Nitrogen-containing iron-based ammonia synthesis catalyst and preparation method thereof

Abstract

The invention discloses a nitrogen-containing iron-based ammonia synthesis catalyst as well as a preparation method and application thereof, and belongs to the technical field of iron-based ammonia synthesis catalysts. The catalyst takes iron as an active component and contains a nitrogen element, wherein the content of nitrogen is 5-30wt%. The preparation method comprises the following steps: taking an iron salt solution as a precursor, adding a precipitator to obtain a precipitate, washing the precipitate, calcining the precipitate in oxygen-containing gas to obtain an oxide precursor, and activating the oxide precursor in ammonia-containing gas to obtain the nitrogen-containing iron-based ammonia synthesis catalyst. Compared with most traditional iron-based ammonia synthesis catalysts containing auxiliaries, the catalyst provided by the invention has the advantages of simple preparation process, no need of addition of auxiliary materials such as cobalt and the like, higher ammonia synthesis activity and wide application prospect.

Description

Technical Field

[0001] The present invention belongs to the technical field of iron-based ammonia synthesis catalysts, and particularly relates to an iron-based ammonia synthesis catalyst containing iron as an active component and nitrogen element and a preparation method thereof. Background Art

[0002] Ammonia (NH 3 ) has become a very promising green energy storage intermediate and carbon-free energy carrier due to its high energy density, high hydrogen content, and no harmful gas generation during decomposition. It is also one of the most important raw materials for producing chemicals and fertilizers and plays an important role in human production and life. Although nitrogen accounts for more than 78% of the atmosphere, due to the extremely large N≡N bond energy, the direct conversion of N 2 into ammonia has always been a huge challenge until the development of the Haber-Bosch method using an iron-based catalyst. However, the Haber-Bosch process is not perfect and has a series of significant problems. This process consumes a large amount of energy during operation. At the same time, it also consumes up to 1% of natural gas resources and inevitably generates a large amount of greenhouse gases during production. Therefore, it has become an urgent task to explore a more environmentally friendly, more sustainable, and higher-performance catalyst and a new synthesis route to reduce the energy consumption of the ammonia synthesis process.

[0003] Transition metal nitrides (TMNs) are a class of potential catalytic materials. Filling N atoms into the transition metal lattice can expand the lattice spacing, increase the valence electron density, and further narrow the metal d-band, thereby optimizing the electronic and defect tolerance properties. Due to their tunable electronic properties, good electrical conductivity, excellent hydrogen dissociation ability, and inherent corrosion resistance, etc., they have been studied in detail in the aspects of hydrogen production by electrolyzing water, hydrodesulfurization of petroleum, Fischer-Tropsch synthesis, and ammonia decomposition reaction. Iron nitride is one of the typical transition metal nitrides. In 2022, Liu Yijiang et al. (Metal-coordinated porouspolydopamine nanospheresderived Fe 3 N-FeCo encapsulated N-doped carbon as ahighly efficient electrocatalyst for oxygen reduction reaction. CatalysisNano Research Energy, 2022) pioneered the rapid in-situ construction of Fe 3 N / FeCo hybrid porous carbon using melamine as a nitrogen source, showing excellent redox catalytic effects, and the synthesized material has better performance than Pt.

[0004] In recent years, studies have shown that nitrides such as CeN, LaN, and YN (Contribution of Nitrogen Vacancies to Ammonia Synthesis over Metal Nitride Catalysts. Journal of the American Chemical Society, 2020) can serve as effective supports or catalysts for ammonia synthesis, where N vacancies can simultaneously activate H 2 and N 2 to stabilize ammonia synthesis, thus proposing a general rule for the design of nitrogen-based ammonia synthesis catalysts: the nitrogen vacancy formation energy dominates the catalytic performance. For Ni-loaded nitrides, H 2 and N 2 are activated at Ni metal and nitrogen sites respectively, proposing the concept of dual active sites, whose activity far exceeds that of other cobalt-based and nickel-based catalysts and can even rival that of ruthenium-based catalysts. However, the above nitrides are difficult to exist stably in air. Summary of the Invention

[0005] The object of the present invention is to provide an iron-based ammonia synthesis catalyst with iron as the active component and containing nitrogen, where the nitrogen content is about 5 - 30 wt%. The present invention has the advantages of simple preparation method, easy control of preparation conditions, and excellent catalytic performance.

[0006] To achieve the above object, the present invention adopts the following technical solutions: The specific steps of the preparation method of the nitrogen-containing iron-based ammonia synthesis catalyst are as follows: (1) Add a precipitant to the precursor solution containing an iron salt to obtain an iron-containing precipitate; (2) Wash the precipitate obtained in step (1) and calcine it in an oxygen-containing gas to obtain an oxide precursor; (3) Heat-treat the oxide precursor obtained in step (2) in an ammonia-containing gas to obtain the nitrogen-containing iron-based ammonia synthesis catalyst.

[0007] The iron salt in step (1) is one of iron nitrate, iron chloride, and iron oxalate; the solvent used to dissolve the precursor is one or two of water, methanol, and ethanol; the precipitant is one of ammonia water, ammonium carbonate, sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate.

[0008] The oxygen-containing gas in step (2) is air, a mixture of oxygen and nitrogen, or a mixture of oxygen and group 0 gas, and the volume content of oxygen in the mixture is 2% - 99%; the calcination temperature is 150 - 750 °C, and the calcination time is 2 - 20 hours.

[0009] The ammonia-containing gas described in step (3) is ammonia, a mixture of ammonia and nitrogen, or a mixture of ammonia and a Group 0 gas. The volume content of ammonia in the mixture is 25%-99%; the heat treatment temperature is 200-900 °C, and the reaction time is 0.5-36 hours.

[0010] The beneficial effects of the present invention are as follows: The present invention provides an iron-based catalyst with iron as the active component and containing nitrogen, wherein the nitrogen content is about 5-30 wt%. The catalyst uses an iron salt solution as the precursor, adds a precipitant to obtain a precipitate for washing, and then calcines in an oxygen-containing gas to obtain an oxide precursor, and subsequently activates and heat-treats in an ammonia-containing gas to obtain the iron-based ammonia synthesis catalyst. The catalyst obtained by the present invention has a high residual nitrogen element and can promote the ammonia synthesis reaction. Description of the Drawings

[0011] Figure 1 XRD patterns of the catalysts prepared in Example 1, 3 and Comparative Example 1, 2.

[0012] Figure 2 For the catalysts of Example 1 and Comparative Example 1, the H 2 -TPR curve and the mass spectrometry signal of m / z = 17. Detailed Embodiments

[0013] In order to make the content described in the present invention easier to understand, the following further illustrates the technical solutions of the present invention with specific examples, but the present invention is not limited thereto.

[0014] Example 1: (1) Weigh 14.467 g of iron nitrate and dissolve it in 100 mL of deionized water. Add an ammonia precipitant, then stir at room temperature, adjust the pH to form a precipitate in the solution, continue stirring for 2 hours, and then let it stand and age overnight to obtain a mixed solution containing an iron precipitate; (2) Wash, centrifuge and dry the iron precipitate obtained in step (1), screen out particles with a mesh size of 32-60 meshes with a sample sieve, and calcine in air at 500 °C for 2 hours to obtain an oxide precursor; (3) Place the oxide precursor obtained in step (2) in ammonia gas and heat-treat at 500 °C for 5 hours to obtain the nitrogen-containing iron-based ammonia synthesis catalyst. Elemental analysis shows that the nitrogen element content in this catalyst is 15.81 wt%.

[0015] Example 2: (1) Weigh 14.467 g of ferric chloride and dissolve it in 100 mL of methanol. Add an ammonium carbonate precipitant, then stir at room temperature, adjust the pH to form a precipitate in the solution, continue stirring for 2 hours, and then let it stand and age overnight to obtain a mixed solution containing an iron precipitate; (2) Wash and centrifugally dry the iron-containing precipitate obtained in step (1), screen out particles with a size of 32 - 60 mesh using a sample sieve, and calcine at 700 °C for 10 hours in a 10% O 2 + 90% Ar mixed gas to obtain the oxide precursor; (3) Place the oxide precursor obtained in step (2) in a 50% NH 3 + 50% Ar mixed gas and heat-treat at 450 °C for 10 hours to obtain the nitrogen-containing iron-based ammonia synthesis catalyst. Elemental analysis shows that the nitrogen content in this catalyst is 11.07 wt%.

[0016] Example 3: (1) Weigh 14.467 g of ferric nitrate and dissolve it in 100 mL of deionized water. Add a sodium hydroxide precipitating agent, then stir at room temperature, adjust the pH to form a precipitate in the solution, continue stirring for 2 hours, and then let it stand and age overnight to obtain a mixed solution of iron-containing precipitate; (2) Wash and centrifugally dry the iron-containing precipitate obtained in step (1), screen out particles with a size of 32 - 60 mesh using a sample sieve, and calcine at 800 °C for 5 hours in oxygen to obtain the oxide precursor; (3) Place the oxide precursor obtained in step (2) in ammonia gas and heat-treat at 600 °C for 15 hours to obtain the nitrogen-containing iron-based ammonia synthesis catalyst. Elemental analysis shows that the nitrogen content in this catalyst is 12.46 wt%.

[0017] Example 4: (1) Weigh 14.467 g of ferric nitrate and dissolve it in 100 mL of deionized water. Add an ammonia water precipitating agent, then stir at room temperature, adjust the pH to form a precipitate in the solution, continue stirring for 2 hours, and then let it stand and age overnight to obtain a mixed solution of iron-containing precipitate; (2) Wash and centrifugally dry the iron-containing precipitate obtained in step (1), screen out particles with a size of 32 - 60 mesh using a sample sieve, and calcine at 500 °C for 4 hours in air to obtain the oxide precursor; (3) Place the oxide precursor obtained in step (2) in ammonia gas and heat-treat at 800 °C for 5 hours to obtain the nitrogen-containing iron-based ammonia synthesis catalyst. Elemental analysis shows that the nitrogen content in this catalyst is 8.72 wt%.

[0018] Example 5: (1) Weigh 14.467 g of ferric nitrate and dissolve it in 100 mL of deionized water. Add an ammonia water precipitating agent, then stir at room temperature, adjust the pH to form a precipitate in the solution, continue stirring for 2 hours, and then let it stand and age overnight to obtain a mixed solution of iron-containing precipitate; (2) Wash and centrifugally dry the iron-containing precipitate obtained in step (1), screen out particles with a size of 32 - 60 mesh using a sample sieve, and calcine at 500 °C in air for 2 hours to obtain the oxide precursor; (3) Place the oxide precursor obtained in step (2) in ammonia gas and heat-treat at 900 °C for 5 hours to obtain the nitrogen-containing iron-based ammonia synthesis catalyst. Elemental analysis shows that the nitrogen content in this catalyst is 5.56 wt%.

[0019] Comparative Example 1: (1) Weigh 14.467 g of iron nitrate and dissolve it in 100 mL of deionized water. Add ammonia precipitation agent, then stir at room temperature, adjust the pH to form a precipitate in the solution, continue stirring for 2 hours, and then let it stand and age overnight to obtain a mixed solution of iron-containing precipitate; (2) Wash and centrifugally dry the iron-containing precipitate obtained in step (1), screen out particles with a size of 32 - 60 mesh using a sample sieve, and calcine at 500 °C in air for 2 hours to obtain the oxide precursor; (3) Place the oxide precursor obtained in step (2) in hydrogen gas and heat-treat at 500 °C for 5 hours. Elemental analysis shows that this catalyst does not contain nitrogen element.

[0020] Comparative Example 2: (1) Weigh 14.467 g of iron nitrate and dissolve it in 100 mL of deionized water. Add ammonia precipitation agent, then stir at room temperature, adjust the pH to form a precipitate in the solution, continue stirring for 2 hours, and then let it stand and age overnight to obtain a mixed solution of iron-containing precipitate; (2) Wash and centrifugally dry the iron-containing precipitate obtained in step (1), screen out particles with a size of 32 - 60 mesh using a sample sieve, and calcine at 500 °C in air for 2 hours to obtain the oxide precursor; (3) First place the oxide precursor obtained in step (2) in hydrogen gas and heat-treat at 500 °C for 5 hours. After cooling to room temperature, then introduce ammonia gas and heat-treat at 500 °C for 5 hours. Elemental analysis shows that the nitrogen content in this catalyst is 5.41 wt%.

[0021] Catalyst activity evaluation: The ammonia synthesis performance evaluation of the prepared iron-based catalysts was all tested in a stainless steel fixed-bed reactor with an inner diameter of 12 mm. When loading the furnace, fill the lower part of the isothermal zone of the reactor with coarse quartz sand. Take 2 mL of fine quartz sand and evenly mix it with 0.3 g of the catalyst and fill it into the isothermal zone. Then pad 5 mL of coarse quartz sand and seal the reactor. The reaction raw material gas is a nitrogen-hydrogen mixed gas obtained by catalytic cracking of liquid ammonia over a nickel catalyst at 810 °C, and the nitrogen-hydrogen ratio is 1:3. The test conditions are: pressure is 1 MPa, reaction temperature is 400 °C, and the outlet mass space velocity is 36000 mL g −1h −1 The catalyst performance results are shown in Table 1.

[0022] Characterization description: Figure 1 It is the XRD pattern of the catalysts prepared in Example 1, Example 3 and Comparative Example 1, Comparative Example 2. It can be seen that there is obvious Fe 2 N and Fe 4 N is generated, indicating that there is obvious nitrogen in the catalyst prepared by the method proposed in the present invention. However, no iron nitride is produced in the sample of Comparative Example 1 without ammonia heat treatment. Although there is an iron nitride phase in the sample of Comparative Example 2 with hydrogen treatment followed by ammonia heat treatment, due to the low nitrogen content, its ammonia synthesis activity is low.

[0023] Figure 2 It is the H 2 -TPR curve and mass spectrometry signal of the catalysts in Example 1 and Comparative Example 1. It can be seen from the figure that compared with the comparative sample, the nitrogen-containing iron catalyst prepared by the present invention consumes more hydrogen, and mass spectrometry analysis shows that ammonia is produced starting from 300 °C during its reduction process, further indicating that the introduction of nitrogen significantly improves the ammonia synthesis performance of the catalyst.

[0024] The above are only the preferred embodiments of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention shall fall within the scope covered by the present invention.

Claims

1. A nitrogen-containing iron-based ammonia synthesis catalyst, characterized in that: The catalyst has iron as an active component and a nitrogen content of 5-30 wt%.

2. A method for preparing the nitrogen-containing iron-based ammonia synthesis catalyst as claimed in claim 1, characterized in that: The following steps are involved: (1) adding a precipitant to a precursor solution containing an iron salt to obtain an iron-containing precipitate; (2) washing the precipitate obtained in step (1), and calcining it in an oxygen-containing gas to obtain an oxide precursor; (3) Heat treating the oxide precursor obtained in step (2) in an ammonia-containing gas to obtain the nitrogen-containing iron-based ammonia synthesis catalyst.

3. The method according to claim 2, characterized in that: In step (1), the iron salt is one of ferric nitrate, ferric chloride, and ferric oxalate; the solvent of the precursor solution is one or two of water, methanol, and ethanol; and the precipitant is one of ammonia water, ammonium carbonate, sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate.

4. The method according to claim 2, characterized in that: In step (2), the oxygen-containing gas is air, a mixture of oxygen and nitrogen, or a mixture of oxygen and a Group 0 gas, and the volume content of oxygen in the mixture is 2-99%; the calcination temperature is 150-750°C, and the time is 2-20 hours.

5. The method according to claim 2, characterized in that: In step (3), the ammonia-containing gas is ammonia, a mixture of ammonia and nitrogen, or a mixture of ammonia and a Group 0 gas, and the volume content of ammonia in the mixture is 25-99%; the heat treatment temperature is 200-900°C, and the time is 0.5-36 hours.

6. Use of the nitrogen-containing iron-based ammonia synthesis catalyst as claimed in claim 1 or the nitrogen-containing iron-based ammonia synthesis catalyst prepared by the method according to any one of claims 2 to 5 in ammonia synthesis reaction.

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