A core-shell catalyst and its preparation method and application

Abstract

The present invention provides a core-shell catalyst, its preparation method, and application. The core-shell catalyst prepared by the present invention has Fe3O4 as the core and TiO2 as the protective shell. The core-shell catalyst can be used in the field of flue gas catalytic oxidation and denitrification. The core-shell catalyst of the present invention has a high specific surface area, an excellent pore structure, and a high magnetic saturation strength. There is a strong interaction between the core and the shell, which is mainly reflected in the electron transfer between the two phases to produce more Fe 2+ The core-shell catalyst of this invention catalyzes H₂O₂ to produce the highly oxidizing ·OH group via the Haber-Weiss reaction, effectively oxidizing NO. Under optimal conditions, combined with alkaline solution absorption, it can achieve a denitrification efficiency of 88%. Furthermore, the catalyst exhibits excellent water resistance and reaction stability, and its preparation method is relatively simple, thus promising promising practical applications.

Description

Technical Field

[0001] The present invention belongs to the technical field of flue gas desulfurization and denitrification, and in particular relates to a core-shell catalyst and a preparation method and application thereof. Background Art

[0002] Large amounts of NOx emitted by coal-fired power plants and waste incineration plants contribute to acid rain, photochemical smog, and PM 2.5 pollution, harming the environment and human health. Currently, the mainstream denitrification method is selective catalytic reduction (SCR). However, SCR technology suffers from system complexity, the potential for ammonia leakage and equipment clogging, and high investment and operating costs. Therefore, it is necessary to develop a low-cost, high-efficiency NOx removal technology.

[0003] In existing research, core-shell structure catalysts have been shown to have excellent catalytic performance. The core-shell structure refers to the coating of nanoparticles with heterogeneous substances on their surfaces through chemical or physical means. It is generally composed of a central core and a shell layer covering the outside. The shell layer can form a closed internal microenvironment to enrich the reactants and increase the reaction rate. The shell layer also has a protective effect on the active components of the core layer, greatly improving the stability of the catalyst, preventing the catalyst from agglomerating, and extending the catalyst life. Currently, core-shell structure materials have been widely used in photocatalysis, electrocatalysis, fuel cells and other fields. In the field of flue gas desulfurization and denitrification, they are mainly used in SCR catalysts.

[0004] Existing MnOx-CeO2@TiO2 core-shell catalysts have been proposed for use in SCR denitrification. The presence of the shell reduces SO2 and H2O attack on the active sites, thereby improving the SCR catalyst's resistance. However, the NO removal efficiency of this catalyst decreases significantly in the presence of SO2, making practical application conditions quite demanding. While the properties of core-shell catalysts make them feasible for H2O2 catalysis, their application in H2O2 catalytic oxidation desulfurization and denitrification has yet to be investigated. Summary of the Invention

[0005] In view of this, the present invention proposes a core-shell catalyst and a preparation method and application thereof to solve the technical problems existing in the prior art.

[0006] In a first aspect, the present invention provides a core-shell catalyst comprising an Fe3O4 core layer and a TiO2 shell layer coated on the Fe3O4 core layer.

[0007] In a second aspect, the present invention further provides a method for preparing the core-shell catalyst, comprising the following steps:

[0008] Fe3O4 is placed in a container, and then an alcohol solvent and acetonitrile are added, dispersed, and then ammonia water is added and stirred to obtain a first mixture;

[0009] adding tetrabutyl titanate to an alcohol solvent and stirring to obtain a second mixture;

[0010] The second mixture was added dropwise to the first mixture, stirred at 40-50° C., and solid-liquid separation was performed to obtain a core-shell catalyst.

[0011] In a third aspect, the present invention further provides an application of the core-shell catalyst or the core-shell catalyst prepared by the preparation method in catalytic oxidation and denitrification of flue gas.

[0012] The core-shell catalyst of the present invention and its preparation method and application have the following beneficial effects compared with the prior art:

[0013] 1. The core-shell catalyst prepared by the present invention has a high specific surface area, an excellent pore structure and a high magnetic saturation strength. There is a strong interaction between the core and the shell, which is mainly reflected in the electron transfer between the two phases to produce more Fe 2+ and oxygen vacancies, promoting the cyclic redox reactions.

[0014] 2. The core-shell catalyst prepared by the present invention can be used for catalytic oxidation denitrification and desulfurization of flue gas. It catalyzes H₂O₂ to produce the highly oxidizing ·OH group via the Haber-Weiss reaction, effectively oxidizing NO. The optimal operating conditions for the core-shell catalyst are: a reaction temperature of 140°C, a flue gas flow rate of 0.5 L / min, an H₂O₂ concentration of 2 mol / L, and an H₂O₂ injection rate of 30 μL / min. Among them, the FT₂ catalyst exhibits the highest denitrification efficiency, achieving 88% under optimal operating conditions when combined with alkaline solution absorption. Furthermore, the core-shell catalyst prepared by the present invention exhibits superior water resistance and reaction stability, surpassing supported catalysts. The core-shell catalyst promotes the H₂O₂ reaction primarily due to the following reasons: the catalyst possesses a larger active surface area and a better pore structure, which facilitates reactant adsorption; the presence of the shell blocks some SO₂, reducing competitive SO₂ adsorption; and the catalyst provides more redox pairs and oxygen vacancies, facilitating the reaction cycle. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0016] Figure 1 This is a schematic structural diagram of the flue gas catalytic oxidation desulfurization and denitrification device of the present invention;

[0017] Figure 2 The morphology and structure diagrams of different catalysts;

[0018] Figure 3 HRTEM image of FT2 catalyst;

[0019] Figure 4 XRD patterns of Fe3O4, FT1, FT2, FT3 and Fe3O4 / TiO2;

[0020] Figure 5 N2 adsorption and desorption curves of different catalysts

[0021] Figure 6 is the pore size distribution diagram of different catalysts;

[0022] Figure 7 Ti 2p maps on the surface of core-shell catalyst and Fe3O4 / TiO2 catalyst;

[0023] Figure 8 is the VSM curve of different catalysts;

[0024] Figure 9 The effect of reaction temperature on the synergistic removal efficiency of NO and SO2;

[0025] Figure 10 The effect of flue gas flow rate on NO removal;

[0026] Figure 11 The effect of water vapor concentration in flue gas on denitrification in catalytic oxidation reaction system;

[0027] Figure 12 The reaction stability of FT2 catalyst and Fe3O4 / TiO2 catalyst. DETAILED DESCRIPTION

[0028] To make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0029] It should be noted that the order of description of the following embodiments is not intended to limit the preferred order of the embodiments. In addition, in the description of this application, the term "including" means "including but not limited to". Various embodiments of the present invention may be presented in the form of a range; it should be understood that the description in the form of a range is merely for convenience and brevity and should not be understood as a rigid limitation on the scope of the invention; therefore, it should be considered that the range description has specifically disclosed all possible sub-ranges and single numerical values ​​within the range. For example, the range description from 1 to 6 should be considered to have specifically disclosed sub-ranges, such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6, etc., as well as single numbers within the range, such as 1, 2, 3, 4, 5 and 6, regardless of the range. In addition, whenever a numerical range is indicated herein, it is meant to include any cited number (fractional or integer) within the indicated range.

[0030] The present invention provides a core-shell catalyst, comprising an Fe3O4 core layer and a TiO2 shell layer coated on the Fe3O4 core layer.

[0031] Based on the same inventive concept, the present invention also provides a method for preparing the above-mentioned core-shell catalyst, comprising the following steps:

[0032] S1. Place Fe3O4 in a container, then add an alcohol solvent and acetonitrile, disperse, add ammonia water, and stir to obtain a first mixture;

[0033] S2. adding tetrabutyl titanate to an alcohol solvent and stirring to obtain a second mixture;

[0034] S3. Add the second mixture dropwise into the first mixture, stir at 40-50° C., and obtain a core-shell catalyst after solid-liquid separation.

[0035] Specifically, in step S3 of the above embodiment, the second mixture is added dropwise to the first mixture, stirred at 40-50°C, and after solid-liquid separation, washed three times with ethanol and deionized water respectively, and then placed at 60-100°C for 5-15 hours to obtain a Fe3O4@TiO2 catalyst, which is a core-shell catalyst.

[0036] In some embodiments, the method for preparing Fe3O4 comprises the following steps:

[0037] S1, Fe 2+ The salt is added to the acid solution to obtain a metal ion solution;

[0038] S2. Add the metal ion solution to the first alkaline solution, heat and stir at 80-100° C. under the protection of inert gas, and obtain Fe 3 O 4 after solid-liquid separation.

[0039] Specifically, in the above embodiments, the inert gas includes but is not limited to nitrogen, argon, helium, neon, etc.

[0040] In the above embodiment, the metal ion solution is added to the first alkali solution, and heated and stirred at 80-100°C under the protection of inert gas to prevent Fe 2+ oxidized, specifically, continuously stirring the solution at a speed of 400 to 600 rpm / min; continuing stirring for 1 to 3 hours after the metal ion solution is added dropwise, stopping heating and cooling to room temperature; repeatedly washing the generated precipitate with deionized water until the pH value of the washing solution is close to neutral; drying the sample at 60 to 100° C. for 10 to 30 hours to obtain Fe3O4 particles.

[0041] In some embodiments, the alcohol solvent comprises ethanol and / or methanol;

[0042] In some embodiments, the molar ratio of Fe3O4 to TiO2 in the core-shell catalyst is 1:(1-3).

[0043] In some embodiments, Fe3O4 is placed in a container, and then an alcohol solvent and acetonitrile are added. After dispersion, in the step of adding ammonia water, the mass volume ratio of Fe3O4, alcohol solvent, acetonitrile, and ammonia water is (0.2-0.6) g: (140-160) mL: (40-60) mL: (2-5) mL, and the mass fraction of ammonia water is 25-30%;

[0044] In some embodiments, since tetrabutyl titanate provides Ti and Fe3O4 provides Fe, the molar ratio of Ti in tetrabutyl titanate to Fe in Fe3O4 is (1-3):1. The mass or volume of tetrabutyl titanate can be calculated based on the mass of Fe3O4.

[0045] In some embodiments, Fe 2+ Adding salt to the acid solution to obtain the metal ion solution specifically includes: adding FeSO4 to the hydrochloric acid solution to obtain the metal ion solution; wherein the mass volume ratio of FeSO4 to the hydrochloric acid solution is (10-20) g: (60-80) mL, and the pH of the hydrochloric acid solution is <1.

[0046] In some embodiments, the preparation method of the first alkaline solution includes: adding NaNO3 and NaOH to water and stirring to obtain the first alkaline solution; wherein the mass ratio of NaNO3, NaOH and water is (3-7):(5-15):(70-80).

[0047] Based on the same inventive concept, the present invention also provides an application of the above-mentioned core-shell catalyst or the core-shell catalyst prepared by the above-mentioned preparation method in catalytic oxidation denitrification of flue gas.

[0048] In some embodiments, the above application includes the following steps:

[0049] S1. Providing a catalytic reactor and placing a core-shell catalyst in the catalytic reactor;

[0050] S2. Providing a mixing tube, wherein the mixing tube is connected to the catalytic reactor and is provided with a heating device;

[0051] S3. The mixing tube is heated by a heating device, and flue gas is introduced into the mixing tube and H2O2 solution is injected into the mixing tube by a syringe. After the H2O2 solution is heated, H2O2 vapor is formed and mixed with the flue gas to form a mixed gas. The mixed gas enters the catalytic reactor for reaction, and the mixed gas after the reaction is introduced into the second alkali solution.

[0052] In some embodiments, the flow rate of flue gas is 0.25-2 L / min, the concentration of H2O2 solution is 1-5 mol / L, the injection rate of the syringe is 10-50 μL / min, the temperature of the catalytic reactor is 100-240°C, and the second alkali solution includes NaOH solution and / or KOH solution.

[0053] In the above embodiment, after the reaction, the mixed gas, the oxidation products (NO2, HNO2, HNO3 and H2SO4) and SO2 are absorbed by the second alkaline solution.

[0054] Specifically, the H2O2 solution in the present invention is an H2O2 aqueous solution.

[0055] Furthermore, the device used in the flue gas catalytic oxidation denitrification process of this application is as follows Figure 1As shown, the flue gas used in this application is mainly NO, and also includes SO2, O2 and N2 (as a balance gas); specifically, the device includes a NO gas cylinder 11, a SO2 gas cylinder 12, an O2 gas cylinder 13, and a N2 gas cylinder 14. The NO gas cylinder 11, the SO2 gas cylinder 12, the O2 gas cylinder 13, and the N2 gas cylinder 14 are all connected to the flue gas mixing pipe 16, and the NO gas cylinder 11, the SO2 gas cylinder 12, the O2 gas cylinder 13, and the N2 gas cylinder 14 are used to store NO, SO2, O2 and N2 respectively. Flowmeters 15 are provided between the NO gas cylinder 11, the SO2 gas cylinder 12, the O2 gas cylinder 13, the N2 gas cylinder 14 and the flue gas mixing tube 16 to control the flow of the corresponding gas; NO, SO2, O2 and N2 enter the flue gas mixing tube 16 and mix to obtain flue gas, and flue gas with different compositions is obtained by controlling the composition of different gases; the flue gas mixing tube 16 is connected to the mixing tube 18 through a three-way valve 17, and the H2O2 solution is injected into the mixing tube 18 through an injection pump 20. The mixing tube 18 is a quartz tube. Wrapped with a heating belt 19, when the flue gas in the flue gas mixing tube 16 and the H2O2 solution enter the mixing tube 18, the mixing tube 18 is heated at the same time (heated to 140-150°C), and the H2O2 solution is heated to become H2O2 vapor. At this time, the flue gas and the H2O2 vapor are mixed to form a mixed gas; the mixing tube 18 is connected to the catalytic reactor 21, and the flue gas and the H2O2 vapor are mixed to form a mixed gas that enters the catalytic reactor 21 and reacts under the action of the core-shell catalyst. The mixed gas after the reaction enters The washing bottle 22 is provided with a second alkaline solution, and the oxidation products generated by the mixed gas after the reaction are absorbed by the second alkaline solution; the washing bottle 22 is also connected to the dryer 23, and the gas absorbed by the washing bottle 22 enters the dryer 23 for drying, and finally enters the flue gas analyzer 24 to measure the concentration of each component of the outlet flue gas; the flue gas analyzer 24 is also connected to the three-way valve 17 through the stop valve 25, and the gas after passing through the flue gas analyzer 24 can also pass through the stop valve 25 and the three-way valve 17 to re-enter the mixing pipe 18 for further reaction.

[0056] Specifically, the removal efficiency of NO and SO2 under different working conditions after the reaction is obtained by the following formula:

[0057]

[0058] Where η is the removal efficiency (%), C in and C out are the inlet and outlet concentrations of NO or SO2, respectively.

[0059] The following further illustrates the core-shell catalysts of the present application, their preparation methods, and applications using specific examples. This section further illustrates the present invention with reference to specific examples, but should not be construed as limiting the present invention. Unless otherwise specified, the techniques employed in the examples are conventional techniques well known to those skilled in the art. Unless otherwise specified, the reagents, methods, and equipment employed in the present invention are conventional in the art.

[0060] Example 1

[0061] The present invention provides a method for preparing a Fe3O4 catalyst, comprising the following steps:

[0062] S1. Dissolve 16.68 g of FeSO4·7H2O in 75 mL of HCl solution (pH 0.5) by stirring to form a metal ion solution;

[0063] S2, 5g NaNO3 and 10g NaOH were dissolved in 75mL ultrasonic deionized water with stirring to form an alkaline solution;

[0064] S3. Pour the alkali solution into a three-necked flask, maintain heating in a water bath at 90°C, slowly drip the metal ion solution into the three-necked flask, continue stirring for 2 hours and introduce nitrogen to prevent the ferrous ions from being oxidized by air; wash the precipitate three times with deionized water, place the obtained sample at 80°C and dry it for 24 hours, grind and sieve to 150 mesh to obtain Fe3O4 particles, which are Fe3O4 catalysts.

[0065] Example 2

[0066] The present invention provides a method for preparing a core-shell catalyst, which comprises the following steps:

[0067] S1. Prepare Fe3O4 particles according to the method in Example 1; put 0.4g of Fe3O4 into a three-necked flask, add 150ml of anhydrous ethanol, 50ml of acetonitrile and 3ml of 28wt% ammonia water, ultrasonicate for 15min, and stir to obtain a first mixture;

[0068] S2. A certain amount of tetrabutyl titanate (the mass or volume of tetrabutyl titanate can be calculated based on the molar ratios of Fe3O4 and TiO2 being 1:1, 1:2, and 1:3, respectively, and 0.4 g of Fe3O4) is stirred and dissolved in 50 ml of anhydrous ethanol to obtain a second mixture;

[0069] S3. Add the second mixture dropwise to the first mixture, continue stirring in a 45°C water bath for 2 hours, perform solid-liquid separation with a magnet, wash three times with ethanol and deionized water respectively, dry the sample at 80°C for 24 hours, grind and sieve to 150 mesh to obtain a core-shell catalyst (Fe3O4@TiO2); according to the amount of tetrabutyl titanate added, the molar ratios of Fe3O4 and TiO2 in the core-shell catalyst Fe3O4@TiO2 finally prepared are 1:1, 1:2, and 1:3, respectively, and the obtained core-shell catalysts Fe3O4@TiO2 are recorded as FT1, FT2, and FT3, respectively.

[0070] Comparative Example 1

[0071] This comparative example provides a method for preparing a Fe3O4 / TiO2 catalyst, comprising the following steps:

[0072] S1. Place a certain amount of TiO2 powder in a three-necked flask, and then add 100 mL of 0.2 mol / L NaOH solution to obtain a mixture;

[0073] S2. Dissolve 2.7 g of FeCl3·6H2O and 1.39 g of FeSO4·7H2O in 100 mL of HCl solution (pH 0.5) by stirring to form a metal ion solution;

[0074] S3. Add the metal ion solution to the mixture and continue stirring for 2 hours in a water bath at 80°C while introducing nitrogen. After the reaction is completed, use a magnet to separate the solid and liquid. Wash the sample with deionized water and dry it at 80°C for 24 hours. Grind and sieve to 150 mesh to obtain Fe3O4 / TiO2 catalyst. The molar ratio of Fe3O4 and TiO2 in the Fe3O4 / TiO2 catalyst is 1:2.

[0075] 1. Catalyst Characterization (Characterization of the Different Catalysts Prepared in Examples 1-2 and Comparative Example 1)

[0076] 1.1 TEM analysis

[0077] The morphology and structure of different catalysts are as follows Figure 2 shown. Figure 2(a)-(c) show the TEM images of FT1, FT2 and FT3. Compared with TiO2, Fe3O4 has a higher ionization energy. Therefore, the darker area in the core is Fe3O4, and the lighter part of the outer layer is TiO2. It can be seen that Fe3O4@TiO2 presents a well-dispersed spherical structure. These spherical particles are about 200nm in diameter, and the outer shell tightly wraps the core particles, forming an obvious core-shell structure. As the molar ratio of TiO2 increases, the thickness of the outer TiO2 shell also gradually increases. The high-angle annular dark field image (HAADF) and elemental mapping (EDS) of the FT2 catalyst are shown in Figure 2. Figure 2 As shown in (d)-(g), the Ti and O signals are localized in the outer layer of the Fe₃O₄@TiO₂ nanoparticles, while the Fe signal is located in the inner layer. This indicates that the Fe₃O₄ nanoparticles are encapsulated by the TiO₂ shell, representing a typical core-shell structure. The TiO₂ shell provides a relatively closed environment, favoring the enrichment of reactants and thus promoting the reaction. Figure 3 HRTEM image of FT2 catalyst. The interplanar spacing of 0.25 nm corresponds to the (311) crystal plane of Fe3O4 (JCPDS PDF#88-0866), and the diffuse diffraction pattern of the outer layer corresponds to amorphous TiO2.

[0078] 1.2XRD analysis

[0079] Figure 4 The XRD patterns of Fe3O4 and FT1, FT2, FT3 and Fe3O4 / TiO2 are shown.

[0080] from Figure 4 As can be seen, the XRD diffraction peaks of Fe3O4 closely resemble those of the core-shell catalysts FT1, FT2, and FT3, and are consistent with those of a standard cubic spinel structure (JCPDS 88-0866). Furthermore, the characteristic diffraction peak intensities of FT1, FT2, and FT3 are all lower than those of Fe3O4, likely due to the weakening of the measured Fe3O4 diffraction peak signal intensity due to the encapsulation of the TiO2 shell. Comparison of the diffraction patterns of FT1, FT2, and FT3 reveals that with increasing TiO2 molar ratio, the shell becomes thicker and the diffraction peak intensity gradually decreases. However, no characteristic diffraction peaks of TiO2 are observed in the XRD patterns of FT1, FT2, and FT3. This indicates that the TiO2 in the core-shell catalyst is uniformly distributed and highly dispersed on the Fe3O4 surface, primarily in an amorphous state, consistent with TEM observations. Amorphous TiO2 is isotropic and possesses open active diffusion channels, which facilitate the diffusion of reactant molecules.

[0081] Figures 5-6 These are the N2 adsorption-desorption curves and pore size distributions of different catalysts.

[0082] from Figures 5-6 It can be seen that the adsorption and desorption isotherms of the catalysts are all typical IV type, and the hysteresis loops are typical H3 type, indicating that there are mesopores on the surface of the catalysts. The adsorption isotherms of the core-shell structure catalysts have a significant increase in adsorption amount in the relative pressure (P / P0) range of 0 to 0.4, indicating that there are a large number of narrow mesopores in the catalysts. Figure 6 It can be seen that the pore size of the core-shell catalyst is primarily concentrated around 4 nm, while the pore size of the Fe₃O₄ / TiO₂ catalyst is concentrated at 7 nm. It can be seen that after the TiO₂ shell is coated, the pore size of the catalyst becomes more concentrated and uniform, which is beneficial for improving the adsorption and desorption capacity of reactants. The smaller pore size of the core-shell catalyst facilitates the increase of specific surface area. Table 1 shows the pore structure parameters such as specific surface area, pore size, and pore volume of each catalyst.

[0083] Table 1 - Specific surface area, average pore diameter and pore volume of catalysts

[0084] <![CDATA[Fe3O4]]> FT1 FT2 FT3 <![CDATA[Fe3O4 / TiO2]]> <![CDATA[Specific surface area (m 2 / g)]]> 50.72 158.12 201.52 235.33 85.36 Pore ​​diameter (nm) 11.39 4.05 3.62 3.38 7.37 <![CDATA[Pore volume (cm 3 / g)]]> 0.124 0.167 0.186 0.224 0.150

[0085] Table 1 shows that the specific surface area and pore volume of the catalyst significantly increase after TiO2 coating Fe3O4, likely due to the loose structure of amorphous TiO2. A larger specific surface area exposes more active sites on the catalyst surface. A larger pore volume provides more channels for NO and H2O2 to enter the core layer. This suggests that the numerous small-diameter channels in the TiO2 shell provide abundant active sites and adsorption interfaces for reactant molecules. Reactants and intermediates can be rapidly transported through these interconnected channels, promoting product dissipation and replenishing fresh reactants, thereby enhancing catalytic activity. Furthermore, with an increase in the n(Ti) ratio in n(Fe):n(Ti), the specific surface area of ​​the core-shell catalyst increases, the average pore diameter decreases, and the pore volume increases. This is likely due to the increased number of small channels in the shell. In contrast, while the pore structure parameters of Fe3O4 / TiO2 improve compared to Fe3O4, the changes in the catalyst's pore structure are limited. The reduction of pore size and the increase of specific surface area are beneficial to reducing the competitive adsorption of SO2 with NO and H2O2 on the active sites.

[0086] 1.4XPS analysis

[0087] XPS analysis of Fe 2p patterns was performed on various catalysts. The binding energies of Fe 2p3 / 2 and the distribution of Fe valence states are shown in Table 2. The relative contents of Fe and O at different valence states on the catalyst surface can be determined based on the areas of the characteristic peaks. XPS probing depths typically within 35 nm indicate that the Fe 2p patterns are detectable, indicating a thin shell thickness for the core-shell catalysts, consistent with observations in TEM images.

[0088] Table 2 - Binding energy and relative content of Fe and O elements on the catalyst surface

[0089]

[0090]

[0091] It can be seen from Table 2 that Fe 2+ The relative content of Fe in FT2 catalyst is significantly higher than that in Fe3O4 catalyst. 2+ / Fe 3+ The highest is 0.661. This shows that the interaction between the core layer and the shell can effectively promote the Fe 2+ The formation of ions. Due to the diversity of transition metal ions, more structural defects will be generated, and smaller-sized TiO2 will precipitate on the core layer using the morphology of the Fe3O4 core as a template, forming more surface defects. At the same time, the crystal structure of amorphous TiO2 itself contains many defects, so it is easy to form a part of Ti 3+ From the equation, we can see that Fe ions and Ti ions can form a redox cycle, Fe 3+ and Ti 3+ The reaction produces Fe 2+ and Ti 4+ , increasing the Fe 2+ With the increase of TiO2 coating, the dispersion of Fe3O4 particles will become worse, which is not conducive to Fe 2+ In addition, the Fe 2+ / Fe 3+ The value is 0.602, indicating that there is also an interaction between Fe3O4 and TiO2 support, which leads to an increase in the electron density around Fe3O4 and promotes the 2+ The generation of ions. The cycle of Fe ions and Ti ions is as follows: Fe 3+ +Ti 3+ →Fe 2+ +Ti 4+ .

[0092] Ti 2p spectra on the surface of core-shell catalyst and Fe3O4 / TiO2 catalyst are shown in Figure 2. Figure 7As shown. The XPS spectrum of Ti 2p mainly contains two energy level peaks, Ti 2p 1 / 2 and Ti 2p3 / 2, which are located near 464.2eV and 458.6eV respectively. The binding energy of Ti2p3 / 2 on the core-shell catalyst surface (458.6eV) is higher than that of Ti 2p3 / 2 of TiO2 (458.2eV), which may be due to the charge transfer between Ti and Fe. Fe has a higher electronegativity than Ti and a stronger ability to attract electrons. The charge transfer between Ti and Fe reduces the electron density around Ti, reduces the shielding effect, and increases the electron binding energy. This shows that there is a synergistic effect between Ti and Fe, which is conducive to the generation of more Fe 2+ In addition, the binding energy of Ti 2p3 / 2 in the core-shell catalyst is higher than that in the Fe3O4 / TiO2 catalyst. This is because the core-shell structure has the largest contact surface between the two phases, resulting in a greater amount of charge transfer, indicating that the core-shell structure has a significant heterojunction synergistic effect.

[0093] The catalyst was subjected to XPS analysis of O1s. O1s can be mainly divided into lattice oxygen (O lat , 530.1eV) and surface adsorbed oxygen (O ads , 531.7eV) two characteristic peaks. Among them, O lat is the lattice oxygen of the catalyst, and O ads The surface active oxygen generated by the adsorbed oxygen and other molecules, the relative content of the two is shown in Table 2. The presence of oxygen vacancies can promote the generation of surface active oxygen, thereby increasing the content of surface adsorbed oxygen, which has better reaction activity due to its higher mobility. ads / (O ads +O lat ) can indirectly reflect the relative content of oxygen vacancies on the catalyst surface. Oxygen vacancies can serve as active sites to activate H2O2 to generate ·OH, undergoing redox reactions. After the reaction products desorb, oxygen vacancies are regenerated on the catalyst surface, forming a dynamic equilibrium process of consumption and replenishment. The relative order of oxygen vacancy concentration of the catalysts is: FT2 > Fe3O4 / TiO2 > Fe3O4 catalyst, indicating that TiO2 loading can promote the increase of oxygen vacancy concentration. Fe 2+ and Ti 3+ The presence of Fe3O4 allows more oxygen vacancies to be generated on the catalyst surface through the oxygen vacancy compensation mechanism. It is worth noting that the oxygen vacancy concentration on the surface of the FT3 catalyst is lower than that of the FT2 catalyst. This may be because the amount of TiO2 that can interact with Fe3O4 is limited. When the TiO2 loading is too high, the newly added TiO2 is simply stacked on the existing TiO2 shell, and the oxygen vacancy concentration may even be reduced due to the agglomeration of TiO2.

[0094] 1.3 VSM Analysis

[0095] The VSM curves of Fe3O4, FT1, FT2, FT3 and Fe3O4 / TiO2 catalysts are as follows Figure 8 As shown, their saturation magnetizations are 70.17, 52.16, 37.50, 30.24, and 22.54 emu / g, respectively. It can be seen that both the core-shell catalyst and the supported catalyst exhibit ferromagnetic properties. As the thickness of the TiO2 shell increases, the Fe3O4 content per unit mass of the core-shell catalyst gradually decreases, and thus the corresponding saturation magnetization decreases. The saturation magnetization of the Fe3O4 / TiO2 catalyst is lower than that of the FT2 catalyst, which may be due to the more uniform dispersion of Fe3O4 on the TiO2 and the fact that some Fe3O4 was not successfully loaded onto the support during the preparation process. The saturation magnetization of the FT2 catalyst is as high as 37.5 emu / g, making it a highly magnetic material. It also exhibits ferromagnetism with almost zero coercivity and remanence, which will facilitate the recovery and reuse of the catalyst.

[0096] 2 Analysis of catalyst denitrification and desulfurization performance

[0097] supply Figure 1 In the flue gas catalytic oxidation denitrification device shown, 0.1 g of catalysts prepared by different methods are added to the catalytic reactor and mixed in the flue gas mixing tube to obtain a flue gas with O2, NO and SO2 concentrations set to 6 vol%, 500 ppm and 1000 ppm, respectively, and the remainder is N2; 500 mL of 0.1 mol / L NaOH solution is provided in the washing bottle 22; during the reaction, the heating belt 19 heats the mixing tube 18 to a temperature of 140°C; the process conditions for reaction control are: the temperature of the catalytic reactor is 100-240°C, the flow rate of the flue gas entering the mixing tube is 0.25-2 L / min, the concentration of the H2O2 solution is 1-5 mol / L, and the injection rate of the injection pump is 10-50 μL / min.

[0098] 2.1 Effect of reaction temperature

[0099] The synergistic removal efficiency of NO and SO2 by different catalysts at different reaction temperatures (when the reaction temperature is changed, the flow rate of flue gas entering the mixing tube is 0.5L / min, the concentration of H2O2 solution is 2mol / L, and the injection rate of the injection pump is 30μL / min) is shown as follows Figure 9As shown. The effect of reaction temperature on the five catalysts follows a similar pattern: as the temperature increases, the NO removal efficiency first gradually increases, then remains relatively stable in the range of 140°C to 200°C, and finally gradually decreases. From the Arrhenius equation, it can be seen that as the reaction temperature increases, the reaction rate of H2O2 catalytic decomposition and NO oxidation will accelerate, which is conducive to the generation of more active free radicals and the oxidation removal of NO. However, the increase in reaction temperature will also lead to an increase in the self-consumption reaction rate between active free radicals and H2O2, reducing the concentration of free radicals in the reaction, thereby reducing the rate of the catalytic reaction, and accompanied by the decomposition of some oxidation products. The specific chemical reactions involved are shown below:

[0100] Fe 2+ +H2O2→Fe 3+ +·OH+OH-(k1=70M / s)

[0101] Fe 3+ +H2O2→Fe 2+ +·OOH+H + (k2=0.001~0.1M / s)

[0102] Ti 3+ +H2O2→Ti 4+ + OH

[0103] Ti 4+ +H2O2→Ti 3+ + OOH

[0104] H2O2+·OH→·OOH+H2O

[0105] OOH→H2O2+O2

[0106] ·OH+·OH→H2O2

[0107] ·OH+·OOH→H2O+O2

[0108] 3HNO2→HNO3+H2O+2NO

[0109] 4HNO3→4NO2+2H2O+O2

[0110] The catalytic efficiency of the core-shell catalysts is higher than that of Fe3O4, indicating that there is a strong interaction between the core and the shell, which promotes the activation and decomposition of H2O2. According to the Haber-Weiss reaction, the iron-based catalyst reacts with H2O2 to produce a large amount of ·OH, thereby promoting the oxidation and removal of NO. Among them, the FT2 catalyst has the highest efficiency. Although the FT3 catalyst has a relatively excellent pore structure, its thick shell may hinder the entry of H2O2 and NO into the core layer for catalytic reaction. The FT2 catalyst has more Fe2+ The presence of oxygen vacancies directly promotes the catalytic decomposition of H2O2, resulting in higher NO removal efficiency. The FT2 catalyst outperforms the Fe3O4 / TiO2 catalyst due to the superior structural properties of the core-shell catalyst and the stronger synergistic effect between Fe and Ti. When the reaction temperature is between 140°C and 200°C, the interactions among these factors are in equilibrium, resulting in the highest NO removal efficiency. This means that in practical applications, the catalyst can be placed after the electrostatic precipitator.

[0111] 2.4 Influence of flue gas flow

[0112] The effect of flue gas flow rate (when changing the flue gas flow rate, the temperature of the catalytic reactor is 140℃, the concentration of H2O2 solution is 2mol / L, and the injection rate of the injection pump is 30μL / min) on the catalytic efficiency of FT2 catalyst and Fe3O4 / TiO2 catalyst is as follows Figure 10 shown. Figure 10 In the table, SO2-All means that the SO2 removal efficiency of all catalysts is 100%, that is, the alkali solution can completely absorb and remove SO2.

[0113] When the gas flow rate increased from 0.25 L / min to 0.5 L / min, the NO removal efficiency of the FT2 catalyst decreased slightly, indicating that the amount of H2O2 relative to NO was in excess at this point, and the flue gas resided on the catalyst long enough to oxidize most of the NO. This ensured stable and efficient denitrification, with the FT2 catalyst achieving a maximum denitrification efficiency of 88%. However, when the gas flow rate exceeded 0.5 L / min, the NO removal efficiency began to decline, plummeting to 70.8% at 2 L / min, while the Fe3O4 / TiO2 catalyst achieved a NO removal efficiency of 75.8%. The increased flue gas flow rate resulted in a deficit of H2O2 relative to NO, preventing the ·OH generated during the reaction from fully oxidizing NO. Furthermore, the increased flue gas flow rate significantly reduced the contact time between the catalyst surface and the reactant molecules (H2O2 and NO), resulting in some H2O2 not being decomposed as it passed through the catalyst, reducing the production of active free radicals. It can be seen that when the gas hourly space velocity (GHSV) exceeds a certain range, the NO removal efficiency of the core-shell catalyst decreases more significantly with increasing GHSV than that of the Fe₃O₄ / TiO₂ catalyst. This is likely due to the adsorption characteristics of the core-shell catalyst. The presence of the smaller pore size shell increases the resistance of reactants to the catalyst core, thereby increasing their adsorption time on the shell structure. During the reaction, some SO₂ is blocked or adsorbed by the TiO₂ shell, while most NO and a small amount of SO₂ penetrate the shell to reach the core surface and be adsorbed. This weakens the inhibitory effect of SO₂ on NO adsorption on the core layer, achieving layered adsorption of SO₂ and NO on the core-shell catalyst.

[0114] Considering the comprehensive catalytic efficiency and cost, the GHSV is 1.5×10 5 h -1 The GHSV of SCR catalyst in actual power plants is generally 10 3 ~10 4 h -1 Therefore, the FT2 catalyst can meet the actual operation requirements, and the amount of catalyst used is less while maintaining a higher denitrification efficiency, which is beneficial to the reduction of the volume of the catalytic reactor and the reduction of system resistance.

[0115] Effect of 2.5H2O concentration

[0116] When the H2O2 solution is heated and evaporated, its volume will expand more than 1100 times. At this time, the H2O content in the flue gas cannot be ignored. The H2O2 concentration and the H2O2 injection rate can be controlled to ensure that the H2O2 / NO ratio remains unchanged, while indirectly controlling the H2O content (volume content) in the flue gas. When the H2O2 concentration is greater than 2 mol / L, the denitrification efficiency of the catalyst does not change much with the increase of the H2O2 concentration. Therefore, when the H2O content is 2% and 6%, although the corresponding H2O2 concentration is different, the effect on the NO removal efficiency is very small. At this time, the main factor affecting the NO removal performance is only the H2O content. When the flue gas flow rate is 0.5L / min, the H2O2 concentration is 2mol / L, and the H2O2 injection rate is 30μL / min, the H2O content in the flue gas is about 6%. Figure 11 The study demonstrates the influence of flue gas water vapor concentration on NO removal in a catalytic oxidation reaction system. When the H2O content in the flue gas changes from 2% to 6%, the NO removal efficiency of the FT2 catalyst decreases by approximately 2%, while the efficiency of the Fe3O4 / TiO2 catalyst decreases by approximately 4%. H2O competes with H2O2 for adsorption on the active sites and oxygen vacancies on the catalyst surface, thereby affecting the generation of active free radicals. Amorphous TiO2 readily adsorbs H2O molecules, resulting in a core-shell catalyst with strong water resistance. Even at a 6% H2O concentration, it maintains a denitrification efficiency exceeding 86%, making it suitable for actual power plant operating conditions. Furthermore, when the H2O concentration is lowered to 2%, both catalysts return to their original denitrification efficiency, demonstrating the catalysts' good reversibility.

[0117] 2.6 Catalyst stability study

[0118] In order to evaluate the reaction stability of FT2 catalyst and Fe3O4 / TiO2 catalyst, a 6-h continuous test was carried out under the same experimental conditions (reaction temperature 140℃; flue gas flow rate 0.5L / min; H2O2 concentration 2mol / L; H2O2 injection rate 60μL / min). The experimental results are shown in Figure 2. Figure 12As can be seen from the figure, the stability of the two catalysts is good within 3 hours. After 6 hours, the denitrification efficiency of the FT2 catalyst decreased by 4.1%, while that of the Fe3O4 / TiO2 catalyst decreased by 8.6%. The reason for the decrease in the denitrification efficiency of the catalyst during long-term operation is that the H2O2 continuously added during the reaction process will denitrify the Fe in the catalyst. 2+ Converted to Fe with lower catalytic activity 3+ , which in turn reduces the catalytic efficiency of H2O2 and leads to a decrease in denitrification efficiency. Due to the presence of the TiO2 shell, the FT2 catalyst reduces the contact and erosion of the active components with SO2 and H2O to a certain extent, so the catalyst has higher stability.

[0119] Based on the characterization and experimental results, we can speculate on the mechanism by which core-shell structure catalysts have higher denitrification efficiency and better stability than supported catalysts. First, core-shell structure catalysts have higher specific surface area and pore volume and smaller pore size, which is conducive to providing abundant active sites and adsorption interfaces. At the same time, the pores of the TiO2 shell increase the accessibility of the active sites at the core-shell interface to the reactant molecules, which has a mass transfer enhancement effect, thereby improving the activity of the catalytic reaction. Secondly, TiO2 has good adsorption capacity, so the presence of the TiO2 shell can adsorb and block part of SO2, reducing the competitive adsorption of SO2 with NO and H2O2 at the core-shell interface, which is beneficial to improving catalytic efficiency and reducing oxidant consumption. Third, the interaction between the core and shell promotes the redistribution of localized electronic states at the interface between the two phases, thereby improving the activity of the two-phase interface. A higher proportion of Fe on the surface of the core-shell catalyst 2+ and Ti 3+ It can induce the formation of more oxygen vacancies, promote the adsorption and activation of gas-phase oxygen, and the active oxygen atoms formed play an important role in the oxidation of NO. The presence of oxygen vacancies can further promote the adsorption and decomposition of H2O2. There is Fe in the core-shell catalyst. 2+ +Ti 4+ and Fe 3+ +Ti 3+ Two pairs of redox pairs make Fe on the catalyst surface 2+ Maintaining the content in a higher range is conducive to the catalytic decomposition of H2O2 to produce more ·OH, thereby improving the removal efficiency of NO.

[0120] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. Application of a core-shell catalyst in catalytic oxidation denitrification of flue gas; The core-shell catalyst comprises an Fe3O4 core layer and a TiO2 shell layer covering the Fe3O4 core layer; The method for preparing the core-shell catalyst comprises the following steps: Fe3O4 is placed in a container, and then an alcohol solvent and acetonitrile are added, dispersed, and then ammonia water is added and stirred to obtain a first mixture; adding tetrabutyl titanate to an alcohol solvent and stirring to obtain a second mixture; The second mixture is added dropwise to the first mixture, stirred at 40-50° C., and solid-liquid separation is performed to obtain a core-shell catalyst; The molar ratio of Fe3O4 to TiO2 in the core-shell catalyst is 1:2; The alcohol solvent includes ethanol and / or methanol; In the step of placing Fe3O4 in a container, then adding an alcohol solvent and acetonitrile, dispersing the Fe3O4, and then adding aqueous ammonia, the mass volume ratio of Fe3O4, alcohol solvent, acetonitrile, and aqueous ammonia is (0.2-0.6) g:(140-160) mL:(40-60) mL:(2-5) mL, and the mass fraction of aqueous ammonia is 25-30%; The application comprises the following steps: Providing a catalytic reactor, and placing a core-shell catalyst in the catalytic reactor; Providing a mixing tube, the mixing tube being in communication with the catalytic reactor, and the mixing tube being provided with a heating device; The mixing tube is heated by a heating device, and flue gas is introduced into the mixing tube at the same time. H2O2 solution is injected into the mixing tube by a syringe. The H2O2 solution is heated to form H2O2 vapor which mixes with the flue gas to form a mixed gas. The mixed gas enters the catalytic reactor for reaction. After the reaction, the mixed gas is introduced into the second alkali solution. The flow rate of the flue gas is 0.5 L / min, the concentration of the H2O2 solution is 2 mol / L, the injection rate of the syringe is 30 μL / min, the temperature of the catalytic reactor is 140° C., and the second alkali solution includes NaOH solution and / or KOH solution.

2. The use according to claim 1, characterized in that The preparation method of Fe3O4 comprises the following steps: Fe 2+ The salt is added to the acid solution to obtain a metal ion solution; Add the metal ion solution to the first alkaline solution, heat and stir at 80-100°C under the protection of inert gas, and obtain Fe3O4 after solid-liquid separation; The preparation method of the first alkali solution comprises: adding NaNO3 and NaOH into water and stirring to obtain the first alkali solution.

3. The use according to claim 2, characterized in that Fe 2+ The method of adding a salt to an acid solution to obtain a metal ion solution specifically comprises: adding FeSO4 to a hydrochloric acid solution to obtain a metal ion solution; wherein the mass volume ratio of FeSO4 to the hydrochloric acid solution is (10-20) g: (60-80) mL, and the pH of the hydrochloric acid solution is less than 1; The mass ratio of NaNO3, NaOH and water is (3-7):(5-15):(70-80).

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