Method for nitridation induction of Fe-Zn-Mn metal oxide catalyst to form single epsilon-iron carbide in CO2 hydrogenation reaction

Abstract

The invention provides a method for nitriding and inducing a Fe-Zn-Mn metal oxide catalyst to form single epsilon-iron carbide in a CO2 hydrogenation reaction, which is characterized in that a Fe-Zn-Mn metal oxide is used as a precursor, and the Fe-Zn-Mn oxide is successfully nitrided by adopting a hydrogen pre-reduction and ammonia nitridation method. The successfully nitrided catalyst is applied to a CO2 hydrogenation reaction, after a long time of reaction, the phase of the catalyst tends to be stable after reduction, carburization and a series of complex reactions in the reaction process, and under the combined action of addition of Zn and Mn assistants and nitridation, the successfully nitrided Fe-Zn-Mn catalyst is induced to form iron carbide only containing epsilon-Fe2C in the reaction. The catalyst disclosed by the invention is applied to CO2 hydrogenation reaction, has relatively high catalytic activity, and shows relatively high olefin selectivity and high stability.

Description

Technical Field

[0001] This invention relates to the field of catalysts for CO2 hydrogenation reactions, and more particularly to a method for forming a single ε-iron carbide in a nitridation-induced Fe-Zn-Mn metal oxide catalyst during the CO2 hydrogenation reaction. Background Technology

[0002] Since carbon dioxide emissions have caused a series of environmental problems, reducing carbon dioxide emissions and mitigating the greenhouse effect is imperative. CO2 hydrogenation can capture CO2 while simultaneously producing high-value-added basic chemical products, showing broad application prospects. However, because CO2 is a typical octet, its activation is relatively difficult, requiring a suitable catalyst to effectively activate CO2 while also improving the selectivity of the target product. CO2 hydrogenation can use reverse water-gas shift (RWGS) (reaction 1) as an intermediate reaction, and then, according to reaction (2), the carbon monoxide produced by RWGS is further hydrogenated in the Fischer-Tropsch synthesis (FTS) reaction:

[0003] CO2 + H2 → CO + H2O (Reaction 1)

[0004] nCO + 2nH₂ → (-CH₂-) n +nH2O Reaction (2)

[0005] Since the reverse water-gas shift reaction is endothermic, high temperatures are typically required for the efficient conversion of CO2 into CO. Iron catalysts, due to their excellent activity in the water-gas shift reaction and their applicability to medium- and high-temperature Fischer-Tropsch synthesis, are well-suited for CO2 hydrogenation. Optimizing the application of iron-based catalysts in CO2 hydrogenation and improving the selectivity of the target product is of significant research value.

[0006] Iron carbide is a typical interstitial compound in which carbon atoms occupy the interstitial spaces formed by the close packing of iron atoms. Based on the position of the carbon atoms in the crystal structure, they can be divided into triangular prism carbides (ε-Fe₂C and ε′-Fe). 2.2C) and octahedral carbides (θ-Fe3C, χ-Fe5C2, and Fe7C3). For iron-based catalysts for CO2 hydrogenation, iron carbide is generally considered to be the active phase in the CO2 hydrogenation reaction. However, in actual production, synthesizing pure-phase iron carbide is very difficult. Furthermore, even if pure-phase iron carbide is synthesized, it is difficult to maintain after the CO2 hydrogenation reaction because different types of iron carbide undergo phase transformations during the reaction. A 2010 article published in the top journal *Journal of the American Chemical Society* (JACS), titled "Stability and reactivity of ε-χ-θiron carbide catalyst phases in Fischer–Tropsch synthesis: Controlling μ," addresses this issue. C The paper presents a very systematic calculation and experiment on the formation conditions of various iron carbides, and the iron carbide transformation relationship diagram is shown in the figure below. Figure 1 As shown, the occurrence of the iron carbide phase transformation (ε–χ–θ phase transformation) depends on temperature and the H2 / CO ratio. Specifically, the high-temperature low-carbon chemical potential (μ...) C That is, a high H2 / CO ratio usually leads to the preferential formation of θ-Fe3C; conversely, a high μ-CO ratio leads to the preferential formation of θ-Fe3C. C Low H2 / CO ratios and moderate temperatures (~250℃) lead to the formation of χ-Fe5C2; ε-carbides, on the other hand, preferentially form at lower temperatures and with higher carbon chemical potentials μ. C The work also points out that although low temperatures favor the formation of ε-iron carbide, the carburizing efficiency at low temperatures is very low. Therefore, kinetic factors dominate the formation of ε-carbides. For example, iron catalysts with small crystallites or catalysts with added structural additives readily form ε-iron carbide. Furthermore, in actual CO2 hydrogenation reactions, changes in reaction conditions can lead to phase transformation of the iron carbide. Additionally, carbon dioxide and water in the CO2 hydrogenation process can cause partial oxidation of the catalyst. In conclusion, maintaining a single type of iron carbide in the catalyst during CO2 hydrogenation is extremely difficult.

[0007] Numerous studies have revealed the unique properties of ε / ε'-iron carbide in CO2 hydrogenation, such as lower CO and methane selectivity and higher olefin selectivity. Unlike χ-Fe5C2 and θ-Fe3C, ε / ε'-iron carbide exhibits relatively poor stability, making its synthesis more difficult. Furthermore, at typical CO2 hydrogenation temperatures (above 250°C), ε / ε'-iron carbide transforms into the more stable χ-Fe5C2 and θ-Fe3C. For iron-based catalysts, manganese and zinc promoters can modulate the catalyst structure to promote the formation of a portion of ε / ε'-iron carbide during CO2 hydrogenation. Additionally, since Fe2N shares the same crystal structure as ε / ε'-iron carbide, Fe2N may also promote its formation during the reaction. However, current research has shown that achieving the formation of a single ε / ε'-iron carbide during CO2 hydrogenation remains challenging.

[0008] In summary, current research can prepare single-phase ε / ε'-iron carbide, but it transforms into a mixture of χ-Fe5C2, θ-Fe3C, and Fe3O4 during the CO2 hydrogenation reaction, ultimately making it difficult to exert the excellent catalytic performance of ε / ε'-iron carbide. Summary of the Invention

[0009] In view of the problems existing in the prior art, the present invention provides a method for nitriding-induced formation of a single ε-iron carbide in the CO2 hydrogenation reaction of Fe-Zn-Mn metal oxide catalyst. The key to the present invention lies in selecting Fe-Zn-Mn metal oxide as a precursor, and successfully nitriding the Fe-Zn-Mn metal oxide after pre-reduction and ammonia nitridation. Under the combined action of the structural aids Zn and Mn and the nitriding treatment, the CO2 hydrogenation reaction can induce the formation of ε-iron carbide containing a single type and exhibit excellent olefin selectivity in the CO2 hydrogenation reaction.

[0010] To achieve this objective, the present invention adopts the following technical solution:

[0011] This invention provides a method for nitridation-induced formation of a single ε-iron carbide in the CO2 hydrogenation reaction of a Fe-Zn-Mn metal oxide catalyst, the method comprising the following steps:

[0012] (1) The Fe-Zn-Mn metal oxide precursor was pre-reduced in a hydrogen atmosphere;

[0013] (2) Nitrogenate the material obtained in step (1) in an ammonia atmosphere;

[0014] (3) The catalyst obtained in step (2) is applied to the CO2 hydrogenation reaction to obtain a catalyst containing only ε-iron carbide.

[0015] Preferably, the Fe-Zn-Mn metal oxide precursor in step (1) is prepared by any one of the following methods: precipitation, sol-gel, or hydrothermal method.

[0016] In this invention, the precipitation method, sol-gel method and hydrothermal method are all carried out using existing operating procedures, and Fe-Zn-Mn metal oxide precursors with a molar ratio of Fe, Zn and Mn of (1-3):1:(1-2) can be obtained.

[0017] Preferably, the molar ratio of Fe, Zn and Mn in the Fe-Zn-Mn metal oxide precursor in step (1) is (1-3):1:(1-2), for example, it can be 1:1:1, 1.2:1:1.2, 1.5:1:1.5, 1.8:1:1.7, 2:1:1.9, 2.5:1:2 or 3:1:2, etc., but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0018] Preferably, the pre-reduction temperature in step (1) is 400 to 500°C, for example, it can be 400°C, 420°C, 440°C, 450°C, 480°C, 490°C or 500°C, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0019] Preferably, the pre-reduction time is 3 to 6 hours, for example, it can be 3 hours, 3.5 hours, 4 hours, 5 hours, 5.5 hours or 6 hours, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0020] The preferred temperature for the pre-reduction of this invention is 400-500℃, and the pre-reduction time is 3-6h, to ensure that the Fe-Zn-Mn metal oxide is fully reduced.

[0021] Preferably, the nitriding temperature in step (2) is 400 to 500°C, for example, it can be 400°C, 420°C, 440°C, 450°C, 480°C, 490°C or 500°C, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0022] Preferably, the nitriding time is 3 to 6 hours, for example, 3 hours, 3.5 hours, 4 hours, 5 hours, 5.5 hours or 6 hours, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0023] The preferred nitriding temperature of this invention is 400–500°C, and the nitriding time is 3–6 h, to ensure that the Fe-Zn-Mn metal oxide precursor after reduction is fully nitrided.

[0024] Preferably, the temperature of the CO2 hydrogenation reaction in step (3) is 260 to 320°C, for example, it can be 260°C, 270°C, 280°C, 300°C, 320°C, etc., but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0025] Preferably, the pressure of the CO2 hydrogenation reaction in step (3) is 2.3 to 4.0 MPa, for example, it can be 2.3 MPa, 2.4 MPa, 2.6 MPa, 2.8 MPa, 3.0 MPa or 4.0 MPa, but it is not limited to the listed values. Other unlisted values ​​within this range are also applicable.

[0026] Compared with the prior art, the present invention has at least the following beneficial effects:

[0027] (1) The required raw materials are simple, readily available, and inexpensive. The Fe-Zn-Mn metal oxide precursor can be synthesized using mature precipitation, sol-gel, or hydrothermal methods. Specifically, Fe-Zn-Mn metal oxide is chosen because the addition of Zn and Mn promoters to the Fe-based catalyst can significantly reduce the grain size, which is beneficial for further hydrogen reduction and ammonia nitriding. Most importantly, the addition of Zn and Mn promoters can promote the formation of ε-iron carbide during the CO2 hydrogenation reaction by modifying the catalyst structure and reducing the grain size. Nitriding treatment can further induce the formation of ε-iron carbide in the Fe-Zn-Mn catalyst and inhibit the formation of other types of iron carbide.

[0028] (2) The reduction and nitriding steps are simple, and both are common gases in chemical industry. Moreover, the preparation process can use the same reactor as the CO2 hydrogenation reaction. Therefore, no additional active phase carbide preparation reaction device is required. The whole process only needs to be carried out in the same reactor for reduction, nitriding and CO2 hydrogenation reaction.

[0029] (3) After CO2 hydrogenation, it contains only a single type of ε-iron carbide, making it suitable for high-temperature and high-pressure (e.g., temperature 260–320℃, pressure 2.3–4.0 MPa) continuous reactors. The reaction stability is extremely high, breaking the traditional literature theory that "at higher carbon chemical potential μ C The theoretical and technical barrier of "ε / ε' iron carbide needs to exist stably under mild conditions below 200℃" is overcome. At the same time, under industrial CO2 hydrogenation reaction conditions, it can exhibit high CO2 conversion rate and high olefin selectivity after a long reaction time (200-300h). Attached Figure Description

[0030] Figure 1 This is a diagram showing the conversion relationship of iron carbide in the background art of this invention;

[0031] Figure 2 This is the XRD pattern of the catalyst after the CO2 hydrogenation reaction in Example 1;

[0032] Figure 3 This is the Mössbauer spectrum of the catalyst before the CO2 hydrogenation reaction in Example 1;

[0033] Figure 4 This is the Mössbauer spectrum of the catalyst after the CO2 hydrogenation reaction in Example 1;

[0034] Figure 5 The Mössbauer spectrum of the catalyst before the CO2 hydrogenation reaction in Comparative Example 1 is shown.

[0035] Figure 6 The Mössbauer spectrum of the catalyst after the CO2 hydrogenation reaction in Comparative Example 1 is shown.

[0036] Figure 7 The Mössbauer spectrum of the catalyst before the CO2 hydrogenation reaction in Comparative Example 2 is shown.

[0037] Figure 8 The Mössbauer spectrum of the catalyst after the CO2 hydrogenation reaction in Comparative Example 2 is shown.

[0038] Figure 9 The Mössbauer spectrum of the catalyst before the CO2 hydrogenation reaction in Comparative Example 3 is shown.

[0039] Figure 10 The Mössbauer spectrum of the catalyst after the CO2 hydrogenation reaction in Comparative Example 3 is shown.

[0040] Figure 11 The images show the XRD patterns of the catalyst before and after the CO2 hydrogenation reaction in Comparative Example 4. Detailed Implementation

[0041] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0042] The present invention will now be described in further detail. However, the examples described below are merely simplified examples of the present invention and do not represent or limit the scope of protection of the present invention. The scope of protection of the present invention is determined by the claims.

[0043] Example 1

[0044] 0.5 g of Fe-Zn-Mn metal oxide was placed in a fixed-bed reactor with a Fe:Zn:Mn ratio of 2:1:1. Hydrogen was first introduced to raise the temperature to 450°C at a rate of 5°C / min, and the temperature of the reaction system was maintained for 240 minutes. Then, nitrogen was introduced to purge the system and the temperature was lowered to 400°C before switching to ammonia. The temperature of the reaction system was maintained for 240 minutes. Then, nitrogen was introduced again to purge the system and the temperature was lowered to the reaction temperature of 280°C. The pressure was increased to 3.0 MPa, and then CO2 and hydrogen were introduced to carry out the reaction. After 216 hours of reaction, the reaction system was cooled to room temperature and the catalyst particles were collected.

[0045] The catalyst particles before and after the CO2 hydrogenation reaction in Example 1 above were characterized:

[0046] Figure 2 The image shows the XRD pattern of the catalyst after the CO2 hydrogenation reaction in Example 1. Figure 2 As shown, the diffraction peaks of the obtained catalyst particles after the reaction are consistent with Fe2N and ε-Fe2C, while the remaining diffraction peaks are located between MnO and FeO, indicating that the corresponding solid solution Mn x Fe 1-x Since Fe₂N and ε / ε' iron carbide have the same crystal structure and cannot be distinguished by powder X-ray diffraction, we performed Mössbauer spectroscopy on the catalyst particles after the reaction. Figure 3 and Figure 4 The images show the Mössbauer spectra of the catalyst before and after the CO2 hydrogenation reaction in Example 1. The comparison shows that the catalyst after the CO2 hydrogenation reaction contains only one set of six-line spectra, which, according to the Mössbauer parameters, corresponds to ε-Fe2C. One set of two-line spectra corresponds to Fe2N, and the other two sets correspond to Mn. x Fe 1-x O and superparamagnetic ions.

[0047] Example 2

[0048] 0.5 g of Fe-Zn-Mn metal oxide was placed in a fixed-bed reactor with a Fe:Zn:Mn ratio of 3:1:1. Hydrogen was first introduced to raise the temperature to 450°C at a rate of 5°C / min, and the temperature of the reaction system was maintained for 240 minutes. Then, nitrogen was introduced to purge the system and the temperature was lowered to 400°C before switching to ammonia. The temperature of the reaction system was maintained for 240 minutes. Then, nitrogen was introduced again to purge the system and the temperature was lowered to the reaction temperature of 280°C. The pressure was increased to 3.0 MPa, and then CO2 and hydrogen were introduced for the reaction. After 216 hours of reaction, the reaction system was cooled to room temperature and the catalyst particles were collected.

[0049] Example 3

[0050] 0.5 g of Fe-Zn-Mn metal oxide was placed in a fixed-bed reactor with a Fe:Zn:Mn ratio of 1:1:1. Hydrogen was first introduced to raise the temperature to 450°C at a rate of 5°C / min, and the temperature of the reaction system was maintained for 240 minutes. Then, nitrogen was introduced to purge the system and the temperature was lowered to 400°C before switching to ammonia. The temperature of the reaction system was maintained for 240 minutes. Then, nitrogen was introduced again to purge the system and the temperature was lowered to the reaction temperature of 280°C. The pressure was increased to 3.0 MPa, and then CO2 and hydrogen were introduced for the reaction. After 216 hours of reaction, the reaction system was cooled to room temperature and the catalyst particles were collected.

[0051] Example 4

[0052] 0.5 g of Fe-Zn-Mn metal oxide was placed in a fixed-bed reactor with a Fe:Zn:Mn ratio of 2:1:1. Hydrogen was first introduced to raise the temperature to 500°C at a rate of 5°C / min, and the temperature of the reaction system was maintained for 240 minutes. Then, nitrogen was introduced to purge the system and the temperature was lowered to 400°C before switching to ammonia. The temperature of the reaction system was maintained for 240 minutes. Then, nitrogen was introduced again to purge the system and the temperature was lowered to the reaction temperature of 300°C. The pressure was increased to 3.0 MPa, and then CO2 and hydrogen were introduced to carry out the reaction. After 216 hours of reaction, the reaction system was cooled to room temperature and the catalyst particles were collected.

[0053] Example 5

[0054] 0.5 g of Fe-Zn-Mn metal oxide was placed in a fixed-bed reactor with a Fe:Zn:Mn ratio of 2:1:1. Hydrogen was first introduced to raise the temperature to 450°C at a rate of 5°C / min, and the temperature of the reaction system was maintained for 300 minutes. Then, nitrogen was introduced to purge the system and the temperature was lowered to 400°C before switching to ammonia. The temperature of the reaction system was maintained for 300 minutes. Then, nitrogen was introduced again to purge the system and the temperature was lowered to the reaction temperature of 290°C. The pressure was increased to 3.0 MPa, and then CO2 and hydrogen were introduced for the reaction. After 216 hours of reaction, the reaction system was cooled to room temperature and the catalyst particles were collected.

[0055] Example 6

[0056] 0.5 g of Fe-Zn-Mn metal oxide was placed in a fixed-bed reactor with a Fe:Zn:Mn ratio of 2:1:1. Hydrogen was first introduced and the temperature was raised to 450 °C at a rate of 5 °C / min. The temperature of the reaction system was maintained at this temperature for 240 minutes. Then, nitrogen was introduced to purge the system and the temperature was lowered to 400 °C. Ammonia was then introduced and the temperature of the reaction system was maintained at this temperature for 240 minutes. Nitrogen was then introduced again to purge the system and the temperature was lowered to the reaction temperature of 320 °C. The pressure was increased to 4.0 MPa and then CO2 and hydrogen were introduced to carry out the reaction. After 216 hours of reaction, the reaction system was cooled to room temperature and the catalyst particles were collected.

[0057] The method provided by this invention for nitriding-induced Fe-Zn-Mn metal oxide catalysts to form single ε-iron carbide in CO2 hydrogenation reaction can successfully nitrid Fe-Zn-Mn metal oxide after hydrogen pre-reduction and ammonia nitridation. After CO2 hydrogenation reaction, it induces the formation of ε-iron carbide containing a single type. The single-pass CO2 conversion rate in the CO2 hydrogenation reaction is more than 25%, and the olefin selectivity is 60% to 70%.

[0058] Comparative Example 1

[0059] 0.5 g of Fe-Zn-Mn metal oxide was placed in a fixed-bed reactor with a Fe:Zn:Mn ratio of 2:1:1. Hydrogen gas was introduced and the temperature was increased to 450 °C at a rate of 5 °C / min. The temperature of the reaction system was maintained at this temperature for 240 minutes. Then, nitrogen gas was switched to purge and the temperature was lowered to the reaction temperature of 280 °C. The pressure was increased to 3.0 MPa, and then CO2 and hydrogen gas were switched to react. After 216 hours of reaction, the reaction system was cooled to room temperature and the catalyst particles were collected.

[0060] The catalyst particles before and after the reaction in Comparative Example 1 were characterized as follows:

[0061] Figure 5 and Figure 6 The images show the Mössbauer spectra of the catalyst before and after the CO2 hydrogenation reaction in Comparative Example 1. Figure 5 As shown, the unnitrided Fe-Zn-Mn catalyst is reduced to α-Fe and Mn before the reaction. x Fe 1-x Mixtures of O; such as Figure 6 As shown, after the CO2 hydrogenation reaction, it transforms into a mixture of Fe3O4, χ-Fe5C2, ε-Fe2C, and superparamagnetic ions. This indicates that the addition of Zn and Mn additives induces the formation of some ε-Fe2C during the CO2 hydrogenation reaction, but cannot simultaneously prevent the formation of χ-Fe5C2.

[0062] Comparative Example 2

[0063] 0.5 g of Fe-Zn-Mn metal oxide was placed in a fixed-bed reactor with a Fe:Zn:Mn ratio of 3:1:1. Ammonia gas was introduced and the temperature was increased to 400 °C at a rate of 5 °C / min. The temperature of the reaction system was maintained for 240 minutes. Then, nitrogen gas was used to purge and cool the system to the CO2 hydrogenation reaction temperature of 280 °C. The pressure was increased to 3.0 MPa, and then CO2 and hydrogen gas were switched to carry out the CO2 hydrogenation reaction. After 216 hours of reaction, the reaction system was cooled to room temperature and the catalyst particles were collected.

[0064] The catalyst particles before and after the reaction in Comparative Example 2 were characterized as follows:

[0065] Figure 7 and Figure 8 The figures show Mössbauer spectra of the catalyst before and after the CO2 hydrogenation reaction in Comparative Example 2. It can be seen that the Fe-Zn-Mn catalyst treated directly with ammonia is zinc manganese iron spinel both before and after participating in the CO2 hydrogenation reaction. Fe-Zn-Mn oxides without hydrogen pre-reduction cannot be directly nitrided by ammonia, and therefore cannot induce the formation of ε-iron carbide in the CO2 hydrogenation reaction. This indicates that the nitriding of Fe-Zn-Mn oxides requires pre-reduction before nitriding with ammonia. This is because ammonia has a weak reducing power, and direct ammonia treatment is insufficient to remove oxygen atoms from the system. Therefore, hydrogen pre-reduction removes oxygen atoms, thereby reducing the resistance to ammonia nitriding.

[0066] Comparative Example 3

[0067] 0.5 g of Fe-Zn metal oxide was placed in a fixed-bed reactor with a Fe:Zn ratio of 1:1. Hydrogen was introduced and the temperature was increased to 450 °C at a rate of 5 °C / min. The temperature of the reaction system was maintained for 240 minutes. Then, nitrogen was introduced to purge the system and the temperature was lowered to 400 °C. Ammonia was then introduced and the temperature of the reaction system was maintained for 240 minutes. Then, nitrogen was introduced again to purge the system and the temperature was lowered to the reaction temperature of 280 °C. The pressure was increased to 3.0 MPa and CO2 and hydrogen were then introduced to carry out the reaction. After 216 hours of reaction, the reaction system was cooled to room temperature and the catalyst particles were collected.

[0068] The catalyst particles before and after the reaction in Comparative Example 3 were characterized as follows:

[0069] Figure 9 and Figure 10 The images show the Mössbauer spectra of the catalyst before and after the CO2 hydrogenation reaction in Comparative Example 3. Figure 9 As shown, the Fe-Zn catalyst with a Fe:Zn ratio of 1:1 is converted into Fe2N and Zn before the reaction. x Fe 1-x Mixtures of O; such as Figure 10 As shown, after the CO2 hydrogenation reaction, it transforms into a mixture of Fe3O4, χ-Fe5C2, and ε-Fe2C. This indicates that when the catalyst does not contain Mn, it can be successfully nitrided through the treatment of hydrogen and ammonia. Although some ε-Fe2C can be induced during the CO2 hydrogenation reaction, the formation of χ-Fe5C2 and Fe3O4 cannot be prevented.

[0070] Comparative Example 4

[0071] 0.5 g of Fe-Zn-Mn metal oxide was placed in a fixed-bed reactor with a Fe:Zn:Mn ratio of 2:1:1. Hydrogen gas was introduced and the temperature was increased to 300 °C at a rate of 5 °C / min. The temperature of the reaction system was maintained for 240 minutes. Then, nitrogen gas was introduced to purge the system and the temperature was lowered to 400 °C. Ammonia gas was then introduced and the temperature of the reaction system was maintained for 240 minutes. Then, nitrogen gas was introduced again to purge the system and the temperature was lowered to the reaction temperature of 300 °C. The pressure was increased to 3.0 MPa and CO2 and hydrogen gas were then introduced to carry out the reaction. After 216 hours of reaction, the reaction system was cooled to room temperature and the catalyst particles were collected.

[0072] The catalyst particles in Comparative Example 4 before and after the reaction were characterized as follows:

[0073] Figure 11 The image shows the XRD pattern of the catalyst after the CO2 hydrogenation reaction in Comparative Example 4. Figure 11 As shown, the Fe-Zn-Mn catalyst with a Fe:Zn:Mn ratio of 2:1:1, after reduction with hydrogen at 300℃ followed by ammonia, failed to successfully nitride the Fe-Zn-Mn catalyst, retaining its spinel structure. Furthermore, subsequent CO2 hydrogenation did not induce the formation of ε-Fe₂C. This indicates that the Fe-Zn-Mn oxide precursor can only guarantee successful subsequent nitride if it is fully reduced in hydrogen.

[0074] The percentage results of each phase composition in the catalysts of Examples 1 and Comparative Examples 1-3 are shown in Table 1.

[0075] Table 1

[0076]

[0077] Note: Spinel represents Fe3O4 or (Mn) x Fe 1-x )3O4; spm Fe is a superparamagnetic ion; "-" indicates that this phase was not detected.

[0078] As can be seen from Table 1, the catalyst of Example 1 can obtain a single ε-iron carbide phase after CO2 hydrogenation reaction, while the catalysts of Comparative Examples 1 to 3 cannot obtain a single ε-iron carbide phase after CO2 hydrogenation reaction.

[0079] The reaction performance results of the catalysts in Examples 1-6 and Comparative Examples 1-3 are shown in Table 2.

[0080] Table 2

[0081]

[0082]

[0083] As can be seen from Table 2, the CO2 conversion rate of the catalyst in Example 1 was 26.22%, and the olefin selectivity was 70.30%; the olefin selectivity of the catalysts in Comparative Examples 1 to 3 was lower than that of the Example.

[0084] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for forming a single ε-iron carbide in the CO2 hydrogenation reaction by nitriding-induced Fe-Zn-Mn metal oxide catalyst, characterized in that, The method includes the following steps: (1) The Fe-Zn-Mn metal oxide precursor was pre-reduced in a hydrogen atmosphere; (2) Nitrogenate the material obtained in step (1) in an ammonia atmosphere; (3) The catalyst obtained in step (2) is applied to the CO2 hydrogenation reaction to obtain a catalyst containing only ε-iron carbide.

2. The method according to claim 1, characterized in that, The Fe-Zn-Mn metal oxide precursor described in step (1) is prepared by any one of the following methods: precipitation, sol-gel, or hydrothermal method.

3. The method according to claim 1 or 2, characterized in that, The molar ratio of Fe, Zn and Mn in the Fe-Zn-Mn metal oxide precursor in step (1) is (1-3):1:(1-2).

4. The method according to any one of claims 1 to 3, characterized in that, The pre-reduction temperature in step (1) is 400-500℃.

5. The method according to any one of claims 1 to 4, characterized in that, The pre-reduction time in step (1) is 3 to 6 hours.

6. The method according to any one of claims 1 to 5, characterized in that, The nitriding temperature in step (2) is 400–500 °C.

7. The method according to any one of claims 1 to 6, characterized in that, The nitriding time in step (2) is 3 to 6 hours.

8. The method according to any one of claims 1 to 7, characterized in that, The temperature of the CO2 hydrogenation reaction in step (3) is 260-320℃, and the pressure of the reaction is 2.3-4.0 MPa.

9. The method according to any one of claims 1 to 8, characterized in that, The CO2 hydrogenation reaction in step (3) takes 200-300 hours, and the reactivity and selectivity remain stable within the 200-hour reaction time.

10. The method according to any one of claims 1 to 9, characterized in that, In step (3), the single-pass CO2 conversion rate in the CO2 hydrogenation reaction is over 25%, and the olefin selectivity is 60% to 70%.

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