Preparation method and application of iron-based catalyst constructed by multi-aid confinement

Abstract

The invention relates to the technical field of catalytic reaction, and discloses a preparation method and application of an iron-based catalyst constructed by multiple additive confinement, and the preparation method comprises the following steps: S1, dissolving an iron precursor, a cerium precursor, a magnesium precursor and a calcium precursor in water, adjusting the pH to 8-10, stirring and mixing to make the system form a precipitate; s2, adding an aluminum precursor, a silicon precursor and a barium precursor into the system, adjusting the pH value to 8-10, stirring and mixing, and cooperatively precipitating again to obtain mixed slurry; and S3, filtering, washing and drying the mixed slurry, roasting in an inert atmosphere, and reducing with hydrogen to obtain the iron-based catalyst containing the Fe-bcc (111) crystal face. According to the present invention, by introducing the synergistic doping of the multiple crystal face auxiliary agents and the confinement auxiliary agent, and combining the two-step precipitation method, the directional stability and the structure confinement protection of the Fe-bcc (111) high-activity crystal face are achieved, such that the ammonia synthesis activity and the high-temperature operation stability of the catalyst are significantly improved.

Description

Technical Field

[0001] This invention relates to the technical field of catalytic reactions, and in particular to a method for preparing and applying an iron-based catalyst constructed with multiple auxiliary agents. Background Technology

[0002] Ammonia synthesis is a fundamental process in the energy and chemical industry, and its core catalysts are mostly iron-based materials. Traditional ammonia synthesis catalysts typically use Fe3O4 or α-Fe as the active phase, with auxiliaries such as K2O, Al2O3, and CaO used to achieve crystal phase stabilization and electronic structure regulation. However, under high temperature and high pressure (400~500℃, 10~15 MPa) reaction conditions, iron particles are prone to sintering, phase transformation, and surface reconstruction, leading to a decrease in the exposure of active sites, a reduction in the ammonia formation rate, and severely affecting the catalyst's lifespan and stability for industrial applications.

[0003] To improve the low-temperature activity and thermal stability of iron-based catalysts, researchers have attempted to optimize their surface structure through methods such as crystal facet manipulation and additive doping. Conventional single-agent systems often only regulate electronic structure or surface alkalinity, making it difficult to simultaneously achieve both crystal facet exposure and anti-sintering performance. Therefore, multi-agent synergistic doping is commonly used to prepare iron-based catalysts. For example, rare earth or alkaline earth elements are used to improve the electronic structure and enhance the activation ability of nitrogen molecules. Patent CN105772024A discloses an iron-ruthenium composite ammonia synthesis catalyst and its preparation method. The catalyst uses metallic ruthenium as the active component, an iron-based catalyst as the support, and / or one or more of alkali metals, alkaline earth metals, and transition elements as the co-catalyst.

[0004] However, the above-mentioned multi-agent synergistic doping still has the following problems: (1) uneven distribution of agents leads to local enrichment or agglomeration, and the crystal facet regulation effect is limited; (2) lack of effective structural confinement mechanism, and grains are prone to grow at high temperature; (3) the crystal facet selectivity and the synergistic mechanism of agents are not yet clear, and the catalytic performance stability is insufficient. Existing studies have shown that Fe-bcc(111) or high-index C7 crystal facets have higher nitrogen adsorption and activation capabilities and are key active faces for ammonia synthesis reactions. However, under conventional preparation conditions, the proportion of such high-activity crystal facets is low, and they are easily reconstructed into thermodynamically stable low-energy faces (such as Fe(110) and Fe(100)) in high-temperature reactions, making it difficult to maintain a high-activity structure for a long time. Therefore, how to achieve directional stabilization of Fe-bcc(111) or C7 crystal facets in iron-based oxide systems through multi-agent synergistic doping, while constructing confinement structures to suppress particle growth and phase transition, is an important research direction in the field of ammonia synthesis catalysis. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a method for preparing and applying an iron-based catalyst constructed with multiple additives in a confined environment. By introducing synergistic doping of various crystal surface additives and confining additives, and combining a two-step precipitation confined construction process, the directional stability and structural confinement protection of the Fe-bcc(111) highly active crystal surface are achieved, thereby significantly improving the ammonia synthesis activity and high-temperature operating stability of the catalyst.

[0006] The objective of this invention is achieved through the following technical solution: This invention provides a method for preparing an iron-based catalyst constructed with multiple auxiliary agents, comprising the following steps: S1. Dissolve the iron precursor, cerium precursor, magnesium precursor and calcium precursor in water, adjust the pH to 8-10, stir and mix to form a precipitate in the system. S2. Add aluminum precursor, silicon precursor and barium precursor to the above system, adjust the pH to 8~10, stir and mix, and precipitate again to obtain a mixed slurry. S3. After the mixed slurry is filtered, washed and dried, it is calcined under an inert atmosphere and then reduced with hydrogen to obtain an iron-based catalyst containing Fe-bcc(111) crystal planes. The mass percentage of the metals contained is: iron 70~88%, cerium 4~10%, magnesium 2~4%, calcium 2~6%, aluminum 1~5%, silicon 2~6%, and barium 0.5~3%.

[0007] This invention achieves spatial separation and synergistic effect of crystal surface aids and confined aids through staged precipitation, avoiding uneven doping or agglomeration of aids. The resulting catalyst contains multiple aids uniformly distributed in the iron oxide lattice and surface in the form of doping or confined coating. Crystal surface aids (cerium, magnesium, calcium) adjust the Fe lattice strain and electronic structure, and the synergistic doping of multiple aids significantly enhances crystal surface selectivity, greatly promoting the preferential exposure of Fe-bcc(111) / C7 crystal faces. Confined aids (aluminum, silicon, barium) construct a stable oxide framework and coating layer on the catalyst surface, effectively inhibiting particle growth and phase transformation, thus achieving a coexistence of stable exposure of highly active crystal faces and anti-sintering properties. This multi-aid confined synergistic effect constructs highly active Fe-bcc(111) / C7 crystal faces, while simultaneously giving the catalyst excellent structural stability and reaction durability. Furthermore, these multiple aids can add electrons to the iron-based catalyst, facilitating the dissociation of N2 and thereby improving the catalytic activity in the ammonia synthesis reaction.

[0008] Therefore, this invention achieves stable exposure and structural enhancement of the Fe-bcc(111) / C7 highly active crystal plane through multi-agent synergistic doping and confined structure construction, solving the technical problem of difficulty in balancing activity and stability in traditional iron-based ammonia synthesis catalysts, and providing a new design idea and feasible path for developing efficient, long-life, and industrially prepared ammonia synthesis catalysts.

[0009] Preferably, the molar ratio of Ce, Mg, and Ca in the cerium, magnesium, and calcium precursors is 0.1~0.9:1:0.3~1.8; and the molar ratio of Al, Si, and Ba in the aluminum, silicon, and barium precursors is 0.1~2.0:1:0.01~0.2. More preferably, the molar ratio of Ce, Mg, and Ca in the cerium, magnesium, and calcium precursors is 0.2~0.6:1:0.4~0.9; and the molar ratio of Al, Si, and Ba in the aluminum, silicon, and barium precursors is 0.5~1.2:1:0.05~0.15.

[0010] By adjusting and controlling the molar ratio and loading of multiple adjuvants, the synergistic effect among the adjuvants can be improved, thereby achieving a better catalytic effect.

[0011] Preferably, in S1, the stirring and mixing is carried out at 40~70℃ for 0.5~3h.

[0012] Preferably, in S1, the iron precursor is ferric nitrate nonahydrate, ferric chloride, or ferric oxalate; the cerium precursor is cerium nitrate hexahydrate, cerium chloride, or cerium ammonium nitrate hexahydrate; the magnesium precursor is magnesium nitrate hexahydrate, magnesium acetate, or magnesium chloride; and the calcium precursor is calcium nitrate tetrahydrate, calcium chloride, or calcium acetate.

[0013] Preferably, in step S2, the stirring and mixing is carried out at 60~90℃ for 1~2 hours, followed by standing and aging for 2~4 hours.

[0014] Preferably, in S2, the aluminum precursor is aluminum nitrate nonahydrate, aluminum chloride, or aluminum isopropanol; the silicon precursor is sodium silicate nonahydrate, ethyl ethoxylate, or ammonium metasilicate; and the barium precursor is barium nitrate or barium chloride.

[0015] Preferably, in S1 and S2, the pH is adjusted using an alkaline solution, which is one or more of ammonia, sodium hydroxide solution and sodium carbonate solution, with a concentration of 0.1~5 mol / L.

[0016] Preferably, in step S3, the calcination temperature is 400~800℃ and the calcination time is 2~6h; the inert atmosphere is a nitrogen atmosphere or an argon atmosphere; the hydrogen reduction temperature is 400~500℃, the pressure is atmospheric pressure, the time is 2~3h, and a mixture of hydrogen and nitrogen is used, with an H2 / N2 molar ratio of 2~3:1.

[0017] Secondly, the present invention also provides an application of the iron-based catalyst prepared by the above method in ammonia synthesis.

[0018] As a preferred embodiment, hydrogen and nitrogen are reacted with an iron-based catalyst to synthesize ammonia under reaction conditions of 350-500°C and 7-15 MPa.

[0019] When the iron-based catalyst prepared by this invention is applied to the ammonia synthesis reaction, compared with the iron-based catalyst prepared by the traditional impregnation method or the single-agent co-precipitation method, the ammonia generation rate of the catalyst of this invention is increased by 30-60% under the same conditions, and the activity decay rate is less than 10% after continuous operation at 600℃ for 72 hours, showing significant anti-sintering performance and high-temperature structural stability.

[0020] Compared with existing technologies, the nanocatalyst of this invention has the following beneficial effects: (1) The synergistic effect of crystal plane additives Ce, Mg and Ca can effectively regulate the electronic structure and lattice distortion of Fe, and promote the stable exposure of highly active Fe-bcc(111) / C7 crystal planes; (2) Confinement agents Al, Si and Ba form a stable framework structure on the surface, which inhibits the migration and agglomeration of Fe particles and improves thermal stability and lifespan. (3) By staged precipitation, the spatial separation and synergistic effect of crystal surface additives and confinement additives are achieved, avoiding uneven doping or agglomeration of additives, accurately constructing Fe-bcc(111) / C7 highly active crystal surfaces, and at the same time making the catalyst have excellent structural stability and reaction durability. (4) The catalyst exhibits excellent and stable ammonia synthesis catalytic performance. At 450℃, the ammonia generation rate is 30-60% higher than that of the traditional Fe-based system. After continuous operation at 600℃ for 72 hours, the activity retention rate exceeds 90%. (5) The preparation process is simple, the reaction conditions are mild, the raw materials are inexpensive, and it is suitable for industrial scale-up and long-term stable operation. Attached Figure Description

[0021] Figure 1 This is a high-resolution transmission electron microscope (HRTEM) image of the catalyst in Example 1.

[0022] Figure 2 This is a high-resolution transmission electron microscope (HRTEM) image of the catalyst in Comparative Example 2. Detailed Implementation

[0023] The technical solution of the present invention will be illustrated below with specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0024] 1. Iron-based catalysts The preparation method of the iron-based catalyst constructed by confinement of multiple auxiliary agents in this invention includes the following steps: S1. Dissolve the iron precursor, cerium precursor, magnesium precursor, and calcium precursor in water, and slowly add 0.1~5 mol / L alkaline solution (one or more of ammonia, sodium hydroxide solution, and sodium carbonate solution) to adjust the pH to 8~10. Stir and mix at 40~70℃ for 0.5~3h to allow the system to form a precipitate. S2. Add aluminum precursor, silicon precursor and barium precursor to the above system, slowly add 0.1~5 mol / L alkaline solution (one or more of ammonia water, sodium hydroxide solution and sodium carbonate solution) to adjust the pH to 8~10, stir and mix at 60~90℃ for 1~2h, and then let it stand and age at the same temperature for 2~4h to precipitate again to obtain mixed slurry; S3. After filtration, the mixed slurry is precipitated and washed with deionized water until neutral. After drying at 100-120℃, it is calcined in an inert atmosphere at a temperature of 400-800℃ for 2-6 hours. Then, it is reduced with hydrogen using a mixture of hydrogen and nitrogen (H2 / N2 molar ratio of 2-3:1) at a temperature of 400-500℃, a pressure of atmospheric pressure, and a time of 2-3 hours to obtain an iron-based catalyst containing Fe-bcc(111) crystal planes. The mass percentage of the metals contained is: iron 70-88%, cerium 4-10%, magnesium 2-4%, calcium 2-6%, aluminum 1-5%, silicon 2-6%, and barium 0.5-3%.

[0025] In a specific embodiment of the present invention, in S1, the iron precursor is ferric nitrate nonahydrate, ferric chloride, or ferric oxalate. The cerium precursor is cerium nitrate hexahydrate, cerium chloride, or cerium ammonium nitrate hexahydrate. The magnesium precursor is magnesium nitrate hexahydrate, magnesium acetate, or magnesium chloride. The calcium precursor is calcium nitrate tetrahydrate, calcium chloride, or calcium acetate.

[0026] In a specific embodiment of the present invention, in S2, the aluminum precursor is aluminum nitrate nonahydrate, aluminum chloride, or aluminum isopropanol. The silicon precursor is sodium silicate nonahydrate, ethyl ethoxylate, or ammonium metasilicate. The barium precursor is barium nitrate or barium chloride.

[0027] 2. Ammonia synthesis reaction The iron-based catalyst prepared above was reduced with hydrogen under a mixed gas of hydrogen and nitrogen (H2 / N2 molar ratio of 3-4:1) at a temperature of 400-500℃ for 2-3 h. Then, hydrogen and nitrogen were reacted with the iron-based catalyst at a temperature of 350-500℃ and a pressure of 7-15 MPa to synthesize ammonia, with a reaction space velocity of 1000-10000 h⁻¹. -1 .

[0028] Example 1 The preparation of iron-based catalysts (promoter system Ce / Mg / Ca + Al / Si / Ba) includes the following steps: S1. Dissolve 20.0 g of ferric nitrate nonahydrate, 0.64 g of cerium nitrate hexahydrate, 1.09 g of magnesium nitrate hexahydrate, and 0.81 g of calcium nitrate tetrahydrate (Ce:Mg:Ca molar ratio of 0.35:1:0.81) in 150 mL of water. Slowly add 0.5 mol / L sodium hydroxide solution to adjust the pH to 9. Stir and mix at 60 °C for 2 h to allow the system to form a precipitate. S2. Add 0.96 g of aluminum nitrate nonahydrate, 1.40 g of sodium silicate nonahydrate and 0.066 g of barium nitrate (Al:Si:Ba molar ratio of 0.52:1:0.051) to the above system, slowly add 0.5 mol / L sodium hydroxide solution to adjust the pH to 9, stir and mix at 70℃ for 1 h, and then let stand and age at the same temperature for 3 h to precipitate again to obtain a mixed slurry; S3. The mixed slurry was filtered to obtain a precipitate, which was washed with deionized water until neutral, dried at 100℃, and then calcined under a nitrogen atmosphere at 600℃ for 4 hours. It was then reduced with hydrogen using a mixture of hydrogen and nitrogen (H2 / N2 molar ratio 3:1) at 450℃, atmospheric pressure, and for 2 hours to obtain an iron-based catalyst with the following metal mass percentages: Fe 80.0%, Ce 6.1%, Mg 3.1%, Ca 3.8%, Al 2.1%, Si 3.9%, Ba 1.0%.

[0029] Example 2 The preparation of iron-based catalysts (promoter system Ce / Mg / Ca + Al / Si / Ba) includes the following steps: S1. Dissolve 20.0 g of ferric nitrate nonahydrate, 0.64 g of cerium nitrate hexahydrate, 1.34 g of magnesium nitrate hexahydrate, and 0.58 g of calcium nitrate tetrahydrate (Ce:Mg:Ca molar ratio of 0.29:1:0.47) in 150 mL of water. Slowly add 0.5 mol / L sodium hydroxide solution to adjust the pH to 9. Stir and mix at 60 °C for 2 h to allow the system to form a precipitate. S2. Add 2.073 g of aluminum nitrate nonahydrate, 2.171 g of sodium silicate nonahydrate and 0.14 g of barium nitrate (Al:Si:Ba molar ratio of 0.72:1:0.07) to the above system, slowly add 0.5 mol / L sodium hydroxide solution to adjust the pH to 9, stir and mix at 70℃ for 1 h, and then let stand and age at the same temperature for 3 h to precipitate again to obtain a mixed slurry; S3. After filtration, the mixed slurry yields a precipitate, which is washed with deionized water until neutral. After drying at 100°C, it is calcined under a nitrogen atmosphere at 600°C for 4 hours. It is then reduced with hydrogen using a mixture of hydrogen and nitrogen (H2 / N2 molar ratio 3:1) at 450°C, atmospheric pressure, and for 2 hours to obtain an iron-based catalyst. The mass percentages of the metals contained are: Fe 76.0%, Ce 5.8%, Mg 3.5%, Ca 2.7%, Al 4.1%, Si 5.9%, and Ba 2.0%.

[0030] Example 3 The preparation of iron-based catalysts (promoter system Ce / Mg / Ca + Al / Si / Ba) includes the following steps: S1. Dissolve 19.5g of ferric nitrate nonahydrate, 0.83g of cerium nitrate hexahydrate, 0.86g of magnesium nitrate hexahydrate, and 0.42g of calcium nitrate tetrahydrate (Ce:Mg:Ca molar ratio of 0.57:1:0.53) in 150 mL of water. Slowly add 0.5 mol / L sodium hydroxide solution to adjust the pH to 9. Stir and mix at 60℃ for 2 h to allow the system to form a precipitate. S2. Add 1.62 g of aluminum nitrate nonahydrate, 1.14 g of sodium silicate nonahydrate and 0.14 g of barium nitrate (Al:Si:Ba molar ratio of 1.08:1:0.13) to the above system, slowly add 0.5 mol / L sodium hydroxide solution to adjust the pH to 9, stir and mix at 70℃ for 1 h, and then let stand and age at the same temperature for 3 h to precipitate again to obtain a mixed slurry; S3. The mixed slurry was filtered to obtain a precipitate, which was washed with deionized water until neutral, dried at 100℃, and then calcined under a nitrogen atmosphere at 600℃ for 4 h. It was then reduced with hydrogen using a mixture of hydrogen and nitrogen (H2 / N2 molar ratio 3:1) at 450℃, atmospheric pressure, and for 2 h to obtain an iron-based catalyst with the following metal mass percentages: Fe 78.8%, Ce 7.9%, Mg 2.4%, Ca 2.1%, Al 3.4%, Si 3.3%, Ba 2.1%.

[0031] Example 4 The preparation of iron-based catalysts (promoter system Ce / Mg / Ca + Al / Si / Ba) includes the following steps: S1. Weigh 20.0 g of ferric nitrate nonahydrate, 0.62 g of cerium nitrate hexahydrate, 1.36 g of magnesium nitrate hexahydrate, and 0.62 g of calcium nitrate tetrahydrate (Ce:Mg:Ca molar ratio of 0.27:1:0.50), dissolve them in 150 mL of water, slowly add 0.5 mol / L sodium hydroxide solution to adjust the pH to 9, stir and mix at 60 °C for 2 h to allow the system to form a precipitate; S2. Add 1.04 g of aluminum nitrate nonahydrate, 0.72 g of sodium silicate nonahydrate and 0.084 g of barium nitrate (Al:Si:Ba molar ratio of 1.09:1:0.13) to the above system, slowly add 0.5 mol / L sodium hydroxide solution to adjust the pH to 9, stir and mix at 70℃ for 1 h, and then let stand and age at the same temperature for 3 h to precipitate again to obtain a mixed slurry; S3. After filtration, the mixed slurry yields a precipitate, which is washed with deionized water until neutral. After drying at 100°C, it is calcined under a nitrogen atmosphere at 500°C for 2 hours. It is then reduced with hydrogen using a mixture of hydrogen and nitrogen (H2 / N2 molar ratio 3:1) at 450°C, atmospheric pressure, and for 2 hours to obtain an iron-based catalyst. The mass percentages of the metals contained are: Fe 81.6%, Ce 5.9%, Mg 3.8%, Ca 3.1%, Al 2.2%, Si 2.1%, and Ba 1.3%.

[0032] Example 5 The preparation of iron-based catalysts (promoter system Ce / Mg / Ca + Al / Si / Ba) includes the following steps: S1. Weigh 20.0 g of ferric nitrate nonahydrate, 0.58 g of cerium nitrate hexahydrate, 1.29 g of magnesium nitrate hexahydrate, and 0.58 g of calcium nitrate tetrahydrate (Ce:Mg:Ca molar ratio of 0.26:1:0.51), dissolve them in 150 mL of water, slowly add 0.5 mol / L sodium hydroxide solution to adjust the pH to 9, stir and mix at 45 °C for 2 h to allow the system to form a precipitate; S2. Add 1.09 g of aluminum nitrate nonahydrate, 0.79 g of sodium silicate nonahydrate and 0.103 g of barium nitrate (Al:Si:Ba molar ratio of 1.05:1:0.14) to the above system, slowly add 0.5 mol / L sodium hydroxide solution to adjust the pH to 9, stir and mix at 70℃ for 1 h, and then let stand and age at the same temperature for 3 h to precipitate again to obtain a mixed slurry; S3. After filtration, the mixed slurry yields a precipitate, which is washed with deionized water until neutral. After drying at 100°C, it is calcined under a nitrogen atmosphere at 700°C for 4 hours. It is then reduced with hydrogen using a mixture of hydrogen and nitrogen (H2 / N2 molar ratio 3:1) at 450°C, atmospheric pressure, and for 2 hours to obtain an iron-based catalyst. The mass percentage of the metals contained is: Fe 81.8%, Ce 5.5%, Mg 3.6%, Ca 2.9%, Al 2.3%, Si 2.3%, Ba 1.6%.

[0033] Comparative Example 1 The difference from Example 1 is that only ferric nitrate solution nonahydrate was used for calcination to obtain a pure iron-based catalyst without additives.

[0034] The preparation of iron-based catalysts includes the following steps: S1. Weigh 20.0g of ferric nitrate nonahydrate and dissolve it in 150mL of water. Slowly add 0.5 mol / L sodium hydroxide solution to adjust the pH to 9. Stir and mix at 60℃ for 2 hours. S2. After filtration, the precipitate is washed with deionized water until neutral, dried at 100°C, and then calcined under a nitrogen atmosphere at 600°C for 4 hours. It is then reduced with hydrogen using a mixture of hydrogen and nitrogen (H2 / N2 molar ratio of 3:1) at 450°C, atmospheric pressure, and for 2 hours to obtain the iron-based catalyst.

[0035] Comparative Example 2 The difference from Example 1 is that only the second precipitation step is performed.

[0036] The preparation of iron-based catalysts (Al / Si / Ba promoter system) includes the following steps:

[0037] S1. Weigh 20.0 g of ferric nitrate nonahydrate and dissolve it in 150 mL of water. Slowly add 0.5 mol / L sodium hydroxide solution to adjust the pH to 9. Stir and mix at 60℃ for 2 h to allow the system to form a precipitate. S2. Add 0.96 g of aluminum nitrate nonahydrate, 1.40 g of sodium silicate nonahydrate and 0.066 g of barium nitrate (Al:Si:Ba molar ratio of 0.52:1:0.051) to the above system, slowly add 0.5 mol / L sodium hydroxide solution to adjust the pH to 9, stir and mix at 70℃ for 1 h, then let stand and age at the same temperature for 3 h to precipitate, and obtain the mixed slurry; S3. After filtration, the mixed slurry yields a precipitate, which is washed with deionized water until neutral. After drying at 100°C, it is calcined under a nitrogen atmosphere at 600°C for 4 hours. Then, it is reduced with hydrogen using a mixture of hydrogen and nitrogen (H2 / N2 molar ratio of 3:1) at 450°C, atmospheric pressure, and for 2 hours to obtain an iron-based catalyst with the following metal mass percentages: Fe: 92.0%, Al: 2.3%, Si: 4.6%, Ba: 1.1%.

[0038] Comparative Example 3 The difference from Example 1 is that only the first step of precipitation is performed.

[0039] The preparation of iron-based catalysts (with Ce / Mg / Ca as a promoter system) includes the following steps: S1. Weigh 20.0 g of ferric nitrate nonahydrate, 0.64 g of cerium nitrate hexahydrate, 1.09 g of magnesium nitrate hexahydrate, and 0.81 g of calcium nitrate tetrahydrate (Ce:Mg:Ca molar ratio of 0.35:1:0.81), dissolve them in 100 mL of water, slowly add 0.5 mol / L sodium hydroxide solution to adjust the pH to 9, stir and mix at 60℃ for 2 h to allow the system to form a precipitate, and obtain a mixed slurry; S2. After filtration, the mixed slurry yields a precipitate, which is washed with deionized water until neutral. After drying at 100°C, it is calcined under a nitrogen atmosphere at 600°C for 4 hours. Then, it is reduced with hydrogen using a mixture of hydrogen and nitrogen (H2 / N2 molar ratio of 3:1) at 450°C, atmospheric pressure, and for 2 hours to obtain an iron-based catalyst with the following metal mass percentages: Fe: 86.1%, Ce: 6.4%, Mg: 3.2%, Ca: 4.3%.

[0040] Comparative Example 4 The difference from Example 1 is that the Ce doping level in the iron-based catalyst is too high.

[0041] The preparation of iron-based catalysts (promoter system Ce / Mg / Ca + Al / Si / Ba) includes the following steps:

[0042] S1. Weigh 20.0 g of ferric nitrate nonahydrate, 2.20 g of cerium nitrate hexahydrate, 1.18 g of magnesium nitrate hexahydrate, and 0.34 g of calcium nitrate tetrahydrate (Ce:Mg:Ca molar ratio of 1.10:1:0.31), dissolve them in 100 mL of water, slowly add 0.5 mol / L sodium hydroxide solution to adjust the pH to 9, stir and mix at 60℃ for 2 h to allow the system to form a precipitate; S2. Add 0.96 g of aluminum nitrate nonahydrate, 1.40 g of sodium silicate nonahydrate and 0.066 g of barium nitrate (Al:Si:Ba molar ratio of 0.52:1:0.051) to the above system, slowly add 0.5 mol / L sodium hydroxide solution to adjust the pH to 9, stir and mix at 70℃ for 1 h, and then let stand and age at the same temperature for 3 h to precipitate again to obtain a mixed slurry; S3. After filtration, the mixed slurry yields a precipitate, which is washed with deionized water until neutral. After drying at 100°C, it is calcined under a nitrogen atmosphere at 600°C for 4 hours. It is then reduced with hydrogen using a mixture of hydrogen and nitrogen (H2 / N2 molar ratio 3:1) at 450°C, atmospheric pressure, and for 2 hours to obtain an iron-based catalyst. The mass percentages of the metals contained are: Fe: 71.0%, Ce: 18.3%, Mg: 2.9%, Ca: 1.5%, Al: 1.8%, Si: 3.6%, Ba: 0.9%.

[0043] Comparative Example 5 The difference from Example 1 is that the metal elements in the first step of precipitation are adjusted to Ce, Ni and Ca, that is, equimolar nickel nitrate hexahydrate is used instead of magnesium nitrate hexahydrate.

[0044] The preparation of iron-based catalysts (promoter system Ce / Ni / Ca+Al / Si / Ba) includes the following steps: S1. Weigh 20.0 g of ferric nitrate nonahydrate, 0.64 g of cerium nitrate hexahydrate, 1.19 g of nickel nitrate hexahydrate, and 0.81 g of calcium nitrate tetrahydrate (Ce:Ni:Ca molar ratio of 0.35:1:0.81), dissolve them in 150 mL of water, slowly add 0.5 mol / L sodium hydroxide solution to adjust the pH to 9, stir and mix at 60 °C for 2 h to allow the system to form a precipitate; S2. Add 0.96 g of aluminum nitrate nonahydrate, 1.40 g of sodium silicate nonahydrate and 0.066 g of barium nitrate (Al:Si:Ba molar ratio of 0.52:1:0.051) to the above system, slowly add 0.5 mol / L sodium hydroxide solution to adjust the pH to 9, stir and mix at 70℃ for 1 h, and then let stand and age at the same temperature for 3 h to precipitate again to obtain a mixed slurry; S3. After filtration, the mixed slurry yields a precipitate, which is washed with deionized water until neutral. After drying at 100°C, it is calcined under a nitrogen atmosphere at 600°C for 4 hours. It is then reduced with hydrogen using a mixture of hydrogen and nitrogen (H2 / N2 molar ratio 3:1) at 450°C, atmospheric pressure, and for 2 hours to obtain an iron-based catalyst. The mass percentages of the metals contained are: Fe: 77.0%, Ce: 5.5%, Ni: 6.7%, Ca: 3.9%, Al: 2.0%, Si: 3.9%, Ba: 1.0%.

[0045] Comparative Example 6 The difference from Example 1 is that the metal elements in the second precipitation step are adjusted to Al, Ni, and Ba, that is, equimolar nickel nitrate hexahydrate is used instead of sodium silicate nonahydrate.

[0046] The preparation of iron-based catalysts (with a promoter system of Ce / Mg / Ca+Al / Ni / Ba) includes the following steps: S1. Weigh 20.0 g of ferric nitrate nonahydrate, 0.64 g of cerium nitrate hexahydrate, 1.09 g of magnesium nitrate hexahydrate, and 0.81 g of calcium nitrate tetrahydrate (Ce:Mg:Ca molar ratio of 0.35:1:0.81), dissolve them in 150 mL of water, slowly add 0.5 mol / L sodium hydroxide solution to adjust the pH to 9, stir and mix at 60 °C for 2 h to allow the system to form a precipitate; S2. Add 0.96 g of aluminum nitrate nonahydrate, 1.43 g of nickel nitrate hexahydrate, and 0.066 g of barium nitrate to the above system (Al:Ni:Ba molar ratio is 0.52:1:0.051). Slowly add 0.5 mol / L sodium hydroxide solution to adjust the pH to 9. Stir and mix at 70℃ for 1 h, then let stand and age at the same temperature for 3 h to allow for synergistic reprecipitation and obtain a mixed slurry. S3. After filtration, the mixed slurry yields a precipitate, which is washed with deionized water until neutral. After drying at 100°C, it is calcined under a nitrogen atmosphere at 600°C for 4 hours. It is then reduced with hydrogen using a mixture of hydrogen and nitrogen (H2 / N2 molar ratio 3:1) at 450°C, atmospheric pressure, and for 2 hours to obtain an iron-based catalyst. The mass percentages of the metals contained are: Fe: 76.7%, Ce: 5.6%, Mg: 2.9%, Ca: 3.8%, Al: 1.9%, Ni: 8.0%, Ba: 1.1%.

[0047] Comparative Example 7 The difference from Example 1 is that all precursors were added at once, without staged precipitation.

[0048] The preparation of iron-based catalysts (with a Ce / Mg / Ca / Al / Si / Ba promoter system) includes the following steps: S1. Weigh 20.0 g of ferric nitrate nonahydrate, 0.64 g of cerium nitrate hexahydrate, 1.09 g of magnesium nitrate hexahydrate, 0.81 g of calcium nitrate tetrahydrate, 0.96 g of aluminum nitrate nonahydrate, 1.40 g of sodium silicate nonahydrate, and 0.066 g of barium nitrate, and dissolve them in 150 mL of water. Slowly add 0.5 mol / L sodium hydroxide solution to adjust the pH to 9. Stir and mix at 60 °C for 3 h, and then let stand and age at the same temperature for 3 h to allow the system to form a precipitate, thus obtaining a mixed slurry. S2. The mixed slurry was filtered to obtain a precipitate, which was washed with deionized water until neutral, dried at 100℃, and then calcined under a nitrogen atmosphere at 600℃ for 4 hours. It was then reduced with hydrogen using a mixture of hydrogen and nitrogen (H2 / N2 molar ratio 3:1) at 450℃, atmospheric pressure, and for 2 hours to obtain an iron-based catalyst with the following metal mass percentages: Fe 80.0%, Ce 6.1%, Mg 3.1%, Ca 3.8%, Al 2.1%, Si 3.9%, Ba 1.0%.

[0049] Comparative Example 8 The difference from Example 1 is that the iron-based catalyst was prepared by the conventional impregnation method.

[0050] The preparation of iron-based catalysts (with a Ce / Mg / Ca / Al / Si / Ba promoter system) includes the following steps: S1. Preparation of Fe2O3 support: Weigh 20.0 g of ferric nitrate nonahydrate and dissolve it in 100 mL of water. Add 0.5 mol / L sodium hydroxide solution dropwise until precipitation is complete. After filtration, washing, and drying, calcine it in air at 500℃ for 4 h to obtain Fe2O3 powder (about 7.0 g).

[0051] S2. Preparation of impregnation solution: Dissolve 0.64 g of cerium nitrate hexahydrate, 1.09 g of magnesium nitrate hexahydrate, 0.81 g of calcium nitrate tetrahydrate, 0.96 g of aluminum nitrate nonahydrate, 1.40 g of sodium silicate nonahydrate and 0.066 g of barium nitrate in deionized water to prepare an impregnation solution with a total volume of 15 mL.

[0052] S3. The impregnation solution was uniformly added dropwise to the Fe2O3 support, allowed to stand at room temperature for 12 hours, dried at 110℃ for 12 hours, and then calcined in air at 400℃ for 2 hours. It was then reduced with hydrogen using a mixture of hydrogen and nitrogen (H2 / N2 molar ratio of 3:1), at a reduction temperature of 450℃, at atmospheric pressure, and for 2 hours to obtain an iron-based catalyst. The mass percentage of the metals contained was: Fe 80.0%, Ce 6.1%, Mg 3.1%, Ca 3.8%, Al 2.1%, Si 3.9%, Ba 1.0%.

[0053] The iron-based catalyst prepared above was ground, compressed into tablets, and sieved to obtain 40-60 mesh particles. Its ammonia synthesis performance was evaluated using a fixed-bed reactor. The fixed-bed reactor was equipped with an iron-based catalyst loading of 0.5 g, a controlled temperature of 450 °C, a pressure of 10 MPa, and a space velocity of 10000 h⁻¹. -1 The raw material gas H2 / N2 molar ratio was 3:1. After the reaction stabilized for 2 hours, the ammonia content in the tail gas was analyzed using the sulfuric acid absorption method, and the ammonia generation rate was calculated. After continuous operation for 72 hours, the ammonia generation rate was measured again, and the activity decay rate was calculated as ((initial NH3 generation rate - NH3 generation rate after 72 hours) / initial NH3 generation rate × 100%). The results are shown in Table 1.

[0054] Table 1. Catalytic reaction results of Examples 1-5 and Comparative Examples 1-8

[0055] like Figure 1 The image shown is an HRTEM image of the iron-based catalyst in Example 1 of the present invention. The image clearly shows that the catalyst has obvious Fe-bcc(111) crystal plane characteristic stripes and the outer auxiliary agent coating layer is complete and continuous, indicating that the synergistic effect of multiple auxiliary agents effectively promotes the stable exposure and structural confinement of highly active crystal planes.

[0056] like Figure 2 The image shown is an HRTEM image of the catalyst in Comparative Example 2. Since only aluminum, silicon and barium were added in Comparative Example 2, only the lattice stripes of the FeO phase were detected in the catalyst, and the Fe-bcc(111) crystal plane features were not observed. This indicates that a highly active Fe crystal plane structure cannot be stably formed when there is a lack of crystal plane induction promoters.

[0057] As shown in Table 1, the results indicate that the multi-auxiliary synergistic doping confined catalysts prepared in this invention (Examples 1-5) exhibit higher activity and thermal stability in the ammonia synthesis reaction, with an ammonia formation rate >140 mmol·g. -1 · h -1The activity decay rate after 72 h was <10%, and the high-temperature operation stability was significantly better than that of Comparative Example 1 (pure iron-based catalyst). Analysis of the data from Comparative Examples 2-6 showed that the element selection and ratio of the promoters in the iron-based catalyst directly affected the initial activity, long-term stability, and thermal stability of the catalyst. Furthermore, the spatial separation and synergistic effect of the crystal facet promoters and confined promoters achieved through staged precipitation helps promote the preferential exposure of the Fe-bcc(111) / C7 crystal facets and avoids uneven doping or agglomeration of the promoters, thereby further improving catalytic activity and long-term stability. Using only a few of the promoters or using an excessive amount of Ce promoter will result in the inability to preferentially expose the Fe-bcc(111) / C7 crystal facets, leading to the destruction of the integrity of crystal facet formation or a low proportion of Fe-bcc(111) crystal facets. Simultaneously, the lack of multiple promoters will also affect the electronic structure of the catalyst, resulting in poor initial catalytic reaction performance.

[0058] Using the same proportion of additives, the catalyst prepared by the conventional one-step precipitation method (Comparative Example 7) could not achieve good confined construction, and a complete crystal form could not be formed, nor could the Fe-bcc(111) crystal face be obtained, resulting in low catalyst activity. The catalyst prepared by the conventional impregnation method (Comparative Example 8), impregnating the already prepared iron oxide support, could no longer regulate the iron oxide support, resulting in poor dispersibility and synergy of the additives, thus leading to poor catalytic activity and stability.

[0059] Therefore, the staged precipitation, appropriate additives, and suitable metal doping in this invention can effectively improve the catalytic performance and stability of the catalyst in the ammonia synthesis reaction, and can be used for a long time even under high temperature and high pressure conditions.

[0060] The above are merely preferred embodiments of the present invention and do not limit the patent scope of the present invention. Any equivalent structural or procedural transformations made using the present invention specification, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of the present invention.

Claims

1. A method for preparing an iron-based catalyst constructed with multiple auxiliary agents, characterized in that, Includes the following steps: S1. Dissolve the iron precursor, cerium precursor, magnesium precursor and calcium precursor in water, adjust the pH to 8-10, stir and mix to form a precipitate in the system. S2. Add aluminum precursor, silicon precursor and barium precursor to the above system, adjust the pH to 8~10, stir and mix, and precipitate again to obtain a mixed slurry. S3. After the mixed slurry is filtered, washed and dried, it is calcined under an inert atmosphere and then reduced with hydrogen to obtain an iron-based catalyst containing Fe-bcc(111) crystal planes. The mass percentage of the metals contained is: iron 70~88%, cerium 4~10%, magnesium 2~4%, calcium 2~6%, aluminum 1~5%, silicon 2~6%, and barium 0.5~3%.

2. The method for preparing the iron-based catalyst confined by multiple auxiliary agents according to claim 1, characterized in that, The molar ratio of Ce, Mg, and Ca in the cerium precursor, magnesium precursor, and calcium precursor is 0.1~0.9:1:0.3~1.8; the molar ratio of Al, Si, and Ba in the aluminum precursor, silicon precursor, and barium precursor is 0.1~2.0:1:0.01~0.

2.

3. The method for preparing the iron-based catalyst confined by multiple auxiliary agents according to claim 1, characterized in that, In S1, the stirring and mixing is carried out at 40~70℃ for 0.5~3h.

4. The method for preparing the iron-based catalyst confined by multiple auxiliary agents according to any one of claims 1-3, characterized in that, In S1, the iron precursor is ferric nitrate nonahydrate, ferric chloride, or ferric oxalate; the cerium precursor is cerium nitrate hexahydrate, cerium chloride, or cerium ammonium nitrate hexahydrate; the magnesium precursor is magnesium nitrate hexahydrate, magnesium acetate, or magnesium chloride; and the calcium precursor is calcium nitrate tetrahydrate, calcium chloride, or calcium acetate.

5. The method for preparing the iron-based catalyst confined by multiple auxiliary agents according to claim 1, characterized in that, In S2, the stirring and mixing is carried out at 60~90℃ for 1~2 hours, followed by standing and aging for 2~4 hours.

6. The method for preparing the iron-based catalyst confined by multiple auxiliary agents according to claim 1 or 5, characterized in that, In S2, the aluminum precursor is aluminum nitrate nonahydrate, aluminum chloride, or aluminum isopropanol; the silicon precursor is sodium silicate nonahydrate, ethyl silicate, or ammonium metasilicate; and the barium precursor is barium nitrate or barium chloride.

7. The method for preparing the iron-based catalyst confined by multiple auxiliary agents according to claim 1, characterized in that, In S1 and S2, the pH is adjusted using an alkaline solution, which is one or more of ammonia, sodium hydroxide solution, and sodium carbonate solution, with a concentration of 0.1~5 mol / L.

8. The method for preparing the iron-based catalyst confined by multiple auxiliary agents according to claim 1 or 7, characterized in that, In S3, the calcination temperature is 400~800℃ and the calcination time is 2~6h; the hydrogen reduction temperature is 400~500℃, the pressure is atmospheric pressure, and the time is 2~3h.

9. The application of an iron-based catalyst prepared by any one of claims 1-8 in ammonia synthesis.

10. The application according to claim 9, characterized in that, include: Ammonia is synthesized by reacting hydrogen and nitrogen with an iron-based catalyst under reaction conditions of 350~500℃ and 7~15 MPa.

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