Iron-based catalyst as well as preparation method and application thereof

Abstract

The invention discloses an iron-based catalyst as well as a preparation method and application thereof. The synthesis method of the iron-based catalyst comprises the following steps: S1, carrying out high-temperature nitriding treatment on an iron-containing catalyst precursor in an ammonia gas atmosphere; wherein the iron-containing catalyst precursor contains an alkali metal element and an alkaline earth element; s2, the activated precursor is placed in a mixed atmosphere to be subjected to in-situ reaction treatment, Fe2N is converted into a Fe7C3-phase iron-based catalyst, and the Fe7C3 catalyst can be obtained, wherein the mixed atmosphere is a mixed atmosphere of a carbon source gas and H2, and the carbon source gas is a mixed gas of one or two of CO2 and CO. The problem of oxidative inactivation possibly occurring in the intermediate transfer process of the catalyst is thoroughly avoided in an in-situ generation mode, so that the initial activity and long-term stability of the catalyst are greatly improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalytic materials, and in particular relates to an iron-based catalyst and a preparation method and application thereof. Background Art

[0002] The hydrogenation of carbon dioxide (CO2) to produce high-value-added olefins is an important approach to achieving carbon resource recycling and mitigating greenhouse gas emissions. However, due to the stable molecular structure of CO2, its direct hydrogenation to olefins requires overcoming high activation energies, necessitating the development of efficient and stable catalysts. Iron-based catalysts are widely used in CO2 hydrogenation to produce hydrocarbons (similar to Fischer-Tropsch synthesis) due to their low cost, abundant resources, and high water-gas shift (WGS) activity. In practical catalytic processes, iron-based catalysts typically exhibit a variety of valence states and crystalline phases, including metallic iron, iron oxide, and iron carbide. Iron carbide phases (such as Fe5C2, Fe3C, and Fe7C3) are widely considered to be the primary active phase in CO2 or CO hydrogenation to olefins. However, the catalytic activity, selectivity, and stability of different iron carbide phases vary significantly under practical reaction conditions. The Fe7C3 phase has garnered significant attention in recent years due to its high intrinsic catalytic activity and ability to significantly promote carbon chain growth. However, the large amount of by-product water generated during the CO2 hydrogenation reaction exhibits a strong competitive oxidative effect on the Fe7C3 phase, causing it to be easily oxidized to inactive iron oxide (Fe3O4), ultimately leading to rapid catalyst deactivation. Therefore, how to obtain a highly active and stable Fe7C3 phase under reaction conditions has become one of the key scientific and technological challenges in improving the performance of iron-based catalysts.

[0003] A limited number of studies have reported on the preparation of Fe7C3-phase catalytic materials. For example, Zhao Huabo et al. (CCS Chem. 2021, 3, 2712) reported a method for synthesizing pure Fe7C3 nanoparticles via thermal decomposition of an organic amine (N,N-dimethyloctadecylamine) ligand. This method requires the use of a highly toxic organic amine solvent and thermal decomposition at approximately 350°C for two hours in an ammonia atmosphere. The conditions are complex and difficult to scale up industrially. Another widely used traditional iron-based catalyst preparation method relies primarily on the carbonization of syngas (CO / H2) at high temperatures for extended periods to form an active iron carbide phase. However, in CO2 hydrogenation reactions, the conventional syngas activation process struggles to form a stable single iron carbide phase due to competitive oxidation by water vapor. Ultimately, the catalyst tends to form an equilibrium state with a Fe3O4@Fe5C2 core-shell structure, making it difficult to maintain stable long-term performance. In addition, patent CN116984008A adopts a simple mechanical ball-milling method of elemental iron powder, elemental boron and elemental carbon to directly mix and carbonize to prepare a boron-containing iron carbide catalyst. Although this method may involve the formation of Fe7C3 phase, due to the lack of clear selection of specific precursors and the lack of precise control of the carbonization process, the Fe7C3 phase prepared by it is difficult to achieve precise control of the crystal phase purity and grain size, resulting in limited stability and catalytic performance of the obtained catalyst.

[0004] Therefore, the existing technology still lacks a simple, efficient and easy-to-scale industrial iron-based catalyst preparation method to achieve efficient synthesis of stable, high-purity Fe7C3 phase and its long-term stable operation in the CO2 hydrogenation reaction to produce olefins. Summary of the Invention

[0005] Based on the above problems existing in the prior art, the purpose of the present invention is to provide a method for in-situ synthesis of iron-based catalysts to overcome the technical bottlenecks of complex process, difficulty in industrial scale-up and poor crystal phase stability in the existing Fe7C3 catalyst preparation process.

[0006] Accordingly, the present invention also provides an iron-based catalyst obtained by the above-mentioned in-situ synthesis method, in particular a catalyst product having a clear Fe7C3 crystal phase structure, and the specific application of the iron-based catalyst in the CO2 hydrogenation reaction to produce olefins.

[0007] In order to achieve the above-mentioned purpose, the technical solutions provided by the present invention are as follows:

[0008] A method for synthesizing an iron-based catalyst comprises the following steps:

[0009] S1: subjecting an iron-containing catalyst precursor to a high-temperature nitriding treatment in an ammonia atmosphere to obtain an activated precursor of iron-containing nitride Fe2N; wherein the iron-containing catalyst precursor contains an alkali metal element and an alkaline earth element;

[0010] S2: placing the activated precursor in a mixed atmosphere for in-situ reaction treatment to convert the Fe2N into an iron-based catalyst in the Fe7C3 phase to obtain the Fe7C3 catalyst; wherein the mixed atmosphere is a mixed atmosphere of carbon source gas and H2, and the carbon source gas is a mixed gas of one or two of CO2 and CO.

[0011] In step S2, the volume ratio of H2 to carbon source gas in the mixed atmosphere is 1:1–5:1, the reaction temperature is 300–380°C, the reaction pressure is 0.1–5 MPa, and the reaction time is 10–100 hours.

[0012] In step S2, the volume ratio of H2 to carbon source gas in the mixed atmosphere is 3:1, the temperature is 340±20°C, the pressure is 1.5~2.5 MPa, and the processing time is 50–100 hours.

[0013] The high-temperature nitriding treatment in step S1 is performed at a temperature of 400-600° C. and for a time of 0.5-5 hours.

[0014] The step S2 of placing the activated precursor in a mixed atmosphere for in-situ reaction treatment is achieved by switching the ammonia atmosphere to a mixed atmosphere.

[0015] The iron-containing catalyst precursor is a catalyst precursor with a Prussian blue-like structure prepared by co-precipitation of iron salt and hexacyanoferrate.

[0016] The alkali metal elements are dispersed in the iron-containing catalyst precursor in the form of compounds, and the mass ratio of the alkali metal elements to Fe is 0.01-0.5:1; the alkaline earth metal elements are dispersed in the catalyst in the form of oxides, and the mass ratio of the alkaline earth metal elements to Fe is 0.01-1:1.

[0017] Wherein, the alkali metal element is K or Na, and the alkaline earth metal element is Mg or Ca.

[0018] Preferably, K is dispersed in the iron-containing catalyst precursor in the form of a compound, with a mass ratio of K to Fe of 0.01–0.5:1; Mg is dispersed in the catalyst in the form of an oxide, with a mass ratio of Mg to Fe of 0.01–1:1. Preferably, the mass ratio of K to Fe is 0.1–0.3:1; and the mass ratio of Mg to Fe is 0.01–1:1.

[0019] More preferably, the mass ratio of K to Fe is 0.1, and that of Mg to Fe is 0.04.

[0020] An iron-based catalyst prepared by the above-mentioned synthesis method of the iron-based catalyst; the main crystal phase of the iron-based catalyst is Fe7C3, and the relative content of the Fe7C3 phase is not less than 80 wt% under XRD detection; the Fe7C3 crystal phase in the iron-based catalyst has a hexagonal structure and a grain size of 20-100 nm.

[0021] The application of the above-mentioned iron-based catalyst in CO2 hydrogenation and CO hydrogenation reactions.

[0022] The beneficial effects of the present invention are specifically embodied as follows:

[0023] (1) The present invention achieves efficient and controllable synthesis of Fe7C3 catalyst by specifically selecting Fe2N nitride as the intermediate phase and using a specific in-situ carbonization process in a CO2 / H2 mixed atmosphere. Unlike traditional mechanical ball milling or processes that preform carbides and then transfer them to a reactor, the present invention completely avoids the oxidative deactivation problem that may occur during the intermediate transfer process of the catalyst through in-situ generation, thereby significantly improving the initial activity and long-term stability of the catalyst.

[0024] (2) The dual-additive system of K and Mg explicitly introduced in the present invention plays a significant synergistic role in the catalyst preparation process: the K additive promotes the selective conversion of the nitride Fe2N to the high-purity Fe7C3 crystalline phase, while the Mg additive effectively captures and neutralizes water vapor in the actual reaction environment, thereby significantly improving the structural stability and service life of the catalyst in the CO2 hydrogenation to olefins reaction.

[0025] (3) The in-situ preparation method of the present invention has a simple process and mild conditions, avoids the use of toxic organic solvents and mechanical ball milling process, and the raw materials used (iron salts, ammonia, CO2, H2, etc.) are low-priced. The entire preparation process can be easily completed in a continuous industrial reaction device, and it is very easy to achieve industrial scale-up production, which has outstanding industrial application prospects.

[0026] (4) The iron-based catalyst preparation method proposed in the present invention overcomes the problems of the existing technology such as complex preparation process, difficulty in controlling the purity and stability of the crystal phase, and difficulty in industrial scale-up. It achieves the synthesis of high-purity Fe7C3 crystal phase with controllable grain size, significantly improving the performance and industrial application potential of the catalyst. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a schematic roadmap of the conversion of an iron-containing catalyst precursor into an Fe7C3 catalyst active phase in a CO2 / H2 atmosphere in the preparation method of the iron-based catalyst of the present invention.

[0028] Figure 2 The XRD patterns of the iron-based catalysts prepared in Examples 1-4 are shown.

[0029] Figure 3 The figure shows the reaction performance of the iron-based catalyst obtained in Example 1-4 in catalyzing CO2 hydrogenation in a CO2 / H2 carbonized atmosphere. Figure 3 a in the figure is the reaction performance diagram of the sample of Example 1; wherein Figure 3 b in the figure is the reaction performance diagram of the sample of Example 2; wherein Figure 3 c in the figure is the reaction performance diagram of the sample of Example 3; wherein Figure 3 d in FIG. 4 is the reaction performance diagram of the sample of Example 4.

[0030] Figure 4 This is a stability test chart of the iron-based catalyst prepared in Example 4. DETAILED DESCRIPTION

[0031] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only intended to illustrate the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0032] Unless otherwise specified, the raw materials and equipment used in the embodiments of the present invention are commercially available.

[0033] See also Figure 1 The present invention provides a method for preparing an iron-based catalyst, comprising the following steps:

[0034] First, an iron-containing catalyst precursor having a Prussian blue-like structure is subjected to a nitridation treatment in an ammonia atmosphere to form a clear nitride intermediate phase Fe2N; wherein the iron-containing catalyst precursor contains alkali metal elements and alkaline earth elements;

[0035] Then, by directly switching to a CO2 / H2 mixed atmosphere, an in-situ carbonization treatment was carried out in the reactor at strictly controlled temperature, pressure and gas ratio to selectively obtain a high-purity and high-stability Fe7C3 active phase catalyst.

[0036] In the above process, specific proportions of alkali metals such as potassium (K) and alkaline earth metals such as magnesium (Mg) as additives were explicitly introduced. The K additive effectively promoted the efficient conversion of Fe2N to the specific crystal phase of Fe7C3, while the Mg additive significantly inhibited the oxidation of the Fe7C3 phase by the reaction water vapor during the CO2 hydrogenation process, thereby achieving structural stability for the long-term operation of the catalyst.

[0037] Specifically, the mixed atmosphere used in the in-situ carbonization process is a mixed gas of CO2 and H2, the volume ratio of which is strictly controlled between H2 / CO2 = 1:1–5:1, the reaction temperature is 300–380°C, the reaction pressure is 0.1–5 MPa, and the reaction time is 10–100 hours; the preferred in-situ carbonization conditions are: H2 / CO2 volume ratio of 3:1, temperature of 340±20°C, pressure of 1.5–2.5 MPa, and treatment time of 50–100 hours.

[0038] The iron-based catalyst obtained by the present invention has Fe7C3 phase as the main crystal phase. XRD detection confirms that the Fe7C3 phase content reaches more than 80 wt%, and has a clear hexagonal crystal structure with a grain size between 20-100 nm. It has the excellent performance of stably maintaining the unchanged crystal phase structure for a long time.

[0039] Unlike existing methods that directly prepare iron carbide through mechanical ball milling of iron powder, elemental boron, and elemental carbon, this invention explicitly proposes for the first time the use of a specific iron-containing precursor with a Prussian blue-like structure. This precursor undergoes a nitridation reaction in an ammonia (NH3) atmosphere to form a specific intermediate phase, Fe2N. This intermediate phase is then selectively synthesized through a carefully controlled in-situ carbonization reaction pathway in a CO2 / H2 mixed atmosphere to yield a single-phase Fe7C3 catalyst with a stable structure and high catalytic activity. This specific precursor selection and in-situ carbonization pathway result in the Fe7C3 crystalline phase with a well-defined grain size distribution, excellent crystal purity, and catalytic stability—technical advantages unattainable by existing mechanical ball milling methods.

[0040] Specifically, the synthesis method of the iron-based catalyst of the present invention comprises the following steps:

[0041] S0: Precursor Preparation: The iron-containing catalyst precursor is prepared using coprecipitation, a technique commonly used in the art. The present invention utilizes a coprecipitation method of an iron salt and hexacyanoferrate (such as potassium ferrocyanide or ammonium ferrocyanide) to prepare a Prussian blue-like precursor to ensure uniform dispersion of the iron component and additive. The molar ratio of the iron salt to the hexacyanoferrate is 1:1 to 1.5.

[0042] S1: Nitriding: After drying and forming the iron-containing catalyst precursor obtained in S0, it undergoes a high-temperature nitriding pretreatment in an ammonia (NH3) stream to produce an activated precursor. Preferably, the precursor is calcined at 400–600°C for 0.5–5 hours in an NH3 atmosphere to convert it into an activated precursor primarily composed of iron nitride, Fe2N. The Fe2N intermediate phase produced by this nitriding treatment creates favorable conditions for the subsequent carbonization to form stable carbide nuclei.

[0043] S2: In-situ Carbonization (Fe2N → Fe7C3): The activated precursor, after the nitridation treatment, is directly switched to a mixed flow of carbon source gas and H2 for in-situ carbonization without exposing it to air. Over a specific carbonization period, the iron nitride gradually transforms into the iron carbide Fe7C3 phase. To ensure sufficient carbonization and stable formation of the Fe7C3 phase, the reaction time is generally 10–100 hours, which can be adjusted appropriately based on the actual reaction pressure and temperature.

[0044] During the conversion process, the carbon source gas reacts in situ with H₂. The generated carbon atoms gradually infiltrate the Fe₂N lattice and replace some nitrogen atoms, forming the iron carbide FeₐC₃ phase. Furthermore, a small amount of H₂O is produced as a byproduct. After carbonization is complete, an iron-based catalyst with FeₐC₃ as the primary crystalline phase is obtained. The step-by-step method of the present invention can also produce a catalyst with FeₐC₃ as the primary phase.

[0045] Preferably, the volume ratio of H2 to CO2 in the mixed atmosphere in step S2 is 1:1–5:1, the reaction temperature is 300–380°C, the reaction pressure is 0.1–5 MPa, and the reaction time is 10–100 hours. A preferred embodiment is to perform the in-situ treatment at 340°C, 2 MPa, and H2 / CO2 = 3:1 for ≥50 hours.

[0046] More preferably, in step S2, the volume ratio of H2 / CO2 in the mixed atmosphere is 3:1, the temperature is 340±20°C, the pressure is 1.5~2.5 MPa, and the treatment time is 50–100 hours.

[0047] The high-temperature nitriding treatment in step S1 is performed at a temperature of 400-600° C. and for a time of 0.5-5 hours.

[0048] Specifically, placing the activated precursor in a mixed atmosphere for in-situ reaction treatment in step S2 is achieved by switching the ammonia atmosphere to a mixed atmosphere.

[0049] The iron-containing catalyst precursor is ferric ferrocyanide. The iron-containing catalyst precursor can be purchased commercially or prepared by itself, and the preparation method is not limited; co-precipitation, sol-gel and other methods commonly used in the art can be used.

[0050] Specifically, the iron-containing catalyst precursor is a catalyst precursor with a Prussian blue-like structure prepared by co-precipitation of iron salt and hexacyanoferrate.

[0051] Further, the iron-containing catalyst precursor also includes alkali metal elements and alkaline earth metal elements, the alkali metal elements are selected from one or more of lithium, sodium, potassium, rubidium, cesium, and the alkaline earth metal elements are selected from one or more of magnesium, calcium, strontium, and barium. The introduced auxiliary agent can effectively control the competition between carbonization and oxidation in the process, so that the Fe7C3 phase can be generated and stably exist.

[0052] The iron-based catalyst of the present application contains alkali metal elements and alkaline earth metal elements, both of which can be introduced in the preparation stage of the above-mentioned precursor, so that the alkali metal elements and alkaline earth metal elements are uniformly dispersed in the iron-based component on a nanometer scale.

[0053] Preferably, the alkali metal auxiliary agent is potassium or sodium, and the alkaline earth metal auxiliary agent is magnesium or calcium. For example, the K auxiliary agent can be introduced by co-precipitation using potassium ferricyanide as a precipitant, or K2CO3 impregnation after the preparation of the precursor; the Mg auxiliary agent can be introduced by adding a soluble magnesium salt during co-precipitation, or using the method of loading Mg(NO3)2 solution in equal volume after the formation of the precursor.

[0054] More preferably, K is uniformly mixed with the iron component in the form of a compound, and the mass ratio of K to Fe is 0.01-0.5:1; Mg is dispersed in the catalyst in the form of an oxide, and the mass ratio of Mg to Fe is 0.01-1:1.

[0055] More preferably, when the mass ratio of K to Fe is about 0.1 and the mass ratio of Mg to Fe is about 0.04, the catalytic effect is best.

[0056] The synergistic mechanism of the two auxiliary agents is as follows: potassium, as an electron donor, changes the surface chemistry of iron, making CO2 molecules more easily adsorbed and dissociated, and reducing the iron-carbon bond energy, thereby accelerating the conversion of Fe2N to Fe7C3. At the same time, K promotes the carbon chain growth pathway of the iron catalyst, increasing the selectivity of C2 + The selectivity of olefins (inhibiting excessive hydrogenation to generate by-products such as methane). The magnesium auxiliary agent mainly exists in the form of magnesium oxide, uniformly distributed on the surface and grain boundaries of iron particles, capturing and neutralizing the generated H2O, slowing down the oxidation corrosion of carbonized iron by water. The addition of Mg also helps to improve the anti-sintering performance of iron species. Under the synergistic action of the two, the Fe7C3 phase can be stably maintained under the CO2 hydrogenation reaction conditions, avoiding the rapid conversion to Fe3O4 and deactivation as in traditional iron catalysts.

[0057] The present application innovatively combines nitrogenation pretreatment and in-situ carbonization two-step method to directly generate Fe7C3 active phase, simplifying the process and being closer to industrial operation. The prepared Fe7C3 active phase does not need to go through the process of transferring from an inert environment to a reaction environment, avoiding oxidation deactivation in the intermediate stage, so that the active phase is in the working atmosphere from the time of self-formation, thereby improving the initial activity.

[0058] In summary, the method is to first pre-treat the iron-containing precursor in an ammonia atmosphere to generate iron nitride Fe2N, and then directly switch to a CO2 / H2 atmosphere to convert Fe2N to iron carbide Fe7C3 active phase in situ. Alkali and alkaline earth metal promoters (such as potassium K and magnesium Mg) are introduced into the catalyst, where K can significantly promote the conversion of iron nitride to carbide and increase the selectivity of olefins in the product, and Mg effectively inhibits the oxidation of Fe7C3 phase by byproduct water during the reaction, enhancing the structural stability of the catalyst.

[0059] Example 1

[0060] In this example, an iron-based catalyst without any promoters is prepared.

[0061] The preparation method of the iron-based catalyst is as follows:

[0062] S0: Precursor preparation: 10.0 g of polyvinylpyrrolidone (PVP, K30) was dissolved in 200 mL of deionized water, and 0.1 mol of ammonium hexacyanoferrate ((NH4)4Fe(CN)6) was added to form solution A; another 0.1 mol of iron (III) nitrate nonahydrate was dissolved in 200 mL of deionized water to form solution B. At room temperature, solution B was slowly added to solution A, and after the addition was completed, stirring was continued for 1 hour to obtain a uniform light blue suspension. After the suspension was aged for 12 hours, it was centrifuged, washed with deionized water for 3 times, and dried at 80°C for 12 hours to obtain a promoter-free iron precursor solid with a Prussian blue-like structure, denoted as Fe precursor.

[0063] S1: Nitriding treatment: The above Fe precursor solid was ground and placed in a tube furnace, after replacing the air with high-purity argon, the atmosphere was switched to ammonia, and the temperature was raised to 400°C at a rate of 5°C / min and held for 5 hours, and then naturally cooled to room temperature to obtain a nitrided activated precursor with Fe2N as the main phase.

[0064] S2: In-situ carbonization: The above nitrided activated precursor was kept in the reactor, and a mixture of CO2 / H2 with a volume ratio of 1:3 (flow rate 40 mL / min, corresponding to a space velocity of about 8000 h -1 ) was introduced, and the temperature was raised to 300°C at a rate of 2°C / min and held for 100 hours, and then naturally cooled after the reaction was completed. The prepared catalyst was taken out under argon protection, denoted as Fe catalyst.

[0065] Example 2

[0066] In this example, an iron-based catalyst containing only the promoter Mg is prepared.

[0067] The preparation steps of the precursor in this example are as follows:

[0068] S0: Preparation of iron-containing precursor:

[0069] S01: Prepare an iron-based precursor by coprecipitation. Dissolve 10.0 g of polyvinylpyrrolidone (PVP, K30) in 200 mL of deionized water, then add 0.1 mol of ((NH4)4Fe(CN)6) to form Solution A. Separately, dissolve 0.1 mol of iron(III) nitrate nonahydrate in 200 mL of water to form Solution B. Slowly add Solution B dropwise to Solution A while stirring. Continue stirring at room temperature for 1 hour to form a light blue, homogeneous suspension. After aging for 12 hours, the resulting coprecipitate is centrifuged, washed with water, and dried in an 80°C waterbath for 12 hours to obtain a precursor solid.

[0070] S02: Introduction of Mg additive: 5.0 g of the dried precursor solid was taken and added to an appropriate amount of deionized water to form a slurry. 0.20 g of anhydrous magnesium nitrate (Mg(NO3)2) was weighed and dissolved in a small amount of water at a Mg / Fe mass ratio of 0.04. This was slowly added to the precursor slurry and stirred at room temperature for 4 hours. The water was then evaporated under reduced pressure in an 80°C water bath to obtain a wet solid. This solid was then dried in a 120°C oven for 8 hours, ground, and sieved to obtain a Mg-modified Fe precursor, designated as the FeMg precursor.

[0071] S1: Nitridation: The FeMg precursor solid was placed in the center of a quartz tube furnace. After replacing the air with high-purity argon, the precursor was heated to 300°C under an argon atmosphere at a rate of 5°C / min and held at that temperature for 1 hour to remove any residue. An ammonia (NH3) atmosphere was then introduced, and the temperature was increased at 5°C / min to 550°C and held at that temperature for 1 hour to nitride the precursor. After this treatment, the precursor was naturally cooled to room temperature to obtain a Mg-containing nitridation catalyst precursor with Fe2N as the main phase.

[0072] S2: In-situ carbonization: The above-mentioned Mg-containing nitriding catalyst precursor is kept in the reaction tube, and a H2 / CO2 mixed gas with a volume ratio of 3:1 (total flow rate 40 mL / min, corresponding to a space velocity of about 8000 h) is introduced. -1 Under a hydrogen reducing atmosphere, the temperature was raised to 340°C at a rate of 2°C / min and the reaction was maintained at this temperature for 72 hours. After the reaction was completed, the mixture was naturally cooled to room temperature and the catalyst product was removed under argon protection and recorded as FeMg catalyst.

[0073] Example 3

[0074] In this example, an iron-based catalyst containing only promoter K was prepared.

[0075] S0: Preparation of a K-promoted iron precursor: Dissolve 10.0 g of polyvinylpyrrolidone (PVP, K30) in 200 mL of deionized water, then add 0.1 mol of potassium hexacyanoferrate(II) (K4[Fe(CN)6]·3H2O) to form Solution A. Separately, dissolve 0.1 mol of iron(III) nitrate nonahydrate in 200 mL of deionized water to form Solution B. At room temperature, slowly add Solution B dropwise to Solution A with stirring. Stirring is continued for 1 hour after the addition is complete, yielding a uniform light blue suspension. After aging for 12 hours, the suspension is centrifuged, washed three times with deionized water, and dried in an oven at 80°C for 12 hours to obtain a K-promoted iron precursor solid with a Prussian blue structure (K content approximately 2.0 wt% and Mg content zero). (Referred to as the FeK precursor).

[0076] S1: Nitridation: The K-containing additive precursor solid was placed in the center of a quartz tube furnace. After replacing the air with high-purity argon, the precursor was heated to 300°C under an argon atmosphere at a rate of 5°C / min and held at that temperature for 1 hour to remove any residue. An ammonia (NH3) atmosphere was then introduced, and the temperature was increased at 5°C / min to 550°C and held at that temperature for 1 hour to nitride the precursor. After this treatment, the precursor was naturally cooled to room temperature to obtain a K-containing nitridation catalyst precursor with Fe2N as the main phase.

[0077] S2: In-situ carbonization: The above-mentioned nitrided catalyst precursor is kept in the reaction tube, and a H2 / CO2 mixed gas with a volume ratio of 3:1 (total flow rate 40 mL / min, corresponding to a space velocity of about 8000 h -1 Under a hydrogen reducing atmosphere, the temperature was raised to 340°C at a rate of 2°C / min and the reaction was maintained at this temperature for 72 hours. After the reaction was completed, the mixture was naturally cooled to room temperature and the catalyst product was removed under argon protection and recorded as FeK catalyst.

[0078] Example 4

[0079] In this example, an iron-based catalyst containing both K and Mg is prepared.

[0080] A method for synthesizing an iron-based catalyst comprises the following steps:

[0081] S0: Preparation of iron-containing precursor:

[0082] S01: A K-containing iron-based precursor was prepared by coprecipitation. 10.0 g of polyvinylpyrrolidone (PVP, K30) was dissolved in 200 mL of deionized water, and 0.1 mol of potassium hexacyanoferrate(II) (K4[Fe(CN)6]·3H2O) was added to form solution A. Separately, 0.1 mol of iron(III) nitrate nonahydrate was dissolved in 200 mL of water to form solution B. Solution B was slowly added dropwise to solution A with stirring. Stirring was continued at room temperature for 1 hour, resulting in a light blue, uniform suspension. After aging for 12 hours, the resulting coprecipitate was centrifuged, washed with water, and dried in an 80°C oven for 12 hours to obtain a K-containing iron precursor solid (structured similar to Prussian blue). The K / Fe mass ratio in this precursor was approximately 0.1, and ICP analysis revealed 45.2 wt% Fe and 4.8 wt% K.

[0083] S02: Introduction of Mg additive: 5.0 g of the dried K-containing iron precursor solid was added to an appropriate amount of deionized water to form a slurry. 0.55 g of anhydrous magnesium nitrate (Mg(NO₃)₂) was weighed and dissolved in a small amount of water at a Mg / Fe mass ratio of 0.04. This was slowly added to the precursor slurry and allowed to steep at room temperature for 4 hours with stirring. The solid was then evaporated under reduced pressure in an 80°C water bath to obtain a wet solid. This solid was then dried in a 120°C oven for 8 hours, ground, and sieved to obtain the Mg-modified FeK precursor (referred to as the FeKMg precursor).

[0084] S1: Nitridation: The FeKMg precursor sample was placed in the center of a quartz tube furnace. First, the temperature was raised to 300°C under an argon flow at a rate of 5°C / min and held at that temperature for 1 hour to remove any surface residue. Anhydrous ammonia (NH3, flow rate 50 mL / min) was then introduced, and the temperature was raised again at 5°C / min to 550°C and held at that temperature for 1 hour to nitride the iron precursor. After nitridation, the sample was cooled to room temperature in the furnace. The resulting sample was primarily composed of the Fe2N phase (characteristic diffraction peaks of Fe2N were detected by XRD), with minor amounts of byproducts such as K3Fe(CN)6. This nitrided sample constituted the activated catalyst precursor.

[0085] S2: In-situ carbonization: The above-mentioned nitrided product was placed in a reactor and introduced into a H2 / CO2 mixed gas with a volume ratio of 3:1 (40 mL / min, space velocity of about 8000 h -1 ), the reaction pressure was 2 MPa, the temperature was increased to 340°C at a rate of 2°C / min and kept constant for 72 hours, and then naturally cooled to obtain the catalyst product, which was recorded as FeKMg catalyst.

[0086] Example 5

[0087] In this example, an iron-based catalyst containing both K and Mg is prepared.

[0088] A method for synthesizing an iron-based catalyst comprises the following steps:

[0089] S0: Preparation of iron-containing precursor:

[0090] S01: A K-containing iron-based precursor was prepared by coprecipitation. 10.0 g of polyvinylpyrrolidone (PVP, K30) was dissolved in 200 mL of deionized water, and 0.15 mol of potassium hexacyanoferrate(II) (K4[Fe(CN)6]·3H2O) was added to form solution A. 0.1 mol of iron(III) nitrate nonahydrate was dissolved in 200 mL of water to form solution B. Solution B was slowly added dropwise to solution A with stirring. Stirring was continued at room temperature for 1 hour, resulting in a light blue, uniform suspension. After aging for 24 hours, the resulting coprecipitate was centrifuged, washed with water, and dried in a water bath at 100°C for 12 hours to obtain a K-containing iron precursor solid (structured similar to Prussian blue). The K / Fe mass ratio in this precursor was approximately 0.3, and ICP analysis revealed 46.1 wt% Fe and 13.8 wt% K.

[0091] S02: Introduction of Mg additive: 5.0 g of the dried K-containing iron precursor solid was added to an appropriate amount of deionized water to form a slurry. 1.4 g of anhydrous magnesium nitrate (Mg(NO3)2) was weighed and dissolved in a small amount of water at a Mg / Fe mass ratio of 0.10. This was slowly added to the precursor slurry and allowed to steep at room temperature for 4 hours with stirring. The solid was then evaporated under reduced pressure in an 80°C water bath to obtain a wet solid. This solid was then dried in a 120°C oven for 12 hours, ground, and sieved to obtain the Mg-modified FeK precursor (referred to as the FeKMg2 precursor).

[0092] S1: Nitridation: Place the FeKMg2 precursor sample in the center of a quartz tube furnace. First, heat the sample to 230°C under an argon flow at a rate of 3°C / min and hold it there for 2 hours to remove any surface residue. Then, switch to anhydrous ammonia (NH3) at a flow rate of 50 mL / min, continue heating the sample at a rate of 4°C / min to 400°C, and hold it there for 0.5 hours to nitride the iron precursor.

[0093] S2: In-situ carbonization: The above-mentioned nitrided product was placed in a reactor and introduced into a H2 / CO2 mixed gas with a volume ratio of 5:1 (30 mL / min, space velocity of about 8000 h -1 ), the reaction pressure was 5 MPa, the temperature was raised to 300°C at a rate of 2°C / min, and after maintaining the temperature for 100 hours, the catalyst product was naturally cooled and recorded as FeKMg2 catalyst.

[0094] Example 6

[0095] In this example, an iron-based catalyst containing both K and Mg is prepared.

[0096] A method for synthesizing an iron-based catalyst, comprising the following steps:

[0097] S0: Preparation of an iron-containing precursor:

[0098] S01: Preparation of an iron-based precursor containing K promoter by coprecipitation. 10.0 g of polyvinylpyrrolidone (PVP, K30) was dissolved in 200 mL of deionized water to form solution A, and 0.13 mol of potassium hexacyanoferrate(II) (K4[Fe(CN)6]·3H2O) was added to form solution A; 0.1 mol of iron(III) nitrate nonahydrate was dissolved in 200 mL of water to form solution B. Solution B was slowly added to solution A under stirring, and stirring was continued at room temperature for 1 hour to form a light blue homogeneous suspension. After standing and aging for 12 hours, the obtained coprecipitate was sequentially separated by centrifugation, washed with water, and dried in a water bath at 80°C for 12 hours to obtain an iron precursor solid containing K (the structure is similar to Prussian blue, the mass ratio of K / Fe in the precursor is about 0.2, and the actual content of Fe is 44.9wt% and the actual content of K is 9.1wt% by ICP analysis).

[0099] S02: Introduction of Mg promoter: 5.0 g of the dried iron precursor solid containing K described above was taken and mixed with an appropriate amount of deionized water to form a slurry. 0.14 g of anhydrous magnesium nitrate (Mg(NO3)2) was weighed according to a mass ratio of Mg / Fe of 0.01, dissolved in a small amount of water, and slowly added to the precursor slurry, and the slurry was stirred and soaked at room temperature for 2 hours. Then, water was removed by evaporation under reduced pressure in a water bath at 80°C to obtain a wet solid. The wet solid was dried in an oven at 120°C for 12 hours, ground and sieved to obtain a FeK precursor modified with Mg (denoted as FeKMg3 precursor).

[0100] S1: Nitridation treatment: The FeKMg3 precursor sample was placed in the center of the quartz tube reactor. First, the temperature was raised to 300°C at a rate of 7°C / min under an argon flow and kept constant for 1 hour to remove surface residual substances. Then, anhydrous ammonia gas (NH3, flow rate 50 mL / min) was introduced, and the temperature was continued to rise to 600°C at a rate of 6°C / min and kept constant for 5 hours to achieve the nitridation of the iron precursor.

[0101] S2: In-situ carbonization: The nitridation product was placed in the reactor, and a mixture of H2 / CO2 (volume ratio 1:1, 50 mL / min, space velocity about 8000 h -1 ) was introduced, and the temperature was raised to 380°C at a rate of 5°C / min and kept constant for 10 hours, and then the catalyst product was obtained by natural cooling, denoted as FeKMg3 catalyst.

[0102] Example 7

[0103] In this example, an iron-based catalyst containing both K and Ca was prepared.

[0104] A method for synthesizing an iron-based catalyst comprises the following steps:

[0105] S0: Preparation of iron-containing precursor:

[0106] S01: A K-containing iron-based precursor was prepared by coprecipitation. 10.0 g of polyvinylpyrrolidone (PVP, K30) was dissolved in 200 mL of deionized water, and 0.13 mol of potassium hexacyanoferrate(II) (K4[Fe(CN)6]·3H2O) was added to form solution A. 0.1 mol of iron(III) nitrate nonahydrate was dissolved in 200 mL of water to form solution B. Solution B was slowly added dropwise to solution A with stirring. Stirring was continued at room temperature for 1 hour, resulting in a light blue, homogeneous suspension. After aging for 12 hours, the resulting coprecipitate was centrifuged, washed with water, and dried in an 80°C water bath for 12 hours to obtain a K-containing iron precursor solid (structured similar to Prussian blue). The K / Fe mass ratio in this precursor was approximately 0.2, and ICP analysis revealed 45.0 wt% Fe and 9.0 wt% K.

[0107] S02: Introduction of the Ca additive: 5.0 g of the dried K-containing iron precursor solid was added to an appropriate amount of deionized water to form a slurry. 9.2 g of anhydrous calcium nitrate (Ca(NO3)2) was weighed and dissolved in a small amount of water at a Ca / Fe mass ratio of 1. This was slowly added to the precursor slurry and allowed to steep at room temperature for 2 hours with stirring. The solid was then evaporated under reduced pressure in an 80°C water bath to obtain a wet solid. This solid was then dried in a 120°C oven for 12 hours, ground, and sieved to obtain the Ca-modified FeK precursor (referred to as the FeKCa precursor).

[0108] S1: Nitridation: Place the FeKCa precursor sample in the center of a quartz tube furnace. First, heat it to 300°C under an argon flow at a rate of 7°C / min and hold it there for 1 hour to remove any surface residue. Then, switch to anhydrous ammonia (NH3) at a flow rate of 50 mL / min, continue heating at a rate of 6°C / min to 600°C, and hold it there for 5 hours to nitride the iron precursor.

[0109] S2: In-situ carbonization: The above-mentioned nitrided product was placed in a reactor and introduced into a H2 / CO mixed gas with a volume ratio of 1:1 (50 mL / min, space velocity of about 8000 h -1 ), the reaction pressure was 0.1 MPa, the temperature was raised to 380°C at a rate of 5°C / min and kept constant for 10 hours, and then naturally cooled to obtain the catalyst product, which was recorded as FeKCa catalyst.

[0110] Example 8

[0111] The present embodiment prepares an iron-based catalyst containing both Na and Mg.

[0112] A method for synthesizing an iron-based catalyst, comprising the following steps:

[0113] S0: Preparation of an iron-containing precursor:

[0114] S01: Preparation of an iron-based precursor containing Na additive by coprecipitation. 10.0 g of polyvinylpyrrolidone (PVP, K30) was dissolved in 200 mL of deionized water to form solution A, and 0.1 mol of sodium hexacyanoferrate(II) (Na4[Fe(CN)6]·3H2O) was added to form solution B. Solution B was slowly added to solution A under stirring, and stirring was continued at room temperature for 1 hour to form a light blue homogeneous suspension. After standing for 12 hours, the obtained coprecipitate was sequentially separated by centrifugation, washed with water, and dried in a water bath at 80°C for 12 hours to obtain an iron precursor containing Na solid (the structure is similar to Prussian blue, the mass ratio of Na / Fe in the precursor is about 0.07, and the actual content of Fe is 44.9wt% and the actual content of Na is 3.2wt% by ICP analysis).

[0115] S02: Introduction of Mg additive: 5.0 g of the dried iron precursor containing Na solid was taken and mixed with an appropriate amount of deionized water to form a slurry. 9.2 g of anhydrous calcium nitrate (Mg(NO3)2) was weighed according to a mass ratio of Mg / Fe 0.1 and dissolved in a small amount of water, and then slowly added to the precursor slurry, and stirred for 2 hours at room temperature. Subsequently, water was removed by evaporation under reduced pressure at 80°C water bath to obtain a wet solid. The wet solid was dried in an oven at 120°C for 12 hours, ground and sieved to obtain a Mg-modified FeNa precursor (denoted as FeNaMg precursor).

[0116] S1: Nitridation treatment: The FeNaMg precursor sample was placed in the center of the quartz tube reactor. First, the temperature was raised to 300°C at a rate of 7°C / min under argon flow and kept constant for 1 hour to remove surface residual substances. Then, anhydrous ammonia gas (NH3, flow rate 50 mL / min) was introduced, and the temperature was continued to rise to 600°C at a rate of 6°C / min and kept constant for 5 hours to realize the nitridation of the iron precursor.

[0117] S2: In-situ carbonization: The nitridation product was placed in the reactor, and a mixture of H2 / CO (volume ratio 1:1, 50 mL / min, space velocity about 8000 h -1 ) was introduced. The temperature was raised to 380°C at a rate of 5°C / min and kept constant for 10 hours, and then the catalyst product was obtained by natural cooling, denoted as FeNaMg catalyst.

[0118] See Figure 2 The Fe catalyst product obtained in Example 1 was mainly composed of the Fe3O4 crystal phase, with no obvious Fe7C3 crystal phase detected. This indicates that in the absence of a promoter, the iron catalyst is easily oxidized and it is difficult to stably form the Fe7C3 phase. The FeMg catalyst obtained in Example 2 lacked the Fe7C3 crystal phase and had a high content of the Fe3O4 oxide crystal phase, indicating that the introduction of the Mg promoter alone could not promote the conversion of the nitrided precursor to Fe7C3 and could not produce the Fe7C3 phase. (Although the Fe-K catalyst product obtained in Example 3 has a certain degree of Fe7C3 crystal phase, the content of Fe3O4 oxide crystal phase is relatively high. This shows that although the introduction of only the K additive can partially promote the conversion of the nitrided precursor to Fe7C3, it is not sufficient to effectively resist the oxidation caused by the water vapor generated during the reaction, and a high-purity Fe7C3 phase cannot be obtained. The FeKMg catalyst product obtained in Example 4 shows that the product has obvious Fe7C3 crystal phase characteristics (in accordance with the PDF#17-0333 standard card), and no obvious Fe3O4 peak is detected. The results show that the synergistic effect of K and Mg additives can effectively promote the conversion of Fe2N to Fe7C3 and inhibit oxidation, thereby achieving the preparation of a stable single Fe7C3 catalyst, showing excellent structural stability and antioxidant properties.

[0119] Test example:

[0120] 1. Catalytic performance evaluation:

[0121] (1) The iron-based catalysts prepared in Examples 1 to 6 were loaded into a fixed-bed reactor (stainless steel tube, inner diameter 10 mm) to evaluate their CO2 hydrogenation performance.

[0122] The above catalysts were respectively subjected to the same CO2 hydrogenation conditions (340 °C, 2 MPa, H2 / CO2=3:1, space velocity 20000 h -1 )reaction.

[0123] Figure 3 The reaction performance of the catalysts obtained in Examples 1 to 4 for catalytic CO2 hydrogenation in a CO2 / H2 carbonized atmosphere is shown in Table 1. The catalytic performance of the catalysts obtained in Examples 1 to 4 for CO2 conversion is shown in Table 1.

[0124] Table 1. Catalytic performance of CO2 conversion of catalysts in Examples 1-6

[0125] from Figure 3 As can be seen from Table 1, the product analysis after the 50-hour reaction showed that the CH4 selectivity of the Fe catalyst product obtained in Example 1 was as high as about 40%, and the C2 +The total selectivity of hydrocarbons (mainly olefins and paraffins) was only about 50%; the product distribution of the FeMg catalyst obtained in Example 2 was similar to that of the Fe catalyst, with no significant improvement; the CH4 selectivity of the FeK catalyst obtained in Example 3 dropped to about 25%, and the C2 + The olefin selectivity was significantly improved, reaching about 45%. The CH4 selectivity of the FeKMg catalyst obtained in Example 4 was the lowest, only about 10%, while the C2 + The olefin selectivity was increased to above 65%. The FeKMg2 and FeKMg3 catalysts obtained in Examples 5 and 6 had a relatively high Fe7C3 phase, and their olefin selectivity was approximately 60%.

[0126] These results demonstrate that the K promoter plays a key role in suppressing methane formation and improving olefin selectivity. While the Mg promoter itself has little effect on selectivity, its combination with K maintains the catalyst's carbide-based phase, indirectly ensuring sustained high selectivity. Therefore, the FeKMg catalyst of the present invention can operate for extended periods at high activity and selectivity, precisely due to the synergistic regulation of the K and Mg promoters.

[0127] (2) The catalytic performance of the FeKMg iron-based catalyst obtained in Example 4 was evaluated.

[0128] Evaluation conditions are: total pressure 2.0 MPa, feed H2 / CO2 ratio 3:1, gas space velocity 20000 mL·h -1 ·g -1 , reaction temperature 340℃; catalyst 0.10 g, 340℃, WHSV=6 L·g cat -1 ·h -1 .

[0129] After reaching stable operating conditions, the gas phase product composition was analyzed online every 50 hours, and the liquid phase / wax product was collected offline. After 400 hours of continuous operation, the results showed that the CO2 single-pass conversion rate decreased slightly from the initial approximately 41.5% to 40.7% (a decrease of <2%), and the C2 + The olefin selectivity always fluctuated within the range of 65–68%, with little significant change (e.g. Figure 4The conversion curve shown remains essentially stable over time. This demonstrates the exceptional stability and resistance of the FeKMg catalyst to water deactivation in high-pressure CO2 hydrogenation reactions. Comparative experiments show that, after 200 hours of operation under the same conditions, the CO2 conversion of the FeK catalyst without the Mg promoter decreased by over 15%, and the Fe3O4 content in the product significantly increased. This demonstrates that the introduction of the Mg promoter effectively delays catalyst deactivation caused by water. This example also demonstrates that the catalyst's combined carbon selectivity for C2–C4 light olefins (ethylene, propylene, and butene) can exceed 50%, significantly exceeding the less than 30% achieved by conventional iron-based catalysts. Figure 4 The 400-h stability evaluation fully verified the superiority of the in-situ synthesized Fe7C3 catalyst described in the present invention in the CO2 to olefin reaction.

[0130] In summary, the above examples fully demonstrate the effectiveness of the present invention. The present invention achieves efficient and stable catalytic performance of iron-based catalysts in the CO2 hydrogenation to olefins reaction by in-situ construction and stabilization of the Fe7C3 active phase in a CO2 / H2 atmosphere. The key lies in a reasonable combination of additives (K and Mg) and a step-by-step activation strategy (nitridation followed by carbonization). These innovations enable the highly active phase Fe7C3 to be generated and exist for a long time under traditionally harsh CO2 hydrogenation conditions. The method of the present invention is simple and easy to implement, and can be extended to other catalytic systems that require in-situ generation and stabilization of metal carbide phases in a reaction atmosphere. The method of the present invention can be used to prepare a nearly single-phase Fe7C3 catalyst in a continuous gas-solid reaction device, which is suitable for industrial scale-up. The prepared Fe7C3-KMg catalyst is used in the preparation of C2 from CO2 hydrogenation. + The catalyst exhibited an olefin selectivity of 67.1% and a CO conversion rate of 41.5% in olefin reactions, with activity decay of no more than 2% over 400 hours of stable operation, demonstrating excellent catalytic performance and stability. The preparation method provided by the present invention is simple and highly efficient. Furthermore, the method and resulting catalyst are also suitable for in-situ carbonization and CO hydrogenation reactions under a CO / H atmosphere.

[0131] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

[0132] Portions not described in detail in this specification belong to the common knowledge in the art. The above examples are provided for the purpose of illustrating the present invention only and are not intended to limit the scope of the present invention. The scope of the present invention is defined by the appended claims. Various equivalent substitutions and modifications made without departing from the spirit and principles of the present invention are intended to be encompassed within the scope of the present invention.

Claims

1. A method for synthesizing an iron-based catalyst, characterized in that: The steps include: S1: subjecting an iron-containing catalyst precursor to a high-temperature nitriding treatment in an ammonia atmosphere to obtain an activated precursor of iron-containing nitride Fe2N; wherein the iron-containing catalyst precursor contains an alkali metal element and an alkaline earth element; S2: placing the activated precursor in a mixed atmosphere for in-situ reaction treatment to convert the Fe2N into an iron-based catalyst in the Fe7C3 phase to obtain the Fe7C3 catalyst; wherein the mixed atmosphere is a mixed atmosphere of carbon source gas and H2, and the carbon source gas is a mixed gas of one or two of CO2 and CO.

2. The method for synthesizing the iron-based catalyst according to claim 1, wherein In step S2, the volume ratio of H2 to carbon source gas in the mixed atmosphere is 1:1–5:1, the reaction temperature is 300–380°C, the reaction pressure is 0.1–5 MPa, and the reaction time is 10–100 hours.

3. The method for synthesizing the iron-based catalyst according to claim 2, wherein In step S2, the volume ratio of H2 to carbon source gas in the mixed atmosphere is 3:1, the temperature is 340±20°C, the pressure is 1.5~2.5 MPa, and the treatment time is 50–100 hours.

4. The method for synthesizing the iron-based catalyst according to claim 1, wherein The high temperature nitriding treatment in step S1 is performed at a temperature of 400-600° C. and for a time of 0.5-5 hours.

5. The method for synthesizing the iron-based catalyst according to claim 1, wherein Placing the activated precursor in a mixed atmosphere for in-situ reaction treatment in step S2 is achieved by switching the ammonia atmosphere to a mixed atmosphere.

6. The method for synthesizing the iron-based catalyst according to claim 1, wherein The iron-containing catalyst precursor is a catalyst precursor with a Prussian blue-like structure prepared by co-precipitation of iron salt and hexacyanoferrate.

7. The method for synthesizing an iron-based catalyst according to claim 1, wherein The alkali metal element is selected from one or more of lithium, sodium, potassium, rubidium, and cesium, and the alkaline earth metal element is selected from one or more of magnesium, calcium, strontium, and barium; the mass ratio of the alkali metal to Fe is 0.01-0.5:1; the mass ratio of the alkaline earth metal to Fe is 0.01-1:

1.

8. The method for synthesizing the iron-based catalyst according to claim 7, wherein: The mass ratio of the alkali metal element to Fe is 0.1, and the mass ratio of the alkaline earth metal element to Fe is 0.

04.

9. An iron-based catalyst, characterized in that Prepared by the synthesis method of the iron-based catalyst according to any one of claims 1 to 8; the main crystalline phase of the iron-based catalyst is Fe7C3, and the relative content of the Fe7C3 phase is not less than 80 wt% under XRD detection; the Fe7C3 crystal phase in the iron-based catalyst has a hexagonal structure and a grain size of 20–100 nm.

10. Use of the iron-based catalyst according to claim 9 in CO2 hydrogenation and CO hydrogenation reactions.

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