Preparation method of ruthenium-based catalyst for synthesizing ammonia under mild condition

Abstract

The invention belongs to the field of catalyst preparation, and provides a preparation method of a cerium oxide supported ruthenium-based catalyst for synthesizing ammonia under mild conditions. The preparation method comprises the following steps: firstly, weighing cerium nitrate, a ruthenium source compound and trimesic acid, and dissolving in a solvent to prepare a precursor solution; then, a solid product is obtained through solvothermal reaction, and composite MOF powder is obtained after centrifugation, washing and vacuum drying; and finally, pyrolyzing and roasting to obtain the cerium oxide loaded ruthenium-based catalyst. According to the method provided by the invention, Ce-MOF is used as a sacrificial template agent to prepare the thermocatalytic ammonia synthesis catalyst with uniform loading and large specific surface area in one step, and the activity of the catalyst reaches up to 11000 micromoles g <-1 > h <-1 > at 400 DEG C, 1 MPa and the air speed of 36000 mL g <-1 > h <-1 >. In addition, other lanthanide elements can be doped to form a composite carrier, the activity is further improved, the preparation process is simple, expensive instruments and equipment are not needed, and large-scale industrial production is easy.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of catalyst materials, and particularly relates to a preparation method of a ruthenium-based catalyst for synthesizing ammonia under mild conditions. BACKGROUND

[0002] Ammonia (NH3) is one of the most widely produced chemicals in the world and is the backbone of the fertilizer industry. Recently, it has been receiving increasing attention as a hydrogen storage carrier due to its high volumetric energy density and ease of storage and transportation, and is considered to be an ideal energy carrier and carbon-free fuel in the future hydrogen economy. However, the Haber-Bosch process commonly used in the current ammonia synthesis industry relies on iron-based catalysts, which requires harsh reaction conditions (400-600℃, 20-40MPa), resulting in huge energy consumption, accounting for 1-2% of the global energy consumption per year, and emitting 500 million tons of carbon dioxide per year.

[0003] With the increasing demand for a carbon-free society, recent research focuses on developing catalysts with higher activity under milder conditions, and it has been proven that Ru-based catalysts are superior to other transition metal catalysts such as Co, Ni and Mo. Cerium oxide (CeO2) has become a good support for ammonia synthesis Ru-based catalysts due to its surface characteristics of oxygen vacancies and strong metal support interaction and good thermal stability. There are a large number of oxygen vacancies in CeO2, and the excess electrons on them can be transferred to Ru through Ru-O-Ce bonds, thereby promoting the activation of N2 on Ru. It has been reported in the literature that the oxygen vacancies of the support CeO2 can promote the hydrogen overflow process, causing the hydrogen cracked on Ru to overflow to the CeO2 support, thereby reducing the phenomenon of hydrogen poisoning of the Ru-based catalyst.

[0004] However, the traditional preparation method of the Ru / CeO2 system usually produces Ru particles of large size and it is difficult to ensure uniform distribution of Ru, and usually the support needs to be prepared first and then Ru is loaded, which has various influencing factors in the process and is challenging in terms of repeatability and scale preparation. SUMMARY

[0005] One of the purposes of the present application is to solve the problem of poor repeatability and uniformity of the preparation of ruthenium-based catalysts in the prior art, and to provide a preparation method of a ruthenium-based catalyst for synthesizing ammonia under mild conditions. The catalyst prepared by the method has high activity, the steps are simple, easy to mass produce, and is conducive to industrial application.

[0006] In order to achieve the above purpose, the technical scheme adopted by the present application is as follows:

[0007] The present application directly obtains a cerium oxide supported ruthenium-based catalyst by synthesizing a Ce-MOF composite material containing Ru, pyrolysis, calcination and reduction "one-step method", and specifically comprises the following steps:

[0008] S1, cerium nitrate, ruthenium source compound and benzene-1, 3, 5-tricarboxylic acid (BTC) are added to a solvent to stir until completely dissolved to prepare a precursor solution, and the molar ratio of cerium nitrate, benzene-1, 3, 5-tricarboxylic acid and ruthenium source compound is 18-20: 12-14: 1-1.2.

[0009] S2, a solid product is obtained by a solvothermal reaction, and a solid powder is obtained by centrifugation, washing and vacuum drying;

[0010] S3, the obtained solid powder is subjected to high-temperature pyrolysis under the protection of an inert atmosphere;

[0011] S4, the pyrolysis product is calcined in a reducing atmosphere to obtain the catalyst.

[0012] Preferably, a lanthanide nitrate is also added to the solvent in step S1. The lanthanide nitrate is one or more of lanthanum nitrate, praseodymium nitrate or samarium nitrate. The molar ratio of the lanthanide nitrate to the ruthenium source compound is 9-10: 1-1.2, and the same molar amount of cerium nitrate is correspondingly reduced. The incorporation of other lanthanide elements into the CeO2 carrier to form a composite carrier can further increase the oxygen vacancy concentration and enhance the ammonia synthesis activity of the catalyst.

[0013] Preferably, in step S1, the nitrate is an analytical reagent, and the solvent is one or more of deionized water, ethanol, N, N-dimethylformamide (DMF).

[0014] Preferably, in step S1, the ruthenium source compound used is one or more of ruthenium trichloride, ruthenium nitrosyl nitrate and ruthenium acetylacetone.

[0015] Preferably, in step S2, the heating temperature of the blast drying oven used for the solvothermal reaction is 100-200℃, and the heating time is 12-24h.

[0016] Preferably, in step S3, the solid-liquid separation is centrifugation, and the centrifugal speed is 10000r·min -1 , and the centrifugal time is 3min.

[0017] Preferably, in step S3, the washing method is to add a solvent, stir uniformly and centrifuge, the centrifugal speed is 10000r·min -1 , the centrifugal time is 3min, the solid product is collected and repeated several times, and the solvent is washed until the supernatant is colorless, and then washed with anhydrous ethanol for 2-4 times.

[0018] Preferably, in step S3, the vacuum drying is carried out in an electric vacuum drying oven, the vacuum degree is 0.05-0.1MPa, the temperature is 80-100℃, and the time is 10-15h.

[0019] Preferably, the step S4 is carried out in a tube furnace or a muffle furnace, and the inert atmosphere is preferably helium, neon, nitrogen or argon. The heating temperature is set to 400-500℃, the heating time is 2-4h, and the temperature rising rate is 2-10℃·min -1 .

[0020] Preferably, the step S5 is carried out in a tube furnace or a muffle furnace, and the reducing atmosphere is preferably hydrogen, hydrogen-argon mixture, nitrogen-hydrogen mixture or carbon monoxide-argon mixture. The heating temperature of the tube furnace or the muffle furnace is 400-500℃, the heating time is 2-4h, and the temperature rising rate is 2-10℃·min -1 .

[0021] The present application has the following beneficial effects:

[0022] 1) The present application synthesizes a CeO2-supported Ru-based catalyst in one step by using Ce-MOF as a sacrificial template, which can conveniently control the loading amount and make the loading uniform. The finally obtained catalyst retains part of the pore structure and has a high specific surface area, which is beneficial to the efficient synthesis of ammonia reaction.

[0023] 2) The present application also provides a method for effectively incorporating other lanthanide elements into a CeO2-supported Ru-based ammonia synthesis catalyst, which is simple, controllable in proportion, and can significantly improve the activity of the catalyst.

[0024] 3) The process of the present application is simple and does not require complex and expensive instruments and equipment. The obtained catalyst has high ammonia synthesis activity under mild conditions, which is beneficial to industrial application. The technical scheme provides a method for preparing catalysts by using a sacrificial template, which can be popularized to other catalyst preparation methods. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 A flow chart for preparing a ruthenium-based ammonia synthesis catalyst by the Ce-MOF template method described in the present application.

[0026] Figure 2 XRD patterns of the catalysts prepared in Examples One, Two and Three of the present application.

[0027] Figure 3 SEM and EDS-Mapping patterns of 3wt% Ru / CeO2 prepared in Example One of the present application.

[0028] Figure 4 SEM and EDS-Mapping patterns of 3wt% Ru / La 0.5 Ce 0.5 O x prepared in Example Two of the present application.

[0029] Figure 5 3wt% Ru / Pr prepared for Example 3 of the present application 0.5 Ce 0.5 O x SEM and EDS-Mapping images of the catalyst prepared for Example 1 of the present application.

[0030] Figure 6 BET specific surface area data chart of the catalysts prepared for Examples 1, 2 and 3 of the present application.

[0031] Figure 7 Synthesis ammonia performance chart of the catalyst prepared for Example 1 of the present application at different space velocities and pressures.

[0032] Figure 8 Synthesis ammonia performance chart of the catalysts prepared for Examples 1, 2 and 3 of the present application at 18000 mL g -1 h -1 space velocity.

[0033] Figure 9 Synthesis ammonia performance comparison chart of the catalysts prepared for Example 1 and Comparative Example 1 of the present application at 36000 mL g -1 h -1 space velocity. DETAILED DESCRIPTION

[0034] In order to make the technical solutions and advantages of the present application clearer, the present application and its beneficial effects will be described in further detail below in conjunction with specific embodiments and the accompanying drawings of the specification, but the embodiments of the present application are not limited thereto.

[0035] Example 1

[0036] A preparation method of a ruthenium-based catalyst for synthesizing ammonia under mild conditions, comprising the following steps:

[0037] S1, 2.0625 g of cerium nitrate hexahydrate, 0.0996 g of ruthenium acetylacetonate and 0.6305 g of BTC were added to 50 mL of DMF and stirred until completely dissolved to prepare a precursor solution.

[0038] S2, the solution was transferred to a high-pressure reaction kettle and placed in a blast drying oven at 130°C for solvothermal reaction for 20 h. The obtained suspension was centrifuged at a speed of 10000 r·min -1 for 3 min to obtain a brown solid product. The solid was washed by centrifugation, first washed with DMF until the supernatant was colorless, then washed with ethanol for 3 times, and finally dried in a vacuum drying oven at 80°C for 12 h.

[0039] S3, the above product was pyrolyzed in a tube furnace under a flowing argon atmosphere, the temperature was set to 450°C, and the time was set to 3 h.

[0040] S4, the pyrolysis product is calcined and reduced under 10% hydrogen-argon mixed gas atmosphere, the temperature is set to 450°C, and the time is set to 3h, to obtain a CeO2-supported ruthenium-based catalyst, the theoretical loading amount is 3wt%, which is marked as 3wt% Ru / CeO2.

[0041] Example Two

[0042] The difference from Example 1 is that lanthanum nitrate is added in step S1, 1.0313g cerium nitrate hexahydrate, 1.0284g lanthanum nitrate hexahydrate, 0.0996g ruthenium acetylacetonate and 0.6305g BTC are added into 50mL DMF and stirred until completely dissolved to prepare a precursor solution.

[0043] Example Two obtains a La-doped CeO2-supported ruthenium-based catalyst, the theoretical loading amount is 3wt%, which is marked as 3wt% Ru / La 0.5 Ce 0.5 O x .

[0044] Example Three

[0045] The difference from Example 1 is that praseodymium nitrate is added in step S1, 1.0313g cerium nitrate hexahydrate, 1.0332g praseodymium nitrate hexahydrate, 0.0996g ruthenium acetylacetonate and 0.6305g BTC are added into 50mL DMF and stirred until completely dissolved to prepare a precursor solution.

[0046] Example Three obtains a Pr-doped CeO2-supported ruthenium-based catalyst, the theoretical loading amount is 3wt%, which is marked as 3wt% Ru / Pr 0.5 Ce 0.5 O x .

[0047] Comparative Example One

[0048] A 3wt% Ru / CeO2catalyst is prepared using a conventional impregnation method, wherein the CeO2support uses a commercial material.

[0049] First, 5mL of tetrahydrofuran and 0.016g of Ru3(CO) 12 are added into an eggplant-shaped flask, and the solid is completely dissolved by ultrasonic. Then, the flask is transferred into a magnetic oil bath, and 0.25g of CeO2support is added. After that, the stirring is started and the oil bath temperature is set to 60°C, and a condenser is connected to prevent the liquid from being evaporated. After waiting for 12h, the solvent is removed using a rotary evaporator, and then dried in a 80°C blast drying oven for 12h. Finally, the solid powder is scraped off, calcined and reduced under 10% hydrogen-argon mixed gas atmosphere, the temperature is set to 450°C, and the time is set to 3h, and the obtained catalyst is marked as 3wt% CeO2-impregnated.

[0050] Specific application example one

[0051] The performance of the above catalysts was evaluated using a fixed bed reactor.

[0052] First, the powder catalyst was pressed into tablets using a tablet press, crushed, and sieved into 20-40 mesh particles. 0.1 g of catalyst was weighed and mixed with quartz sand of the same mesh size (catalyst to quartz sand mass ratio of 1:10). This step was to overcome the potential external diffusion effect of the catalyst and to avoid the catalyst powder blocking the pipeline.

[0053] Subsequently, the catalyst bed was padded with a quartz tube and quartz sand to the constant temperature heating zone of the reaction tube. After the catalyst was packed, a layer of quartz sand was added on top of the bed to prevent the catalyst from being blown away by the gas flow. Before starting the test, the gas tightness of the device was checked.

[0054] Then, high-purity N2 and H2 steel cylinders were used to provide synthesis gas, and the flow rates of the two gases were adjusted to maintain a space velocity of 18000 or 36000 mL g -1 h -1 -1min-1 according to the flow ratio of 1:3. The catalyst bed temperature was set to 400℃, and the pressure was set to 0.1 MPa or 1 MPa by adjusting the back pressure valve. The concentration of ammonia in the outlet tail gas was tested at the set temperature and pressure, which was converted into the synthesis ammonia reaction rate of the catalyst.

[0055] Finally, the concentration of ammonia in the outlet tail gas was obtained by chemical absorption method. A 5 mmol L -1 of dilute sulfuric acid solution was prepared as the absorption liquid, and during the test, the tail gas was passed into a known volume of acid solution, and stirring was applied to ensure complete absorption. After a period of time, the concentration of NH4 + in the absorption liquid was measured by ion chromatography (Qingdao Shenghe, CIC-D100), which was converted into the synthesis ammonia reaction rate, and the formula was as follows:

[0056]

[0057] Where: C NH4 + is the concentration of NH4 + in the absorption liquid (mg L -1 );

[0058] V 酸 is the volume of dilute sulfuric acid (L);

[0059] M is the molar mass of NH4 + (g mol -1 );

[0060] m is the mass of the catalyst (g);

[0061] t is the absorption time (h);

[0062] r is the rate of ammonia synthesis (μmol g) -1 h -1 ).

[0063] The catalysts prepared in Examples 1 to 3 and Comparative Example 1 were subjected to activity evaluation and compositional characterization according to specific application example 1. The experimental results are shown in [Figure 1]. Figures 2 to 8 .

[0064] from Figure 2 It can be seen that the catalysts prepared in Examples 1, 2, and 3 are mainly composed of CeO2, and the grain size is relatively small. Due to the incorporation of other lanthanide elements, the diffraction peaks have shifted to lower angles. No Ru diffraction peaks were observed in the XRD patterns, indicating that Ru is uniformly distributed on the catalyst surface and has not aggregated.

[0065] from Figure 3 It can be seen that the Ru element is uniformly distributed in the 3wt% Ru / CeO2 catalyst prepared in Example 1, and no aggregation occurs.

[0066] from Figure 4 It can be seen that Ru element in the 3wt% Ru / La prepared in Example 2 0.5 Ce 0.5 O x The catalyst was uniformly distributed without aggregation, and the incorporated La element was clearly observed.

[0067] from Figure 5 It can be seen that Ru element in the 3wt% Ru / Pr prepared in Example 3 0.5 Ce 0.5 O x The catalyst was uniformly distributed without aggregation, and the incorporated Pr element was clearly observed.

[0068] from Figure 6 It can be seen that the specific surface area of ​​the 3wt% Ru / CeO2 catalyst prepared in Example 1 is as high as 98m². 2 g -1 After incorporating La or Pr, the specific surface area decreased to 80 m². 2 g -1 Around, but still much larger than the 15m of commercial CeO2. 2 g -1 A high specific surface area not only increases the opportunity for the catalyst to come into contact with the reactants, but also enhances the diffusion process of gas molecules on the catalyst surface.

[0069] from Figure 7It can be seen that the ammonia synthesis reaction rate of the 3wt% Ru / CeO2 catalyst prepared in Example 1 at 400℃ increases with increasing space velocity, and at a space velocity of 36000 mL g / L... -1 h -1 Under a pressure of 1.0 MPa, the reaction rate reached 11000 μmol / g. -1 h -1

[0070] from Figure 8 It can be seen that the catalysts prepared in Examples 1, 2, and 3 perform well at 400℃, 0.1 MPa, and 18000 mL g. -1 h -1 All of them exhibited high reactivity for ammonia synthesis at space velocities, with the 3wt% Ru / Pr prepared in Example 3 being particularly effective. 0.5 Ce 0.5 O x The ammonia synthesis reaction rate is as high as 2480 μmol g. -1 h -1 .

[0071] from Figure 9 It can be seen that the catalytic activity of the 3wt% Ru / CeO2-impregnated catalyst prepared in Comparative Example 1 is poor, with a space velocity of 36000 mL g. -1 h -1 At that time, the ammonia synthesis reaction rate at 400℃ and 1.0MPa was only 4560 μmol g. -1 h -1 This is less than the 11000 μmol g of 3wt% Ru / CeO2 prepared according to this method in Example 1. -1 h -1 This half of the results demonstrates the advantages of this method.

[0072] This invention provides a method for the one-step synthesis of CeO2-supported ruthenium-based catalysts using Ce-MOF as a sacrificial template agent. This method allows for convenient control of the loading amount and ensures uniform loading. Furthermore, the final catalyst retains some of the porous structure of the original MOF, exhibiting a high specific surface area, which is beneficial for catalytic ammonia synthesis under mild conditions, specifically at 400°C, 1 MPa, and 36000 mL g. -1 h -1 The activity at space velocity is as high as 11000 μmol g -1 h -1 Other lanthanide elements can be added to form a uniform composite support with CeO2, increasing the oxygen vacancy concentration and further enhancing the catalyst activity.

[0073] Those skilled in the art can make various modifications and variations to the above embodiments based on the disclosure and teachings of this specification. Therefore, the present application should not be limited to the above specific embodiments, and any obvious modifications, replacements or variations made by those skilled in the art based on the present application shall fall within the scope of the present application. In addition, although some specific terms are used in this specification, these terms are only for the convenience of description and do not constitute any limitation on the present application.

Claims

1. A method for preparing a ruthenium-based catalyst for ammonia synthesis under mild conditions, comprising the following steps: S1, adding cerium nitrate, a ruthenium source compound and trimesic acid into a solvent and stirring until completely dissolved to prepare a precursor solution, the molar ratio of cerium nitrate, trimesic acid and the ruthenium source compound being 18-20: 12-14: 1-1.

2. S2, obtaining a solid product by a solvothermal reaction, and obtaining a solid powder by centrifugation, washing and vacuum drying; S3, high-temperature pyrolysis of the obtained solid powder under protection of an inert atmosphere; S4, calcining the pyrolysis product under a reducing atmosphere to obtain the catalyst.

2. The production method according to claim 1, characterized by, In step S1, a lanthanide nitrate is also added to the solvent, and the molar ratio of the lanthanide nitrate to the ruthenium source compound is 9-10: 1-1.

2.

3. The preparation method according to claim 2, characterized in that, The lanthanide nitrate is one or more of lanthanum nitrate, praseodymium nitrate or samarium nitrate.

4. The method of claim 1, wherein, The solvent in step S1 is one or more of water, ethanol and DMF.

5. The preparation method according to claim 1, characterized in that, The ruthenium source compound in step S1 is one or more of ruthenium trichloride, ruthenium nitrosyl nitrate and ruthenium acetylacetonate.

6. The method of claim 1, wherein, The temperature of the solvothermal reaction in step S2 is 100-200℃, and the reaction time is 12-24h.

7. The preparation method according to claim 1, characterized in that, The temperature of the vacuum drying in step S2 is 80-100℃, and the time is 10-15h.

8. The method of claim 1, wherein, The high-temperature pyrolysis temperature of step S3 is 400-500℃, the heating time is 2-4h, and the heating rate is 2-10℃·min -1 The inert atmosphere is one of helium, neon, nitrogen or argon.

9. The method of claim 1, wherein, The baking temperature of step S4 is 400-500℃, the heating time is 2-4h, and the temperature rising rate is 2-10℃·min -1 The reducing atmosphere is one of hydrogen, hydrogen-argon mixture, nitrogen-hydrogen mixture, or carbon monoxide-argon mixture.

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