An in-situ formed FeO x Fe3C catalyst, its preparation method and application

Abstract

The application relates to the technical field of catalyst preparation, in particular to an in-situ formed FeO x / Fe3C catalyst as well as a preparation method and application thereof. The FeO x / Fe3C catalyst provided by the application is a heterogeneous composite structure composed of surface iron oxide (FeO x ) and bulk iron carbide (Fe3C), breaks through the limitation of only focusing on a single phase, systematically reveals the dynamic structure evolution law from the surface to the bulk, and clarifies the internal correlation with the reaction mechanism. Based on the synergistic circulation mechanism from the surface to the bulk, the FeO x / Fe3C catalyst exhibits excellent catalytic performance and stability in the reverse water gas shift reaction, which is superior to most of the catalyst systems reported at present.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of catalyst preparation, and particularly relates to an in-situ formed FeO x / Fe3C catalyst, a preparation method and application thereof. BACKGROUND

[0002] The information disclosed in the background of the present application is only intended to increase the understanding of the overall background of the present application, and should not necessarily be regarded as acknowledging or implying in any form that the information constitutes prior art known to those skilled in the art.

[0003] To cope with climate change and promote the green transformation of industry, it is of great strategic significance to develop economic and sustainable CO2 utilization technology. The reverse water gas shift (RWSG) reaction is a key step in activating inert CO2 molecules into a key intermediate CO. The generated CO can be used as an important component of synthesis gas, which can be further processed by the Fischer-Tropsch synthesis process to produce various high-value chemicals and liquid fuels, which is crucial for the resource utilization of CO2. However, the large-scale industrial application of this reaction has long been hindered by the bottleneck of developing high-performance and economical catalysts. Supported platinum, ruthenium, palladium and other active metal type catalysts have become the main catalytic materials for catalyzing the RWGS reaction because they can form active interfaces between active metals and supports, thus exhibiting active catalytic properties. However, the catalytic properties of the active interface are influenced by both the active metal and the support, which makes the agglomeration of the active metal and the sintering of the support lead to the rapid deactivation of the catalytic interface. Especially for high-temperature RWGS reactions, the harsh reaction conditions (high temperature, strong reducing atmosphere) are more likely to cause the destruction of the active interface structure. In addition, supported active metal type catalysts require the participation of active metals, which makes the preparation process of the catalyst more complex and costly.

[0004] Based on the outstanding advantages of abundant resources, low cost and environmental friendliness, iron-based catalysts have become the most potential candidate materials to replace noble metals. The unique electronic structure of iron enables it to effectively activate the inert C=O bond and promote the dissociation of H2 molecules, which lays a theoretical foundation for its RWGS application. However, in the real CO2 hydrogenation reaction environment, iron-based catalysts exhibit much more complexity than traditional cognition. The competitive adsorption and reaction of reactant molecules (CO2 and H2) on the catalyst surface can induce continuous and dynamic structural reorganization of the catalyst surface and even bulk phase. This dynamic reconstruction process leads to complex transformation between carbonized state (such as Fe3C, Fe5C2), oxidized state and metallic state of iron phase, making it difficult for traditional and initial or steady-state structure-based "structure-activity" relationship research methods to accurately capture the real active site, which seriously hinders the clear understanding of its catalytic mechanism and the directional design of high-performance catalysts.

[0005] Current researches mainly focus on the macro-morphology evolution of catalyst or the isolated effect of bulk phase / surface phase, and the structural differences between the surface phase and the bulk phase of the catalyst under reaction conditions and the synergistic effect thereof are generally ignored. The dynamic heterogeneous structure formed by the synergistic effect of the surface phase and the bulk phase is likely to be the key to regulating the activation path of the reaction molecules and the final performance. SUMMARY

[0006] Therefore, the application provides an in-situ formed FeO x / Fe3C catalyst and a preparation method and application thereof. x The FeO x / Fe3C catalyst provided by the application exhibits excellent catalytic performance and stability in the reverse water gas shift reaction based on the synergistic cycle mechanism from the surface to the bulk phase.

[0007] To achieve the above-mentioned purpose, the application is implemented by the following technical solutions:

[0008] In a first aspect, the application provides an in-situ formed FeO x / Fe3C catalyst, which is a spherical particle with uniform size. x The FeO x / Fe3C catalyst particle is a heterogeneous composite structure composed of a surface iron oxide (FeO x x) and a bulk phase carbonized iron (Fe3C).

[0009] In the formula, 1 ≤ x ≤ 1.5.

[0010] Further, the particle size of the FeO x / Fe3C catalyst is 60-220 nm.

[0011] The FeO x / Fe3C catalyst provided by the application is a dynamic heterogeneous structure formed by the synergistic effect of the surface phase and the bulk phase. The in-situ formed dynamic FeO x / Fe3C heterogeneous structure can realize efficient exchange between carbon, oxygen atoms and reactants as a catalytically active phase. x On the disordered FeO x (1 ≤ x ≤ 1.5) surface, CO2 is dissociated into gaseous CO and surface O; the carbon atoms generated by the further decomposition of part of CO penetrate into the Fe3C lattice. At the same time, the C atoms in the Fe3C bulk phase can migrate to the FeO x surface and combine with the surface O to generate CO, so that the Fe3C acts as a carbon atom storage bank. In this process, H2 can effectively remove the residual oxygen on the surface to prevent the catalyst from being excessively oxidized.

[0012] In a second aspect, the application provides the FeO xA preparation method of / Fe3C catalyst comprises the following steps:

[0013] (1) Fe2O3 powder is subjected to nitriding treatment under a pure ammonia atmosphere to prepare a Fe2N precursor; after cooling, the Fe2N precursor is subjected to passivation treatment using an O2 / Ar gas stream;

[0014] (2) The Fe2N precursor is heated to 300±10 ℃, 400±10 ℃, 500±10 ℃ and 600±10 ℃ under a mixed atmosphere of CO2 and H2, and maintained at each temperature for 0.5-1.5 h, to obtain a FeO x / Fe3C catalyst.

[0015] Further, in step (1), the pure ammonia gas flow rate is 40-60 mL / min; preferably 50 mL / min.

[0016] Further, in step (1), the nitriding treatment temperature is 630-680 ℃; the nitriding treatment time is 3-5 h; and the heating rate is 3-8 ℃ / min; preferably, the nitriding treatment temperature is 650 ℃; the nitriding treatment time is 4 h; and the heating rate is 5 ℃ / min.

[0017] Further, the Fe2O3 powder is prepared by calcining iron nitrate powder at a heating rate of 3-8 ℃ / min at 380-420 ℃ for 3-5 h; preferably, the iron nitrate powder is calcined at a heating rate of 5 ℃ / min at 3400 ℃ for 4 h.

[0018] Further, the passivation treatment is specifically performed as follows: the prepared Fe2N precursor is passivated using an O2 / Ar gas stream with a concentration of 1-2% for 0.5-1 h after cooling to room temperature; preferably, the prepared Fe2N precursor is passivated using an O2 / Ar gas stream with a concentration of 1% for 0.5 h after cooling to room temperature. The passivation treatment can effectively prevent self-combustion of the Fe2N precursor in air.

[0019] Further, in step (2), the volume ratio of CO2 to H2 is 2-4:1, preferably 3:1.

[0020] Further, in step (2), the total gas flow rate of the mixed atmosphere of CO2 and H2 is 60-70 mL / min.

[0021] Under the RWGS reaction conditions, the Fe2N precursor undergoes dynamic structural evolution, and finally forms a heterogeneous composite structure composed of a surface iron oxide (FeO x ) and a bulk phase carbonized iron (Fe3C).

[0022] In a third aspect, the present application provides the FeOx Application of Fe3C catalyst in reverse water-gas shift reaction.

[0023] FeO x The Fe3C heterostructure catalyst exhibits high activity, high selectivity, wide operating atmosphere ratio, and excellent cycling and long-term stability in the reverse water-gas shift reaction. It demonstrates excellent catalytic performance in the reverse water-gas shift reaction at 600 °C, achieving a CO formation rate of 1.1 × 10⁻⁶. -2 mol·g -1 ·s -1 It is superior to most catalyst systems reported in the past.

[0024] Compared with the prior art, the present invention has achieved the following beneficial effects:

[0025] The FeO provided by this invention x / Fe3C catalyst is generated in situ from Fe2N precursor under RWGS reaction conditions. In disordered FeO x (1 ≤ x ≤ 1.5) On the surface, CO2 dissociates into gaseous CO and surface O; some CO further decomposes, producing carbon atoms that diffuse into the Fe3C lattice. C atoms in the Fe3C bulk phase can migrate to FeO. x On the surface, Fe3C combines with surface O to generate CO, thus serving as a carbon atom reservoir. During this process, H2 effectively removes residual oxygen from the surface, preventing excessive oxidation of the catalyst. Based on this synergistic recycling mechanism from the surface to the bulk phase, FeO... x The Fe3C catalyst exhibits high activity, high selectivity, and excellent long-term stability in the reverse water-gas shift reaction, with overall performance far exceeding that of conventional iron-based catalysts. Furthermore, compared to active metals such as platinum, ruthenium, and palladium, this catalyst significantly reduces costs, demonstrating tremendous potential for industrial applications. Attached Figure Description

[0026] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0027] Figure 1 The images show TEM and HAADF-STEM images of the Fe2N precursor; where (a) is the TEM image and (b) is the HAADF-STEM image.

[0028] Figure 2 Fe2N precursor and prepared FeO x XRD pattern and Raman spectrum of Fe3C catalyst; where (a) is the XRD pattern and (b) is the Raman spectrum;

[0029] Figure 3 FeO x HRTEM and HAADF-STEM images of the Fe3C catalyst; where (a) is the HRTEM image and (b) is the HAADF-STEM image.

[0030] Figure 4 FeO x XPS image of Fe3C catalyst during RWGS reaction;

[0031] Figure 5 FeO x RWGS activity evaluation diagram of Fe3C catalyst; where (a) is FeO x / Plot of CO2 conversion rate of Fe3C catalyst at different temperatures; (b) Plot of FeO x / Graph of CO selectivity test of Fe3C catalyst at different temperatures;

[0032] Figure 6 FeO x / Stability evaluation diagram of Fe3C catalyst. Detailed Implementation

[0033] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0034] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0035] Example 1

[0036] (1) Catalyst preparation method

[0037] 2 g of ferric nitrate was calcined in a muffle furnace at 400 °C for 4 h at a heating rate of 5 °C / min to obtain Fe2O3 powder.

[0038] 200 mg of Fe2O3 powder was placed in a quartz tube and nitrided at 650 °C for 4 h under a pure ammonia gas flow (50 mL / min), with the heating rate also being 5 °C / min. After cooling to room temperature, the prepared Fe2N was passivated with a 1% O2 / Ar gas flow for 0.5 h.

[0039] 20 mg of the prepared Fe₂N precursor was placed in a quartz tube, and a mixture of 23% CO₂ / 69% H₂ / 8% N₂ (CO₂:H₂ = 3:1) was introduced at a total gas flow rate of 66.67 mL / min. The temperature was then gradually increased to 300 °C, 400 °C, 500 °C, and 600 °C under this atmosphere, and maintained at each temperature for 1 h to obtain FeO. x / Fe3C catalyst.

[0040] (2) Catalyst performance test

[0041] The catalytic performance of the catalyst was evaluated in a fixed-bed reactor using a mixed gas containing 23% CO2 / 69% H2 / 8% N2 (66.7 mL / min) at atmospheric pressure. 20 mg of catalyst powder was mixed with 500 mg of inert SiO2 and packed into a quartz tube. Two rounds of testing were conducted using an RWGS reaction environment ranging from 300 °C to 600 °C. At each test temperature, the RWGS reaction was stabilized for 60 min before analyzing the gaseous products. The gaseous products were analyzed using an online gas chromatograph equipped with a thermal conductivity detector (TCD). The carbon dioxide conversion, carbon monoxide selectivity, and carbon balance were calculated using the following formulas:

[0042] (1)

[0043] (2)

[0044] (3)

[0045] in This indicates the amount of CO2 in the intake air, while , , These represent the amounts of CO, CO2, and CH4 in the gas, respectively. Used to test the apparent activation energy (…). E a The reactor used was the same as the one used to evaluate the activity. To obtain accurate kinetic data, the catalyst had to undergo a round of reactant gas treatment within the range of 300 °C to 600 °C. In the kinetic tests, the reaction rate data of the catalyst was obtained by varying the gas flow rate while maintaining the CO2 conversion within the range of 5% to 15%. The equations for the total gas hourly space velocity (GHSV) and the reaction rate are as follows:

[0046] (4)

[0047] (5)

[0048] in, Ftot denoted as the total flow rate of the feed gas (mL / h), and m as the mass of the catalyst (g). The flow rate of CO2 is expressed in mol / s. X The conversion rate of CO2 S C represents the selectivity of carbon dioxide, and C represents the carbon balance.

[0049] Results and Discussion

[0050] Figure 1 The images show TEM and HAADF-STEM images of the Fe2N precursor; where (a) is the TEM image and (b) is the HAADF-STEM image. Figure 1 As shown, the Fe2N precursor catalyst was obtained by high-temperature nitriding at 650 °C under a pure NH3 atmosphere, followed by passivation in a 1% O2 / Ar mixed gas. This precursor exhibits an irregular shape with dimensions of several hundred nanometers. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images revealed a clearly visible disordered surface layer of approximately 2.8 nm thickness on the catalyst surface. In the bulk region, the interplanar spacing was measured to be 0.22 nm, consistent with the (002) crystal plane of Fe2N, indicating a well-crystallized Fe2N structure in the bulk phase.

[0051] Figure 2 Fe2N precursor and prepared FeO x XRD pattern and Raman spectrum of Fe3C catalyst; where (a) is the XRD pattern and (b) is the Raman spectrum. Figure 2 As shown, XRD characterization revealed that the diffraction peaks of the catalyst before the reaction were consistent with the standard card for Fe₂N. Raman spectroscopy analysis indicated that at 220 cm⁻¹... -1 (A) 1g ) and 280 cm -1 (E) g The characteristic vibrational peaks detected at the [location] are attributed to Fe2O3, proving that the main component of the disordered layer on the surface is amorphous Fe2O3. No Fe2O3 diffraction signal was observed in the XRD pattern, indicating that its content is below the XRD detection limit, further confirming that Fe2O3 exists only as a thin layer on the surface. After using the above Fe2N precursor for the RWGS reaction, the catalyst underwent significant structural reconstruction. The diffraction peaks of Fe2N completely disappeared after the reaction, while a new set of diffraction peaks appeared, perfectly corresponding to the Fe3C standard card. This indicates that Fe2N underwent complete carbonization and phase transformation during the reaction, and the bulk structure transformed into Fe3C. Raman spectroscopy further confirmed that while forming the Fe3C bulk structure, the catalyst surface still retained the characteristic peaks of Fe2O3, indicating that a layer of Fe2O3 still covered the catalyst surface after the reaction.

[0052] Figure 3 FeO x HRTEM and HAADF-STEM images of the Fe3C catalyst; where (a) is the HRTEM image and (b) is the HAADF-STEM image. Figure 3 As shown, in terms of morphology, the catalyst after the reaction transformed from the original irregular particles into uniformly sized, regular spherical particles with a particle size distribution in the range of 60-220 nm. HAADF-STEM images clearly reveal the structure of the catalyst after the reaction. The bulk phase is an Fe3C structure with clearly defined lattice fringes, while its surface is covered with an amorphous disordered layer with a thickness of approximately 4.8 nm. Raman spectroscopy confirmed that the main component of this disordered layer is also Fe2O3. Figure 4 FeO x XPS spectra of Fe3C catalyst during the RWGS reaction. The spectra show the presence of Fe on the catalyst surface. 2+ and Fe 3+ The Fe species. This confirms that under actual reaction conditions, the catalyst surface is mainly composed of FeO. x It exists in the form of (1 ≤ x ≤ 1.5). Based on the above characterization results, the Fe2N precursor underwent a dynamic structural evolution under RWGS reaction conditions, ultimately forming a structure composed of surface iron oxides (FeO). x A heterogeneous composite structure consisting of FeO and bulk iron carbide (Fe3C). The catalyst resulting from this reaction was named FeO. x / Fe3C catalyst.

[0053] Figure 5 FeO x RWGS activity evaluation diagram of Fe3C catalyst; where (a) is FeO x / Plot of CO2 conversion rate of Fe3C catalyst at different temperatures; (b) Plot of FeO x / CO selectivity test graph of Fe3C catalyst at different temperatures. In the RWGS reaction, the intrinsic activity of the catalyst is a key indicator for evaluating its industrial application potential. The prepared FeO x / Fe3C catalysts exhibit significant advantages in this regard. At concentrations up to 200,000 mL / g cat At high space velocities of [value missing] / h, this catalyst exhibits CO2 conversion rates close to thermodynamic equilibrium values ​​at reaction temperatures of 500 °C and 600 °C, demonstrating significantly superior performance compared to various reference iron-based catalysts, such as [example missing]. Figure 5 As shown, this demonstrates its superior intrinsic catalytic activity. While achieving high conversion rates, the catalyst exhibits extremely high selectivity for the target product. The CO selectivity consistently remains above 98%, clearly indicating that the reaction primarily targets CO production, with side reactions effectively suppressed.

[0054] The stability of catalysts at high temperatures is a key limiting factor for their industrial application. For FeO... x The stability of the Fe3C catalyst was evaluated. Fe2N precursor was reacted once in a mixed atmosphere containing 23% CO2 / 69% H2 / 8% N2 (66.7 mL / min) to obtain FeO. x After / Fe3C, stability testing was conducted. In a fixed-bed reactor, the catalyst stability was evaluated at atmospheric pressure using a mixed gas containing 23% CO2 / 69% H2 / 8% N2 (66.7 mL / min). 20 mg of catalyst powder was mixed with 500 mg of inert SiO2 and packed into a quartz tube. Stability was assessed at 600 °C and 200,000 mL / g. cat The test lasted for 200 hours under the harsh conditions of a high airspeed of / h. Figure 6 FeO x / Fe3C catalyst stability evaluation diagram. (See diagram below.) Figure 6 As shown, the catalyst exhibits excellent long-term stability. Although there was a slight adjustment in activity within the initial 15 hours, the catalyst performance entered a highly stable plateau period during the subsequent 185-hour test, ultimately maintaining approximately 85% of its initial activity. This dynamic stabilization process indicates that this heterostructure, after initial adaptive adjustment, can enter a highly stable state and possesses the potential to withstand long-term high-temperature industrial reaction conditions. In summary, FeO x The Fe3C catalyst exhibits high activity, high selectivity, and long-term stability in the reverse water-gas shift reaction, with overall performance far exceeding that of conventional iron-based catalysts, demonstrating great potential for industrial applications.

[0055] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. An in-situ formed FeO x / Fe3C catalyst, characterized in that FeO x The Fe3C catalyst consists of uniformly sized spherical particles, while FeO... x / Fe3C catalyst particles are surface iron oxide FeO x A heterogeneous composite structure formed with bulk iron carbide Fe3C; wherein, 1≤x≤1.5; FeO x / The Fe3C catalyst particles have a diameter of 60-220 nm; The FeO x / Fe3C catalyst is generated in situ from Fe2N precursor under reverse water-gas shift reaction conditions.

2. The FeO as described in claim 1 x The method for preparing Fe3C catalyst is characterized by, Includes the following steps: (1) Fe2O3 powder was nitrided under a pure ammonia atmosphere to prepare Fe2N precursor; after cooling, Fe2N precursor was passivated by O2 / Ar gas flow. (2) The Fe2N precursor was heated to 300±10 ℃, 400±10 ℃, 500±10 ℃ and 600±10 ℃ in a mixed atmosphere of CO2 and H2, and maintained at each temperature for 0.5-1.5 h to obtain FeO. x / Fe3C catalyst.

3. The preparation method according to claim 2, characterized in that, In step (1), the flow rate of pure ammonia gas is 40-60 mL / min.

4. The preparation method according to claim 2, characterized in that, In step (1), the nitriding temperature is 630-680℃; the nitriding time is 3-5 h; and the heating rate is 3-8 ℃ / min.

5. The preparation method according to claim 2, characterized in that, The method for preparing Fe2O3 powder is as follows: calcining ferric nitrate powder at 380-420 ℃ for 3-5 h with a heating rate of 3-8 ℃ / min.

6. The preparation method according to claim 2, characterized in that, The specific operation of passivation treatment is as follows: after the prepared Fe2N precursor is cooled to room temperature, it is passivated with 1-2% O2 / Ar gas flow for 0.5-1 h.

7. The preparation method according to claim 2, characterized in that, In step (2), the volume ratio of CO2 to H2 is 2-4:

1.

8. The preparation method according to claim 2, characterized in that, In step (2), the total gas flow rate of the mixed atmosphere of CO2 and H2 is 60-70 mL / min.

9. The FeO as described in claim 1 x Application of Fe3C catalyst in reverse water-gas shift reaction.

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