A Fe-based ammonia synthesis catalyst and its preparation method

Abstract

This invention discloses a method for preparing an Fe-based catalyst promoted by an auxiliary agent and its application in green ammonia, belonging to the field of catalyst material preparation technology. The preparation method includes the following steps: carbonizing an iron-containing precursor to obtain a carbide, and then ball-milling it with an auxiliary agent to obtain the Fe-based catalyst. The ammonia synthesis performance under mild conditions is controlled by adjusting the molar ratio of the active components Fe to C and the loading of the auxiliary agent. The highest ammonia synthesis rate is obtained when the molar ratio of Fe to C is 5:2, the auxiliary agent is K, and the auxiliary agent loading is 1.5 wt.%, reaching a reaction rate of 19.4 mmol / L at 400 °C and 1 MPa. NH3 g cat ‑1 h ‑1 Since the catalyst does not contain precious metals, it can effectively reduce the cost of the catalyst. The preparation method is simple and easy to operate, the raw materials are cheap and readily available, and the stability is good, so it has great industrial application value.

Description

Technical Field

[0001] This invention belongs to the field of catalyst material preparation technology, specifically relating to a method for preparing an additive-promoted Fe-based catalyst and its application in green ammonia synthesis. Background Technology

[0002] Ammonia is an important chemical raw material, widely used in industries such as plastics, rubber, environmental protection, and metallurgy, and is fundamental to the chemical industry. At the same time, ammonia has a high energy density (12.3 GJ·m³). -3 Hydrogen, with its advantages of easy storage and transportation, high hydrogen content (17.6 wt.%), and no carbon dioxide production during decomposition, is considered a green hydrogen storage material and a new type of energy carrier. However, the harsh reaction conditions for industrial ammonia synthesis (400-500 ℃, 20-30 MPa) result in high energy consumption and high pollution, leading to increasingly serious environmental pollution problems, and the greenhouse effect and carbon emission issues urgently need to be addressed.

[0003] Replacing fossil fuels with sustainable energy sources for ammonia synthesis can significantly reduce carbon emissions and facilitate carbon neutrality. Therefore, sustainable energy sources such as hydropower, wind power, solar power, and tidal power are gaining increasing attention. However, limitations in cost, seasonality, volatility, dispersion, and energy storage methods restrict their large-scale industrial application. In recent years, based on the maturity of green power generation technologies, water electrolysis for H2 production has gradually matured. The Haber-Bosch process for ammonia synthesis (sustainable energy power generation → water electrolysis for hydrogen production → ammonia synthesis → liquid ammonia storage and transportation → hydrogen energy) driven by electrolysis has become a new technological path to achieve zero carbon emissions. However, current water electrolysis hydrogen production systems require an output pressure ≤5.0 MPa (mainly concentrated in the 1.6~3.2 MPa range), and the resulting H2, after deep dehydration and deoxygenation, reaches a temperature of approximately 350~400 ℃. Current industrial ammonia synthesis using fossil resources as raw materials falls far short of this goal. Therefore, there is an urgent need to develop ammonia synthesis technologies under mild conditions (reaction conditions: ~400 ℃, 1.6~3.2 MPa).

[0004] Developing a complementary and integrated technology that combines renewable energy-powered electrolysis for hydrogen production with advanced ammonia synthesis is one of the most feasible approaches to achieving clean and efficient utilization of renewable energy, optimizing hydrogen source structure, and ensuring safe storage and transportation of hydrogen. The most crucial aspect of developing advanced ammonia synthesis technology remains the development of highly efficient catalysts. Developing catalysts with high ammonia synthesis rates at lower temperatures and pressures is a key research focus for efficient ammonia synthesis under mild conditions. Summary of the Invention

[0005] Given the challenges of traditional Fe-based catalysts requiring high temperatures and pressures, the present invention aims to provide a method for preparing an additive-promoted Fe-based catalyst and its application in green ammonia synthesis. By controlling the molar ratio of the active components Fe to C and the additive loading, a significant improvement in ammonia synthesis performance under mild conditions is achieved.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] This invention provides a method for preparing a Fe-based mild green ammonia catalyst, comprising the following steps:

[0008] (1) The Fe salt precursor solution was added to the Na2CO3 solution to form a suspension, and the hydrothermal reaction was carried out. The hydroxide was obtained by filtration and drying.

[0009] (2) Grind the hydroxide from step (1) and calcine it in air to obtain Fe2O3;

[0010] (3) Calcine the Fe2O3 from step (2) under a reducing atmosphere to obtain Fe5C2;

[0011] (4) After grinding the Fe5C2 and the additives from step (3) evenly, the mixture is ball-milled at 350 rpm for 2 h and then reduced at 400 °C for 2 h under H2 atmosphere to obtain the Fe-based ammonia synthesis catalyst.

[0012] The Fe salt precursor mentioned in step (1) is one of ferric nitrate, ferric chloride, and ferric acetate, preferably ferric nitrate nonahydrate. The concentration of the Na2CO3 solution is 10-30 wt.%, preferably 15-20 wt.%. The Fe(NO3)3 solution is added to the Na2CO3 solution, and the resulting suspension is stirred magnetically for 5-15 min, preferably 10 min. The hydrothermal reaction temperature is 150-190 ℃, preferably 180 ℃; the hydrothermal reaction time is 8-24 h, preferably 20 h. The drying temperature is 50-80 ℃, preferably 60 ℃; the drying time is 8-24 h, preferably 12 h.

[0013] In step (2), the calcination temperature is 350-450 ℃, preferably 400 ℃; the calcination time is 3-7 h, preferably 5 h.

[0014] The reducing atmosphere described in step (3) is one of carbon monoxide, ammonia, and hydrogen, preferably carbon monoxide, with a CO concentration of 99.9%; the calcination temperature is 350-450 ℃, preferably 400 ℃, and the time is 1-3 h, preferably 2 h. During calcination, the heating rate is 1-4 ℃ min. -1 Preferably 2 ℃ min -1 .

[0015] The additive mentioned in step (4) is one of K, Ba, and La, and the mass of the additive is 1.0-5.0% of the total mass of the catalyst.

[0016] The present invention also provides the application of the above-mentioned Fe-based mild green ammonia catalyst in the catalytic synthesis of ammonia, preferably as a catalyst for the synthesis of ammonia, and more preferably as a catalyst for the thermal catalytic mild synthesis of ammonia.

[0017] The temperature for the ammonia synthesis method is 300-450 ℃, preferably 400 ℃.

[0018] The pressure of the ammonia synthesis method is 0.2-5 MPa, preferably 1 MPa.

[0019] The total mass hourly space velocity of H2 and N2 is 10,000-100,000 mL g. -1 h -1 Preferably 60,000 mL g -1 h -1 .

[0020] The K content, based on the mass of the catalyst, is 1-8 wt.%, preferably 1.0-5.0 wt.%.

[0021] The beneficial effects of this invention are as follows:

[0022] (1) This invention synthesizes Fe-based catalysts with different K contents as additives, using iron carbide as a support, and adjusts the ammonia synthesis performance of the catalysts by mechanical ball milling. The Fe-based catalysts of this invention have excellent catalytic performance, providing a solution for the research and application of mild ammonia synthesis catalysts.

[0023] (2) The relative loading of additive K and Fe and the synthesis method in the catalyst provided by the present invention play a crucial role in the performance of ammonia synthesis. By changing the loading of additive K and Fe and the synthesis method, a novel high-efficiency Fe-based ammonia synthesis catalyst promoted by additive K was prepared, which exhibited excellent catalytic performance in mild ammonia synthesis reaction.

[0024] (3) This invention provides a new approach to ammonia synthesis catalysts. The preparation method is relatively simple, easy to operate, and low in cost. The catalyst is easy to shape and has a high ammonia synthesis rate and good activity under low pressure conditions, which has broad prospects for industrial application. Attached Figure Description

[0025] Figure 1 The ammonia synthesis performance of the catalysts obtained in Examples 1-5 under reaction conditions of 400 °C and 1 MPa;

[0026] Figure 2The catalyst obtained in Example 3 exhibits catalytic stability under reaction conditions of 400 °C and 1 MPa. Detailed Implementation

[0027] The technical solution of the present invention will be further described in detail below with reference to specific embodiments. It should be understood that the following embodiments are merely illustrative and explanatory of the present invention, and should not be construed as limiting the scope of protection of the present invention. All technologies implemented based on the above content of the present invention are covered within the scope of protection intended by the present invention.

[0028] Unless otherwise stated, the raw materials and reagents used in the following examples are commercially available products or can be prepared by known methods.

[0029] Example 1:

[0030] Preparation of Fe5C2

[0031] (1) Prepare a 0.24 g / 100 mL solution of ferric nitrate, and disperse 8 mL of Fe(NO3)3 solution into 7 mL of 0.212 g / mL Na2CO3 solution;

[0032] (2) The suspension was stirred with a magnetic stirrer for 10 min and then hydrothermated at 180 °C for 20 h to obtain hydroxide; the oxide was obtained by calcining in air at 400 °C for 5 h.

[0033] (3) The above oxides were treated in a reducing atmosphere of CO at 400 °C for 2 h to finally obtain Fe5C. 2。

[0034] Example 2:

[0035] Preparation of 1% K-Fe5C2 catalyst

[0036] (1) Take 0.6 g Fe5C2 and 0.006 g KNO3 respectively and put them into a mortar and grind them manually for 20 minutes;

[0037] (2) The above mixture was placed in a ball mill and continuously ground at 350 rpm for 2 h. The catalyst was then reduced at 400 ℃ for 2 h in an H2 atmosphere to obtain the 1K-Fe5C2 catalyst. The loading of promoter K in this catalyst was 1 wt.%.

[0038] Example 3:

[0039] Preparation of 1.5% K-Fe5C2 catalyst: 0.6 g Fe5C2 and 0.009 g KNO3 were placed in a mortar and manually ground for 20 min. The resulting mixture was then continuously ground in a ball mill at 350 rpm for 2 h. The catalyst was then reduced at 400 ℃ for 2 h under H2 atmosphere to obtain the 1.5K-Fe5C2 catalyst. The K loading of the promoter in this catalyst was 1.5 wt.%.

[0040] Preparation of 1.5% Ba-Fe5C2 catalyst: 0.6 g Fe5C2 and 0.023 g Ba(NO3)2 were placed in a mortar and manually ground for 20 min. The resulting mixture was then continuously ground in a ball mill at 350 rpm for 2 h. The catalyst was reduced at 400 ℃ for 2 h under H2 atmosphere to obtain 1.5K-Fe5C2 catalyst. The Ba loading of the promoter in this catalyst was 1.5 wt.%.

[0041] Preparation of 1.5% La-Fe5C2 catalyst: 0.6 g Fe5C2 and 0.029 g La(NO3)3 were placed in a mortar and manually ground for 20 min. The resulting mixture was then continuously ground in a ball mill at 350 rpm for 2 h. The catalyst was reduced at 400 ℃ for 2 h under H2 atmosphere to obtain 1.5K-Fe5C2 catalyst. The La loading of the promoter in this catalyst was 1.5 wt.%.

[0042] Example 4:

[0043] Preparation of 3% K-Fe5C2 catalyst

[0044] (1) Take 0.6 g Fe5C2 and 0.018 g KNO3 respectively and put them into a mortar and grind them manually for 20 minutes;

[0045] (2) The mixture obtained above was placed in a ball mill and continuously ground at 350 rpm for 2 h. The catalyst was reduced at 400 °C for 2 h under H2 atmosphere to obtain 3K-Fe5C2 catalyst. The K loading of the promoter in the catalyst here is 3 wt.%.

[0046] Example 5:

[0047] Preparation of 5% K-Fe5C2 catalyst

[0048] (1) Take 0.6 g Fe5C2 and 0.03 g KNO3 respectively and put them into a mortar and grind them manually for 20 minutes;

[0049] (2) The mixture obtained above was continuously ground in a ball mill at 350 rpm for 2 h. The catalyst was then reduced at 400 °C for 2 h under H2 atmosphere to obtain the 5K-Fe5C2 catalyst. The K loading of the promoter in this catalyst was 5 wt.%.

[0050] Example 6:

[0051] Preparation of Fe2O3 catalyst

[0052] (1) Prepare a 0.24 g / 100 mL solution of ferric acetate, and disperse 8 mL of Fe(NO3)3 solution into 7 mL of 0.212 g / mL Na2CO3 solution;

[0053] (2) The suspension was stirred with a magnetic stirrer for 10 min and then hydrothermated at 180 °C for 20 h to obtain hydroxide; Fe2O3 was obtained by calcining in air at 400 °C for 5 h.

[0054] Catalyst performance evaluation

[0055] Weigh 0.2 g of each of the catalysts prepared in Examples 1-6, and then heat them at a mass hourly space velocity (WHSV) of 60,000 mL / g. -1 h -1 The conversion rate of ammonia synthesis was determined in a continuous flow micro-fixed bed reactor. The reaction gas composition was a mixture of 25% N2 and 75% H2. The conversion rate of the catalyst for ammonia synthesis was tested at 400 °C and 1 MPa. The NH3 concentration in the tail gas was updated by ion chromatography (Thermo Scientific, DIONEX, ICS-600). The test results are shown below. Figure 1-2 .

[0056] from Figure 1 It can be seen that the ammonia synthesis rate of the catalyst follows the following rule: 1.5K-Fe5C2>1K-Fe5C2≈3K-Fe5C2>5K-Fe5C2, indicating that the content of alkali metal promoter is closely related to the ammonia synthesis reaction rate. 1.5K-Fe5C2 exhibits the best ammonia synthesis reaction rate, reaching 19.4 mmol. NH3 g cat -1 h -1 .

[0057] from Figure 2 It can be seen that the ammonia synthesis rate of the catalyst did not show a significant decreasing trend within 100 h, remaining basically stable. This indicates that the catalyst has good catalytic stability.

[0058] In summary, potassium (K) promoters generally promote the growth of Fe-based catalysts. The effect of K loading on the ammonia synthesis rate exhibits a distinct volcano-shaped curve, thus determining the optimal K loading content to be 1.5 wt.%.

[0059] The embodiments of the present invention have been described above. However, the present invention is not limited to the above embodiments. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing an Fe-based ammonia synthesis catalyst, characterized in that: The Fe-based ammonia synthesis catalyst was prepared by depositing the additives onto Fe5C2 using a mechanical ball milling method. Includes the following steps: (1) The Fe salt precursor solution was added to the Na2CO3 solution to form a suspension, and the suspension was subjected to hydrothermal reaction at 180 °C. The hydroxide was obtained by filtration and drying. (2) Grind the hydroxide from step (1) and calcine it at 400 °C for 5 h in air atmosphere to obtain Fe2O3; (3) Calcine the Fe2O3 from step (2) under a reducing atmosphere to obtain Fe5C2; (4) After grinding the Fe5C2 and the additives from step (3) evenly, the Fe5C2 was ball-milled at 350 rpm for 2 h and then reduced at 400℃ for 2 h under H2 atmosphere to obtain the Fe-based ammonia synthesis catalyst. The additive mentioned in step (4) is one of K, Ba, or La, and the mass of the additive is 1.0-5.0% of the total mass of the catalyst.

2. The method according to claim 1, characterized in that: The Fe salt precursor mentioned in step (1) is one of ferric nitrate, ferric chloride, or ferric acetate.

3. The method according to claim 1, characterized in that: The reducing atmosphere described in step (3) is carbon monoxide, and the CO concentration is 99.9%; the calcination temperature is 350-450 ℃, and the time is 1-3 h.

4. An Fe-based ammonia synthesis catalyst prepared by the method according to any one of claims 1-3.

5. The application of an Fe-based ammonia synthesis catalyst prepared by the method according to any one of claims 1-3, characterized in that: Fe-based ammonia synthesis catalysts are applied to the ammonia synthesis reaction in water electrolysis.

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