An ammonia decomposition catalyst and a method for producing the same

Abstract

The application belongs to the technical field of ammonia decomposition process, and particularly relates to an ammonia decomposition catalyst and a preparation method thereof. The preparation method comprises the following steps: S1, mixing and impregnating a biological template with reagent A to obtain an optimized biological template; S2, mixing and impregnating the optimized biological template with reagent B, drying and then calcining to obtain a catalyst carrier; S3, mixing and impregnating the catalyst carrier with reagent C to obtain a catalyst precursor; and S4, activating the catalyst precursor to obtain the ammonia decomposition catalyst. The reagent A contains carboxylic acid and hydrogen ions, the impregnation time is 1-24 hours, and the impregnation temperature is 20-80 DEG C. The biological template after the impregnation treatment of the reagent A can better adsorb metal ions, and the prepared metal oxide catalyst carrier has more uniform and complete micro-morphology and greater specific surface area, so that the catalytic efficiency and stability of the ammonia decomposition catalyst are improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of ammonia decomposition process, and particularly relates to an ammonia decomposition catalyst and a preparation method thereof. BACKGROUND

[0002] Hydrogen energy is one of the most potential clean energies, and has the outstanding advantages of high efficiency, cleanness and zero carbon environmental protection. In hydrogen production technologies, ammonia decomposition hydrogen production has the characteristics of high raw material conversion efficiency and no pollution, and is an ideal hydrogen source. The traditional ammonia decomposition reaction still needs a reaction temperature of 800-850 DEG C under the action of a catalyst, and has high energy consumption, so researchers hope to design a catalyst capable of realizing low-temperature catalytic ammonia decomposition.

[0003] Ruthenium metal nanoparticles are a kind of excellent catalytic active ingredients, and can act at low temperature and low pressure. The ruthenium metal nanoparticles need to be supported by a metal oxide with rich pore structure as a catalyst carrier, and the carrier structure morphology also affects the catalytic effect of ruthenium. The biomimetic template method is a synthesis method of the metal oxide carrier, and the principle is to use a natural biological material with a special structure as a template, to adsorb, deposit and remove metal ions on the template, so that the metal oxide obtains a microstructure similar to the biological template. In the previous patent CN 104289217A of the inventors, a preparation method of a biological morphology composite oxide supported ruthenium-based ammonia synthesis catalyst is proposed, and the patent uses Chinese fir, filter paper, cotton fiber, amino acid or protein as a biological template, and directly mixes and impregnates the biological template with a metal salt solution to obtain a catalyst carrier. The inventors further found that although the preparation method directly using the biological template is simple and fast, the adsorption of metal ions is easily saturated due to the limitation of the number of active groups on the surface of the biological template, the particle size of the prepared metal oxide carrier is uneven, the specific surface area of the carrier is limited, the loading capacity of the active component is limited, and the catalyst is easily aged and deactivated. SUMMARY

[0004] Therefore, the inventors hope to propose an optimized ammonia decomposition catalyst and a preparation method thereof, to improve the adsorption capacity of the biological template for metal ions by optimizing the biological template, and to improve the microstructure of the metal oxide carrier and increase the specific surface area, so that the obtained catalyst product has a longer service life. On this basis, the ammonia decomposition catalyst also has the same or even better catalytic effect as the biological template method before optimization.

[0005] The application is implemented by the following technical solutions:

[0006] A preparation method of an ammonia decomposition catalyst, characterized in that at least the following steps are included:

[0007] S1, impregnate the biological template with reagent A, and obtain an optimized biological template after drying treatment;

[0008] S2, the optimized biomorphic template is mixed with reagent B for impregnation, dried and treated, and then calcined to obtain a catalyst carrier;

[0009] S3, the catalyst carrier is mixed with reagent C for impregnation and dried to obtain a catalyst precursor;

[0010] S4, the catalyst precursor is activated to obtain the ammonia decomposition catalyst.

[0011] The reagent A contains a carboxylic acid, the mixing and impregnation in S1 is performed for 1-24 h, and the temperature for the mixing and impregnation in S1 is 20-80°C.

[0012] The reagent B is a solution containing at least magnesium ions and barium ions;

[0013] The catalyst carrier is a Ba-MgO carrier;

[0014] The reagent C contains 0.2wt%-5wt% ruthenium element.

[0015] S1 is a pretreatment step, which aims to optimize the microstructure of the biomorphic template; S2 is a preparation step of a metal oxide carrier, in which metal ions are adsorbed and deposited on the surface of the optimized biomorphic template through impregnation, and a biomorphic metal oxide Ba-MgO carrier is obtained after drying and calcination; S3 is a loading step of an active component, in which the active component ruthenium contained in reagent C is dispersed and loaded on the Ba-MgO carrier; and S4 is an activation step, in which the catalytic activity of ruthenium is fully released.

[0016] As a preferred embodiment, the reagent A contains a hydrogen ion donor reagent for adjusting the abundance of hydrogen ions in the reagent A.

[0017] As a preferred embodiment, the reagent A is a 1:4 (v:v) mixed solution of 3 mol / L hydrochloric acid solution and 3 mol / L acetic acid solution.

[0018] As a preferred embodiment, the concentration of magnesium ions in the reagent B is 0.1-0.8 mol / L, and the molar ratio of magnesium ions to barium ions is 100-180:1.

[0019] As a preferred embodiment, the reagent B is an acetate solution.

[0020] As a preferred embodiment, the temperature for the mixing and impregnation in S1 is 50-80°C.

[0021] As a preferred embodiment, the mixing and impregnation in S1 is performed for 1-5 h.

[0022] As a preference, the biological template is plant fiber. Cellulose in plant fiber has a multi-level network structure and a high specific surface area, and is an excellent biological template; cellulose is a macromolecular polysaccharide composed of glucose, and has hydroxyl groups in the structure, which can be combined with inorganic substances and replicate the morphology; cellulose mainly contains carbon, hydrogen and oxygen elements, and does not contain other toxic elements that can poison the catalyst, and as a biological template, it is easy to remove and does not affect the catalytic activity of the final catalyst.

[0023] As a preference, the reagent C is a tetrahydrofuran solution of Ru3(CO) 12 . The reagent C is used to load the catalytically active ruthenium element onto the catalyst carrier. During the impregnation process, the ruthenium element in the form of Ru3(CO) 12 is loaded onto the carrier, and after the subsequent activation process, Ru3(CO) 12 can decompose in the low temperature zone, and the ruthenium remains in the carrier, and the remaining elements become CO and are removed, without introducing other adverse elements.

[0024] The application also includes an ammonia decomposition catalyst, which is characterized by being prepared by the preparation method of any one of the above.

[0025] The application has the following beneficial effects:

[0026] 1. Catalyst performance

[0027] The application provides a Ba-MgO carrier ruthenium-based ammonia decomposition catalyst based on a pretreated biological template, which can realize high-activity catalysis at low temperature and normal pressure, and greatly reduces the difficulty of ammonia decomposition reaction. Experimental results show that the ammonia decomposition catalyst obtained by the application can improve the conversion rate of ammonia decomposition to 90% under the conditions of 450℃, normal pressure and 7800h -1 , and the ammonia decomposition catalyst prepared under the preferred parameters can even improve the conversion rate to more than 99%. The ammonia decomposition catalyst obtained by the application also has stability, and the catalytic performance does not change obviously after 100 hours of continuous testing under the conditions of 450℃ and 10000h -1 .

[0028] 2. Preparation method

[0029] Compared with the prior art, the application improves the structure of the biological template by adding a pretreatment step. The application compares the parameters of the pretreatment step and its preferences through extensive experiments, and these parameters also form a mutually promoting relationship, so that the biological template can adsorb and combine more metal ions, and finally construct a metal oxide carrier with a larger specific surface area and better uniformity. The principle includes:

[0030] 2.1. The biological template is a cellulose polymer, and the hydrogen ions in reagent A can penetrate the beta-1, 4-glycosidic bond of the cellulose molecule to break the long cellulose chain and expose the hydroxyl groups inside the polymer; the magnesium ions and barium ions are combined with the hydroxyl groups on the cellulose, which is conducive to the more accurate and sufficient replication of the biological template form by the Ba-MgO metal oxide carrier;

[0031] 2.2. Reagent A is an acidic solution containing carboxylic acid and hydrogen ions, wherein the hydrogen ions open the long cellulose chain, and the carboxyl groups can enter the interior of the cellulose polymer to make the cellulose swell, which is conducive to the further penetration of hydrogen ions; when only carboxylic acid exists, the degree of ionization of hydrogen ions is low, and the opening degree of the long cellulose chain is limited; when only strong acid hydrogen ions exist, the hydrogen ions are easy to cause excessive hydrolysis of cellulose, and the hydrogen ions cannot sufficiently penetrate the interior of the long chain; when the carboxyl groups and hydrogen ions synergistically act, they can promote each other to fully expose the hydroxyl groups on the cellulose;

[0032] 2.3. Reagent B contains acetate, which is easy to combine with cellulose and can be removed together with the biological template during calcination; the acetate also has a large ionic radius, which can leave more vacancies during the removal of the biological template, thereby increasing the specific surface area of the biological form Ba-MgO carrier, and further improving the ammonia decomposition activity of the catalyst;

[0033] 2.4. The temperature of the mixing and impregnation in S1 is controlled at 60-80 DEG C, and the time is controlled at 2-3 h, which can make the degree of hydrolysis of cellulose in the best state, neither causing excessive hydrolysis of cellulose, nor promoting the synergistic effect of hydrogen ions and carboxyl groups, thereby improving the ammonia decomposition activity of the catalyst;

[0034] 2.5. The foregoing technical means 2.1-2.4 optimize the proportion and dispersion of the promoter barium in the carrier by treating the biological template, which is conducive to the dispersion of the active component ruthenium in the subsequent step, and helps to improve the strong interaction between ruthenium and the biological form Ba-MgO carrier, thereby improving the activity and stability of the catalyst.

[0035] In addition, the preparation method proposed in the present application reduces the use amount of strong acid and shortens the pretreatment time by mild heating treatment and the addition of carboxylic acid, and completes the incorporation of the promoter barium and the preparation of the carrier in one step, which is simple in steps, practical, and has industrialization prospect. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 is a catalyst stability test diagram;

[0037] Figure 2 is an FE-SEM diagram of the biological template morphology in each stage of the experiment;

[0038] Figure 3FE-SEM images of each biological template and biological morphology Ba-MgO carrier;

[0039] Figure 4 FE-SEM images of cotton fibers after treatment with different reagent A. DETAILED DESCRIPTION

[0040] The present application will be further described below with reference to the accompanying drawings and specific examples. Those skilled in the art will be able to implement the present application based on these descriptions. In addition, the embodiments of the present application involved in the following description are generally only embodiments of a part of the present application, rather than all embodiments. Therefore, all other embodiments obtained by those skilled in the art based on the embodiments in the present application without making creative efforts shall fall within the scope of protection of the present application.

[0041] 1. Experimental scheme

[0042] 1.1 Preparation of reagents

[0043] 3 mol / L hydrochloric acid solution; 3 mol / L acetic acid solution;

[0044] Magnesium acetate-barium acetate solution, in this experiment, the preparation method is as follows: magnesium acetate and barium acetate are weighed according to n(Mg 2+ )∶n(Ba 2+ )=166∶1, and dissolved with distilled water to obtain a 0.3 mol / L aqueous solution of Mg 2+ .

[0045] Magnesium nitrate-barium nitrate solution, the preparation method is as follows: magnesium nitrate and barium nitrate are weighed according to n(Mg 2+ )∶n(Ba 2+ )=166∶1, and dissolved with distilled water to obtain a 0.3 mol / L aqueous solution of Mg 2+ .

[0046] Tetrahydrofuran solution of Ru3(CO) 12 with a ruthenium content of 2wt%;

[0047] In other embodiments, the magnesium salt-barium salt solution can be other chloride salts or other acid forms, the molar ratio of magnesium ions to barium ions can be in the range of 100-180:1, the concentration of magnesium ions can be in the range of 0.1-0.8 mol / L; the ruthenium content in the Ru3(CO) 12 tetrahydrofuran solution can be in the range of 0.2wt%-5wt%.

[0048] 1.2 Design of experimental parameters

[0049] Table 1 summarizes the parameter differences between the examples and the comparative examples. The order of the numbers of the examples is related to the parameter optimization ideas.

[0050] Table 1 Parameter design of the examples and the comparative examples

[0051]

[0052]

[0053] 1.3 Experimental process

[0054] Comparative Example 1

[0055] This comparative example and Comparative Example 2 form a control relationship with “biological template” as a variable. Comparative Example 2 is relatively preferred.

[0056] S1, mix a sufficient amount of wheat straw with deionized water, immerse at room temperature for 24 hours; wash the wheat straw to neutral with deionized water, and dry at 110°C;

[0057] S2, take 400 mL of magnesium acetate-barium acetate solution, and then take the same mass of wheat straw obtained in S1 as magnesium acetate, mix and immerse for 48 hours, and then dry at 110°C for 12 hours; calcine in a muffle furnace at 600°C for 3 hours to obtain a biological Ba-MgO carrier;

[0058] S3, mix the biological Ba-MgO carrier obtained in S2 with a Ru3(CO) 12 tetrahydrofuran solution with a ruthenium content of 2wt% for 12 hours; evaporate the solvent in a water bath, and dry at 60°C in an oven;

[0059] S4, vacuum treatment at 450°C in a tube furnace for 3 hours, and cool to room temperature in a H2 atmosphere;

[0060] S5, tabletting, crushing, and sieving to obtain an available form of ammonia decomposition catalyst.

[0061] Comparative Example 2

[0062] The difference between this comparative example and Comparative Example 1 is that the biological template used in S1 of this comparative example is cotton fiber; the other steps and parameters are the same as those of Comparative Example 1.

[0063] This comparative example and Comparative Example 1 form a control relationship with “biological template” as a variable; the results show that cotton fiber is relatively preferred as a “biological template”.

[0064] Comparative Example 3

[0065] The difference between this comparative example and Comparative Example 2 is that the reagent B used in S2 of this example is a magnesium nitrate-barium nitrate solution; the other steps and parameters are the same as those of Comparative Example 2.

[0066] This example and Comparative Example 2 form a control relationship with "the salt form of Reagent B" as the variable; the results show that acetate is relatively preferred for Reagent B.

[0067] Example 1

[0068] This example differs from Comparative Example 2 in that Reagent A is used in the impregnation with the biomatrix in Example S1; the Reagent A used is a 3 mol / L hydrochloric acid solution; the other steps and parameters are the same as in Comparative Example 2.

[0069] This example and Comparative Example 2 form a control relationship with "whether impregnation is performed with Reagent A" as the variable; the results show that impregnation with Reagent A is relatively preferred.

[0070] Example 2

[0071] This example differs from Example 1 in that the Reagent A used in Example S1 is a 3 mol / L acetic acid solution; the other steps and parameters are the same as in Example 1.

[0072] This example and Example 1 form a control relationship with "the hydrogen ion concentration of Reagent A" as the variable; the results show that a higher hydrogen ion concentration is relatively preferred. Hydrogen ions can destroy the -C-O-glycosidic bonds between adjacent glucose monomers on cellulose, causing the cellulose long chain to break, exposing more active hydroxyl groups.

[0073] Example 3

[0074] This example differs from Example 1 in that the Reagent A used in Example S1 is impregnated at 80°C for 2 hours; the other steps and parameters are the same as in Example 1.

[0075] This example and Example 1 form a control relationship with "the impregnation temperature of Reagent A" and "the impregnation time of Reagent A" as the variables; the results show that short-term heating treatment can improve the ammonia conversion rate of the obtained catalyst in a working environment below 450°C. The reason for this is that heating can promote the swelling of cellulose and make it loose, which is conducive to the entry of hydrogen ions into the interior of the cellulose long chain.

[0076] Example 4

[0077] This example differs from Example 3 in that the Reagent A used in Example S1 is impregnated at 80°C for 3 hours; the other steps and parameters are the same as in Example 3.

[0078] This example forms a control relationship with Example 3 with "length of impregnation under heating condition" as the variable; the results show that further prolonging the impregnation time under heating condition will reduce the ammonia conversion rate of the catalyst in the working environment below 450°C. The reason can be that long-time heating causes excessive hydrolysis of cellulose, and the internal structure is destroyed.

[0079] Example 5

[0080] The difference between this example and Example 2 is that the use condition of reagent A in this example S1 is 80°C for 24 hours of impregnation; other steps and parameters are the same as those in Example 2.

[0081] This example forms a control relationship with Example 2 with "impregnation temperature during acetic acid treatment" as the variable; the results show that when treated with the same acid, a higher impregnation temperature is relatively preferred.

[0082] Example 6

[0083] The difference between this example and Example 3 is that the component of reagent A in this example S1 is 3 mol / L hydrochloric acid solution: 3 mol / L acetic acid solution 1:4 configuration; other steps and parameters are the same as those in Example 3.

[0084] This example forms a control relationship with Example 3 with "whether the carboxyl group is contained in the acidic environment" as the variable; the results show that the addition of the carboxyl group is a preferred condition.

[0085] Example 7

[0086] The difference between this example and Example 6 is that the component of reagent A in this example S1 is 3 mol / L hydrochloric acid solution: 3 mol / L acetic acid solution 1:4 configuration; other steps and parameters are the same as those in Example 6.

[0087] This example forms a control relationship with Example 6 with "content of carboxyl group in the acidic environment" as the variable; the results show that when the content of hydrogen ions is sufficient, the increase of the content of the carboxyl group is a preferred condition.

[0088] Example 8

[0089] The difference between this example and Example 7 is that the impregnation temperature of reagent A in this example S1 is 60°C; other steps and parameters are the same as those in Example 7.

[0090] This example forms a control relationship with Example 7 with "impregnation temperature of reagent A" as the variable; in combination with the conclusion of Example 5, the component of reagent A is a relatively major factor affecting the catalytic efficiency of the product of the example, and the impregnation temperature is a relatively secondary factor.

[0091] 2. Experimental results

[0092] 2.1 Catalyst catalytic efficiency evaluation

[0093] The ammonia decomposition catalysts prepared in Comparative Examples 1-3 and Examples 1-8 were evaluated for activity in a fixed bed reactor. The reaction tube had an inner diameter of 8 mm, and the length of the dynamic isothermal zone was 30 mm, with the catalyst being loaded in the isothermal zone. The reaction temperature was 300-500°C, the reaction pressure was atmospheric pressure, the reaction volume space velocity was 7800 h -1 -1, and the raw material gas was 99.999% high-purity ammonia. The ammonia concentration at the reactor outlet (expressed as a volume percentage, vol%) was determined using the sulfuric acid solution neutralization method, and was used as an evaluation index for the activity of the catalyst. The experimental results are shown in Table 2.

[0094] Table 2 Comparison of ammonia conversion at different temperatures for each of the comparative examples and examples

[0095]

[0096] As shown in Table 2, Example 7 had the best catalytic efficiency at 450°C, reaching 99.65%. The corresponding optimal technical solution was: in S1, cotton fibers were used as the biomorphic template, 3 mol / L hydrochloric acid:3 mol / L acetic acid 1:4 was used as reagent A, and the impregnation was performed at 80°C for 2 hours; in S2, a magnesium acetate-barium acetate solution was used as reagent B; the other steps and parameters were the same as in Comparative Example 1. Example 1 still achieved a catalytic efficiency of more than 90% at a temperature of 500°C, at which time the corresponding pretreatment parameters were room temperature (20°C) and 3 mol / L hydrochloric acid for 24 hours. It can be seen that increasing the impregnation temperature can simultaneously achieve the effects of shortening the impregnation time and improving the catalytic efficiency, so the preferred value range of this technical solution can be further expanded, i.e., the impregnation temperature range is 50-80°C, and the mixed impregnation time is 1-5 hours.

[0097] 2.2 Catalyst stability evaluation

[0098] Based on the experimental results of 2.1, the ammonia decomposition catalyst prepared in Example 7 had the best activity, and was tested for stability. The results are shown in Table 3. Figure 1 At 450°C and 10000 h -1 , the conversion rate of the catalyst was stably maintained at about 98%, and the catalytic performance did not change significantly within 100 hours. The principle may be that the biomorphic Ba-MgO carrier has more uniform and complete nanoparticles, has a larger specific surface area, can better disperse the active component ruthenium, and thus effectively inhibits the sintering of ruthenium at high temperatures, leading to catalyst deactivation.

[0099] 2.3 Morphology characterization

[0100] Figure 2Fig. 2 is the FE-SEM images of the biological templates and the biomorphic Ba-MgO supports in different stages of the experiment. 2-A is the original cotton fiber, 2-B is the cotton fiber treated by the mixture of hydrochloric acid and acetic acid, and 2-C is the biomorphic Ba-MgO support prepared by using 2-B as the biological template. The SEM images show that after the treatment of the mixture of hydrochloric acid and acetic acid, the impurities on the surface of the cotton fiber are removed, and more wrinkles are exposed. The biomorphic Ba-MgO support prepared by using the treated cotton fiber as the biological template has a more uniform spherical nano-morphology, and can better load and disperse the active component ruthenium.

[0101] Figure 3 Fig. 3 is the FE-SEM images of the biological templates and the biomorphic Ba-MgO supports in different stages of the experiment. 3-A is the original wheat straw, 3-B is the wheat straw immersed in 3 mol / L hydrochloric acid at room temperature for 24 hours, 3-C is the biomorphic Ba-MgO support prepared by using 3-B as the biological template, 3-D is the original cotton fiber, 3-E is the cotton fiber immersed in 3 mol / L hydrochloric acid at 60°C for 2 hours, and 3-F is the biomorphic Ba-MgO support prepared by using 3-E as the biological template.

[0102] Figure 4 Fig. 4 is the FE-SEM images of the cotton fibers treated by different reagent A. 4-A is the treatment result of 1 mol / L acetic acid, 4-B is the treatment result of 3 mol / L acetic acid, 4-C is the treatment result of 1 mol / L hydrochloric acid, and 4-D is the treatment result of 3 mol / L hydrochloric acid.

Claims

1. A method for preparing an ammonia decomposition catalyst, characterized in that, It should include at least the following steps: S1. The biological template is mixed with reagent A and impregnated, and then dried to obtain the optimized biological template; S2. The optimized biological template is mixed and impregnated with reagent B, dried, and then calcined to obtain a catalyst support. S3. The catalyst support is mixed with reagent C and impregnated, and then dried to obtain the catalyst precursor. S4. Activate the catalyst precursor to obtain the ammonia decomposition catalyst; Reagent A is a mixture of 3 mol / L hydrochloric acid solution and 3 mol / L acetic acid solution, with a mixing volume ratio V 3mol / L盐酸溶液 :V 3mol / L乙酸溶液 The ratio is 1:1 to 1:4; the duration of the mixed impregnation in S1 is 1 to 24 hours, and the temperature of the mixed impregnation in S1 is 20 to 80°C; The reagent B is a mixed solution of magnesium acetate and barium acetate; The catalyst support is a Ba-MgO support; The reagent C is Ru3(CO) with a ruthenium content of 2wt%. 12 Tetrahydrofuran solution; The biological template is plant fiber.

2. The preparation method according to claim 1, characterized in that, The concentration of magnesium ions in reagent B is 0.1–0.8 mol / L, and the molar ratio of magnesium ions to barium ions is 100–180:

1.

3. The preparation method according to claim 1, characterized in that, The temperature for the mixed impregnation described in S1 is 50–80°C.

4. The preparation method according to claim 1, characterized in that, The duration of the mixed impregnation described in S1 is 1 to 5 hours.

5. An ammonia decomposition catalyst, characterized in that, Prepared by the preparation method according to any one of claims 1 to 4.

Images