Preparation method and application of iron-based catalyst with multi-aid limited construction

Abstract

The application relates to the technical field of catalytic reactions, and discloses a preparation method and application of an iron-based catalyst with multi-adjuvant limited construction, which comprises the following steps: S1, dissolving iron precursors, cerium precursors, magnesium precursors and calcium precursors in water, adjusting the pH to 8-10, stirring and mixing, and making the system form a precipitate; S2, adding aluminum precursors, silicon precursors and barium precursors to the system, adjusting the pH to 8-10, stirring and mixing, and synergistically precipitating again to obtain a mixed slurry; and S3, filtering, washing and drying the mixed slurry, calcining the mixed slurry under an inert atmosphere, and then reducing the mixed slurry by hydrogen to obtain an iron-based catalyst containing Fe-bcc (111) crystal faces. The application realizes directional stability and structure limited protection of the Fe-bcc (111) high-activity crystal face by introducing the synergistic doping of multiple crystal face adjuvants and limited adjuvants and combining a two-step precipitation method, so that the ammonia synthesis activity and high-temperature operation stability of the catalyst are significantly improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of catalytic reaction, and in particular to a preparation method and application of an iron-based catalyst with multi-adjuvant limited construction. BACKGROUND

[0002] Ammonia synthesis reaction is an important basic process in the field of energy chemical industry, and the core catalyst thereof is mainly iron-based material. The traditional ammonia synthesis catalyst usually adopts Fe3O4 or a-Fe as an active phase, and realizes crystal phase stability and electronic structure adjustment through adjuvants such as K2O, Al2O3, CaO and the like. However, under the high-temperature and high-pressure (400-500 ℃, 10-15 MPa) reaction condition, iron particles are prone to sintering, phase transition and surface reconstruction, which leads to a decrease in active site exposure degree, a decrease in ammonia generation rate, and a serious influence on the service life and industrial application stability of the catalyst.

[0003] In order to improve the low-temperature activity and thermal stability of the iron-based catalyst, researchers try to optimize the surface structure through crystal face regulation and adjuvant doping and the like. The conventional single adjuvant system can only regulate the electronic structure or surface basicity, and it is difficult to simultaneously consider the crystal face exposure and sintering resistance, and therefore, the iron-based catalyst is usually prepared by using multi-adjuvant synergistic doping. For example, rare earth or alkaline earth elements are used to improve the electronic structure and improve the nitrogen molecule activation capacity, and the patent for invention with publication number CN105772024A discloses an iron-ruthenium composite ammonia synthesis catalyst and a preparation method thereof. The catalyst is an iron-ruthenium composite ammonia synthesis catalyst, which is prepared by using metal ruthenium as an active component, an iron-based catalyst as a carrier, and an adjuvant catalyst composed of one or more of alkali metal, alkaline earth metal and transition element.

[0004] However, the above multi-adjuvant synergistic doping still has the following problems: (1) uneven distribution of adjuvants leads to local enrichment or agglomeration, and the crystal face regulation effect is limited; (2) there is a lack of effective structure limiting mechanism, and the crystal grains are prone to grow at high temperature; (3) the crystal face selectivity and adjuvant synergistic mechanism are not clear, and the catalytic performance stability is insufficient. Existing researches have shown that Fe-bcc (111) or high-index C7 site crystal face has higher nitrogen adsorption and activation capacity, and is a key active face for ammonia synthesis reaction. However, under the conventional preparation condition, the proportion of the high-activity crystal face is low, and the high-activity structure is difficult to maintain for a long time in the high-temperature reaction, because the high-activity crystal face is easily reconstructed into a thermodynamically stable low-energy face (such as Fe (110), Fe (100)). Therefore, how to realize directional stability of Fe-bcc (111) or C7 site crystal face through multi-adjuvant synergistic doping in the iron-based oxide system, and construct a limiting structure to inhibit the growth and phase transition of the particles, is an important research direction in the field of ammonia synthesis catalyst at present. SUMMARY

[0005] In order to solve the above technical problems, the application provides a preparation method and application of an iron-based catalyst constructed by multiple additive limitation, which realizes directional stability and structure limitation protection of Fe-bcc(111) high-activity crystal surface by introducing multiple crystal surface additives and limitation additives in a synergistic doping manner and combining a two-step precipitation limitation construction process, so that the ammonia synthesis activity and high-temperature operation stability of the catalyst are significantly improved.

[0006] The object of the application is achieved by the following technical solutions.

[0007] The application provides a preparation method of an iron-based catalyst constructed by multiple additive limitation, which comprises the following steps.

[0008] S1, dissolving iron precursors, cerium precursors, magnesium precursors and calcium precursors in water, adjusting the pH to 8-10, and stirring and mixing to form a precipitate in the system;

[0009] S2, adding aluminum precursors, silicon precursors and barium precursors to the above system, adjusting the pH to 8-10, stirring and mixing, and synergistically precipitating again to obtain a mixed slurry;

[0010] S3, after the mixed slurry is filtered, washed and dried, it is calcined in an inert atmosphere, and then reduced by hydrogen to obtain an iron-based catalyst containing Fe-bcc(111) crystal surface, and the mass percentage of the contained metals is: iron 70-88 %, cerium 4-10 %, magnesium 2-4 %, calcium 2-6 %, aluminum 1-5 %, silicon 2-6 %, and barium 0.5-3 %.

[0011] The application realizes spatial separation and synergistic effect of crystal surface additives and limitation additives by stage-by-stage precipitation, avoids uneven doping or agglomeration of the additives, and uniformly distributes multiple additives in the form of doping or limitation coating in the iron oxide crystal lattice and surface. The crystal surface additives (cerium, magnesium and calcium) adjust the Fe lattice strain and electronic structure through multiple additive synergistic doping, the selectivity of the crystal surface is significantly enhanced, and the preferential exposure of the Fe-bcc(111) / C7 crystal surface is significantly promoted; the limitation additives (aluminum, silicon and barium) construct a stable oxidation skeleton and coating layer on the surface of the catalyst, effectively inhibit the particle growth and phase change, and thus realize the coexistence of stable exposure and sintering resistance of the high-activity crystal surface. The multiple additive limitation synergistic effect realizes the construction of the Fe-bcc(111) / C7 high-activity crystal surface, and at the same time, the catalyst has excellent structural stability and reaction durability. In addition, the multiple additives can increase the electrons of the iron-based catalyst, which is helpful for the dissociation of N2, so as to improve the catalytic activity in the ammonia synthesis reaction.

[0012] Therefore, the application realizes stable exposure and structure strengthening of Fe-bcc(111) / C7 high-activity crystal surface through multi-additive synergistic doping and limited structure construction, solves the technical problem that activity and stability are difficult to be considered in traditional iron-based ammonia synthesis catalysts, and provides a new design idea and implementable path for developing ammonia synthesis catalysts with high efficiency, long service life and industrial preparation.

[0013] Preferably, 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. More preferably, the molar ratio of Ce, Mg and Ca in the cerium precursor, magnesium precursor and calcium precursor is 0.2-0.6:1:0.4-0.9; the molar ratio of Al, Si and Ba in the aluminum precursor, silicon precursor and barium precursor is 0.5-1.2:1:0.05-0.15.

[0014] By adjusting and controlling the molar ratio and loading amount of the multi-additives, the synergistic effect between the multi-additives can be better, thereby better catalytic effect can be achieved.

[0015] Preferably, in S1, the stirring and mixing is stirring at 40-70℃ for 0.5-3h.

[0016] 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.

[0017] Preferably, in S2, the stirring and mixing is stirring at 60-90℃ for 1-2h, and then standing and aging for 2-4h.

[0018] Preferably, in S2, the aluminum precursor is aluminum nitrate nonahydrate, aluminum chloride or aluminum isopropyl alcohol; the silicon precursor is sodium silicate nonahydrate, ethyl silicate or ammonium metasilicate; and the barium precursor is barium nitrate or barium chloride.

[0019] Preferably, in S1 and S2, the pH adjustment is using an alkali solution, the alkali solution is one or more of ammonia, sodium hydroxide solution and sodium carbonate solution, and the concentration is 0.1-5 mol / L.

[0020] Preferably, in S3, the calcination temperature is 400-800℃, the calcination time is 2-6h; the inert atmosphere is nitrogen atmosphere or argon atmosphere; the hydrogen reduction temperature is 400-500℃, the pressure is normal pressure, the time is 2-3h, the mixed gas of hydrogen and nitrogen is used, and the H2 / N2 molar ratio is 2-3:1.

[0021] In a second aspect, the application further provides an application of the iron-based catalyst prepared by the preparation method in ammonia synthesis.

[0022] Preferably, the method comprises: under the reaction conditions of a temperature of 350-500℃ and a pressure of 7-15 MPa, contacting hydrogen, nitrogen and the iron-based catalyst to react and synthesize ammonia.

[0023] Compared with the iron-based catalyst prepared by the traditional impregnation method or the single-assistant co-precipitation method, the ammonia generation rate of the catalyst prepared by the application is increased by 30-60% under the same conditions, and the activity decay rate is less than 10% after 72h of continuous operation at a high temperature of 600℃, which shows significant sintering resistance and high-temperature structural stability.

[0024] Compared with the prior art, the nano catalyst in the application has the following beneficial effects:

[0025] (1) The synergistic effect of the crystal face assistants Ce, Mg and Ca can effectively regulate the electronic structure and lattice distortion of Fe, and promote the stable exposure of the high-activity Fe-bcc(111) / C7 crystal face;

[0026] (2) The limited domain assistants Al, Si and Ba form a stable skeleton structure on the surface, inhibit the migration and agglomeration of Fe particles, and improve the thermal stability and service life;

[0027] (3) The spatial separation and synergistic effect of the crystal face assistants and the limited domain assistants are realized by the staged precipitation, the assistants are prevented from being unevenly doped or agglomerated, the Fe-bcc(111) / C7 high-activity crystal face is precisely constructed, and the catalyst has excellent structural stability and reaction durability;

[0028] (4) The ammonia synthesis catalytic performance of the catalyst is excellent and stable, the ammonia generation rate is increased by 30-60% compared with the traditional Fe-based system at 450℃, and the activity retention rate is more than 90% after 72h of continuous operation at a high temperature of 600℃;

[0029] (5) The preparation process is simple, the reaction conditions are mild, the raw materials are cheap, and the process is suitable for industrial amplification and long-term stable operation. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 FIG. 2 is a high-resolution transmission electron microscope (HRTEM) image of the catalyst in Example 1.

[0031] Figure 2 High resolution transmission electron microscopy (HRTEM) image of the catalyst in Comparative Example 2. DETAILED DESCRIPTION

[0032] The technical solutions of the present application are described below with specific examples, but the scope of protection of the present application is not limited thereto.

[0033] 1. Iron-based catalyst

[0034] The preparation method of the iron-based catalyst constructed by multi-aid limited in the present application comprises the following steps:

[0035] S1, dissolving the iron precursor, cerium precursor, magnesium precursor and calcium precursor in water, slowly adding 0.1-5 mol / L alkali solution (one or more of ammonia, sodium hydroxide solution and sodium carbonate solution) to adjust the pH to 8-10, stirring and mixing at 40-70℃ for 0.5-3h, so that the system forms a precipitate;

[0036] S2, adding aluminum precursor, silicon precursor and barium precursor to the above system, slowly adding 0.1-5 mol / L alkali solution (one or more of ammonia, sodium hydroxide solution and sodium carbonate solution) to adjust the pH to 8-10, stirring and mixing at 60-90℃ for 1-2h, then aging at the same temperature for 2-4h, and precipitating again to obtain a mixed slurry;

[0037] S3, filtering the mixed slurry to obtain a precipitate, washing it with deionized water until it is neutral, drying at 100-120℃, and then calcining in an inert atmosphere, with a calcination temperature of 400-800℃ and a calcination time of 2-6h; then reducing it with hydrogen, using a mixed gas of hydrogen and nitrogen (H2 / N2 molar ratio of 2-3:1), with a reduction temperature of 400-500℃, a pressure of normal pressure, and a time of 2-3h, to obtain an iron-based catalyst containing Fe-bcc(111) crystal face, with the mass percentage of the contained metals being: iron 70-88 %, cerium 4-10 %, magnesium 2-4 %, calcium 2-6 %, aluminum 1-5 %, silicon 2-6 %, and barium 0.5-3 %.

[0038] In the specific embodiments of the present application, 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.

[0039] In the specific embodiments of the present application, in S2, the aluminum precursor is aluminum nitrate nonahydrate, aluminum chloride or aluminum isopropyl alcohol. The silicon precursor is sodium silicate nonahydrate, ethyl silicate or ammonium metasilicate. The barium precursor is barium nitrate or barium chloride.

[0040] 2. Ammonia synthesis reaction

[0041] The iron-based catalyst prepared above is reduced by hydrogen, and the reduction is carried out under a mixed gas of hydrogen and nitrogen (H2 / N2 molar ratio of 3-4:1) at a reduction temperature of 400-500°C for 2-3 h. Then, hydrogen, nitrogen and the iron-based catalyst are contacted to react to synthesize ammonia under reaction conditions of a temperature of 350-500°C and a pressure of 7-15 MPa, and the reaction space velocity is 1000-10000 h -1 .

[0042] Example 1

[0043] The preparation of the iron-based catalyst (promoter system Ce / Mg / Ca + Al / Si / Ba) includes the following steps:

[0044] S1, 20.0 g of iron 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) are dissolved in 150 mL of water, 0.5 mol / L sodium hydroxide solution is slowly added to adjust the pH to 9, and the system is stirred and mixed at 60°C for 2 h to form a precipitate;

[0045] S2, 0.96 g of aluminum nitrate nonahydrate, 1.40 g of sodium silicate nonahydrate and 0.066 g of barium nitrate are added to the above system (Al:Si:Ba molar ratio of 0.52:1:0.051), 0.5 mol / L sodium hydroxide solution is slowly added to adjust the pH to 9, and the system is stirred and mixed at 70°C for 1 h, then aged at the same temperature for 3 h, and a second precipitate is obtained to form a mixed slurry;

[0046] S3, the mixed slurry is filtered to obtain a precipitate, which is washed to neutral with deionized water, dried at 100°C, and calcined under a nitrogen atmosphere at a calcination temperature of 600°C for 4 h; then reduced by hydrogen, using a mixed gas of hydrogen and nitrogen (H2 / N2 molar ratio of 3:1) at a reduction temperature of 450°C and a pressure of atmospheric pressure for 2 h, to obtain an iron-based catalyst containing Fe 80.0%, Ce 6.1%, Mg 3.1%, Ca 3.8%, Al 2.1%, Si 3.9% and Ba 1.0% by mass.

[0047] Example 2

[0048] The preparation of the iron-based catalyst (promoter system Ce / Mg / Ca + Al / Si / Ba) includes the following steps:

[0049] S1, dissolve 20.0 g of iron 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 is 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 the mixture at 60°C for 2 h, and make the system form a precipitate;

[0050] 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 is 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 the mixture at 70°C for 1 h, then stand and age at the same temperature for 3 h, and cooperatively precipitate again to obtain a mixed slurry;

[0051] S3, after the mixed slurry is filtered to obtain a precipitate, it is washed with deionized water until neutral, dried at 100°C, and then calcined under a nitrogen atmosphere, with a calcination temperature of 600°C and a calcination time of 4 h; and then reduced by hydrogen, using a mixed gas of hydrogen and nitrogen (H2 / N2 molar ratio is 3:1), with a reduction temperature of 450°C, a pressure of normal pressure, and a time of 2 h, to obtain an iron-based catalyst, which contains the following metals by mass percentage: Fe 76.0%, Ce 5.8%, Mg 3.5%, Ca 2.7%, Al 4.1%, Si 5.9%, and Ba 2.0%.

[0052] Example 3

[0053] The preparation of the iron-based catalyst (adjuvant system Ce / Mg / Ca + Al / Si / Ba) includes the following steps:

[0054] S1, dissolve 20.0 g of iron 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 is 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 the mixture at 60°C for 2 h, and make the system form a precipitate;

[0055] 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 is 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 the mixture at 70°C for 1 h, then stand and age at the same temperature for 3 h, and cooperatively precipitate again to obtain a mixed slurry;

[0056] S3, the mixed slurry was filtered to obtain a precipitate, which was washed with deionized water until neutral, dried at 100°C, and then calcined under a nitrogen atmosphere at a calcination temperature of 600°C for 4 h; and then reduced by hydrogen, using a mixed gas of hydrogen and nitrogen (H2 / N2 molar ratio of 3:1) at a reduction temperature of 450°C, under normal pressure for 2 h, to obtain an iron-based catalyst, which contained the following metals in the following mass percentages: Fe 78.8%, Ce 7.9%, Mg 2.4%, Ca 2.1%, Al 3.4%, Si 3.3%, and Ba 2.1%.

[0057] Example 4

[0058] The preparation of the iron-based catalyst (adjuvant system Ce / Mg / Ca + Al / Si / Ba) included the following steps:

[0059] S1, 20.0 g of iron 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) were dissolved in 150 mL of water, and a 0.5 mol / L sodium hydroxide solution was slowly added dropwise to adjust the pH to 9, and the mixture was stirred at 60°C for 2 h to form a precipitate;

[0060] S2, 1.04 g of aluminum nitrate nonahydrate, 0.72 g of sodium silicate nonahydrate, and 0.084 g of barium nitrate were added to the above system (Al:Si:Ba molar ratio of 1.09:1:0.13), a 0.5 mol / L sodium hydroxide solution was slowly added dropwise to adjust the pH to 9, and the mixture was stirred at 70°C for 1 h, and then aged at the same temperature for 3 h to form a second precipitate, to obtain a mixed slurry;

[0061] S3, the mixed slurry was filtered to obtain a precipitate, which was washed with deionized water until neutral, dried at 100°C, and then calcined under a nitrogen atmosphere at a calcination temperature of 500°C for 2 h; and then reduced by hydrogen, using a mixed gas of hydrogen and nitrogen (H2 / N2 molar ratio of 3:1) at a reduction temperature of 450°C, under normal pressure for 2 h, to obtain an iron-based catalyst, which contained the following metals in the following mass percentages: Fe 81.6%, Ce 5.9%, Mg 3.8%, Ca 3.1%, Al 2.2%, Si 2.1%, and Ba 1.3%.

[0062] Example 5

[0063] The preparation of the iron-based catalyst (adjuvant system Ce / Mg / Ca + Al / Si / Ba) included the following steps:

[0064] S1, 20.0 g of iron 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) were weighed into 150 mL of water, 0.5 mol / L sodium hydroxide solution was slowly added dropwise to adjust the pH to 9, and the mixture was stirred at 45°C for 2 h to form a precipitate;

[0065] S2, 1.09 g of aluminum nitrate nonahydrate, 0.79 g of sodium silicate and 0.103 g of barium nitrate (Al:Si:Ba molar ratio of 1.05:1:0.14) were added to the above system, 0.5 mol / L sodium hydroxide solution was slowly added dropwise to adjust the pH to 9, and the mixture was stirred at 70°C for 1 h, then aged at the same temperature for 3 h to form a second precipitate, and a mixed slurry was obtained;

[0066] S3, the mixed slurry was filtered to obtain a precipitate, which was washed with deionized water until neutral, dried at 100°C, and calcined under a nitrogen atmosphere, with a calcination temperature of 700°C and a calcination time of 4 h; then reduced by hydrogen, using a mixture of hydrogen and nitrogen (H2 / N2 molar ratio of 3:1), with a reduction temperature of 450°C, a pressure of normal pressure, and a time of 2 h, to obtain an iron-based catalyst, which contained the following metals in the following mass percentages: Fe 81.8%, Ce 5.5%, Mg 3.6%, Ca 2.9%, Al 2.3%, Si 2.3%, and Ba 1.6%.

[0067] Comparative Example 1

[0068] The difference from Example 1 is that only iron nitrate nonahydrate solution was used for calcination to obtain a pure iron-based catalyst without additives.

[0069] The preparation of the iron-based catalyst includes the following steps:

[0070] S1, 20.0 g of iron nitrate nonahydrate was weighed into 150 mL of water, 0.5 mol / L sodium hydroxide solution was slowly added dropwise to adjust the pH to 9, and the mixture was stirred at 60°C for 2 h;

[0071] S2, the precipitate was obtained after filtration, which was washed with deionized water until neutral, dried at 100°C, and calcined under a nitrogen atmosphere, with a calcination temperature of 600°C and a calcination time of 4 h; then reduced by hydrogen, using a mixture of hydrogen and nitrogen (H2 / N2 molar ratio of 3:1), with a reduction temperature of 450°C, a pressure of normal pressure, and a time of 2 h, to obtain an iron-based catalyst.

[0072] Comparative Example 2

[0073] The difference from Example 1 is that only the second step of precipitation was performed.

[0074] The preparation of the iron-based catalyst (promoter system Al / Si / Ba) comprises the following steps:

[0075] S1, 20.0 g of iron nitrate nonahydrate was weighed and dissolved in 150 mL of water, 0.5 mol / L sodium hydroxide solution was slowly added dropwise to adjust the pH to 9, and the mixture was stirred at 60°C for 2 h to form a precipitate;

[0076] S2, 0.96 g of aluminum nitrate nonahydrate, 1.40 g of sodium silicate nonahydrate, and 0.066 g of barium nitrate (molar ratio of Al:Si:Ba is 0.52:1:0.051) were added to the above system, 0.5 mol / L sodium hydroxide solution was slowly added dropwise to adjust the pH to 9, and the mixture was stirred at 70°C for 1 h, then aged at the same temperature for 3 h, and precipitated to obtain a mixed slurry;

[0077] S3, the mixed slurry was filtered to obtain a precipitate, which was washed with deionized water until neutral, dried at 100°C, and then calcined under a nitrogen atmosphere, with a calcination temperature of 600°C and a calcination time of 4 h; then reduced by hydrogen, using a mixed gas of hydrogen and nitrogen (molar ratio of H2 / N2 is 3:1), with a reduction temperature of 450°C, a pressure of normal pressure, and a time of 2 h, to obtain an iron-based catalyst, which contains the following metals in the following mass percentages: Fe: 92.0%, Al: 2.3%, Si: 4.6%, and Ba: 1.1%.

[0078] Comparative Example 3

[0079] The difference from Example 1 is that only the first step of precipitation is performed.

[0080] The preparation of the iron-based catalyst (promoter system Ce / Mg / Ca) comprises the following steps:

[0081] S1, 20.0 g of iron nitrate nonahydrate, 0.64 g of cerium nitrate hexahydrate, 1.09 g of magnesium nitrate hexahydrate, and 0.81 g of calcium nitrate tetrahydrate (molar ratio of Ce:Mg:Ca is 0.35:1:0.81) were weighed and dissolved in 100 mL of water, 0.5 mol / L sodium hydroxide solution was slowly added dropwise to adjust the pH to 9, and the mixture was stirred at 60°C for 2 h to form a precipitate, obtaining a mixed slurry;

[0082] S2, the mixed slurry was filtered to obtain a precipitate, which was washed with deionized water until neutral, dried at 100°C, and then calcined under a nitrogen atmosphere, with a calcination temperature of 600°C and a calcination time of 4 h; then reduced by hydrogen, using a mixed gas of hydrogen and nitrogen (molar ratio of H2 / N2 is 3:1), with a reduction temperature of 450°C, a pressure of normal pressure, and a time of 2 h, to obtain an iron-based catalyst, which contains the following metals in the following mass percentages: Fe: 86.1%, Ce: 6.4%, Mg: 3.2%, and Ca: 4.3%.

[0083] Comparative Example 4

[0084] The difference from Example 1 is that the Ce doping amount in the iron-based catalyst is too high.

[0085] The preparation of the iron-based catalyst (adjuvant system Ce / Mg / Ca + Al / Si / Ba) includes the following steps:

[0086] S1, weigh 20.0 g of iron 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 is 1.10:1:0.31) into 100 mL of water, slowly add 0.5 mol / L sodium hydroxide solution to adjust the pH to 9, and stir the mixture at 60°C for 2 h to form a precipitate;

[0087] S2, add 0.96 g of aluminum nitrate nonahydrate, 1.40 g of sodium silicate nonahydrate, and 0.066 g of barium nitrate to the above system (Al:Si: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 the mixture at 70°C for 1 h, then stand at the same temperature for 3 h to form a second precipitate, and obtain a mixed slurry;

[0088] S3, filter the mixed slurry to obtain a precipitate, wash it with deionized water until it is neutral, dry it at 100°C, and then calcine it under a nitrogen atmosphere at a calcination temperature of 600°C for 4 h; then reduce it with hydrogen, using a mixture of hydrogen and nitrogen (H2 / N2 molar ratio is 3:1) at a reduction temperature of 450°C, a pressure of atmospheric pressure, and a time of 2 h, to obtain an iron-based catalyst, which contains the following metals in the following mass percentages: Fe: 71.0%, Ce: 18.3%, Mg: 2.9%, Ca: 1.5%, Al: 1.8%, Si: 3.6%, and Ba: 0.9%.

[0089] Comparative Example 5

[0090] The difference from Example 1 is that the metal elements in the first step of precipitation are adjusted to Ce, Ni, and Ca, i.e., using equimolar nickel nitrate hexahydrate instead of magnesium nitrate hexahydrate.

[0091] The preparation of the iron-based catalyst (adjuvant system Ce / Ni / Ca + Al / Si / Ba) includes the following steps:

[0092] S1, 20.0 g of iron 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) were weighed into 150 mL of water, 0.5 mol / L sodium hydroxide solution was slowly added dropwise to adjust the pH to 9, and the mixture was stirred at 60°C for 2 h to form a precipitate;

[0093] S2, 0.96 g of aluminum nitrate nonahydrate, 1.40 g of sodium silicate nonahydrate and 0.066 g of barium nitrate were added to the above system (Al: Si: Ba molar ratio of 0.52:1:0.051), 0.5 mol / L sodium hydroxide solution was slowly added dropwise to adjust the pH to 9, and the mixture was stirred at 70°C for 1 h, then aged at the same temperature for 3 h, and precipitated again to obtain a mixed slurry;

[0094] S3, the mixed slurry was filtered to obtain a precipitate, which was washed with deionized water until neutral, dried at 100°C, and calcined under a nitrogen atmosphere, with a calcination temperature of 600°C and a calcination time of 4 h; then reduced by hydrogen, using a mixture of hydrogen and nitrogen (H2 / N2 molar ratio of 3:1), with a reduction temperature of 450°C, a pressure of normal pressure, and a time of 2 h, to obtain an iron-based catalyst, which contained the following metals in the following mass percentages: Fe: 77.0%, Ce: 5.5%, Ni: 6.7%, Ca: 3.9%, Al: 2.0%, Si: 3.9%, and Ba: 1.0%.

[0095] Comparative Example 6

[0096] The difference from Example 1 is that in the second step of precipitation, the metal elements are adjusted to Al, Ni and Ba, i.e. using equimolar nickel nitrate hexahydrate instead of sodium silicate nonahydrate.

[0097] The preparation of the iron-based catalyst (Ce / Mg / Ca+Al / Ni / Ba additive system) includes the following steps:

[0098] S1, 20.0 g of iron 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) were weighed into 150 mL of water, 0.5 mol / L sodium hydroxide solution was slowly added dropwise to adjust the pH to 9, and the mixture was stirred at 60°C for 2 h to form a precipitate;

[0099] S2, 0.96 g of aluminum nitrate nonahydrate, 1.43 g of nickel nitrate hexahydrate and 0.066 g of barium nitrate (molar ratio of Al: Ni: Ba is 0.52:1:0.051) were added to the above system, a 0.5 mol / L sodium hydroxide solution was slowly added dropwise to adjust the pH to 9, the mixture was stirred at 70°C for 1 h, and then was aged at the same temperature for 3 h, and then was precipitated again to obtain a mixed slurry;

[0100] S3, the precipitate obtained after the mixed slurry was filtered was washed with deionized water until neutral, and then was dried at 100°C, and then was calcined under a nitrogen atmosphere, the calcination temperature was 600°C, and the calcination time was 4 h; and then the calcined product was reduced by hydrogen, a mixed gas of hydrogen and nitrogen (molar ratio of H2 / N2 is 3:1) was used, the reduction temperature was 450°C, the pressure was normal pressure, and the time was 2 h, to obtain an iron-based catalyst, the mass percentage of the metals contained in the catalyst was as follows: Fe: 76.7%, Ce: 5.6%, Mg: 2.9%, Ca: 3.8%, Al: 1.9%, Ni: 8.0%, and Ba: 1.1%.

[0101] Comparative Example 7

[0102] The difference from Example 1 is that all the precursors were added at one time, and no stage-by-stage precipitation was performed.

[0103] The preparation of the iron-based catalyst (the assistant system Ce / Mg / Ca / Al / Si / Ba) included the following steps:

[0104] S1, 20.0 g of iron 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 were dissolved in 150 mL of water, a 0.5 mol / L sodium hydroxide solution was slowly added dropwise to adjust the pH to 9, the mixture was stirred at 60°C for 3 h, and then was aged at the same temperature for 3 h to form a precipitate, to obtain a mixed slurry;

[0105] S2, the precipitate obtained after the mixed slurry was filtered was washed with deionized water until neutral, and then was dried at 100°C, and then was calcined under a nitrogen atmosphere, the calcination temperature was 600°C, and the calcination time was 4 h; and then the calcined product was reduced by hydrogen, a mixed gas of hydrogen and nitrogen (molar ratio of H2 / N2 is 3:1) was used, the reduction temperature was 450°C, the pressure was normal pressure, and the time was 2 h, to obtain an iron-based catalyst, the mass percentage of the metals contained in the catalyst was as follows: Fe 80.0%, Ce 6.1%, Mg 3.1%, Ca 3.8%, Al 2.1%, Si 3.9%, and Ba 1.0%.

[0106] Comparative Example 8

[0107] The difference from Example 1 is that the iron-based catalyst is prepared by a conventional impregnation method.

[0108] The preparation of the iron-based catalyst (promoter system Ce / Mg / Ca / Al / Si / Ba) comprises the following steps:

[0109] S1, Preparation of Fe2O3 carrier: 20.0 g of iron nitrate nonahydrate was dissolved in 100 mL of water, and 0.5 mol / L sodium hydroxide solution was added dropwise until the precipitation was complete. After filtration, washing and drying, the Fe2O3 powder (about 7.0 g) was obtained by calcination at 500°C in air for 4h.

[0110] S2, Preparation of impregnation solution: 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 were dissolved in deionized water to prepare an impregnation solution with a total volume of 15 mL.

[0111] S3, The impregnation solution was uniformly added to the Fe2O3 carrier, and after standing at room temperature for 12h, it was dried at 110°C for 12h, and then calcined at 400°C in air for 2h. Then it was reduced by hydrogen, using a mixture of hydrogen and nitrogen (H2 / N2 molar ratio of 3:1), the reduction temperature was 450°C, the pressure was normal pressure, and the time was 2h. The iron-based catalyst was obtained, and the mass percentage of the contained metals was: Fe 80.0%, Ce 6.1%, Mg 3.1%, Ca 3.8%, Al 2.1%, Si 3.9%, and Ba 1.0%.

[0112] The iron-based catalyst prepared above was ground, tabletted and sieved, and 40-60 mesh particles were taken. The ammonia synthesis performance evaluation was carried out using a fixed bed reactor. The loading amount of the iron-based catalyst in the fixed bed reactor was 0.5g, the temperature was controlled at 450°C, the pressure was 10 MPa, the space velocity was 10000 h -1 , and the molar ratio of raw gas H2 / N2 was 3:1. After 2h of stable reaction, the ammonia content in the tail gas was analyzed by sulfuric acid absorption method, and the ammonia generation rate was calculated. After continuous operation for 72h, the ammonia generation rate was measured again, and the activity decay rate was calculated ((initial NH3 generation rate-72h after NH3 generation rate) / initial NH3 generation rate x 100%). The results are shown in Table 1.

[0113] Table 1 Catalytic reaction results of Examples 1-5 and Comparative Examples 1-8

[0114]

[0115] As Figure 1The 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.

[0116] 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.

[0117] 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 -1 The 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.

[0118] 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.

[0119] 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.

[0120] The above merely provides the preferred embodiments of the present application, and is not intended to limit the patent scope of the present application. Any equivalent structure or equivalent process transformation made according to the present application specification, or direct or indirect application in other related technical fields, are also included in the patent protection scope of the present application.

Claims

1. A process for the preparation of a multi-promoter confined architecture iron-based catalyst, characterized in that, It comprises the following steps: S1, dissolving iron precursor, cerium precursor, magnesium precursor and calcium precursor in water, adjusting pH to 8-10, stirring and mixing to form a precipitate; S2, adding aluminum precursor, silicon precursor and barium precursor to the above system, adjusting pH to 8-10, stirring and mixing, and synergistically precipitating again to obtain a mixed slurry; S3, after filtering, washing and drying the mixed slurry, calcining under an inert atmosphere, and then reducing by hydrogen, an iron-based catalyst containing Fe-bcc(111) crystal surface is obtained, and the mass percentage of the contained metals 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 process for preparing the multi-promoter confined construction iron-based catalyst according to claim 1, characterized by, 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 process for preparing the multi-promoter confined construction iron-based catalyst according to claim 1, characterized by, In S1, the stirring and mixing is stirring at 40-70℃ for 0.5-3h.

4. The process for preparing a multi-promoter confined architecture iron-based catalyst according to one of claims 1 to 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 process for preparing the multi-promoter confined construction iron-based catalyst according to claim 1, characterized by, In S2, the stirring and mixing is stirring at 60-90℃ for 1-2h, followed by standing and aging for 2-4h.

6. The process for preparing the multi-additive confined construction iron-based catalyst according to claim 1 or 5, characterized by, In S2, the aluminum precursor is aluminum nitrate nonahydrate, aluminum chloride or aluminum isopropyl alcohol; the silicon precursor is sodium silicate nonahydrate, ethyl silicate or ammonium metasilicate; and the barium precursor is barium nitrate or barium chloride.

7. The process for preparing the multi-promoter confined construction iron-based catalyst according to claim 1, characterized by, In S1 and S2, the pH adjustment is performed by using an alkali solution, and the alkali solution is one or more of ammonia, sodium hydroxide solution and sodium carbonate solution, and the concentration is 0.1-5 mol / L.

8. The process for preparing the multi-promoter confined construction iron-based catalyst according to claim 1 or 7, characterized in that, In S3, the calcination temperature is 400-800℃, the calcination time is 2-6h; the hydrogen reduction temperature is 400-500℃, the pressure is normal pressure, and the time is 2-3h.

9. An iron-based catalyst prepared by the preparation method of any one of claims 1-8 for use in ammonia synthesis.

10. Use according to claim 9, characterized in that, It comprises: Under the reaction conditions of a temperature of 350-500℃ and a pressure of 7-15 MPa, hydrogen, nitrogen and the iron-based catalyst are contacted to react to synthesize ammonia.

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