Preparation method and application of Fe-based oxygen carrier

Abstract

The oxygen carrier has the advantages that the nano metal particles are out of solution; and the oxygen carrier prepared by the improved method has the phenomenon that the reduced lattice cations migrate to the surface of the catalyst material. The application is oriented to reduce CO generation and realize the synergy of the reverse water gas shift-Fischer-Tropsch synthesis reaction in the process of CO2 hydrogenation to olefins, the improved hydrothermal synthesis method is used to prepare the oxygen-catalyst dual functional particles, the catalytic activity of LaFeO3 perovskite is regulated, CO generation is reduced, and the yield of low-carbon olefins is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of preparation and application of oxygen-carrying and catalytic bifunctional particles in chemical looping combustion technology, and to the application of perovskite oxygen carriers in the preparation of ethylene, propylene and butene by CO2 hydrogenation. BACKGROUND

[0002] Fe-based perovskite oxygen carriers of ABO3 type have attracted much attention due to their adjustable structure. By doping A and B sites in the bulk phase, the oxygen activity of the lattice is adjusted by changing the lattice defects and oxygen vacancies, thereby improving the efficiency of chemical looping combustion. Among various perovskite materials, LaFeO3 is considered an attractive oxygen carrier candidate, mainly because of its high oxygen mobility and ability to accommodate a large number of vacancies in the structure at relatively high reaction temperatures. Due to different operating temperature conditions, the removal of oxygen and the total amount of available oxygen during CH4 oxidation can cause LaFeO3 to be reduced from Fe 3+ to Fe 2+ , but further reduction seems to be inhibited. Studies on the kinetics of oxygen removal in CH4 reduction of LaFeO3 reveal the involvement of surface oxygen and oxygen vacancies. Initially, the removal of oxygen generates vacancies on the surface of the oxygen carrier, which will help to promote the occurrence of the reaction. Subsequently, as the vacancy concentration increases, the activity of oxygen becomes a rate-determining step. LaFeO3 has two unequal atomic positions for oxygen, the first oxygen (O1) is coordinated based on two La ions and two Fe ions, located in the La ion layer, and the second oxygen (O2) is coordinated based on three La ions and two Fe ions, located between the Fe ion layers. As an excellent oxygen carrier, Fe-based perovskite is still a difficult point if its catalytic activity is to be adjusted by taking advantage of its own characteristics.

[0003] Due to the stability of perovskite oxides, it is difficult to expose more active metals, and the low pore volume and low specific surface area caused by high calcination temperature are unfavorable factors for perovskite oxides, which inhibit the capture ability of reactants, resulting in low catalytic efficiency. The research group tried B-site doping strategy and found that it can effectively improve the CO2 hydrogenation activity, but the reverse water gas shift activity is high, generating a large amount of CO. At the same time, limited by the A-S-F distribution rule of Fischer-Tropsch, the C5 + product distribution is wide, and the conversion of C2-C4 products is low. In order to reduce CO generation and realize the synergy of reverse water gas shift-Fischer-Tropsch synthesis reaction in the process of CO2 hydrogenation to olefins, it is still necessary to further develop new oxygen-carrying and catalytic particles, break the stability of perovskite, and improve the catalytic activity.

[0004] Perovskite-type oxides have the ability to be doped with high activity elements. Another prominent ability of Fe-based perovskite is the nanosized metal particle out-diffusion. By high-temperature reduction treatment or in a reaction environment with a reducing atmosphere, the perovskite usually exists in partial metal reduction. Therefore, the phenomenon of lattice cations migration to the surface of the catalyst material exists, and this metal segregation is considered to be the "anchoring" of active metal particles, which can maximize the catalytic activity of single atoms or metal oxide nanoparticles, which is extremely beneficial to the activation of stable CO2 molecules. Selecting metals that are easy to out-diffuse and can reduce the reduction energy barrier and doped into Fe-based perovskite. At the same time, it is embedded in the inert carbon material in an in-situ growth manner. Compared with traditional co-precipitation, sol-gel and other technologies, small size and high dispersion of catalyst nanoparticles can be formed. How to use the unique structural characteristics of perovskite, innovative preparation methods and improved oxygen carriers are a path worth exploring.

[0005] The research group found in the early stage that Zn-containing iron-based catalysts showed good catalytic activity and low-carbon olefin selectivity in the CO2 hydrogenation reaction to low-carbon olefins. In the study of Fe-Zn-K series catalysts, it was found that with the addition of Zn, the phase structure of the catalyst changed, and ZnFe2O4 phase was formed in the catalyst precursor. And it was confirmed that the Zn species played a role in promoting dispersion and anti-sintering, which helped to improve the initial catalytic activity in the CO2 hydrogenation reaction, and it was relatively easy to generate carbonitride and iron carbide species and iron oxide species. ZnFe2O4 plays an important role in adjusting CO2 hydrogenation activity and product distribution. And the research found that the hydrogenation and chain growth ability of Zn-containing catalysts is closely related to the structural characteristics of iron carbide. ZnO can adsorb reaction intermediates and promote the occurrence of RWGS reaction. ZnO and ZnFe2O4 can act as structure promoters by inhibiting the sintering of iron carbide. Fe and Zn species cooperate with each other to promote the exposure of active sites and promote the CO2 hydrogenation to low-carbon olefins process. Zn as a promoter can accelerate the FTS reaction behavior, improve the adsorption of CO2, improve the catalytic activity and stability of Fe-based catalysts, and improve the selectivity of low-carbon olefins and inhibit the generation of long-chain hydrocarbons. Combined with the previous research of this paper, doping metals in perovskite can effectively prevent metal sintering, and provide higher CO2 conversion and low-carbon olefin yield during the reaction. In Chapter 5, Cu in LaGdFeCuO3 catalyst synthesized by hydrothermal method reduces the reduction energy barrier of Fe-based perovskite, promotes the high dispersion of Fe, and the close contact between the surface of the metal ions promotes the hydrogenation process of CO species, inhibits the formation of CH4 and C5 + , which is conducive to the formation of C2 = -C4 =The carbon carrier prepared by the hydrothermal method of glucose can be added as a lubricant additive, and a highly graphitized friction film can be formed at the metal-carrier friction interface during the reaction process, thereby effectively reducing the loss of active metals. As an amorphous carbon, the carbon carrier has sp2 and sp3 hybrid carbon in the structure, and compared with carbon atom crystals, the carbon can fill the gaps in the reaction interface and play a role in adsorbing and transferring reactants. However, the morphology of the carbon-coated material is variable, and the carbon-coated material prepared by the hydrothermal method of glucose can be obtained by calcination and changes in raw material composition. The existing literature reports that the metal oxide with partially reduced surface is mainly focused on the synthesis of methanol by CO2 hydrogenation. There are few reports on the application of carbon-coated perovskite catalysts to the synthesis of low-carbon olefins by CO2 hydrogenation or the technical route of single chemical chain CO2 capture. In addition to the preparation method, the non-stoichiometric control of A or B site cations is also an effective method to adjust the oxygen vacancy content. Element doping can construct catalyst surface defects, promote CO2 adsorption, and promote the catalytic synthesis of low-carbon olefins from CO2 hydrogenation. According to the previous research results, this chapter takes Zn as the variable, uses the hydrothermal synthesis method, and investigates the performance of carbon-coated LaGdFeZnO3 in catalyzing CO2 hydrogenation to low-carbon olefins. SUMMARY

[0006] The present application is directed to the shortcomings in the prior art, and aims to reduce CO generation and realize the synergy of the reverse water gas shift-Fischer-Tropsch synthesis reaction in the process of CO2 hydrogenation to olefins. The improved hydrothermal synthesis method is used to prepare oxygen-carrying-catalytic dual-function particles, and the catalytic activity of LaFeO3 perovskite is adjusted to reduce CO generation and improve the yield of low-carbon olefins.

[0007] The present application uses inexpensive glucose as a carbon source to prepare LaGdFeZnO3 composite particles with Gd modification at site A and Zn partial substitution at site B by a hydrothermal method, and investigates the performance of carbon-coated LaGdFeZnO3@C series oxygen carriers in CO2 hydrogenation to low-carbon olefins. The results show that the modification of Gd adjusts the phase of the particles, promotes the exudation of active metals and the migration of lattice oxygen. The LaGdFeZnO3 prepared by the hydrothermal method has a special morphology, and the carbon chain growth ability is weak due to the steric hindrance effect. The product distribution is mainly C2-C4 hydrocarbons, and the catalytic performance of CO2 hydrogenation is better than that of the sol-gel method. After Zn doping, with the increase of the Fe / C molar ratio, the catalytic activity shows a first increase and then a decrease trend; when Fe / Zn=0.5, it shows good olefin selectivity. The post-reaction sample has phases such as Fe2O3 and Fe3O4, which further confirms that the perovskite catalyst prepared by the hydrothermal method improves the metal / perovskite interface effect, reduces the structural stability, and effectively improves the catalytic activity of CO2 hydrogenation.

[0008] The above research provides a certain theoretical basis for the design and research of Fe-based oxygen-carrying-catalysis bifunctional particles coupled with CO2 capture by chemical looping combustion and catalytic hydrogenation.

[0009] The catalyst has suitable CO2 adsorption and dissociation capacity, inhibits the re-adsorption of olefins, reduces the secondary reaction of olefins, and improves the selectivity of olefins. In order to break through the product distribution of A-S-F, high selectivity is obtained for low-carbon olefins and byproduct methane, C5 ＋ and other byproducts provide a strong cornerstone. The catalyst can effectively inhibit the secondary hydrogenation reaction of primary olefins, and regulate the production of high-value C2-C4 low-carbon olefins. The Fe-based perovskite has active main lattice Fe, which effectively promotes the dissolution or migration of active Fe ions out of the iron oxide in the perovskite matrix, reduces the reduction energy barrier by doping Zn, and cooperatively activates CO2 between metal oxides, which helps to improve the ability of perovskite catalyst to prepare low-carbon olefins.

[0010] In order to achieve the above-mentioned purpose of the present application, the technical scheme adopted by the present application is as follows:

[0011] (1) La: Gd: Fe = 1-3: 1: 1, Fe: Zn = 0.1-2, glucose: (La+Gd+Fe+Zn) = 3 stoichiometric ratio, dissolved in water, stirred until uniform, and then placed in a high-pressure reaction kettle.

[0012] (2) The reaction kettle is placed in an oven, and set at 180 °C for 8-12 h, and cooled to room temperature.

[0013] (3) The black precipitate in the reaction kettle is separated and washed with deionized water. The precipitate is placed in an 80 °C oven for drying.

[0014] (4) After drying, the sample is placed in a muffle furnace, heated to 400 °C at a rate of 5 °C / min and held for 1 h, then heated to 750 °C at a rate of 2 °C / min and held for 5 h, cooled and ground to obtain the target particles.

[0015] The metal ions are preferably lanthanum nitrate, gadolinium nitrate, iron nitrate and zinc nitrate.

[0016] The separation in step (3) is preferably centrifugal separation. The applicant investigated filtration separation and centrifugal separation. It was found that the particles prepared by centrifugal separation were relatively better in catalytic performance, and the centrifugal conditions were 3500r, 2min. Since the water is not completely removed, the drying time is relatively slow compared with filtration separation. Since the filter cake formed by filtration separation is directly dried, the drying time can be accelerated. However, the number of washing times needs to be controlled to avoid the loss of metal elements such as Gd and La.

[0017] The calcination in step (4) is carried out under a nitrogen atmosphere or an air atmosphere. The morphology formed under the air atmosphere is a special morphology.

[0018] The oxygen-carrying and catalytic bifunctional particles prepared according to the preparation method are applied to the preparation of low-carbon olefins by CO2 hydrogenation, and the reaction conditions are H2 / CO2 = 1-4, 150-400℃, 2-4 MPa, 1000-4000 h –1 .

[0019] The oxygen carrier can also be modified by impregnation of elements such as Zr, Co, Mg, Mn, Na, Zn, K to optimize the catalytic activity of the oxygen carrier. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 is the XRD pattern before reaction of Examples 1-4. Figure 2 is the XRD pattern after reaction of Examples 1-4. Figure 3 is the SEM pattern of Examples 1-4. (a) LaFeZnO3 (b) LaGdFeZn 0.5 O3(c) LaGdFeZn 0.75 O3(d) LaGdFeZnO3. Figure 4 is the CO2 hydrogenation evaluation data of Examples 1-4. DETAILED DESCRIPTION

[0021] The application will be further described in the following embodiments. The embodiments are implemented on the premise of the application technology, and the detailed implementation and specific operation process are given to illustrate the creativity of the application, but the protection scope of the application is not limited to the following embodiments.

[0022] Based on the information contained in the present application, various changes to the precise description of the application can be made by those skilled in the art without departing from the spirit and scope of the appended claims. It is to be understood that the scope of the application is not limited to the defined processes, properties or components, as these embodiments and other descriptions are merely illustrative of the specific aspects of the application. Indeed, various modifications of the embodiments of the application, as would be apparent to those skilled in the art or the relevant art, are intended to be within the scope of the appended claims.

[0023] In order to better understand the application without limiting the scope of the application, all numbers expressing quantities, percentages, and other numerical values used in the specification, are to be understood as being modified in all instances by the term "about". Accordingly, unless indicated to the contrary, the numerical parameters set forth in the specification and attached claims are approximations. It will be apparent to those skilled in the art that various modifications can be made to the embodiments described without departing from the scope of the application as set forth in the appended claims.

[0024] Example 1

[0025] According to the stoichiometric ratio of La:Fe = 2:1, Fe:Zn = 1, glucose: (La+Gd+Fe+Zn) = 3, the corresponding lanthanum nitrate, gadolinium nitrate, iron nitrate, zinc nitrate was weighed and dissolved in deionized water, stirred for 2 h, and the uniformly stirred mixed solution was transferred to a high-pressure reaction kettle. Reaction at 180 °C for 10 h, cool to room temperature. Finally, the black precipitate in the reaction kettle was centrifuged and washed with deionized water, then the black powder was placed in an 80 °C oven for drying for 12 h, then placed in a muffle furnace (under air atmosphere), heated to 400 °C at a rate of 5 °C / min and kept for 1 h, then heated to 750 °C at a rate of 2 °C / min and kept for 5 h, after cooling, grinding, the target catalyst sample was obtained. Denoted as sample 1, LaFeZnO3.

[0026] Example 2

[0027] According to the stoichiometric ratio of La:Gd:Fe = 2:1:1, Fe:Zn = 0.5, glucose: (La+Gd+Fe+Zn) = 3, the corresponding lanthanum nitrate, gadolinium nitrate, iron nitrate, zinc nitrate was weighed and dissolved in deionized water, stirred for 2 h, and the uniformly stirred mixed solution was transferred to a high-pressure reaction kettle. Reaction at 180 °C for 10 h, cool to room temperature. Finally, the black precipitate in the reaction kettle was centrifuged and washed with deionized water, then the black powder was placed in an 80 °C oven for drying for 12 h, then placed in a muffle furnace (under air atmosphere), heated to 400 °C at a rate of 5 °C / min and kept for 1 h, then heated to 750 °C at a rate of 2 °C / min and kept for 5 h, after cooling, grinding, the target catalyst sample was obtained. Denoted as sample 2, LaGdFeZn 0.5 O3.

[0028] Example 3

[0029] According to the stoichiometric ratio of La:Gd:Fe = 2:1:1, Fe:Zn = 0.75, glucose: (La+Gd+Fe+Zn) = 3, the corresponding lanthanum nitrate, gadolinium nitrate, iron nitrate, zinc nitrate was weighed and dissolved in deionized water, stirred for 2 h, and the uniformly stirred mixed solution was transferred to a high-pressure reaction kettle. Reaction at 180 °C for 10 h, cool to room temperature. Finally, the black precipitate in the reaction kettle was centrifuged and washed with deionized water, then the black powder was placed in an 80 °C oven for drying for 12 h, then placed in a muffle furnace (under air atmosphere), heated to 400 °C at a rate of 5 °C / min and kept for 1 h, then heated to 750 °C at a rate of 2 °C / min and kept for 5 h, after cooling, grinding, the target catalyst sample was obtained. Denoted as sample 3, LaGdFeZn 0.75 O3.

[0030] Example 4

[0031] According to the stoichiometric ratio of La:Gd:Fe = 2:1:1, Fe:Zn = 1, glucose: (La+Gd+Fe+Zn) = 3, the corresponding lanthanum nitrate, gadolinium nitrate, iron nitrate and zinc nitrate are weighed and dissolved in deionized water, stirred for 2 h, and then transferred to a high-pressure reaction kettle. The mixed solution is stirred uniformly and reacted at 180 °C for 10 h. After cooling to room temperature, the black precipitate in the reaction kettle is centrifuged and washed with deionized water. Then the black powder is placed in an 80 °C oven for drying for 12 h, and then placed in a muffle furnace (in an air atmosphere), heated at a rate of 5 °C / min to 400 °C and kept at this temperature for 1 h, and then heated at a rate of 2 °C / min to 750 °C and kept at this temperature for 5 h. After cooling and grinding, the target catalyst sample is obtained. It is recorded as sample 4, LaGdFeZnO3.

[0032] Catalyst performance test and characterization:

[0033] In order to make the catalyst better react and not block the reaction tube, the catalyst prepared in the above examples 1-4 of the present application is all made into 20-40 mesh catalyst particles.

[0034] The catalyst is evaluated by using a micro fixed bed reactor, and the process conditions are as follows: 20-40 mesh catalyst 0.5-5 mL, reaction temperature 280-400 °C, reaction pressure 0.5-8 MPa, raw material gas H2 / CO2=1 or 2, space velocity 500-5000 h -1 .

[0035] For example, the catalyst prepared in example 1 is evaluated in a micro fixed bed reactor, and the specific operation steps are as follows: 1.0 mL of the sample catalyst prepared in example 1 is weighed and loaded into the middle constant temperature zone of the reaction tube, the raw material gas H2 / CO2=3, the temperature is 320 °C, the pressure is 2.0 MPa, the space velocity (GHSV) is 1000 h -1 , after reaching the steady state, sampling analysis is carried out, and sampling is carried out every 3 h. The raw material gas and the product are quantitatively and qualitatively analyzed by gas chromatography. According to the “determination of H2, N2, CO, CO2 and C1-C8 hydrocarbons in coal-based fischer-tropsch synthesis tail gas and gas chromatography method” methane correlation method, the CO conversion rate and the selectivity of each component are calculated.

[0036] Figure 1The XRD phase of the sample after Zn doping is reflected. Among them, the LaFeZnO3 phase peak is relatively chaotic, and LaFeO3, La(OH)3, La2O3, and Fe2O3 phases appear. It shows that the system is in a non-stoichiometric ratio, La is excessive, and La2O3 phase is detected. Because elements with similar radii, same valence state and same coordination number in the structure are more likely to be replaced. Therefore, Gd can easily replace La at the A site of the perovskite structure. This substitution may cause disorder in the perovskite structure, thereby hindering the growth of the LaFeO3 crystal form.

[0037] Figure 2 The XRD spectrum of the sample after CO2 hydrogenation reaction is shown. ZnFe2O4, ZnO, and Fe2O3 phases appear. When the Zn doping molar ratio is 0.5, there is no obvious change before and after the reaction. The LaFeO3 perovskite structure is still retained. With the increase of Zn content, ZnFe2O4 phase appears. Zn reduces the size of iron particles, forms ZnFe2O4 to increase the catalytic activity in the process of CO2 hydrogenation, and helps to generate olefins. The formation of ZnFe2O4 also affects the structural stability and properties of Fe species. The strong interaction between Zn and Fe inhibits the formation of iron carbide, mainly in the form of ZnFe2O4 phase, which inhibits the sintering of Fe species during the activation process. However, due to its alkalinity, it increases the CO2 adsorption capacity, and the content of Zn acts as a structure promoter, reducing the size of Fe species. Under the confinement of ZnFe2O4 and perovskite lattice, the restriction on Fe increases, and the phase transformation of Fe3O4 is relatively difficult to occur. When the Zn doping molar ratio is 0.75 and 1, this process becomes complex, and ZnO appears at the same time. Fe3O4 characteristic peaks are also detected. Because the carbon coating process can reduce the carbonization conditions, promote the phase transformation of transition metals and their compounds, and catalyze the transformation of amorphous carbon into graphitized carbon structure, the introduction of Zn may change the molecular structure of organic carbon in amorphous carbon, promote the appearance of Fe3O4, ZnO, La2O3, and other oxide species. After the reaction, the characteristic diffraction peak of the catalyst becomes weak, indicating that the outer amorphous carbon may be stripped to the metal-perovskite interface to form a carbon-coated structure during the reaction. This effectively reduces the interaction force between the precipitated metal and the perovskite, thereby promoting the exudation of iron oxide, and thus improving the CO2 hydrogenation catalytic activity. The appearance of Fe3O4, ZnO and other phases confirms that Fe and Zn participate in the reaction together in the CO2 hydrogenation reaction, and Zn plays a good synergistic effect. Combined with the analysis of CO2 hydrogenation catalytic activity evaluation data, the Fe-based perovskite with carbon coating exists Fe oxide segregation phenomenon, and the formed Fe3O4 in the reaction improves the CO2 hydrogenation catalytic activity.

[0038] Figure 3SEM images of samples 1-4. From the figure, after Zn doping, the hydrothermal catalyst LaFeZnO3 series catalysts are stacked in sheets, the particles are closely combined, the catalyst surface is composed of randomly curled graphite sheets, and the particle size is 10-30 nm. GdFeZnO3 has an irregular morphology, and the surface roughness is high, indicating that the carbon-coated GdFeZnO3 structure is not conducive to the formation of regular spherical structures. LaGdFeZn 0.75 O3 has an olive ball type morphology, and the particle branch structure is tight, with less hole exposure. LaGdFeZn 0.5 The LaGdFeZnO3 sample is in block form, with small particles densely dispersed on the surface of the block, and the dispersion is relatively uniform. The SEM image also shows the agglomeration phenomenon of carbon particles without a clear morphology. During the preparation of the catalyst by the hydrothermal method, the LaGdFeZnO3 series catalysts are formed under the action of the carbon plate, and the limited nanometer space provided by the carbon template with interconnected pore channels, the target catalyst morphology grows in this space, and during the process of selective removal of carbon by high temperature calcination, the structure and morphology have changed greatly. These changes in morphology are attributed to the templating effect of carbon templates in the perovskite synthesis process, and the carbon-coated LaGdFeZnO3 series catalysts exhibit framework restriction, avoiding the agglomeration of porous nanoparticles. The wrinkled surface of the Zn-doped catalyst increases the effective collision area of the catalyst surface with the reactants, while increasing the number of entire active sites, thereby promoting the reaction process. There are a large number of particles that are closely combined to form a block structure, the structure is porous, the pore size is different, and the full contact of CO2 and H2 helps to expose more unsaturated active sites on the material, thereby improving the catalytic activity, which is supported by the catalytic activity data. Solvothermal leads to the surface reconstruction of the initial perovskite oxide and promotes the formation of layered nanostructures. These nanometer structures similar to flower clusters are beneficial to increase the reaction specific surface area and are beneficial to expose more active metal, thereby improving the catalytic activity of the catalyst surface.

[0039] Figure 4The CO2 hydrogenation performance of LaGdFeZnO3 series catalysts is reflected in the figure. As can be seen from the figure, the complexity of the catalyst structure improves the catalytic performance. The carbon-coated Gd-modified LaFeZnO3 is more easily contacted with CO2 and H2 molecules, and the reactant conversion rate and product generation rate are significantly higher than that of single oxide catalytic CO2 hydrogenation process. The adsorption step in the catalytic process is significantly accelerated, and the increase of active metal sites promotes the RWGS-FT reaction, and the CO selectivity is significantly reduced compared with the carbon-coated catalyst. The CO2 conversion rate increases from 16.3% to 26.54%. Different iron phases play different roles in the CO2 hydrogenation reaction. Many researchers believe that Fe3O4 is responsible for the RWGS reaction, while metallic iron and iron carbide are effective for the CO hydrogenation to produce hydrocarbons. The carbon-coated pore structure provides a reaction channel for CO2 and H2 molecules, which is beneficial to the flow and mass transfer of molecules. Fe2O3 and Fe3O4 reduced at high temperature are anchored on the LaGdFeZnO3 matrix, and the electron-donating ability of Gd promotes CO dissociation and weakens H2 adsorption. The interaction between Gd and Fe involves charge transfer to the surface Fe atoms, which shows the characteristics of improving the selectivity of low-carbon olefins. Due to the strong interaction between Zn and Fe, a uniform oxide is formed, which synergizes and improves the product distribution. In the LaFeZnO3 catalyst prepared by sol-gel method, the reactants are first adsorbed on the surface of the catalyst, and then the reverse water gas RWGS reaction occurs, and CO products dominate. Combined with the XRD analysis of the catalyst after the reaction, the reaction process promotes the phase change of Fe, and the FT activity starts from the CO decomposition reaction of iron and carbon, and the possible formation of iron carbide (Fe5C2). Unfortunately, it is difficult to detect iron carbide, and it is necessary to optimize the Zn content and the amount of glucose added to balance the carburizing rate, so as to prevent the oxidation of active iron carbide and avoid excessive carbon deposition to block the active sites. During the reaction process, the iron precursor is in complete contact with the alkali metal oxide, which is very effective for the production of low-carbon olefins. The carbon-coated structure weakens the degree of secondary hydrogenation and catalytic reaction of the product, increases the surface coverage of carbon oxides in the FT synthesis process, and thus increases the selectivity of low-carbon olefins. In addition, the amount of ethylene, propylene and butene produced by the catalyst containing Gd is slightly higher than that of the catalyst without Gd. It can be assumed that CH4 is not completely decomposed into CHx particles and molecular hydrogen on the Gd atom, but is decomposed into carbon and atomic hydrogen on the iron atom. It can be said that the Gd particles are formed on the iron nanoparticles, and the existence of Gd atoms on the surface promotes the generation of new active sites and enhances the adsorption of CO2. Gd promotes the activity and olefin selectivity of the product, so the existence of Gd plays an important role in improving the catalytic activity.

[0040] Meanwhile, the addition of Zn further promoted Fe dispersion, exposed more active sites, and addressed the difference in binding energies between MC and MH (M = Fe, Zn). Fe and Zn increased product yield during the adsorption and desorption of CO2 and H2. Zn led to the formation of ZnO and ZnFe2O4 spinel phases, thereby increasing Fe-Zn interactions and altering CO2 adsorption behavior. The ZnFe2O4 phase stabilized active sites on the active metal surface during the hydrogenation reaction. Changes in the catalyst surface and Zn phase structure due to potassium modification activated both the RWGS reaction and CO2 hydrogenation to hydrocarbons. Simultaneous doping with Fe and Zn ions increased the oxygen vacancy content in the oxide, further enhancing the catalyst's catalytic activity. Results showed that the carbon-coated LaGdFeZnO3 catalyst for producing low-carbon olefins had an alkene / alkane (O / P) value of 1.84-2.11 and a C2... = -C4 = It accounted for 30.78-33.54 wt% of the total hydrocarbon distribution. Perovskite catalysts significantly suppressed C5... + Hydrocarbon formation, C5 + Product yield is below 7%. Reduced carbon chain growth capability and lattice constraint cause product distribution to deviate from the ASF distribution. Simultaneously, ZnO possesses lattice oxygen vacancies, composed of electron pairs in the lattice, which promote CC coupling. With increasing Zn content, the CH4 content decreases slightly, while the C2-C4 hydrocarbon product distribution shows no significant change. Partial surface reduction of ZnO generates oxygen vacancies, providing strong binding with the adsorbed species CO2, which is beneficial for CO2 hydrogenation; however, when CO content is dominant, hydrocarbon selectivity is very low. Excessive Zn doping on iron catalysts has the opposite effect, leading to a reduction in the production of target olefins.

[0041] The figure shows that Zn inhibits the secondary hydrogenation reaction of olefins, enhances CO2 desorption, and exhibits higher selectivity for short-chain olefins. With increasing Zn doping concentration, the activity of LaGdFeZnO3 shows a trend of first increasing and then decreasing. These results indicate that the perovskite surface is rich in chemical properties, and changes in the reaction chemical potential on the catalyst surface promote the increase of substate metal ions and segregation effects, thus affecting the catalytic activity of CO2 hydrogenation. Combined with H2-TPR analysis, the LaGdFeZnO3 series catalysts exhibit high stability in reducing media, while the catalytic activity depends on the exposed iron content, indicating that the content of migrated iron oxides is relatively consistent, exhibiting similar catalytic activity. Because the oxygen vacancies formed in the LaGdFeZnO3 lattice and the migrated iron oxides are the key active components of the reaction, the addition of Zn, while further promoting Fe dispersion and exposing more active sites, also inhibits the Fe reduction process. The formation of ZnFe2O4 promotes Fe… x O yThe species is reduced to metallic Fe. The appropriate Zn loading enhances the dispersion of the metal / metal oxide, which makes the catalyst surface have more active sites, thus promoting the preparation of low-carbon olefins by CO2 hydrogenation. The iron oxide is anchored in the perovskite matrix, and the catalytic activity data should have significant changes, assuming that the degree of carbon coverage and plugging is very small, but in fact, the catalytic performance of Fe-Zn metal catalysts is quite similar, and the similarity of catalytic activity shows that the proportion of iron oxide participating in the reaction is very small, and it is difficult to effectively play a catalytic activity role. Under the conditions of a reaction temperature of 320 °C, a reaction pressure of 2.0 MPa, and a reaction space velocity of 1000 h -1 The C2 = -C4 = olefins account for about 40 wt% of the total hydrocarbon distribution, and the olefin / alkane ratio O / P is 1.84-2.22. When Fe:Zn=0.5, the catalyst has good CO2 hydrogenation performance. Based on the above activity behavior and the structural characteristics of the prepared hydrothermal LaGdFeZnO3 catalyst, it is considered that Zn promotes the high catalytic activity of the iron-based perovskite catalyst, which is beneficial to the preparation of low-carbon olefins by CO2 hydrogenation.

Claims

1. A method for preparing a Fe-based oxygen carrier, characterized by Comprising the following steps: (1) Take lanthanum nitrate, gadolinium nitrate, iron nitrate, zinc nitrate and glucose according to the molar ratio of La:Gd:Fe=1-3:1:1, Fe:Zn=0.1-2, glucose:(La+Gd+Fe+Zn)=3 stoichiometric ratio, dissolve in water, stir until uniform, and then put into a high-pressure reaction kettle; (2) Put the reaction kettle into an oven, set at 160-180°C for 8-12h, and cool to room temperature; (3) Separate the black precipitate of the reaction kettle and wash with deionized water; put the precipitate into an 80°C oven for drying; (4) After drying, the sample is placed in a muffle furnace for calcination, which is calcined in an air atmosphere, with the conditions of 5°C / min heating to 400°C and constant temperature for 1h, then 2°C / min rate heating to 750°C and constant temperature for 5h, grinding after cooling to obtain the target particles.

2. The preparation method of the Fe-based oxygen carrier according to claim 1, characterized in that The separation in step (3) is centrifugal separation.

3. The use of the oxygen carrier prepared by the method of claim 1 in the preparation of low-carbon olefins by CO2 hydrogenation, under the reaction conditions of H2 / CO2 = 1-4, 150-400 ℃, 2-4 MPa, 1000-4000 h. -1 .

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