CN112058277A - High-activity catalyst for ammonia synthesis and preparation method thereof - Google Patents

Abstract

The invention discloses a highly active catalyst for ammonia synthesis and a preparation method thereof. Cobalt is first grown on a cerium oxide carrier in situ, and then ruthenium is introduced, thereby using ruthenium and cobalt bimetals as active components. The high-activity catalyst for ammonia synthesis is specifically obtained by mixing the cerium precursor and the cobalt precursor and dissolving in water, mixing with the alkaline solution and stirring uniformly, and obtaining a solid solution through a hydrothermal reaction, and then washing, drying, and stirring to obtain a solid solution. After calcination, the obtained sample was impregnated with ruthenium precursor solution, and finally prepared by reduction. The catalyst obtained by the invention has higher ammonia synthesis activity and better application prospect.

Description

A kind of high activity catalyst for ammonia synthesis and preparation method thereof

technical field

The invention belongs to the technical field of ammonia synthesis catalysts, and in particular relates to a high-activity catalyst for ammonia synthesis and a preparation method thereof.

Background technique

Ammonia is an important chemical intermediate in the production of agricultural fertilizers, plastics, drugs, explosives and other chemicals. Ammonia is an almost ideal hydrogen storage material due to its appreciable hydrogen capacity of 17.6 wt% and its easy condensing into a liquid state for storage or transport. At the same time, the high energy density of 4.3 kWh h ^–1 and zero carbon emissions make ammonia an ideal energy carrier and even an alternative to hydrogen in the future energy economy. Therefore, the importance of ammonia in the field of new energy has become increasingly significant, and the development of environmentally beneficial ammonia synthesis methods has attracted extensive attention.

It is well known that the dissociation of _N2 is a rate-controlling step in ammonia synthesis. Based on the relationship of the volcano curve, Ru metal with good nitrogen adsorption and dissociation ability is considered to be the most ideal active component for ammonia synthesis catalysts, but the high cost and stability of ruthenium-based catalysts are still controversial. While in the volcano-type curve, the binding energy of pure Co to nitrogen species is much lower _than that of Ru, which means that the adsorption energy of Co to nitrogen is small, while the energy barrier of N dissociation is larger, so when Co alone acts as the active metal , the ammonia synthesis activity of the catalyst is weak.

In order to prepare high-efficiency catalysts, it is necessary to break the relationship between the adsorption energies of catalysts for reactants and intermediates. Chen et al. (Chen, P.; Gao, W.; Wang, P. et al. Barium Hydride-Mediated NitrogenTransfer and Hydrogenation for Ammonia Synthesis: A Case Study of Cobalt. ACS Catal. 2017, 7(5):3654-3661. ) prepared a dual-site catalyst (LiH–TM), and the activity of the catalyst was best when Co metal was present in the LiH–TM catalyst. The patent (CN 107376996A) reports a ruthenium-cobalt bimetallic nano-supported catalyst for hydrogen hydrolysis of ammonia borane and its preparation method. Adding MIL-110 aqueous solution, and finally adding NaBH4 solution, a catalyst with MIL _- 110 as carrier and Ru-Co as active component was prepared, named as RuCo@MIL-110 catalyst, in which the molar ratio of ruthenium metal to cobalt metal is 1:1, the catalyst exhibits good catalytic performance in the hydrolysis of hydrogen by ammonia borane, with extremely high anti-toxicity and cycle stability.

In the preparation process of the existing ruthenium-cobalt bimetallic catalyst, two active metal precursors are usually mixed and added to the carrier, and the difference in the interaction between the two metals and the carrier will affect the catalytic effect of the active metal. In the present invention, cobalt and ruthenium metals are introduced into the ammonia synthesis catalyst supported by cerium oxide to prepare a double active center catalyst, which can change the interaction between the metal supports, and the synergistic effect of the double active centers is expected to improve the reaction performance of ammonia synthesis.

SUMMARY OF THE INVENTION

In view of the deficiencies of the existing Ru catalysts, the present invention provides a high-activity catalyst for ammonia synthesis and a preparation method thereof, which can efficiently catalyze the synthesis of ammonia.

To achieve the above object, the present invention adopts the following technical solutions:

A high-activity catalyst for ammonia synthesis, which is to first grow cobalt on a cerium oxide carrier in situ, and then introduce ruthenium to form a catalyst with ruthenium and cobalt bimetals as active components; Calculated on the basis of mass, the loading of cobalt is 0.003-1.7 wt %, and the loading of ruthenium is 0.1-20 wt %.

The preparation method of the high activity catalyst comprises the following steps:

1) The cobalt precursor and the cerium precursor are mixed and dissolved in deionized water to prepare a mixed solution;

2) After the mixed solution obtained in step 1) is mixed with the alkaline solution and stirred evenly, hydrothermally reacts for a period of time to obtain a solid solution, which is washed and dried and then calcined;

3) The sample obtained in step 2) is impregnated with a ruthenium precursor solution, and the highly active catalyst is obtained after reduction.

The cerium precursor in step 1) is any one of cerium oxalate, cerium nitrate, and cerium acetate; the cobalt precursor is any one of cobalt nitrate, cobalt oxalate, and cobalt chloride; Ce in the obtained mixed solution The concentration of ions is 0.1-0.5 mol/L.

The volume ratio of the alkaline solution and the mixed solution used in step 2) is 8:1-2:1; the alkaline solution is an aqueous sodium hydroxide solution or an aqueous potassium hydroxide solution, and its concentration is 6-10 mol/L.

The stirring time in step 2) is 0.5-8 hours; the temperature of the hydrothermal reaction is 60-180 °C, and the time is 1-36 hours; the drying temperature is 60-120 °C, and the time is 0.5- 18 hours; the calcination temperature is 250-600°C, and the time is 0.5-6 hours.

The ruthenium precursor solution described in step 3) is prepared with any one of ruthenium chloride, ruthenium carbonyl, and ruthenium nitrosyl nitrate as a solute, and any one or both of methanol, ethanol, and water as a solvent. ; The concentration of Ru ions in the ruthenium precursor solution is 1 g/L-20 g/L, and the volume ratio of alcohol in the solvent is not less than 50%.

In step 3), a hydrogen-containing mixed gas is used for reduction, wherein the volume ratio of hydrogen is 1%-100%, and the rest is argon or nitrogen; the temperature of the reduction is 150-600 ° C, and the time is 0.1-36 hours .

Significant advantages of the present invention:

In the preparation process of the ruthenium-cobalt bimetallic catalyst, cobalt is first grown on the cerium oxide carrier in situ, and then the ruthenium precursor is introduced after washing and calcining. The catalyst prepared by this method has appropriate metal carrier interaction. Compared with single-metal supported Ru/CeO ₂ , Co/CeO ₂ catalysts and the ruthenium-cobalt bimetallic catalyst prepared by the co-addition of traditional bimetals, the catalyst prepared by the present invention exhibits better ammonia synthesis performance and has better performance. prospects for industrial applications.

Description of drawings

Figure 1 shows the Raman spectra of the cerium oxide carrier and the catalysts 0.5RuCo _0.15 /Ce and 0.5Ru/CeO ₂ prepared in Example 2 and Comparative Example 1.

2 is the H ₂ -TPD mass spectra of catalysts 0.5RuCo _0.15 /Ce and 0.5Ru/CeO ₂ prepared in Example 2 and Comparative Example 1.

Detailed ways

In order to make the content of the present invention easier to understand, the technical solutions of the present invention will be further described below with reference to specific embodiments, but the present invention is not limited thereto.

Example 1:

Take 2.538 g of cerium acetate and 23 mg of cobalt nitrate and dissolve them in 20 mL of deionized water to form a mixed solution, and take 38.4 g of NaOH and dissolve them in 140 mL of distilled water to form an alkaline solution; then mix and stir the two solutions for 1 hour. The solid solution was obtained after 6 hours of hydrothermal reaction; after the solid solution was washed to neutrality, dried at 100 °C overnight, and calcined at 400 °C for 1 hour, the obtained sample was impregnated with 10 g/L carbonyl ruthenium alcohol solution for 3 hour; then in the H ₂ -Ar mixed gas with a hydrogen volume concentration of 10%, the catalyst 1RuCo _0.1 /Ce was obtained after reduction at 600 ° C for 4 hours. Calculated on the basis of the mass of cerium oxide, ruthenium metal and cobalt metal in the catalyst The addition amount is 1% and 0.1%, respectively.

Example 2:

Take 1.736 g of cerium nitrate and 2.6 mg of cobalt chloride and dissolve them in 30 mL of deionized water to form a mixed solution, and take 20 g of KOH and dissolve them in 60 mL of distilled water to form an alkaline solution; then mix and stir the two solutions for 0.5 hours. The solid solution was obtained after hydrothermal reaction at 100 °C for 24 hours; after the solid solution was washed to neutrality, dried at 80 °C for 2 hours, and calcined at 550 °C for 2 hours, the obtained sample was treated with 15 g/L nitrosyl nitric acid The ruthenium alcohol solution was soaked in ruthenium alcohol solution for 2 hours; then the catalyst was reduced with pure hydrogen at 500 °C for 2 hours to obtain a catalyst of 0.5RuCo _0.15 /Ce. Calculated based on the mass of cerium oxide, the addition amounts of ruthenium metal and cobalt metal in the catalyst were 0.5 %, 0.15%.

Example 3:

Take 3.472 g of cerium nitrate and 2.3 mg of cobalt nitrate and dissolve them in 40 mL of deionized water to form a mixed solution, and take 38.4 g of NaOH and dissolve them in 140 mL of distilled water to form an alkaline solution; then mix and stir the two solutions for 2 hours. The solid solution was obtained after thermal reaction for 24 hours; after the solid solution was washed to neutrality, dried at 80 °C for 2 hours, and calcined at 550 °C for 2 hours, the obtained sample was impregnated with 5 g/L ruthenium chloride alcohol solution for 2 hours. 10% H ₂ -3.3% N ₂ -86.7% Ar mixed gas at 350 ℃ for 4 hours to obtain catalyst 0.5RuCo _0.05 /Ce, calculated on the basis of the mass of cerium oxide, the ruthenium metal in the catalyst The addition amount of cobalt metal is 0.5% and 0.05%, respectively.

Example 4:

Dissolve 4.352 g of cerium oxalate and 7.5 mg of cobalt acetate in 40 mL of deionized water to form a mixed solution, and take 38.4 g of KOH and dissolve it in 140 mL of distilled water to form an alkaline solution; then mix and stir the two solutions for 2 hours. The solid solution was obtained after thermal reaction for 24 hours; after the solid solution was washed to neutrality, dried at 80 °C for 2 hours, and calcined at 550 °C for 2 hours, the obtained sample was impregnated with 5 g/L ruthenium chloride alcohol solution for 4 hours. Then, the catalyst 3RuCo ₁ /Ce was obtained after reduction with pure hydrogen at 400 °C for 6 hours. Based on the mass of ceria, the additions of ruthenium metal and cobalt metal in the catalyst were 3% and 1%, respectively.

Comparative Example 1:

Dissolve 2.538 g of cerium acetate in 20 mL of deionized water to form a cerium acetate solution, and dissolve 38.4 g of NaOH in 140 mL of deionized water to form an alkaline solution; then mix and stir the two solutions for 2 hours, and perform a hydrothermal reaction at 100 °C for 24 After 1 hour, a solid solution was obtained; after the solid solution was washed to neutrality, dried at 80 °C for 2 hours, and calcined at 550 °C for ₂ hours, the obtained CeO carrier was impregnated with a 10 g/L ruthenium carbonyl alcohol solution for 3 hours; Then, in H ₂ -Ar mixed gas with a hydrogen volume concentration of 10%, a catalyst of 0.5Ru/CeO ₂ was obtained after reduction at 550 °C for 4 hours. Calculated on the basis of the mass of ceria, the addition amount of ruthenium metal in the catalyst was: 0.5%.

Comparative Example 2:

Dissolve 2.327 g of cerium nitrate in 40 mL of deionized water to form a cerium nitrate solution, and dissolve 38.4 g of NaOH in 140 mL of deionized water to form an alkaline solution; then mix and stir the two solutions for 2 hours, and perform a hydrothermal reaction at 100 °C for 24 A solid solution was obtained after 1 hour; after the solid solution was washed to neutrality, dried at 80 °C for 2 hours, and calcined at 500 °C for ₂ hours, the obtained CeO carrier was impregnated with 3 g/L cobalt nitrate alcohol solution for 4 hours; then A catalyst of 0.5Co/CeO ₂ was obtained after reduction in a H ₂ -Ar mixed gas with a hydrogen volume concentration of 10% at 600 °C for 4 hours. Calculated on the basis of the mass of cerium oxide, the addition amount of cobalt metal in the catalyst was 0.5 %.

Comparative Example 3:

Dissolve 2.538 g of cerium acetate in 20 mL of deionized water to form a cerium acetate solution, and dissolve 38.4 g of NaOH in 140 mL of deionized water to form an alkaline solution; then mix and stir the two solutions for 2 hours, and perform a hydrothermal reaction at 80 °C for 24 After 2 hours, a solid solution was obtained; after the solid solution was washed to neutrality, dried at 80 °C for 2 hours, and calcined at 550 °C for ₂ hours, the obtained CeO carrier was impregnated with 5 g/L cobalt chloride alcohol solution for 2 hours After being calcined in air at 550 °C for 4 hours, immersed in 10 g/L ruthenium carbonyl alcohol solution for 2 hours; finally, the mixture was reduced at 350 °C for 4 hours with H ₂ -Ar mixed gas with a hydrogen volume concentration of 10%. The catalyst is 0.5Ru/Co _0.15 /CeO ₂ , calculated on the basis of the mass of cerium oxide, and the addition amounts of ruthenium metal and cobalt metal in the catalyst are 0.5% and 0.15% respectively.

Comparative Example 4:

Take 2.538 g of cerium acetate and 23 mg of cobalt nitrate and dissolve them in 20 mL of ethanol to form a mixed solution, and take 38.4 g of NaOH and dissolve them in 140 mL of deionized water to form an alkaline solution; then mix and stir the two solutions for 1 hour. The solid solution was obtained after hydrothermal reaction at ℃ for 6 hours; the solid solution was washed to neutrality, dried at 100 ℃ overnight, and calcined at 400 ℃ for 1 hour; then the obtained sample was reduced with pure hydrogen at 500 ℃ for 6 hours to obtain the catalyst Co _0.15 Ce, calculated on the basis of the mass of cerium oxide, the addition amount of cobalt metal in the catalyst is 0.15%.

Comparative Example 5:

Take 1.736 g of cerium nitrate and 52 mg of cobalt chloride and dissolve it in 30 mL of deionized water to form a mixed solution, and take 20 g of KOH and dissolve it in 60 mL of deionized water to form an alkaline solution; then mix and stir the two solutions for 0.5 hours. The solid solution was obtained after hydrothermal reaction at 120 °C for 24 hours; after the solid solution was washed to neutrality, dried at 60 °C for 2 hours, and calcined at 500 °C for 2 hours, the obtained sample was treated with 15 g/L nitrosyl nitric acid The ruthenium alcohol solution was immersed for 3 hours; then the catalyst 1RuCo ₃ /Ce was obtained after reduction with pure hydrogen at 450 ℃ for 2 hours. Calculated on the basis of the mass of cerium oxide, the addition amount of ruthenium metal and cobalt metal in the catalyst was 1% respectively , 3%.

Comparative Example 6:

Dissolve 1.736 g of cerium nitrate in 30 mL of deionized water, add 15 g/L of ruthenium nitrosyl nitrate solution to form a mixed solution, and dissolve 20 g of KOH in 60 mL of distilled water to form an alkaline solution; then mix the two solutions After mixing and stirring for 0.5 hours, a solid solution was obtained after hydrothermal reaction at 100 °C for 24 hours; after the solid solution was washed to neutrality, it was dried at 80 °C for 2 hours, and calcined at 550 °C for 2 hours. L of cobalt nitrate alcohol solution was immersed for 2 hours; then reduced with pure hydrogen at 500 °C for 2 hours to obtain a catalyst 0.15CoRu _0.5 /Ce, calculated on the basis of the mass of cerium oxide, the addition amount of ruthenium metal and cobalt metal in the catalyst 0.5% and 0.15% respectively.

Figure 1 shows the Raman spectra of the cerium oxide carrier and the catalysts 0.5RuCo _0.15 /Ce and 0.5Ru/CeO ₂ prepared in Example 2 and Comparative Example 1. It can be seen from the figure that a new peak appears at 969 cm for ^0.5Ru /CeO ₂ compared with CeO ₂ , which can be attributed to the formation of Ru-O-Ce bonds. And compared with 0.5Ru/CeO ₂ , the Raman peak of 0.5RuCo _0.15 /CeO ₂ is stronger at 969 cm ^-1 , indicating that the addition of Co enhances the interaction of metal supports in the catalyst.

2 is the H ₂ -TPD mass spectra of catalysts 0.5RuCo _0.15 /Ce and 0.5Ru/CeO ₂ prepared in Example 2 and Comparative Example 1. As can be seen from the figure, the addition of Co changes the desorption path of hydrogen in the catalyst. The introduction of Co increases the amount of hydrogen desorption at high temperature in the catalyst, while the amount of water desorption decreases, which indicates that the introduction of Co can increase the active sites for ammonia synthesis in the catalyst. Therefore, the introduction of Co changes the interaction between the metal supports and the adsorption properties of the catalyst, thereby enhancing the activity of the catalyst.

The catalysts obtained in Examples 1-4 and Comparative Examples 1-6 were evaluated for catalytic activity in a high-pressure activity testing device. The reactor was a fixed bed with an inner diameter of 12 mm. During the test, 0.2 g of catalyst was mixed with larger particle size quartz sand and packed in the isothermal zone of the reactor. The reaction gas is a mixture of nitrogen and hydrogen obtained from ammonia high-temperature catalytic cracking, and the ratio of hydrogen to nitrogen is 3:1; the reaction conditions are: pressure 1 MPa, reaction temperature 400 ℃, reaction space velocity 3.6×10 ⁴ cm ³ g ^-1 h ^{- 1} , and the results are shown in Table 1.

Table 1 Ammonia synthesis reaction rates of catalysts

As can be seen from Table 1, under the conditions of the preparation method of the present invention, adding appropriate cobalt can improve the ammonia synthesis performance of the catalyst; at the same time, within the same addition range of ruthenium and cobalt, the ammonia synthesis rate of the catalyst obtained in the embodiment is higher than that of other methods. The ruthenium-based, cobalt-based and ruthenium-cobalt catalysts have high ammonia synthesis rates, which proves that they have good ammonia synthesis catalytic activity and have good application prospects.

The above descriptions are only preferred embodiments of the present invention, and all equivalent changes and modifications made according to the scope of the patent application of the present invention shall fall within the scope of the present invention.

Claims (8)

1. A method for preparing a high-activity catalyst for ammonia synthesis is characterized by comprising the following steps: the method comprises the following steps:

1) mixing a cobalt precursor and a cerium precursor, and dissolving the mixture in deionized water to prepare a mixed solution;

2) mixing the mixed solution obtained in the step 1) with an alkaline solution, uniformly stirring, carrying out a hydrothermal reaction for a period of time to obtain a solid solution, washing and drying the solid solution, and calcining the solid solution;

3) dipping the sample obtained in the step 2) by using a ruthenium precursor solution, and reducing to obtain the high-activity catalyst.

2. The method for preparing a high-activity catalyst according to claim 1, wherein: in the step 1), the cerium precursor is any one of cerium oxalate, cerium nitrate and cerium acetate; the cobalt precursor is any one of cobalt nitrate, cobalt oxalate and cobalt chloride; the concentration of Ce ions in the mixed solution is 0.1-0.5 mol/L.

3. The method for preparing a high-activity catalyst according to claim 1, wherein: the volume ratio of the alkaline solution to the mixed solution in the step 2) is 8:1-2: 1; the alkaline solution is sodium hydroxide aqueous solution or potassium hydroxide aqueous solution, and the concentration of the alkaline solution is 6-10 mol/L.

4. The method for preparing a high-activity catalyst according to claim 1, wherein: the stirring time in the step 2) is 0.5-8 hours; the temperature of the hydrothermal reaction is 60-180 ℃ and the time is 1-36 hours; the drying temperature is 60-120 ℃, and the drying time is 0.5-18 hours; the calcining temperature is 250-600 ℃ and the time is 0.5-6 hours.

5. The method for preparing a high-activity catalyst according to claim 1, wherein: the ruthenium precursor solution in the step 3) is prepared by taking any one of ruthenium chloride, ruthenium carbonyl and ruthenium nitrosyl nitrate as a solute and taking any one or two of methanol, ethanol and water as a solvent; the concentration of Ru ions in the ruthenium precursor solution is 1 g/L-20 g/L, and the volume ratio of alcohol in the solvent is not lower than 50%.

6. The method for preparing a high-activity catalyst according to claim 1, wherein: reducing by using a hydrogen-containing mixed gas in the step 3), wherein the volume of the hydrogen accounts for 1-100%, and the balance is argon or nitrogen; the temperature of the reduction is 150 ℃ and 600 ℃, and the time is 0.1-36 hours.

7. The method for preparing a high-activity catalyst according to claim 1, wherein: calculated by taking the mass of cerium oxide as a reference, the loading amount of cobalt in the high-activity catalyst is 0.003-1.7 wt%, and the loading amount of ruthenium is 0.1-20 wt%.

8. A high activity catalyst for ammonia synthesis obtainable by the process according to any one of claims 1 to 7.

Images