A catalyst for the synthesis of ammonia and a method for its preparation

Abstract

The application provides a catalyst for synthesizing ammonia and a preparation method thereof, and the composition of the catalyst comprises: an active carbon base, and metal ruthenium and a metal additive supported on the active carbon base; wherein the mass ratio of the metal ruthenium to the active carbon is 0.5-5%, and the mass ratio of the metal additive to the active carbon is 0.5-12%; the metal additive is selected from alkali metals or alkaline earth metals; and the metal ruthenium is sub-nano ruthenium clusters with a particle size of 0.2-1.0 nm. The metal additive is used to modify the metal ruthenium clusters, the strong interaction between the carbon base and the ruthenium clusters and the inhibition of the metal additive on the agglomeration of the ruthenium clusters are utilized, and the ruthenium clusters are fully utilized to efficiently synthesize ammonia, so that the ruthenium clusters can efficiently catalyze the synthesis of ammonia under mild conditions; wherein the ammonia synthesis yield can reach 26.5 mmol / g / h under the reaction conditions of 400 DEG C and 5 MPa, and the catalyst activity does not decrease after continuous reaction for 360 hours.

Description

Technical Field

[0001] This invention relates to the field of catalysts and their preparation technology, and more particularly to a catalyst for ammonia synthesis and its preparation method. Background Technology

[0002] Ammonia is one of the world's most produced chemical products, widely used in agriculture, chemical industry, electronics industry, food industry, military and many other fields. In recent years, ammonia has also shown great application potential in the field of hydrogen energy, due to its advantages such as high hydrogen content, high solvent density, no carbon, and ease of storage and transportation, making it considered an ideal hydrogen energy carrier. Currently, industrial ammonia synthesis mainly uses the traditional Haber-Bosch process, using ash (H2) from fossil fuels as raw material, and employing iron-based catalysts under high temperature and high pressure conditions of 450-525 ℃ and 15-32 MPa. This results in huge energy consumption and CO2 emissions, necessitating improvements to the ammonia synthesis process. Summary of the Invention

[0003] To address the aforementioned problems in the existing technology, this invention provides a catalyst for ammonia synthesis and its preparation method. The provided catalyst has low development cost and a simple preparation process, and exhibits high activity and high stability under mild conditions (300-450 °C), which is of great significance for promoting the development of green ammonia synthesis processes.

[0004] The specific details of the invention are as follows:

[0005] In a first aspect, the present invention provides a catalyst for ammonia synthesis, the catalyst comprising: an activated carbon substrate, and ruthenium metal and a metal promoter supported on the activated carbon substrate; wherein,

[0006] The mass ratio of ruthenium to activated carbon is 0.5-5%, and the mass ratio of the metal additive to activated carbon is 0.5-12%.

[0007] The metal additive is selected from alkali metals or alkaline earth metals;

[0008] The ruthenium metal is a sub-nanometer ruthenium cluster with an average particle size of 0.2-1.0 nm.

[0009] Optionally, the metal additive includes one or more of potassium, rubidium, cesium, and barium.

[0010] Optionally, the average particle size of the activated carbon is 0.1-10 mm.

[0011] Optionally, when the catalyst catalyzes the synthesis of ammonia from N2 and H2, the volume ratio of N2 to H2 is 1:3-10:1, and the gas space velocity is 1000-50000 h⁻¹. -1The reaction temperature is 300-450 ℃ and the reaction pressure is 0.1-10 MPa.

[0012] In a second aspect, the present invention provides a method for preparing the catalyst for ammonia synthesis as described in the first aspect above, the preparation method comprising:

[0013] Activated carbon was calcined in a hydrogen atmosphere and then added to a nitric acid solution. The surface was oxidized and modified by continuous stirring to obtain pretreated activated carbon.

[0014] The ruthenium precursor solution was dropped onto the surface of the pretreated activated carbon. After being left to stand under vacuum for a period of time, the resulting solid was dispersed in deionized water, and a precipitant was added to the suspension. The solution was heated to 50-90 °C and stirred continuously for 6-15 h. The solid material was collected and placed in a reducing atmosphere for high-temperature calcination to obtain activated carbon loaded with ruthenium clusters.

[0015] A metal precursor solution was dropped onto the surface of the activated carbon supported on ruthenium clusters. After being left to stand under vacuum for a period of time, the carbon was dried to obtain the catalyst for ammonia synthesis.

[0016] The ruthenium precursor solution has a pH of 1-3 and a ruthenium concentration of 9-91 mgRu / mL; the volume-to-mass ratio of the ruthenium precursor solution to the activated carbon is 0.5-1.6 mL / g.

[0017] The volume ratio of the deionized water to the mass of the activated carbon is 20-300 mL / g.

[0018] The metal concentration in the metal precursor solution is 12.5-200 mg / mL; the volume-to-mass ratio of the metal precursor solution to the activated carbon is 0.5-1.6 mL / g.

[0019] Optionally, the metal additive precursor solution is one or more selected from potassium nitrate solution, potassium hydroxide solution, rubidium nitrate solution, rubidium hydroxide solution, cesium nitrate solution, cesium hydroxide solution, and barium nitrate solution.

[0020] Optionally, the precipitant is selected from at least one of urea, ammonia, potassium hydroxide, sodium hydroxide, potassium carbonate, and sodium carbonate;

[0021] The molar ratio of the precipitant to ruthenium in the ruthenium precursor solution is 20:1 to 300:1.

[0022] Optionally, the calcination temperature of the activated carbon in a hydrogen atmosphere is 600-900℃, and the calcination time is 6-20 h.

[0023] The surface oxidation modification is performed at a temperature of 50-90 ℃ for 6-20 h.

[0024] Optionally, the temperature at which the collected solid material is placed in a reducing atmosphere for high-temperature calcination is 300-450 °C for 3-12 h.

[0025] Optionally, the drying is carried out in a vacuum environment at a temperature of 60-110°C.

[0026] Compared with the prior art, the present invention has the following advantages:

[0027] This invention provides a catalyst for ammonia synthesis, the catalyst comprising: an activated carbon substrate, and ruthenium metal and a metal promoter supported on the activated carbon substrate; wherein the mass ratio of ruthenium metal to activated carbon is 0.5-5%, and the mass ratio of the metal promoter to activated carbon is 0.5-12%; the metal promoter is selected from alkali metals or alkaline earth metals; the ruthenium metal is a sub-nanometer ruthenium cluster with a particle size of 0.2-1.0 nm.

[0028] The catalyst for ammonia synthesis provided by the present invention modifies the ruthenium cluster with a metal promoter, enabling it to efficiently catalyze ammonia synthesis under mild conditions; wherein, the ammonia synthesis yield can reach 26.5 mmol / g / h under reaction conditions of 400 °C and 5 MPa.

[0029] This invention uses porous activated carbon as a support to load ruthenium clusters and metal promoters. By utilizing the strong interaction between the carbon substrate and the clusters of ruthenium and the inhibition of ruthenium cluster aggregation by the metal promoters, not only can ruthenium clusters be fully utilized for efficient ammonia synthesis, but also good long-term stability is exhibited under mild ammonia synthesis conditions, and the catalyst activity does not decrease after 360 hours of continuous reaction.

[0030] The catalyst for ammonia synthesis provided by this invention uses inexpensive commercial chemicals such as activated carbon and urea as raw materials, resulting in low cost. Furthermore, the catalyst uses ruthenium clusters as the active component, significantly reducing the ruthenium loading and further lowering the catalyst cost. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1A flowchart illustrating the preparation method of the catalyst for ammonia synthesis provided in an embodiment of the present invention is shown;

[0033] Figure 2 The ammonia synthesis catalysts provided in embodiments and comparative examples of the present invention exhibit ammonia synthesis activity at different temperatures.

[0034] Figure 3 The ammonia synthesis activity of the catalyst provided in Example 4 of the present invention under different pressures is shown.

[0035] Figure 4 The ammonia synthesis performance of the catalyst provided in Example 4 of the present invention as a function of reaction time is shown.

[0036] Figure 5 A transmission electron microscope (TEM) image of the catalyst for ammonia synthesis provided in Example 4 of the present invention is shown.

[0037] Figure 6 A transmission electron microscope (TEM) image of a catalyst for ammonia synthesis provided in the comparative example of the present invention is shown. Detailed Implementation

[0038] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the present invention or its application or use. Based on the embodiments of the present invention, any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention. Furthermore, all other embodiments obtained by those skilled in the art without inventive effort are within the protection scope of the present invention.

[0039] Specific experimental steps or conditions are not specified in the embodiments; they can be performed according to the conventional experimental steps or conditions described in the prior art. Reagents and other instruments used, unless otherwise specified, are all commercially available conventional reagent products. Furthermore, the accompanying drawings are merely illustrative diagrams of the embodiments of the present invention and are not necessarily drawn to scale. The same reference numerals in the drawings denote the same or similar parts, and therefore, repeated descriptions of them will be omitted. Some block diagrams shown in the drawings are functional entities and do not necessarily correspond to physically or logically independent entities.

[0040] Techniques, methods, and devices known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and devices should be considered part of this specification.

[0041] In the description of this invention, it should be understood that the use of terms such as "first" and "second" to define components is merely for the purpose of distinguishing the corresponding components. Unless otherwise stated, the above terms have no special meaning and therefore should not be construed as limiting the scope of protection of this invention.

[0042] Furthermore, the technical features involved in the different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0043] Before providing a detailed description of the catalyst for ammonia synthesis and its preparation method provided by this invention, it is necessary to explain the relevant technologies as follows:

[0044] The key to reducing energy consumption and cost in industrial ammonia synthesis lies in developing ammonia synthesis catalysts with excellent performance under mild conditions. Under mild conditions (≤ 400 ℃, ≤ 10 MPa), iron-based catalysts exhibit poor ammonia synthesis activity, while ruthenium-based catalysts possess advantages such as high activity and insensitivity to hydrogen and ammonia concentrations, making them ideal materials for ammonia synthesis. However, existing ruthenium-based catalysts generally use ruthenium nanoparticles as active sites, requiring a relatively large ruthenium mass fraction, leading to high catalyst costs and limiting their practical application. Therefore, this invention focuses on developing a catalyst using sub-nano clusters of ruthenium (size less than 1.0 nm) as the active component. Sub-nano clusters of ruthenium offer advantages such as high metal dispersion and high metal utilization, effectively reducing the amount of precious metals used and lowering catalyst costs. Furthermore, sub-nano clusters of ruthenium possess a completely different electronic structure from ruthenium nanoparticles, potentially lowering the activation energy of ammonia synthesis and improving activity through an association mechanism. Moreover, the strong interaction between sub-nano clusters of ruthenium and the support can significantly enhance catalyst activity and stability. Finally, the introduction of alkali metals or alkaline earth metals can further promote the ammonia synthesis reaction of ruthenium.

[0045] The specific implementation details are as follows:

[0046] Firstly, this invention addresses the problem in existing technologies that ammonia synthesis catalysts have low activity under mild conditions and are expensive to prepare. It utilizes porous activated carbon as a support, loading ruthenium metal and metal promoters to form an ammonia synthesis catalyst, the composition of which is expressed as: aRu-bAL / AC c ;

[0047] Wherein, AL represents a metal additive, specifically selected from alkali metals or alkaline earth metals, including one or more of potassium, rubidium, cesium, and barium; AC represents activated carbon; a represents the mass ratio of ruthenium to activated carbon, specifically between 0.5% and 5%; b represents the mass ratio of the metal additive to activated carbon, specifically between 0.5% and 12%; c represents the average particle size of activated carbon, specifically between 0.1% and 10 mm. As an example, 4Ru-4Ba / AC 0.1mm The loading of ruthenium and barium in the catalyst is 4% of the mass of activated carbon, and the average particle size of the activated carbon is 0.1 mm.

[0048] The metallic ruthenium is specifically composed of sub-nanometer ruthenium clusters with an average particle size of 0.2-1.0 nm. The metal additives are selected from alkali metals or alkaline earth metals. Because sub-nanometer ruthenium clusters have a smaller particle size than ruthenium nanoparticles, they have a higher surface atomic ratio under the same loading, thus providing more reactive sites and resulting in higher metal utilization and better catalytic activity. The addition of alkali / alkaline earth metals (such as K and Ba) facilitates electron injection into ruthenium via electron transfer, weakens N2 adsorption strength, and acts as a structural aid, improving the sintering stability of the active material during preparation.

[0049] In practical implementation, since ruthenium exists in the form of sub-nanometer ruthenium clusters, it can provide more reactive sites. Therefore, while ensuring that the catalytic performance of the catalyst meets the reaction requirements, the mass ratio of ruthenium in the catalyst can be effectively reduced. In this invention, the mass ratio of ruthenium to activated carbon is 0.5-5%. Compared with this proportion of ruthenium in conventional ruthenium-based catalysts (generally greater than 5%), the use of ruthenium is significantly reduced, and the catalyst preparation cost is lowered.

[0050] In specific implementation, considering the important role of metal additives as electron promoters and structural aids, the present invention controls the mass ratio of metal additives to activated carbon to be 0.5-12%, that is, controls the mass ratio of ruthenium to metal additives to be 1-10:1-24, thereby suppressing the aggregation of ruthenium clusters during use.

[0051] In practical implementation, the feasibility of industrial applications is considered. For example, if the activated carbon particle size is too small, it will cause dust problems, requiring special processing equipment and increasing costs. Also, excessively fine particles will agglomerate during the preparation process, affecting dispersibility. If the particle size is too large, it will mainly affect the uniformity of metal loading. Based on the above considerations, the particle size of the activated carbon used in this invention is controlled within 0.1-10 mm.

[0052] In some embodiments, the catalyst for ammonia synthesis provided by the present invention, when catalyzing the ammonia synthesis reaction of N2 and H2, specifically includes the following process: loading aRu-bAL / AC into a fixed-bed reactor. c The catalyst is used for ammonia synthesis under mild conditions; it can be diluted with silica sand of the same size as needed. The reaction space velocity ranges from 1000 to 50000 h⁻¹. -1 The reaction temperature range is 300-450 ℃, the reaction pressure range is 0.1-10 MPa, and the ratio of N2 to H2 in the hydrogen and nitrogen gas used as raw materials can be between 1:3 and 10:1.

[0053] In practical implementation, compared to the high-temperature and high-pressure catalytic conditions (450-525 ℃, 15-32 MPa) required by iron-based catalysts, the ruthenium-based catalyst supported on sub-nanometer ruthenium clusters provided by this invention can achieve the catalytic reaction of ammonia synthesis under mild conditions (300-450 ℃, 0.1-10 MPa). The 4Ru-4Ba / AC provided by this invention... 0.1mm The catalyst synthesized ammonia at 400 °C and 5 MPa with a yield of 26.5 mmol / g / h, and the catalyst activity did not decrease even after 360 hours of continuous reaction. This indicates that the catalysis provided by this invention not only fully utilizes ruthenium clusters for efficient ammonia synthesis, but also exhibits good long-term stability under mild ammonia synthesis conditions.

[0054] Secondly, the present invention provides a method for preparing the catalyst for ammonia synthesis as described in the first aspect above. Figure 1 A flowchart illustrating the preparation method of the catalyst for ammonia synthesis provided in an embodiment of the present invention is shown, as follows: Figure 1 As shown, the preparation method includes:

[0055] S1. After calcining the activated carbon in a hydrogen atmosphere, add it to a nitric acid solution and stir continuously to carry out surface oxidation modification to obtain pretreated activated carbon.

[0056] In practice, this step first involves reducing and calcining activated carbon with an average particle size of 0.1-10 mm in a tube furnace under atmospheric pressure and pure hydrogen atmosphere. The calcination temperature is 600-900℃, the heating rate is 3-10℃ / min, the calcination time is 6-20 h, and the gas flow rate is 10-100 mL / min. At high temperature, impurities in the activated carbon component are reduced and removed in the hydrogen atmosphere. Then, the obtained activated carbon is dispersed in a dilute nitric acid solution, and the suspension is heated and stirred for 6-20 h, maintaining the temperature at 50-90℃. The resulting solid is washed 5-10 times with deionized water and then vacuum dried at 60-110℃ to obtain pretreated activated carbon. Under the action of nitric acid, the surface of the activated carbon is oxidized, acquiring negatively charged functional groups such as carboxyl groups (-COOH), which facilitate electrostatic adsorption with positively charged Ru(OH)3 colloids, thereby chemical anchoring and increasing the deposition density of the ruthenium precursor.

[0057] S2. The ruthenium precursor solution is dropped onto the surface of the pretreated activated carbon. After being left to stand under vacuum for a period of time, the resulting solid is dispersed in deionized water, and a precipitant is added to the suspension. The solution is heated to 50-90 °C and stirred continuously for 6-15 h. The solid material is collected and placed in a reducing atmosphere for high-temperature calcination to obtain activated carbon loaded with ruthenium clusters.

[0058] In practice, this step first involves preparing the ruthenium precursor solution. Specifically, the ruthenium precursor is dissolved in a certain amount of deionized water, and the pH is adjusted to 1-3 with hydrochloric acid to obtain the ruthenium precursor solution. In this process, the ruthenium precursor can be selected from at least one of ruthenium trichloride, ruthenium nitrite, ruthenium acetylacetonate, and ruthenium dodecylcarbonyl, and the concentration of ruthenium in the ruthenium precursor solution is between 9-91 mg Ru / mL.

[0059] Then, the obtained ruthenium precursor solution was dropped onto the surface of pretreated activated carbon and allowed to stand under vacuum at room temperature for 1-3 hours to allow the ruthenium precursor solution to be fully absorbed by the pretreated activated carbon. During this process, the amount of ruthenium precursor solution used was positively correlated with the mass of activated carbon, and the volume-to-mass ratio of ruthenium precursor solution to activated carbon was 0.5-1.6 mL / g.

[0060] Further, the solid material that absorbed the ruthenium precursor solution was dispersed in deionized water to form a suspension, and a precipitant was added to the suspension. After stirring continuously at 50-95 °C for 6-15 h, heating and stirring were stopped, the suspension was cooled to room temperature, and the obtained solid was separated by vacuum filtration to obtain activated carbon loaded with nano-colloidal particles. This step is crucial for the final formation of ruthenium clusters. First, in this step, the solid material that absorbed the ruthenium precursor solution was dispersed in deionized water to form a suspension. The volume of deionized water was determined according to the mass of activated carbon, and the ratio of the volume of deionized water to the mass of activated carbon was between 20-300 mL / g. The precipitant is selected from at least one of urea, ammonia, potassium hydroxide, sodium hydroxide, potassium carbonate, and sodium carbonate; the amount of precipitant used satisfies the molar ratio of precipitant to ruthenium in the ruthenium precursor solution being 20:1 to 300:1; the addition of the precipitant changes the pH value of the suspension, inducing the reaction of ruthenium precursor hydrolysis to generate Ru(OH)3 colloidal particles to occur slowly. This invention controls the pH value of the precipitation reaction environment, so that the nucleation rate of colloidal particles on the activated carbon surface is much lower than the growth rate, thereby transforming the ruthenium precursor into smaller and monodisperse nanocolloidal particles.

[0061] Furthermore, activated carbon loaded with nano-colloidal particles is placed in a tube furnace and reduced at 300-450 °C for 3-12 hours. The atmosphere used can be a nitrogen-hydrogen mixture (hydrogen content 10-90 vol.%), the gas pressure inside the tube is 0.1-3 MPa, and the heating rate is 5 °C / min. After calcination, activated carbon loaded with ruthenium clusters is obtained.

[0062] S3. The metal auxiliary precursor solution is dropped onto the surface of the activated carbon supported on the ruthenium cluster, and after being left to stand under vacuum for a period of time, it is dried to obtain the catalyst for ammonia synthesis.

[0063] In specific implementation, the metal auxiliary agent precursor solution is one or more of potassium nitrate solution, potassium hydroxide solution, rubidium nitrate solution, rubidium hydroxide solution, cesium nitrate solution, cesium hydroxide solution, and barium nitrate solution. In this step, the amount of metal auxiliary agent precursor solution is determined according to the mass and particle size of activated carbon. Specifically, the volume-to-mass ratio of the metal auxiliary agent precursor solution to activated carbon is 0.5-1.6 mL / g; the metal concentration in the metal auxiliary agent precursor solution is 12.5-200 mg / mL.

[0064] After the metal additive precursor solution was dropwise added to the surface of the activated carbon supporting the ruthenium cluster, it was allowed to stand under vacuum at room temperature for 1-3 h, then heated to 60-110 °C and vacuum dried for 6-12 h. After cooling to room temperature, aRu-bAL / AC was obtained. c .

[0065] To enable those skilled in the art to more clearly understand the present invention, the following examples will provide a detailed description of a catalyst for ammonia synthesis and its preparation method.

[0066] Example 1

[0067] Porous activated carbon with an average size of 5 mm was placed in a tube furnace under a pure hydrogen atmosphere and reduced at 900 °C for 14 h at a heating rate of 5 °C / min and a gas flow rate of 50 mL / min. The resulting activated carbon was dispersed in a 5 mol / L nitric acid solution and heated and stirred at 80 °C for 12 h. The resulting solid was washed 10 times with deionized water and then dried under vacuum at 110 °C for 12 h to obtain pretreated activated carbon.

[0068] 2 μL of 1mol / L dilute hydrochloric acid was added to 0.2 mL of ruthenium precursor solution (ruthenium concentration in Ru(NO3)3(NO) solution was 15 mg / mL) to adjust the pH of the ruthenium precursor solution to 2. The ruthenium precursor solution was then added dropwise to 0.3 g of pretreated activated carbon and allowed to stand under vacuum for 1 h. The resulting solid material was dispersed in 30 mL of deionized water and stirred for 10 min. 71 mg of urea was then added to form a suspension. The suspension was heated to 80 °C and stirred for 10 h. After filtration, the solid was placed in a vacuum drying oven and dried under vacuum to obtain activated carbon loaded with the ruthenium precursor.

[0069] Activated carbon loaded with ruthenium precursor was reduced in a tube furnace at 400 °C for 3 hours under a nitrogen-hydrogen mixture atmosphere at a heating rate of 5 °C / min to obtain 1Ru / AC activated carbon loaded with ruthenium clusters. 5mm .

[0070] A certain amount of the metal auxiliary agent precursor CsNO3 was weighed and dissolved in deionized water. 0.18 ml of the resulting metal auxiliary agent precursor solution (Cs concentration of 50 mg / mL) was added dropwise to the above solid. The mixture was allowed to stand under vacuum for 3 h at room temperature, and then dried under vacuum to obtain 1Ru-3Cs / AC. 5mm .

[0071] Example 2

[0072] Commercial porous activated carbon with an average size of 1 mm was placed in a tube furnace under a pure hydrogen atmosphere and reduced at 900 °C for 14 h at a heating rate of 5 °C / min and a gas flow rate of 50 mL / min. The resulting activated carbon was dispersed in a 5 mol / L nitric acid solution and heated and stirred at 80 °C for 12 h. The resulting solid was washed 10 times with deionized water and then dried under vacuum at 110 °C for 12 h to obtain pretreated activated carbon.

[0073] Add an appropriate amount of dilute hydrochloric acid to 0.25 mL of a ruthenium precursor solution (ruthenium concentration in Ru(NO3)3(NO) solution was 15 mg / mL) to adjust the pH of the ruthenium precursor solution to 2. Add the above ruthenium precursor solution dropwise to 0.3 g of pretreated activated carbon, and allow to stand under vacuum for 1 h. Disperse the resulting solid material in 30 mL of deionized water, stir for 10 min, and then add 142 mg of urea. Heat the suspension to 80 °C and maintain the temperature with stirring for 10 h. After filtration and separation, place the solid in a vacuum drying oven and dry under vacuum to obtain activated carbon loaded with the ruthenium precursor.

[0074] The above solid was reduced in a tube furnace at 400 °C for 3 hours under a nitrogen-hydrogen mixture atmosphere at a heating rate of 5 °C / min to obtain ruthenium-supported activated carbon 2Ru / AC. 1mm .

[0075] A certain amount of the metal precursor KNO3 was weighed and dissolved in deionized water. 0.18 mL of the resulting metal precursor solution (K concentration 50 mg / mL) was added dropwise to the solid. The mixture was allowed to stand under vacuum for 3 h at room temperature, and then dried under vacuum to obtain 2Ru-3K / AC. 1mm .

[0076] Example 3

[0077] Commercial porous activated carbon with an average size of 0.1 mm was placed in a tube furnace under a pure hydrogen atmosphere and reduced at 800 °C for 14 h at a heating rate of 5 °C / min and a gas flow rate of 50 mL / min. The resulting activated carbon was dispersed in a 5 mol / L nitric acid solution and heated and stirred at 80 °C for 12 h. The resulting solid was washed 10 times with deionized water and then dried under vacuum at 110 °C for 12 h to obtain pretreated activated carbon.

[0078] Add an appropriate amount of dilute hydrochloric acid to 0.3 mL of a ruthenium precursor solution (ruthenium concentration in Ru(NO3)3(NO) solution was 15 mg / mL) to adjust the pH of the ruthenium precursor solution to 2. Add the above ruthenium precursor solution dropwise to 0.3 g of pretreated activated carbon, and allow to stand under vacuum for 1 h. Disperse the resulting solid material in 30 mL of deionized water, stir for 10 min, and then add 284 mg of urea. Heat the suspension to 80 °C and stir for 12 h. After filtration, place the solid in a vacuum drying oven and dry under vacuum to obtain activated carbon loaded with the ruthenium precursor.

[0079] The above solid was reduced in a tube furnace at 400 °C for 3 hours under a nitrogen-hydrogen mixture atmosphere at a heating rate of 5 °C / min to obtain ruthenium-supported activated carbon 4Ru / AC. 0.1mm .

[0080] A certain amount of the metal precursor KNO3 was weighed and dissolved in deionized water. 0.18 ml of the resulting metal precursor solution (K concentration 67 mg / mL) was added dropwise to the solid. The mixture was allowed to stand under vacuum for 3 h at room temperature, and then dried under vacuum to obtain 4Ru-4K / AC. 0.1mm .

[0081] Example 4

[0082] Commercial porous activated carbon with an average size of 0.1 mm was placed in a tube furnace under a pure hydrogen atmosphere and reduced at 800 °C for 12 h at a heating rate of 5 °C / min and a gas flow rate of 50 mL / min. The resulting activated carbon was dispersed in a 5 mol / L nitric acid solution and heated and stirred at 80 °C for 12 h. The resulting solid was washed 5-10 times with deionized water and then dried under vacuum at 110 °C for 12 h to obtain pretreated activated carbon.

[0083] Add an appropriate amount of dilute hydrochloric acid to 0.3 mL of a ruthenium precursor solution (ruthenium concentration in Ru(NO3)3(NO) solution was 15 mg / mL) to adjust the pH of the ruthenium precursor solution to 2. Add the above ruthenium precursor solution dropwise to 0.3 g of pretreated activated carbon, and allow to stand under vacuum for 1 h. Disperse the resulting solid material in 30 mL of deionized water, stir for 10 min, and then add 284 mg of urea. Heat the suspension to 80 °C and maintain the temperature with stirring for 10 h. After filtration and separation, place the solid in a vacuum drying oven and dry under vacuum to obtain activated carbon loaded with the ruthenium precursor.

[0084] The above solid was reduced in a tube furnace at 400 °C for 3 hours under a nitrogen-hydrogen mixture atmosphere at a heating rate of 5 °C / min to obtain ruthenium-supported activated carbon 4Ru / AC. 0.1mm .

[0085] A certain amount of the metal auxiliary agent precursor Ba(NO3)2 was weighed and dissolved in deionized water. 0.18 ml of the resulting metal auxiliary agent precursor solution (Ba concentration of 67 mg / mL) was added dropwise to the above solid. The mixture was allowed to stand under vacuum for 3 h at room temperature, and then dried under vacuum to obtain 4Ru-4Ba / AC. 0.1mm .

[0086] Comparative Example 1

[0087] Commercial porous activated carbon with an average size of 5 mm was placed in a tube furnace under a pure hydrogen atmosphere and reduced at 00 °C for 14 h at a heating rate of 5 °C / min and a gas flow rate of 50 mL / min. The resulting activated carbon was dispersed in a 5 mol / L nitric acid solution and heated and stirred at 80 °C for 12 h. The resulting solid was washed 10 times with deionized water and then dried under vacuum at 110 °C for 12 h to obtain pretreated activated carbon.

[0088] Weigh 0.3 g of pretreated activated carbon, add 0.2 mL of Ru(NO3)3(NO) solution, then add 2 μL of 1 mol / L dilute hydrochloric acid. After standing, dry under vacuum at 110 ℃ for 12 h. After cooling, obtain activated carbon loaded with ruthenium precursor. Reduce the above solid in a tube furnace at 400 ℃ for 3 h under a nitrogen-hydrogen mixture atmosphere (hydrogen content 10 vol.%) and a heating rate of 5 ℃ / min to obtain activated carbon 1RuNP / AC loaded with ruthenium nanoparticles. 5mm .

[0089] Comparative Example 2

[0090] Commercial porous activated carbon with an average size of 0.1 mm was placed in a tube furnace under a pure hydrogen atmosphere and reduced at 900 °C for 14 h at a heating rate of 5 °C / min and a gas flow rate of 50 mL / min. The resulting activated carbon was dispersed in a 5 mol / L nitric acid solution and heated and stirred at 80 °C for 12 h. The resulting solid was washed 10 times with deionized water and then dried under vacuum at 110 °C for 12 h to obtain pretreated activated carbon.

[0091] Weigh 0.3 g of pretreated activated carbon, add 0.3 mL of Ru(NO3)3(NO) solution, then add 2 μL of 1 mol / L dilute hydrochloric acid. After standing, dry under vacuum at 110 °C for 12 h. After cooling, obtain activated carbon loaded with ruthenium precursor. Reduce the above solid in a tube furnace at 450 °C for 3 h under a nitrogen-hydrogen mixture atmosphere (hydrogen content 10 vol.%) and a heating rate of 5 °C / min to obtain activated carbon 4RuNP / AC loaded with ruthenium nanoparticles. 0.1mm A certain amount of Ba(NO3)2 was weighed and dissolved in deionized water. The resulting solution was added dropwise to the above solid, and the mixture was allowed to stand under vacuum for 3 h at room temperature. After vacuum drying, 4RuNP-4Ba / AC was obtained. 0.1mm .

[0092] Performance testing:

[0093] (1) Effect of temperature on ammonia yield of catalyst

[0094] The catalysts obtained in Examples 1-4 and Comparative Examples 1-2 were used to synthesize ammonia, and the specific operations are as follows:

[0095] The catalysts obtained in Examples 1-4 and Comparative Examples 1-2 were respectively loaded into fixed-bed tubular reactors for ammonia synthesis. A nitrogen-hydrogen mixture (nitrogen-hydrogen ratio 1:3) was introduced at room temperature at a rate of 60 mL / min. After stabilizing at 400 °C for 1 h at a rate of 5 °C / min, the tail gas absorbent was analyzed by ion chromatography. The ammonia concentration in the tail gas was measured after each temperature adjustment and stabilization for 1 h. The reaction space velocity was 10000 h⁻¹. -1 The reaction pressure is 1 MPa, and the reaction temperature range is 375-425 ℃.

[0096] Figure 2 The following examples illustrate the ammonia synthesis catalysts provided by the present invention and comparative examples at different temperatures, such as... Figure 2 As shown, Example 1 (1Ru-3Cs / AC) 5mm Example 2 (2Ru-3K / AC) 1mm Example 3 (4Ru-4K / AC) 0.1mm Example 4 (4Ru-4Ba / AC) 0.1mm Comparative Example 1 (1RuNP / AC) 5mm ) and Comparative Example 2 (4RuNP-4Ba / AC) 0.1mm The ammonia synthesis activity of the catalyst at different temperatures (reaction conditions: reaction pressure 1 MPa, N2:H2 ratio in the feed gas = 1:3, reaction space velocity 10000 h⁻¹) -1 Example 4 shows a ruthenium cluster catalyst 4Ru-4Ba / AC with 0.1 mm size activated carbon as a support and ruthenium and barium loadings of 4% of the activated carbon mass. 0.1mm The ammonia synthesis activity is much higher than that of the ruthenium nanoparticle catalyst 4RuNP-4Ba / AC provided in Comparative Example 2. 0.1mm The ammonia synthesis activity; and, when the reaction temperature is 350 °C and the reaction pressure is 1 MPa, 4Ru-4Ba / AC 0.1mm The ammonia synthesis activity is 4RuNP-4Ba / AC 0.1mm The activity is three times greater; when the reaction pressure is 1 MPa and the reaction temperature is in the range of 300-425℃, the ruthenium cluster catalyst 1Ru-3Cs / AC provided in Example 1 has three times the activity. 5mm The ammonia synthesis activity is slightly better than that of the ruthenium nanoparticle catalyst 4RuNP-4Ba / AC provided in Comparative Example 2. 0.1mm .

[0097] Depend on Figure 2 It can be seen that the catalytic material of the supported ruthenium cluster containing metal promoter of the present invention not only has high ammonia synthesis activity, but also reduces the cost of ruthenium carbon catalyst by improving the utilization rate of ruthenium metal.

[0098] (2) Effect of pressure on ammonia yield of catalyst

[0099] Select the 4RuNP-4Ba / AC provided in Example 4 0.1mm The effect of pressure on the ammonia yield of the catalyst was tested, and the specific procedures are as follows:

[0100] 4Ru-4Ba / AC is packed in a fixed-bed tubular reactor. 0.1mm The catalyst was used for ammonia synthesis. A nitrogen-hydrogen mixture (nitrogen:hydrogen ratio 1:3) was introduced at room temperature at a rate of 60 mL / min. After stabilization at 400 °C for 1 h at a rate of 5 °C / min, the tail gas absorbent was analyzed by ion chromatography. The reaction space velocity was 10000 h⁻¹. -1 The reaction pressure range is 0.1-7 MPa.

[0101] Figure 3 The ammonia synthesis activity of the catalyst provided in Example 4 of this invention under different pressures is shown. Figure 3 It can be seen that the reaction pressure affects Example 4 (4Ru-4Ba / AC) 0.1mm The effect of catalyst on ammonia synthesis performance (reaction conditions: reaction temperature 400 ℃, reaction pressure 0.1, 1, 3, 5, 7 MPa, N2:H2 ratio of the nitrogen-hydrogen mixture used as feed gas = 1:3, reaction space velocity 10000 h⁻¹) -1 At 400°C, 4Ru-4Ba / AC of Example 4 0.1mm The ammonia synthesis activity of the catalyst increases with increasing reaction pressure, indicating that the catalyst does not exhibit significant hydrogen poisoning and meets the requirements for industrial ammonia synthesis. The stability test results for catalysts in other examples are similar, all showing no significant hydrogen poisoning.

[0102] (3) Changes in the ammonia synthesis performance of the catalyst with reaction time

[0103] Select the 4RuNP-4Ba / AC provided in Example 4 0.1mm The effect of pressure on the catalyst's performance in ammonia synthesis as a function of reaction time was tested. The reaction temperature was 400℃, the reaction pressure was 1 MPa, the nitrogen-hydrogen mixture used as feed gas had an N2:H2 ratio of 1:3, and the reaction space velocity was 10000 h⁻¹. -1 The ammonia synthesis reaction was carried out continuously for 350 hours, during which the ammonia production rate was tested intermittently.

[0104] Figure 4 The ammonia synthesis performance of the catalyst provided in Example 4 of this invention as a function of reaction time is shown in the following figures. Figure 4 As shown, Example 4 provides 4Ru-4Ba / AC 0.1mmThe catalyst exhibits very stable catalytic activity, remaining essentially unchanged after 350 hours of reaction. Stability test results for catalysts in other examples are similar, all demonstrating good stability.

[0105] Furthermore, Figure 5 A transmission electron microscope (TEM) image of the catalyst for ammonia synthesis provided in Example 4 of the present invention is shown, as follows: Figure 5 As shown, the catalyst for ammonia synthesis provided in Example 4 does not contain obvious ruthenium nanoparticles; Figure 6 Transmission electron microscopy (TEM) images of the catalysts for ammonia synthesis provided in the comparative examples of this invention are shown, such as... Figure 6 As shown, ruthenium nanoparticles can be observed in the catalyst for ammonia synthesis provided in Comparative Example 2.

[0106] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. In addition, those skilled in the art can combine and integrate the different embodiments or examples described in this specification.

[0107] For the sake of simplicity, the method embodiments are described as a series of actions. However, those skilled in the art should understand that the present invention is not limited to the described order of actions, as some steps can be performed in other orders or simultaneously according to the present invention. Furthermore, those skilled in the art should also understand that the embodiments described in the specification are preferred embodiments, and the actions and components involved are not necessarily essential to the present invention.

[0108] The catalyst for ammonia synthesis and its preparation method provided by the present invention have been described in detail above. Specific examples have been used to illustrate the principle and implementation of the present invention. The description of the above embodiments is only for the purpose of helping to understand the method and core idea of ​​the present invention. At the same time, for those skilled in the art, there will be changes in the specific implementation and application scope based on the idea of ​​the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A catalyst for ammonia synthesis, characterized in that, The catalyst comprises: an activated carbon substrate, and ruthenium metal and a metal promoter supported on the activated carbon substrate; wherein, The mass ratio of ruthenium to activated carbon is 0.5-5%, and the mass ratio of the metal additive to activated carbon is 0.5-12%. The metal additive is selected from one of potassium, rubidium, cesium, and barium; The ruthenium metal is a sub-nanometer ruthenium cluster with an average particle size of 0.2-1.0 nm; The average particle size of the activated carbon is 0.1-10 mm; The catalyst is obtained by a method comprising the following steps: Activated carbon was calcined in a hydrogen atmosphere and then added to a nitric acid solution. Surface oxidation modification was carried out by continuous stirring to obtain pretreated activated carbon. The ruthenium precursor solution was dropped onto the surface of the pretreated activated carbon. After being left to stand under vacuum for a period of time, the resulting solid was dispersed in deionized water, and a precipitant was added to the suspension. The solution was heated to 50-90 °C and stirred continuously for 6-15 h. The solid material was collected and placed in a reducing atmosphere for high-temperature calcination to obtain activated carbon loaded with ruthenium clusters. A metal precursor solution was dropped onto the surface of the activated carbon supported on ruthenium clusters. After being left to stand under vacuum for a period of time, the carbon was dried to obtain the catalyst for ammonia synthesis. The ruthenium precursor solution has a pH of 1-3 and a ruthenium concentration of 9-91 mg Ru / mL; the volume-to-mass ratio of the ruthenium precursor solution to the activated carbon is 0.5-1.6 mL / g. The volume ratio of the deionized water to the mass of the activated carbon is 20-300 mL / g; The metal concentration in the metal precursor solution is 12.5-200 mg / mL; the volume-to-mass ratio of the metal precursor solution to the activated carbon is 0.5-1.6 mL / g. The molar ratio of the precipitant to ruthenium in the ruthenium precursor solution is 20:1 to 300:

1.

2. The catalyst for ammonia synthesis according to claim 1, characterized in that, When the catalyst catalyzes the synthesis of ammonia from N2 and H2, the volume ratio of N2 to H2 is 1:3-10:1, and the gas space velocity is 1000-50000 h⁻¹. -1 The reaction temperature is 300-450 ℃ and the reaction pressure is 0.1-10 MPa.

3. The catalyst for ammonia synthesis according to claim 1, characterized in that, The metal additive precursor solution is one of potassium nitrate solution, potassium hydroxide solution, rubidium nitrate solution, rubidium hydroxide solution, cesium nitrate solution, cesium hydroxide solution, and barium nitrate solution.

4. The catalyst for ammonia synthesis according to claim 1, characterized in that, The precipitant is selected from at least one of urea, ammonia, potassium hydroxide, sodium hydroxide, potassium carbonate, and sodium carbonate.

5. The catalyst for ammonia synthesis according to claim 1, characterized in that, The roasting temperature for roasting activated carbon in a hydrogen atmosphere is 600-900℃, and the roasting time is 6-20 h. The surface oxidation modification is performed at a temperature of 50-90 ℃ for 6-20 h.

6. The catalyst for ammonia synthesis according to claim 1, characterized in that, The solid material is collected and placed in a reducing atmosphere for high-temperature calcination at a temperature of 300-450 °C for 3-12 h.

7. The catalyst for ammonia synthesis according to claim 1, characterized in that, The drying process is carried out in a vacuum environment at a temperature of 60-110°C.

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