the total volume to the modified γ-Al2O3 carrier loaded with TiO2 being 1 mL:1 g. After each addition, ultrasonic treatment was performed for 10 min (frequency 40 kHz, power 100 W), and then stirred at 200 rpm for 30 min. Finally, the solution was allowed to stand for 12 h, dried at 100 ° C for 12 h, and then calcined at 500 ° C for 3 h to obtain a urea hydrolysis catalyst for ammonia production.

[0056] The particle size of the urea hydrolysis catalyst for ammonia production obtained in this example is 0.5 mm, and the specific surface area is 180 m 2 / g, an average pore size of 6nm, a porosity of 0.43, and XRD confirmed the crystal structures of TiO2 and La2O3. The catalyst was stored in a sealed container, away from direct sunlight and high humidity.

[0057] The urea hydrolysis catalyst for ammonia production obtained in this embodiment has a urea hydrolysis efficiency of 96.5% when operated at 300° C. for 1000 h in flue gas containing 81.5% N2, 4.4% O2, and 9.8% H2O (all by volume and free of SO2, alkali metals, and dust); an activity retention rate of 91.2% when operated at 500° C. for 2000 h in flue gas containing 81.5% N2, 4.4% O2, and 9.8% H2O (all by volume and free of SO2, alkali metals, and dust); and an activity retention rate of 91.2% when operated at 500° C. for 2000 h in flue gas containing 500 ppm SO2, 100 ppm total KNa alkali metal content, and 10 g / m3 dust. 3 In complex flue gas, after running at 300℃ for 1000h, the activity decreased by 6.1% and the pore blockage rate was 8.5%.

[0058] Example 2

[0059] The preparation method of the catalyst for producing ammonia by hydrolysis of urea in this embodiment comprises the following steps:

[0060] 1) Surface modification of carrier

[0061] Select particle size 1mm, specific surface area 250m 2 / g, an average pore diameter of 8nm, and a porosity of 0.55 SiO2 as a carrier;

[0062] The SiO2 carrier was placed in a muffle furnace and heated to 700°C, and calcined at this temperature for 4 h.

[0063] The calcined SiO2 support was added to a mixed solution of silane coupling agent KH-570 and anhydrous ethanol in a mass ratio of 1:50 (mass ratio of support to silane coupling agent was 25:1), and ultrasonically treated for 35 min (frequency 50 kHz, power 110 W). After removal, the solution was dried at 100°C for 4 h to obtain a modified SiO2 support.

[0064] 2) Gradient loading of active components

[0065] The modified support was immersed in a 0.2 mol / L zirconium nitrate solution at a volume ratio of 1 g to 0.6 mL of the transition metal salt solution. The support was ultrasonically treated for 12 min (frequency 50 kHz, power 110 W), dried at 110°C for 11 h, and then calcined at 300°C for 4 h.

[0066] The modified support was then immersed in a 0.6 mol / L zirconium nitrate solution at a volume ratio of 1 g to 0.6 mL of the transition metal salt solution, and ultrasonicated for 12 min (frequency 50 kHz, power 110 W). The support was then removed and dried at 110°C for 11 h, and then calcined at 400°C for 4 h.

[0067] The modified SiO2 support was immersed in a 1.0 mol / L zirconium nitrate solution at a volume ratio of 1 g to the transition metal salt solution of 1 g:0.6 mL. The support was ultrasonically treated for 12 min (frequency 50 kHz, power 110 W). After removal, the support was dried at 110°C for 11 h and then calcined at 550°C for 4 h to obtain a modified SiO2 support loaded with ZrO2.

[0068] 3) Uniform loading of auxiliary components

[0069] 0.5 mol / L yttrium nitrate solution was added dropwise to the modified SiO2 carrier loaded with ZrO2 three times (equally divided), with the mass ratio of the total volume to the modified SiO2 carrier loaded with ZrO2 being 1.25 mL:1 g. After each addition, ultrasonic treatment was performed for 12 min (frequency 50 kHz, power 110 W), and then stirred at 250 rpm for 35 min. Finally, the mixture was allowed to stand for 18 h, taken out and dried at 110 ° C for 11 h, and then calcined at 550 ° C for 4 h to obtain a urea hydrolysis catalyst for ammonia production.

[0070] The particle size of the urea hydrolysis catalyst for ammonia production obtained in this example is 1 mm, and the specific surface area is 212 m 2 / g, an average pore size of 7nm, a porosity of 0.51, and the ZrO2 and Y2O3 crystal structures were confirmed using XRD. The catalyst was stored in a sealed container, away from direct sunlight and high humidity.

[0071] The catalyst for preparing ammonia by urea hydrolysis obtained in this embodiment has a urea hydrolysis efficiency of 97.2% when operated at 350° C. for 1000 h in flue gas containing 81.5% N2, 4.4% O2, and 9.8% H2O (all percentages by volume and free of SO2, alkali metals, and dust). The catalyst has an activity retention rate of 92.5% when operated at 400° C. for 2000 h in flue gas containing 81.5% N2, 4.4% O2, and 9.8% H2O (all percentages by volume and free of SO2, alkali metals, and dust). The catalyst has a urea hydrolysis efficiency of 97.2% when operated at 350° C. for 1000 h in flue gas containing 81.5% N2, 4.4% O2, and 9.8% H2O (all percentages by volume and free of SO2, alkali metals, and dust). The catalyst has an activity retention rate of 92.5% when operated at 400° C. for 2000 h in flue gas containing 500 ppm SO2, 100 ppm total KNa alkali metal content, and 10 g / m3 dust. 3 In complex flue gas, after running at 300℃ for 1000h, the activity decreased by 5.8% and the pore blockage rate was 7.9%.

[0072] Comparative Example 1

[0073] The preparation method of the catalyst for producing ammonia by hydrolysis of urea in this comparative example comprises the following steps:

[0074] 1) Select particle size 0.5mm, specific surface area 220m 2 / g, an average pore size of 7nm, and a porosity of 0.49 as a carrier; the γ-Al2O3 carrier was placed in a muffle furnace and heated to 600°C, and calcined at a constant temperature for 3h to obtain a modified carrier;

[0075] 2) immersing the modified support in a 0.8 mol / L titanium nitrate solution at a volume ratio of 1 g to 0.5 mL of the transition metal salt solution, ultrasonically treating the support for 10 min (frequency 40 kHz, power 100 W), drying the support at 100° C. for 12 h, and calcining the support at 500° C. for 3 h to obtain a TiO2-loaded modified γ-Al2O3 support;

[0076] 3) A 0.3 mol / L lanthanum nitrate solution was added to the TiO2-loaded modified γ-Al2O3 support in a volume-to-mass ratio of 1 mL:1 g. The solution was first ultrasonically treated for 10 min (frequency 40 kHz, power 100 W) and then stirred at 200 rpm for 30 min. The solution was allowed to stand for 12 h, dried at 100°C for 12 h, and then calcined at 500°C for 3 h to obtain a urea hydrolysis catalyst for ammonia production.

[0077] The particle size of the urea hydrolysis catalyst for ammonia production obtained in this comparative example is 0.5 mm, and the specific surface area is 161 m 2 / g, an average pore size of 5.5nm, and a porosity of 0.35. Example 1 optimizes the TiO2 distribution through multi-level gradient deposition. The low-concentration solution, assisted by a silane modifier and ultrasound, penetrates into the deep pores of the particles to form a uniform base layer; medium and high concentrations are sequentially deposited in the middle pores and the surface to form a decreasing gradient. Therefore, Example 1 can ensure uniform pore filling. In this comparative example, a single high-concentration impregnation is used without silane modification. TiO2 is quickly adsorbed on the surface, blocking shallow pores, resulting in low utilization of internal pores. In addition, this comparative example uses a single high-temperature calcination to form large-grain TiO2. Therefore, compared with Example 1, the specific surface area, pore size, and porosity of this comparative example are significantly reduced.

[0078] The urea hydrolysis catalyst for ammonia production obtained in this comparative example has a urea hydrolysis efficiency of 88.3% when operated at 300°C for 1000h in flue gas containing 81.5% N2, 4.4% O2, and 9.8% H2O (all by volume and free of SO2, alkali metals, and dust); an activity retention rate of 78.6% when operated at 400°C for 2000h in flue gas containing 81.5% N2, 4.4% O2, and 9.8% H2O (all by volume and free of SO2, alkali metals, and dust); and an activity retention rate of 78.6% when operated at 400°C for 2000h in flue gas containing 500ppm SO2, 100ppm total alkali metal content of KNa, and 10g / m3 dust. 3In complex flue gas, after running at 300℃ for 1000h, the activity decreased by 18.7% and the pore blockage rate was 25.4%.

[0079] The above embodiments are merely examples for clarification and are not intended to limit the implementation methods. Those skilled in the art will appreciate that other variations or modifications may be made based on the above descriptions. It is not necessary and impossible to enumerate all implementation methods here, and any obvious variations or modifications derived therefrom are still within the scope of protection of the present invention.

Claims

1. A method for preparing a catalyst for producing ammonia by hydrolysis of urea, characterized in that: The following steps are involved: 1) After calcining the support, adding a mixed solution of a silane coupling agent and anhydrous ethanol, performing ultrasonic treatment, taking out and drying to obtain a modified support; 2) immersing the modified support in a transition metal salt solution multiple times, and after each immersion, first ultrasonically treating the support, then removing the support, drying it, and calcining it, and repeating this process multiple times to obtain a modified support loaded with a transition metal oxide; 3) Adding a rare earth metal salt solution or an alkali metal salt solution to the modified support loaded with the transition metal oxide in portions, first ultrasonically treating and then stirring after each addition, and after all additions are completed, allowing the solution to stand, taking it out, drying it, and calcining it to obtain a urea hydrolysis catalyst for ammonia production.

2. The method for preparing a catalyst for producing ammonia by hydrolysis of urea according to claim 1, wherein: The transition metal salt solution is one or more of titanium nitrate solution, zirconium nitrate solution, and cerium nitrate solution, with a concentration of 0.05 to 1.0 mol / L; the modified support is immersed in the transition metal salt solution 2 to 4 times, and the concentration of the transition metal salt solution is gradually increased with an increase gradient of 0.2 to 0.4 mol / L.

3. The method for preparing a catalyst for producing ammonia by hydrolysis of urea according to claim 1, wherein: The modified support is immersed in transition metal salt solution three times, the concentration of the first transition metal salt solution is 0.1-0.2 mol / L, the concentration of the second transition metal salt solution is 0.4-0.6 mol / L, and the concentration of the third transition metal salt solution is 0.8-1.0 mol / L.

4. The method for preparing a catalyst for producing ammonia by hydrolysis of urea according to claim 1, wherein: In step 2), each time the immersion is performed, the volume ratio of the modified support mass to the transition metal salt solution is 1 g: (0.5-1) mL, the single ultrasonic time is 5-15 min, the calcination temperature is 300-600° C., and the single calcination time is 2-4 h.

5. The method for preparing a catalyst for producing ammonia by hydrolysis of urea according to claim 1, wherein: The rare earth metal salt solution is one of a lanthanum nitrate solution and a yttrium nitrate solution, and the alkali metal salt solution is one of a potassium nitrate solution and a sodium nitrate solution, and the concentration of both is 0.05 to 0.8 mol / L; the rare earth metal salt solution or the alkali metal salt solution is added in 2 to 3 times, and the mass ratio of the total added volume to the modified support loaded with the transition metal oxide is (1 to 2) mL:1 g.

6. The method for preparing a catalyst for producing ammonia by hydrolysis of urea according to claim 1, wherein: In step 3), the single ultrasonic time is 5 to 15 minutes, the single stirring time is 20 to 40 minutes, the stirring speed is 100 to 300 rpm, the standing time is 12 to 24 hours, the calcination temperature is 400 to 600° C., and the calcination time is 2 to 4 hours.

7. The method for preparing a catalyst for producing ammonia by hydrolysis of urea according to claim 1, wherein: The carrier is γ-Al2O3 or SiO2, with a particle size of 0.1 to 2 mm and a specific surface area of ​​≥200 m 2 / g, an average pore size of 5 to 10 nm, and a porosity of 0.4 to 0.6; the particle size of the urea hydrolysis ammonia catalyst is 0.1 to 2 mm, and the specific surface area is 150 to 250 m 2 / g, the average pore diameter is 4-10nm, and the porosity is 0.35-0.

55.

8. The method for preparing a catalyst for producing ammonia by hydrolysis of urea according to claim 1, wherein: In step 1), the calcination temperature of the carrier is 600-800° C., the calcination time is 2-4 hours, the mixed solution of the silane coupling agent and anhydrous ethanol is composed of the silane coupling agent and anhydrous ethanol in a mass ratio of 1:(10-50), the mass ratio of the carrier to the silane coupling agent is (10-50):1, and the ultrasonic time is 20-40 minutes.

9. The method for preparing a catalyst for producing ammonia by hydrolysis of urea according to claim 1, wherein: The silane coupling agent is KH-550 or KH-570; the frequency of the ultrasonic treatment is 40-60 kHz, and the power is 80-120 W; in step 1), the drying temperature is 80-100° C., and the drying time is 4-6 hours; in step 2) and step 3), the drying temperature is 80-120° C., and the drying time each time is 8-12 hours.

10. A catalyst for producing ammonia by hydrolysis of urea prepared by the method according to any one of claims 1 to 9.

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