Catalyst for synthesizing ammonia and preparation method thereof

Abstract

The invention provides a catalyst for synthesizing ammonia and a preparation method of the catalyst. The catalyst comprises an activated carbon substrate, and metal ruthenium and a metal additive which are loaded on the activated carbon substrate, wherein the mass ratio of the metal ruthenium to the activated carbon is 0.5-5%, and the mass ratio of the metal additive to the activated carbon is 0.5-12%; the metal additive is selected from alkali metal or alkaline earth metal; the metal ruthenium is a sub-nano ruthenium cluster, and the particle size of the metal ruthenium is 0.2-1.0 nm. According to the method, the metal ruthenium cluster is modified through the metal additive, and the ruthenium cluster is fully utilized to efficiently synthesize the ammonia by utilizing the strong interaction of the carbon substrate on the ruthenium cluster and the inhibition of the metal additive on the agglomeration of the ruthenium cluster, so that the ammonia can be efficiently catalyzed and synthesized under a mild condition; wherein the synthesis ammonia yield can reach 26.5 mmol / g / h under the reaction conditions of 400 DEG C and 5 MPa, and the activity of the catalyst is not reduced after continuous reaction for 360 hours.

Description

Technical Field

[0001] The present invention relates to the technical field of catalysts and preparation, and particularly relates to a catalyst for ammonia synthesis and a preparation method thereof. Background Art

[0002] Ammonia is one of the largest chemical products in the world and is widely used in many fields such as agriculture, chemical industry, electronics industry, food industry, and military. In recent years, ammonia has also shown great application potential in the field of hydrogen energy. Because of its high hydrogen content, high solvent density, carbon-free, and easy storage and transportation, it is considered an ideal hydrogen energy carrier. Currently, industrial ammonia synthesis mainly uses the traditional Haber-Bosch process, with grey H2 from fossil fuels as raw materials, and uses iron-based catalysts to synthesize ammonia under high temperature and high pressure conditions of 450-525 °C and 15-32 MPa, which leads to huge energy consumption and CO2 carbon emissions. There is an urgent need to improve the ammonia synthesis process. Summary of the Invention

[0003] Aiming at the above problems in the prior art, the present invention provides a catalyst for ammonia synthesis and a preparation method thereof. The provided catalyst has a low development cost and a simple preparation process, and has high activity and high stability under mild conditions (300-450 °C), which is of great significance for promoting the development of green ammonia synthesis technology.

[0004] The specific content of the invention is as follows: In a first aspect, the present invention provides a catalyst for ammonia synthesis. The composition of the catalyst includes: an activated carbon substrate, and ruthenium metal and metal promoters supported on the activated carbon substrate; wherein, the mass ratio of the ruthenium metal to the activated carbon is 0.5-5%, and the mass ratio of the metal promoter to the activated carbon is 0.5-12%; the metal promoter is selected from alkali metals or alkaline earth metals; the ruthenium metal is sub-nanometer ruthenium clusters with an average particle size of 0.2-1.0 nm.

[0005] Optionally, the metal promoter includes one or more of potassium, rubidium, cesium, and barium.

[0006] Optionally, the average particle size of the activated carbon is 0.1-10 mm.

[0007] 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, the reaction gas space velocity is 1000-50000 h -1 , the reaction temperature is 300-450 °C, and the reaction pressure is 0.1-10 MPa.

[0008] Second aspect, the present invention provides a preparation method of the catalyst for ammonia synthesis described in the first aspect above, the preparation method comprising: After calcining activated carbon in a hydrogen atmosphere, it is added to a nitric acid solution, and surface oxidation modification is carried out by continuous stirring to obtain pretreated activated carbon; The ruthenium precursor solution is dropped onto the surface of the pretreated activated carbon. After standing in vacuum for a period of time, the obtained solid is dispersed in deionized water, and a precipitating agent is added to the suspension. The solution is heated to 50-90 °C and continuously stirred for 6-15 h, then the solid material is collected and placed in a reducing atmosphere for high-temperature calcination to obtain activated carbon loaded with ruthenium clusters; The metal promoter precursor solution is dropped onto the surface of the activated carbon loaded with ruthenium clusters. After standing in vacuum for a period of time, drying treatment is carried out to obtain the catalyst for ammonia synthesis; Wherein, the pH of the ruthenium precursor solution is 1-3, and the ruthenium concentration in the ruthenium precursor solution is 9-91 mgRu / mL; the volume-mass ratio of the ruthenium precursor solution to the activated carbon is 0.5-1.6 mL / g; The ratio of the volume of the deionized water to the mass of the activated carbon is 20-300 mL / g mL / g; The metal concentration in the metal promoter precursor solution is 12.5-200 mg / mL; the volume-mass ratio of the metal promoter precursor solution to the activated carbon is 0.5-1.6 mL / g.

[0009] Optionally, the metal promoter 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.

[0010] Optionally, the precipitating agent is selected from at least one of urea, ammonia water, potassium hydroxide, sodium hydroxide, potassium carbonate and sodium carbonate; The molar ratio of the precipitating agent to ruthenium in the ruthenium precursor solution is 20:1~300:1.

[0011] Optionally, the calcination temperature for calcining the activated carbon in a hydrogen atmosphere is 600-900 °C, and the calcination time is 6-20 h; The temperature of the surface oxidation modification is 50-90 °C, and the time is 6-20 h.

[0012] Optionally, the temperature for collecting the solid material and placing it in a reducing atmosphere for high-temperature calcination is 300-450 °C, and the time is 3-12 h.

[0013] Optionally, the drying is carried out in a vacuum environment, and the drying temperature is 60-110 °C.

[0014] Compared with the prior art, the present invention has the following advantages: The present invention provides a catalyst for synthesizing ammonia. The composition of the catalyst includes: an activated carbon substrate, and ruthenium metal and metal promoters supported on the activated carbon substrate; wherein, the mass ratio of the ruthenium metal to the activated carbon is 0.5-5%, and the mass ratio of the metal promoter to the activated carbon is 0.5-12%; the metal promoter is selected from alkali metals or alkaline earth metals; the ruthenium metal is sub-nano ruthenium clusters with a particle size of 0.2-1.0 nm.

[0015] For the above-mentioned catalyst for synthesizing ammonia provided by the present invention, the metal ruthenium clusters are modified by the metal promoter, enabling it to 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 °C and 5 MPa.

[0016] The present invention uses porous activated carbon as a carrier to support ruthenium clusters and metal promoters. By utilizing the strong interaction between the carbon substrate and the ruthenium clusters and the inhibition of the metal promoter on the agglomeration of ruthenium clusters, not only is the ruthenium clusters 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 continuous reaction for 360 hours.

[0017] For the above-mentioned catalyst for synthesizing ammonia provided by the present invention, the raw materials used are inexpensive commercial chemical products such as activated carbon and urea, with a low cost. In addition, the catalyst uses ruthenium clusters as the active component, significantly reducing the ruthenium loading and further reducing the catalyst cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0019] Figure 1 Shows the flowchart of the preparation method of the catalyst for synthesizing ammonia provided by the embodiment of the present invention; Figure 2 Shows the ammonia synthesis activity of the catalysts for synthesizing ammonia provided by the embodiments and comparative examples of the present invention at different temperatures; Figure 3 Shows the ammonia synthesis activity of the catalyst for synthesizing ammonia provided in Embodiment 4 of the present invention at different pressures; Figure 4Shows the change of ammonia synthesis performance of the catalyst for ammonia synthesis provided in Embodiment 4 of the present invention with the reaction time; Figure 5 Shows the transmission electron microscope image of the catalyst for ammonia synthesis provided in Embodiment 4 of the present invention; Figure 6 Shows the transmission electron microscope image of the catalyst for ammonia synthesis provided in the comparative example of the present invention. Detailed implementation manners

[0020] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. The following description of at least one exemplary embodiment is actually only illustrative and in no way limits the present invention and its application or use. Based on the embodiments of the present invention, any product identical or similar to the present invention obtained by anyone under the inspiration of the present invention or by combining the features of the present invention with other existing technologies falls within the protection scope of the present invention. And all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present invention.

[0021] For the technologies, methods and equipment known to those of ordinary skill in the relevant fields, they may not be discussed in detail, but under appropriate circumstances, the said technologies, methods and equipment should be regarded as part of the description of the present invention.

[0022] In the description of the present invention, it should be understood that using terms such as "first" and "second" to limit components is only for the convenience of differentiating the corresponding components. Without further statement, the above terms have no special meaning, so they cannot be understood as limiting the protection scope of the present invention.

[0023] In addition, the technical features involved in different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0024]

[0025] ​Before elaborating on a catalyst for ammonia synthesis and its preparation method provided by the present invention in detail, it is necessary to make the following description of related technologies: The key to reducing the energy consumption and cost of industrial ammonia synthesis is to develop an ammonia synthesis catalyst with excellent performance under mild conditions. Under mild conditions (≤ 400 °C, ≤ 10 MPa), the ammonia synthesis activity of iron-based catalysts is poor, while ruthenium-based catalysts have the advantages of high activity and insensitivity to the concentrations of hydrogen and ammonia, making them ideal materials for ammonia synthesis. However, existing ruthenium-based catalysts generally use ruthenium nanoparticles as active sites, usually requiring a relatively large mass fraction of ruthenium, resulting in a high cost of the catalyst and limiting its practical application. Based on this, the present invention conducts research and development on a catalyst with sub-nanometer cluster ruthenium (size less than 1.0 nm) as the catalyst active component. Sub-nanometer cluster ruthenium has the advantages of high metal dispersion and high metal utilization rate, which can effectively reduce the amount of precious metal used and lower the catalyst cost. In addition, sub-nanometer cluster ruthenium has an electronic structure completely different from that of ruthenium nanoparticles, and is expected to reduce the activation energy of ammonia synthesis through the association mechanism and improve the activity. Further, the strong interaction between sub-nanometer cluster ruthenium and the carrier can significantly enhance the activity and stability of the catalyst. Finally, the introduction of alkali metals or alkaline earth metals can further promote the ammonia synthesis reaction of ruthenium.

[0026] The specific implementation content is as follows: In the first aspect, aiming at the problems existing in the prior art that the ammonia synthesis catalyst has low activity and high preparation cost under mild conditions, the present invention uses porous activated carbon as the carrier to load metal ruthenium and metal promoters to form an ammonia synthesis catalyst, and its composition is expressed as: aRu-bAL / AC c ; wherein, AL represents a metal promoter, 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 metal ruthenium to activated carbon, specifically ranging from 0.5% to 5%; b represents the mass ratio of the metal promoter to activated carbon, specifically ranging from 0.5% to 12%; c represents the average particle size of activated carbon, specifically ranging from 0.1 mm to 10 mm. As an example, 4Ru-4Ba / AC 0.1mm represents that the loadings of ruthenium and barium in the catalyst are both 4% of the mass of activated carbon, and the average particle size of activated carbon is 0.1 mm.

[0027] The ruthenium metal is specifically sub-nanometer ruthenium clusters with an average particle size of 0.2 - 1.0 nm. The metal promoter is selected from alkali metals or alkaline earth metals. Since sub-nanometer ruthenium clusters have a smaller particle size than ruthenium nanoparticles, at the same loading amount, sub-nanometer ruthenium clusters have a higher proportion of surface atoms than ruthenium nanoparticles. Therefore, more reactive sites can be provided, resulting in higher metal utilization and better catalytic activity. The addition of alkali / alkaline earth metals (such as K, Ba) helps to inject electrons into ruthenium through electron transfer, weaken the N2 adsorption strength, and act as a structural promoter to improve the sintering stability of the active substances during the preparation process.

[0028] In specific implementation, based on the fact that ruthenium metal exists in the form of sub-nanometer ruthenium clusters, more reactive sites can be provided. Therefore, while ensuring that the catalytic performance of the catalyst meets the reaction requirements, the mass ratio of ruthenium metal in the catalyst can be effectively reduced. In the present invention, the mass ratio of ruthenium metal to activated carbon is 0.5 - 5%, which is significantly reduced compared to the proportion of ruthenium metal in conventional ruthenium-based catalysts (generally greater than 5%), thus reducing the preparation cost of the catalyst.

[0029] In specific implementation, considering the important role of the metal promoter as an electron promoter and a structural promoter, the present invention controls the mass ratio of the metal promoter to activated carbon to be 0.5 - 12%, that is, controls the mass ratio of ruthenium metal to the metal promoter to be 1 - 10:1 - 24, thereby inhibiting the agglomeration of ruthenium clusters during use.

[0030] In specific implementation, considering the feasibility of industrial application, if the particle size of the activated carbon is too small, it will cause dust problems and require special treatment equipment, increasing costs, and, too fine particles will agglomerate during the preparation process, affecting the 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 the present invention is controlled to be 0.1 - 10 mm.

[0031] In some embodiments, the catalyst for synthesizing ammonia provided by the present invention, when catalyzing the synthesis of ammonia reaction of N2 and H2, the specific process of catalyzing the synthesis of ammonia reaction of N2 and H2 includes: loading aRu-bAL / AC c catalyst in a fixed-bed reactor for mild-condition ammonia synthesis reaction, and the catalyst can be diluted with quartz sand of the same size as needed. The reaction space velocity range is 1000 - 50000 h -1 , the reaction temperature range is 300 - 450 °C, the reaction pressure range is 0.1 - 10 MPa, and the ratio of N2 and H2 in the hydrogen-nitrogen gas as the raw material can be between 1:3 and 10:1.

[0032] In specific implementation, compared with the high-temperature and high-pressure catalytic conditions (450 - 525 °C, 15 - 32 MPa) required by iron-based catalysts, the ruthenium-based catalyst loaded with sub-nanometer ruthenium clusters provided by the present invention can achieve the catalytic reaction of ammonia synthesis under mild conditions (300 - 450 °C, 0.1 - 10 MPa). The 4Ru-4Ba / AC 0.1mm catalyst provided by the present invention synthesizes ammonia under the reaction conditions of 400 °C and 5 MPa, and the yield can reach 26.5 mmol / g / h. Moreover, after continuous reaction for 360 hours, the activity of the catalyst does not decrease. This shows that the catalyst provided by the present invention not only makes full use of ruthenium clusters for efficient ammonia synthesis, but also exhibits good long-term stability under mild ammonia synthesis conditions.

[0033] In the second aspect, the present invention provides a preparation method of the catalyst for ammonia synthesis described in the first aspect above. Figure 1 The flowchart of the preparation method of the catalyst for ammonia synthesis provided by the embodiment of the present invention is shown, as Figure 1 shown, the preparation method includes: S1. After calcining activated carbon in a hydrogen atmosphere, it is added to a nitric acid solution, and continuous stirring is carried out for surface oxidation modification to obtain pretreated activated carbon; In specific implementation, in this step, activated carbon with an average particle size of 0.1 - 10 mm is first placed in a tube furnace for reduction calcination. The atmosphere used is normal-pressure pure hydrogen, the calcination temperature is 600 - 900 °C, the heating rate is 3 - 10 °C / min, the calcination time is 6 - 20 h, and the gas flow rate is 10 - 100 mL / min; at high temperature, the impurities contained in the activated carbon component are reduced and removed in a 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, and the temperature is maintained at 50 - 90 °C; the obtained solid is washed 5 - 10 times with deionized water and then placed in a vacuum drying at 60 - 110 °C to obtain pretreated activated carbon. Under the action of nitric acid, the surface of the activated carbon is oxidized, with negatively charged groups such as oxygen-containing functional groups (-COOH) etc., so as to facilitate subsequent electrostatic adsorption with positively charged Ru(OH)3 colloid, thereby undergoing chemical anchoring and improving the deposition density of ruthenium precursors.

[0034] S2. The ruthenium precursor solution is dropped onto the surface of the pretreated activated carbon. After standing in vacuum for a period of time, the obtained solid is dispersed in deionized water, and a precipitant is added to the suspension. After heating the solution to 50 - 90 °C and continuously stirring 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; In specific implementation, this step first involves the preparation of a 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 nitrosyl nitrate, ruthenium acetylacetonate, and dodecacarbonyltriruthenium. The concentration of ruthenium in the ruthenium precursor solution is between 9 - 91 mg Ru / mL.

[0035] Then, the obtained ruthenium precursor solution is dropped onto the surface of the pretreated activated carbon and vacuum - left standing at room temperature for 1 - 3 h, so that the ruthenium precursor solution is fully absorbed by the pretreated activated carbon. In this process, the amount of the ruthenium precursor solution is positively correlated with the mass of the activated carbon, and the volume - to - mass ratio of the ruthenium precursor solution to the activated carbon is 0.5 - 1.6 mL / g.

[0036] Furthermore, the solid material absorbed with the ruthenium precursor solution is dispersed in deionized water to form a suspension, and a precipitant is added to the suspension. After continuously stirring at 50 - 95 °C for 6 - 15 h, the heating and stirring are stopped. After the suspension cools to room temperature, the solid obtained by suction filtration is vacuum - dried to obtain the activated carbon loaded with nano - colloid particles. This step is crucial for the final formation of ruthenium clusters. First, in this step, the solid material absorbed with the ruthenium precursor solution is dispersed in deionized water to form a suspension. The volume of deionized water is determined according to the mass of the activated carbon, and the ratio of the volume of deionized water to the mass of the activated carbon is between 20 - 300 mL / g. The precipitant is selected from at least one of urea, ammonia water, potassium hydroxide, sodium hydroxide, potassium carbonate, and sodium carbonate. The amount of the precipitant satisfies that the molar ratio of the precipitant to ruthenium in the ruthenium precursor solution is 20:1 - 300:1. The addition of the precipitant changes the pH value of the suspension and induces the slow occurrence of the reaction of ruthenium precursor hydrolysis to generate Ru(OH)3 colloid particles. By controlling the pH value of the precipitation reaction environment in the present invention, the nucleation rate of colloid particles on the surface of the activated carbon is much lower than the growth rate, so that the ruthenium precursor is transformed into nano - colloid particles with smaller particle size and monodispersity.

[0037] Furthermore, the activated carbon loaded with nano - colloid particles is placed in a tubular furnace and reduced at 300 - 450 °C for 3 - 12 hours. The atmosphere used can be a nitrogen - hydrogen mixed gas (hydrogen content 10 - 90 vol.%), the pressure inside the tube is 0.1 - 3 MPa, and the heating rate is 5 °C / min. After roasting, the activated carbon loaded with ruthenium clusters is obtained.

[0038] S3. Drop the metal promoter precursor solution onto the surface of the activated carbon loaded with ruthenium clusters, vacuum - leave standing for a period of time, and then carry out drying treatment to obtain the catalyst for ammonia synthesis. In specific implementation, the metal promoter 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 dosage of the metal promoter precursor solution is determined according to the mass and particle size of the activated carbon. Specifically, the volume-mass ratio of the metal promoter precursor solution to the activated carbon is 0.5 - 1.6 mL / g; the metal concentration in the metal promoter precursor solution is 12.5 - 200 mg / mL.

[0039] After the metal promoter precursor solution is dropped onto the surface of the activated carbon loaded with ruthenium clusters, it is left standing under vacuum at room temperature for 1 - 3 h, then heated to 60 - 110 °C and dried under vacuum for 6 - 12 h. After cooling to room temperature, it is taken out to obtain aRu-bAL / AC c 。

[0040] To enable those skilled in the art to understand the present invention more clearly, the following examples are now used to describe in detail a catalyst for ammonia synthesis and its preparation method according to the present invention.

[0041] Example 1 Porous activated carbon with an average size of 5 mm is placed in a tubular furnace under a pure hydrogen atmosphere and reduced at 900 °C for 14 h, with a heating rate of 5 °C / min and a gas flow rate of 50 mL / min. The obtained activated carbon is dispersed in 5 moL / L nitric acid solution and heated with stirring at 80 °C for 12 h. The obtained solid is washed 10 times with deionized water and then dried under vacuum at 110 °C for 12 h to obtain pretreated activated carbon.

[0042] 2 μL of 1 mol / L dilute hydrochloric acid is added to 0.2 mL of ruthenium precursor solution (the ruthenium concentration in the Ru(NO3)3(NO) solution is 15 mg / mL) to adjust the pH value of the ruthenium precursor solution to 2; the above ruthenium precursor solution is dropped onto 0.3 g of pretreated activated carbon, and after standing under vacuum for 1 h, the obtained solid material is dispersed in 30 mL of deionized water, stirred for 10 min, and then 71 mg of urea is added to form a suspension. The suspension is heated to 80 °C and kept stirring for 10 h. After filtration and separation, the solid is placed in a vacuum drying oven and dried under vacuum to obtain activated carbon loaded with ruthenium precursor.

[0043] The activated carbon loaded with ruthenium precursor in the tube is placed in the tubular furnace and reduced at 400 °C for 3 h. The atmosphere used is a nitrogen-hydrogen mixture, and the heating rate is 5 °C / min to obtain activated carbon loaded with ruthenium clusters 1Ru / AC 5mm 。

[0044] Weigh a certain amount of the metal promoter precursor CsNO3 and dissolve it in deionized water. Then, add 0.18 ml of the obtained metal promoter precursor solution (the concentration of Cs is 50 mg / mL) dropwise to the above solid, and let it stand in vacuum at room temperature for 3 h. After vacuum drying, 1Ru-3Cs / AC is obtained. 5mm 。

[0045] Example 2 Place commercial porous activated carbon with an average size of 1 mm in a tube furnace under a pure hydrogen atmosphere, and reduce it at 900 °C for 14 h, with a heating rate of 5 °C / min and a gas flow rate of 50 mL / min. Disperse the obtained activated carbon in 5 moL / L nitric acid solution, heat and stir it at 80 °C for 12 h. Wash the obtained solid with deionized water 10 times, and then vacuum dry it at 110 °C for 12 h to obtain the pretreated activated carbon.

[0046] Add appropriate dilute hydrochloric acid to 0.25 mL of the ruthenium precursor solution (the ruthenium concentration in the Ru(NO3)3(NO) solution is 15 mg / mL) to make the pH value of the ruthenium precursor solution 2. Drop the above ruthenium precursor solution onto 0.3 g of the pretreated activated carbon, and after standing in vacuum for 1 h, disperse the obtained 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 keep stirring for 10 h. After filtration and separation, place the solid in a vacuum drying oven and vacuum dry it to obtain the activated carbon loaded with the ruthenium precursor.

[0047] Reduce the above solid in the tube furnace at 400 °C for 3 h, using a nitrogen-hydrogen mixed gas as the atmosphere and a heating rate of 5 °C / min to obtain the activated carbon loaded with ruthenium clusters 2Ru / AC. 1mm 。

[0048] Weigh a certain amount of the metal promoter precursor KNO3 and dissolve it in deionized water. Then, add 0.18 ml of the obtained metal promoter precursor solution (the concentration of K is 50 mg / mL) dropwise to the above solid, and let it stand in vacuum at room temperature for 3 h. After vacuum drying, 2Ru-3K / AC is obtained. 1mm 。

[0049] Example 3 Place commercial porous activated carbon with an average size of 0.1 mm in a tube furnace under a pure hydrogen atmosphere, and reduce it at 800 °C for 14 h, with a heating rate of 5 °C / min and a gas flow rate of 50 mL / min. Disperse the obtained activated carbon in 5 moL / L nitric acid solution, heat and stir it at 80 °C for 12 h. Wash the obtained solid with deionized water 10 times, and then vacuum dry it at 110 °C for 12 h to obtain the pretreated activated carbon.

[0050] Add an appropriate amount of dilute hydrochloric acid to 0.3 mL of ruthenium precursor solution (ruthenium concentration in Ru(NO3)3(NO) solution is 15 mg / mL) to adjust the pH value of the ruthenium precursor solution to 2. Drop the above ruthenium precursor solution onto 0.3 g of pretreated activated carbon. After standing in vacuum for 1 h, disperse the obtained 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 while maintaining the temperature. After separation by suction filtration, place the solid in a vacuum drying oven and obtain the activated carbon loaded with ruthenium precursor after vacuum drying.

[0051] Reduce the above solid in a tube furnace at 400 °C for 3 hours. The atmosphere used is a nitrogen-hydrogen mixture, and the heating rate is 5 °C / min to obtain the activated carbon loaded with ruthenium clusters 4Ru / AC. 0.1mm 。

[0052] Weigh a certain amount of metal promoter precursor KNO3 and dissolve it in deionized water. Drop 0.18 ml of the obtained metal promoter precursor solution (K concentration is 67 mg / mL) onto the above solid, stand in vacuum at room temperature for 3 h, and obtain 4Ru-4K / AC after vacuum drying. 0.1mm 。

[0053] Example 4 Place commercial porous activated carbon with an average size of 0.1 mm in a tube furnace under a pure hydrogen atmosphere, reduce it at 800 °C for 12 h, with a heating rate of 5 °C / min and a gas flow rate of 50 mL / min. Disperse the obtained activated carbon in 5 moL / L nitric acid solution, heat and stir at 80 °C for 12 h. Wash the obtained solid with deionized water 5 - 10 times and then dry it in vacuum at 110 °C for 12 h to obtain the pretreated activated carbon.

[0054] Add an appropriate amount of dilute hydrochloric acid to 0.3 mL of ruthenium precursor solution (ruthenium concentration in Ru(NO3)3(NO) solution is 15 mg / mL) to adjust the pH value of the ruthenium precursor solution to 2. Drop the above ruthenium precursor solution onto 0.3 g of pretreated activated carbon. After standing in vacuum for 1 h, disperse the obtained 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 10 h while maintaining the temperature. After separation by suction filtration, place the solid in a vacuum drying oven and obtain the activated carbon loaded with ruthenium precursor after vacuum drying.

[0055] Reduce the above solid in a tube furnace at 400 °C for 3 hours. The atmosphere used is a nitrogen-hydrogen mixture, and the heating rate is 5 °C / min to obtain the activated carbon loaded with ruthenium clusters 4Ru / AC. 0.1mm 。

[0056] Weigh a certain amount of the metal promoter precursor Ba(NO3)2 and dissolve it in deionized water. Add 0.18 ml of the obtained metal promoter precursor solution (the concentration of Ba is 67 mg / mL) dropwise to the above solid, and let it stand in vacuum at room temperature for 3 h. After vacuum drying, 4Ru-4Ba / AC is obtained. 0.1mm 。

[0057] Comparative Example 1 Place commercial porous activated carbon with an average size of 5 mm in a tube furnace under a pure hydrogen atmosphere, reduce it at 00 °C for 14 h, with a heating rate of 5 °C / min and a gas flow rate of 50 mL / min. Disperse the obtained activated carbon in 5 moL / L nitric acid solution, heat and stir it at 80 °C for 12 h. Wash the obtained solid 10 times with deionized water, and then dry it in vacuum at 110 °C for 12 h to obtain pretreated activated carbon.

[0058] Weigh 0.3 g of the pretreated activated carbon, add 0.2 mL of Ru(NO3)3(NO) solution, and then add 2 μL of 1 mol / L dilute hydrochloric acid. Let it stand and then dry it in vacuum at 110 °C for 12 h. After cooling, activated carbon loaded with ruthenium precursor is obtained. Reduce the above solid in a tube furnace at 400 °C for 3 h, using a nitrogen-hydrogen mixed gas (hydrogen content 10 vol.%) as the atmosphere, with a heating rate of 5 °C / min, to obtain activated carbon loaded with ruthenium nanoparticles 1RuNP / AC 5mm 。

[0059] Comparative Example 2 Place commercial porous activated carbon with an average size of 0.1 mm in a tube furnace under a pure hydrogen atmosphere, reduce it at 900 °C for 14 h, with a heating rate of 5 °C / min and a gas flow rate of 50 mL / min. Disperse the obtained activated carbon in 5 moL / L nitric acid solution, heat and stir it at 80 °C for 12 h. Wash the obtained solid 10 times with deionized water, and then dry it in vacuum at 110 °C for 12 h to obtain pretreated activated carbon.

[0060] Weigh 0.3 g of the pretreated activated carbon, add 0.3 mL of Ru(NO3)3(NO) solution, and then add 2 μL of 1 mol / L dilute hydrochloric acid. Let it stand and then dry it in vacuum at 110 °C for 12 h. After cooling, activated carbon loaded with ruthenium precursor is obtained. Reduce the above solid in a tube furnace at 450 °C for 3 h, using a nitrogen-hydrogen mixed gas (hydrogen content 10 vol.%) as the atmosphere, with a heating rate of 5 °C / min, to obtain activated carbon loaded with ruthenium nanoparticles 4RuNP / AC 0.1mm ; Weigh a certain amount of Ba(NO3)2 and dissolve it in deionized water. Add the obtained solution dropwise to the above solid, let it stand in vacuum at room temperature for 3 h, and after vacuum drying, 4RuNP-4Ba / AC is obtained0.1mm 。

[0061] Performance test: (1)Effect of temperature on ammonia production rate of catalyst Carry out the ammonia synthesis reaction on the catalysts obtained in the above Examples 1-4 and Comparative Examples 1-2 respectively. The specific operation is as follows: Load the catalysts obtained in Examples 1-4 and Comparative Examples 1-2 into a fixed-bed tubular reactor respectively, carry out the ammonia synthesis reaction, introduce a nitrogen-hydrogen mixed gas of 60 mL / min (nitrogen-hydrogen ratio is 1:3) at room temperature, heat up to 400 °C at a rate of 5 °C / min and stabilize for 1 h, and then detect the tail gas absorption liquid by ion chromatography. After each adjustment of the reaction temperature, stabilize for 1 h and then measure the ammonia concentration in the tail gas. The reaction space velocity is 10000 h -1 , the reaction pressure is 1 MPa, and the reaction temperature range is 375 - 425 °C.

[0062] Figure 2 Shows the ammonia synthesis activity of the catalysts for ammonia synthesis provided in the examples and comparative examples of the present invention at different temperatures. As Figure 2 shown, the ammonia synthesis activities of the catalysts of 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 ) at different temperatures (reaction conditions: reaction pressure is 1 MPa, N2: H2 = 1:3 in the raw material gas, reaction space velocity is 10000 h -1 ). When Example 4 uses activated carbon with a size of 0.1 mm as the carrier, and the loading amounts of ruthenium and barium are both 4% of the mass of the activated carbon, the ammonia synthesis activity of the ruthenium cluster catalyst 4Ru-4Ba / AC 0.1mm is much higher than that of the ruthenium nanoparticle catalyst 4RuNP-4Ba / AC provided in Comparative Example 2 0.1mm . And when the reaction temperature is 350 °C and the reaction pressure is 1 MPa, the ammonia synthesis activity of 4Ru-4Ba / AC 0.1mm is 3 times that of 4RuNP-4Ba / AC 0.1mm . When the reaction pressure is 1 MPa and the reaction temperature is in the range of 300 - 425 °C, the ammonia synthesis activity of the ruthenium cluster catalyst 1Ru-3Cs / AC provided in Example 1 5mm is slightly better than that of the ruthenium nanoparticle catalyst 4RuNP-4Ba / AC provided in Comparative Example 2 0.1mm.

[0063] Depend on Figure 2 It can be seen that the catalytic material of the present invention containing metal promoters and loaded with ruthenium clusters not only has a higher activity in synthesizing ammonia, but also can reduce the cost of the ruthenium-carbon catalyst by improving the utilization rate of ruthenium metal.

[0064] (2) Effect of pressure on catalyst ammonia yield Select 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 operation was as follows: Loading 4Ru-4Ba / AC in a fixed bed tubular reactor 0.1mm The catalyst was used for ammonia synthesis reaction. A nitrogen-hydrogen mixture (nitrogen-hydrogen ratio of 1:3) was introduced at room temperature at a rate of 60 mL / min. The temperature was raised to 400 °C at a rate of 5 °C / min and stabilized for 1 h. The tail gas absorption liquid was then detected by ion chromatography. The reaction space velocity was 10000 h -1 , the reaction pressure range is 0.1-7 MPa.

[0065] Figure 3 The ammonia synthesis activity of the catalyst for synthesizing ammonia provided in Example 4 of the present invention at different pressures is shown. Figure 3 It can be seen that the reaction pressure has an effect on the 0.1mm ) The effect of catalyst on the ammonia synthesis performance (reaction conditions: reaction temperature is 400 ℃, reaction pressure is 0.1, 1, 3, 5, 7 MPa, the nitrogen-hydrogen mixture as raw material is N2: H2= 1:3, reaction space velocity is 10000 h -1 ), at 400°C, the 4Ru-4Ba / AC of Example 4 0.1mm The ammonia synthesis activity of the catalyst increases with the increase of reaction pressure, indicating that the catalyst has no obvious hydrogen poisoning phenomenon and meets the requirements of industrial ammonia synthesis. The stability test results of the catalysts of other examples are similar to the above, and there is no obvious hydrogen poisoning.

[0066] (3) Changes in the catalyst's ammonia synthesis performance over reaction time Select 4RuNP-4Ba / AC provided in Example 4 0.1mm The effect of pressure on the performance of the catalyst for ammonia synthesis as a function of reaction time was tested. Under the conditions of a reaction temperature of 400°C and a reaction pressure of 1 MPa, the nitrogen-hydrogen mixture as the raw material had a ratio of N2:H2=1:3 and a reaction space velocity of 10000 h -1 The ammonia synthesis reaction was carried out continuously for 350 hours, during which the ammonia production rate was tested at irregular intervals.

[0067] Figure 4shows the change of the ammonia synthesis performance of the catalyst for ammonia synthesis provided in Example 4 of the present invention with the reaction time. The results are as follows Figure 4 shown, the 4Ru-4Ba / AC catalyst provided in Example 4 0.1mm has very stable catalytic activity, and the activity remains basically unchanged after 350 hours of reaction. The stability test results of the catalysts in other examples are similar to the above, and all have good stability.

[0068] Furthermore, Figure 5 shows the transmission electron microscope image of the catalyst for ammonia synthesis provided in Example 4 of the present invention. As Figure 5 shown, there are no obvious ruthenium nanoparticles in the catalyst for ammonia synthesis provided in Example 4; Figure 6 shows the transmission electron microscope image of the catalyst for ammonia synthesis provided in the comparative example of the present invention. As Figure 6 shown, obvious ruthenium nanoparticles can be observed in the catalyst for ammonia synthesis provided in Comparative Example 2.

[0069] In the description of this specification, the descriptions with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples", etc. mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples. In addition, those skilled in the art can combine and combine the different embodiments or examples described in this specification.

[0070] For the method embodiments, for the sake of simple description, they are all expressed as a series of action combinations. However, those skilled in the art should know that the present invention is not limited by the described action sequence, because according to the present invention, certain steps can be performed in other sequences or simultaneously. Secondly, those skilled in the art should also know that the embodiments described in the specification are all preferred embodiments, and the actions and components involved are not necessarily essential to the present invention.

[0071] The above has introduced in detail a catalyst for ammonia synthesis and its preparation method provided by the present invention. Specific examples are used in this article to elaborate on the principle and implementation manner of the present invention. The description of the above embodiments is only used to help understand the method and its core idea of the present invention; at the same time, for those of ordinary skill in the art, according to the idea of the present invention, there will be changes in the specific implementation manner and application scope. In summary, the content of this specification should not be construed as a limitation to the present invention.

Claims

1. A catalyst for synthesizing ammonia, characterized in that, The composition of the catalyst includes: an activated carbon substrate, and ruthenium metal and metal promoters supported on the activated carbon substrate; wherein, the mass ratio of the ruthenium metal to the activated carbon is 0.5-5%, and the mass ratio of the metal promoter to the activated carbon is 0.5-12%; the metal promoter is selected from alkali metals or alkaline earth metals; the ruthenium metal is sub-nanometer ruthenium clusters with an average particle size of 0.2-1.0 nm.

2. The catalyst for ammonia synthesis according to claim 1, characterized in that, The metal promoter includes one or more of potassium, rubidium, cesium, and barium.

3. The catalyst for ammonia synthesis according to claim 1, characterized in that, The average particle size of the activated carbon is 0.1-10 mm.

4. The catalyst for synthesizing ammonia 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, the reaction gas space velocity is 1000 - 50000 h -1 , the reaction temperature is 300 - 450 °C, and the reaction pressure is 0.1 - 10 MPa.

5. A method for preparing a catalyst for ammonia synthesis according to any one of claims 1-4 above, characterized in that, The preparation method includes: Placing the activated carbon in a hydrogen atmosphere for roasting, adding it to a nitric acid solution, and continuously stirring for surface oxidation modification to obtain pretreated activated carbon; Dropping the ruthenium precursor solution onto the surface of the pretreated activated carbon, standing in vacuum for a period of time, dispersing the obtained solid in deionized water, adding a precipitant to the suspension, heating the solution to 50-90 °C and continuously stirring for 6-15 h, collecting the solid material and placing it in a reducing atmosphere for high-temperature roasting to obtain activated carbon loaded with ruthenium clusters; Dropping the metal promoter precursor solution onto the surface of the activated carbon loaded with ruthenium clusters, standing in vacuum for a period of time, and then performing a drying treatment to obtain the catalyst for ammonia synthesis; wherein, the pH of the ruthenium precursor solution is 1-3, and the ruthenium concentration in the ruthenium precursor solution is 9-91 mg Ru / mL; the volume-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 promoter precursor solution is 12.5-200 mg / mL; the volume-mass ratio of the metal promoter precursor solution to the activated carbon is 0.5-1.6 mL / g.

6. The preparation method according to claim 5, characterized in that, The metal promoter 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.

7. The preparation method according to claim 5, characterized in that, The precipitant is selected from at least one of urea, ammonia water, potassium hydroxide, sodium hydroxide, potassium carbonate, and sodium carbonate; the molar ratio of the precipitant to ruthenium in the ruthenium precursor solution is 20:1-300:

1.

8. The preparation method according to claim 5, wherein, The roasting temperature for placing the activated carbon in a hydrogen atmosphere for roasting is 600-900 °C, and the roasting time is 6-20 h; the temperature for the surface oxidation modification is 50-90 °C, and the time is 6-20 h.

9. The preparation method according to claim 5, characterized in that The temperature for collecting the solid material and placing it in a reducing atmosphere for high-temperature roasting is 300-450 °C, and the time is 3-12 h.

10. The preparation method according to claim 5, characterized in that, The drying is carried out in a vacuum environment, and the drying temperature is 60-110 °C.

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