A cerium-doped core-shell catalyst and its preparation method and application

Abstract

The present invention provides a cerium-doped core-shell catalyst and its preparation method and application. The cerium-doped core-shell catalyst of the present invention comprises a Fe3O4 core layer and a TiO2 shell layer, wherein the TiO2 shell layer is a Ce-doped TiO2 shell layer; the cerium-doped core-shell catalyst of the present invention has a high specific surface area and an excellent pore structure, and Ce doping introduces Ce into the core-shell catalyst. 4+ / Ce 3+ Redox pairing increases the Fe 2+ 、Ti 3+ and oxygen vacancy concentration; after doping a certain proportion of Ce, the catalyst still has good magnetic separation. XPS analysis shows that the Fe 2+ and Ti 3+ The decrease in concentration is smaller than that of the undoped Ce catalyst, indicating that its cyclic reaction rate is higher; the denitration efficiency, water resistance and stability of the cerium-doped core-shell catalyst prepared by the present invention are improved. Under the optimal working conditions, the denitration efficiency of the FTC5 catalyst can reach 92%.

Description

Technical Field

[0001] The present invention belongs to the technical field of desulfurization and denitrification, and in particular relates to a cerium-doped 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] Although the denitrification efficiency of core-shell catalysts has been greatly improved, there is still room for further improvement. my country's rare earth element reserves rank first in the world. Among them, cerium dioxide (CeO2) has been widely used as an active component of catalysts and in catalyst modification due to its rich surface oxygen vacancy defects and excellent redox properties. However, there is currently no modification of core-shell catalysts by Ce doping and their application in the field of H2O2 catalytic oxidation denitrification. Summary of the Invention

[0005] In view of this, the present invention proposes a cerium-doped 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 method for preparing a cerium-doped core-shell catalyst, comprising the following steps:

[0007] Put Fe3O4 in a container, then add alcohol solvent and acetonitrile, disperse it, add ammonia water, stir, and obtain a first mixed material;

[0008] Adding the cerium source and the titanium source into the alcohol solvent and stirring to obtain a second mixed material;

[0009] The second mixed material is dropped into the first mixed material, stirred at 40-50° C., and after solid-liquid separation, a cerium-doped core-shell catalyst is obtained.

[0010] Preferably, in the method for preparing the cerium-doped core-shell catalyst, the cerium source comprises at least one of cerium chloride, cerium sulfate, and cerium nitrate;

[0011] And / or, the titanium source includes at least one of ethyl titanate, isopropyl titanate, and tetrabutyl titanate.

[0012] Preferably, the preparation method of the cerium-doped core-shell catalyst and the preparation method of Fe3O4 comprise the following steps:

[0013] Fe 2+ The salt is added to the acid solution to obtain a metal ion solution;

[0014] The metal ion solution is added to the first alkaline solution, and heated and stirred at 80-100° C. under the protection of inert gas. After solid-liquid separation, Fe3O4 is obtained.

[0015] Preferably, the method for preparing the cerium-doped core-shell catalyst comprises the following steps: 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;

[0016] The preparation method of the first alkali solution comprises: adding NaNO3 and NaOH to water and stirring to obtain the first alkali solution; wherein the mass ratio of NaNO3, NaOH and water is (3-7):(5-15):(70-80).

[0017] Preferably, in the preparation method of the cerium-doped core-shell catalyst, in the step of placing Fe3O4 in a container, then adding an alcohol solvent and acetonitrile, dispersing, and then 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%.

[0018] Preferably, in the preparation method of the cerium-doped core-shell catalyst, the molar ratio of cerium to titanium in the cerium-doped core-shell catalyst is (1-10):100;

[0019] And / or, the alcohol solvent includes ethanol and / or methanol.

[0020] In a second aspect, the present invention further provides a cerium-doped core-shell catalyst prepared by the above-mentioned preparation method.

[0021] In a third aspect, the present invention further provides a cerium-doped core-shell catalyst prepared by the preparation method or the use of the cerium-doped core-shell catalyst in catalytic oxidation denitrification of flue gas.

[0022] Preferably, the application comprises the following steps:

[0023] Providing a catalytic reactor, placing a cerium-doped core-shell catalyst in the catalytic reactor;

[0024] Providing a mixing tube, the mixing tube being in communication with the catalytic reactor, and the mixing tube being provided with a heating device;

[0025] 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. 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.

[0026] Preferably, in the application, the flow rate of the flue gas is 0.25 to 2 L / min, the concentration of the H2O2 solution is 1 to 5 mol / L, the injection rate of the syringe is 10 to 50 μL / min, the temperature of the catalytic reactor is 100 to 240°C, and the second alkali solution includes NaOH solution and / or KOH solution.

[0027] The cerium-doped core-shell catalyst of the present invention and its preparation method and application have the following beneficial effects compared with the prior art:

[0028] 1. The cerium-doped core-shell catalyst prepared by the present invention has a high specific surface area and an excellent pore structure; Ce doping introduces Ce 4+ / Ce 3+ Redox pairing increases the Fe 2+ 、Ti 3+ and oxygen vacancy concentration; after doping a certain proportion of Ce, the catalyst still has good magnetic separation. XPS analysis shows that the Fe 2+ and Ti 3+ The decrease in concentration was smaller than that of the undoped Ce catalyst, indicating a higher cyclic reaction rate. XPS analysis showed that a small amount of SO2 was oxidized to sulfate species during the reaction and was mainly absorbed by the titanium dioxide shell, demonstrating that the shell has an adsorption and barrier effect on SO2.

[0029] 2. The denitration efficiency, water resistance and stability of the cerium-doped core-shell catalyst prepared by the present invention are all improved. Under the optimal working conditions, the denitration efficiency of the FTC5 catalyst can reach 92%. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] 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.

[0031] Figure 1 This is a schematic structural diagram of the flue gas catalytic oxidation denitrification device of the present invention;

[0032] Figure 2 TEM and HRTEM images of FTC5 catalyst;

[0033] Figure 3 XRD patterns of FT2, FTC3, FTC5 and FTC10 catalysts;

[0034] Figure 4 N2 adsorption and desorption curves of different catalysts;

[0035] Figure 5 is the pore size distribution diagram of different catalysts;

[0036] Figure 6 Ce 3d spectra on different catalyst surfaces;

[0037] Figure 7 Ti 2p maps of different catalyst surfaces;

[0038] Figure 8 VSM curves of different catalysts;

[0039] Figure 9 H2-TPR spectra of different catalysts;

[0040] Figure 10 NO and NO with different catalysts x Removal efficiency diagram;

[0041] Figure 11 The effect of GHSV on the desulfurization and denitrification performance of the catalyst;

[0042] Figure 12 The effect of H2O concentration on the denitrification performance of the catalyst;

[0043] Figure 13 It is a reaction stability test of the catalyst;

[0044] Figure 14 This is the EPR spectrum of DMPO capturing free radicals in the catalyst and H2O2 system;

[0045] Figure 15 XPS spectra of FTC5 catalyst before and after use.

[0046] Figure 16 is the S 2p spectrum of the FTC5 catalyst after use. DETAILED DESCRIPTION

[0047] 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.

[0048] 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.

[0049] The present invention provides a method for preparing a cerium-doped core-shell catalyst, comprising the following steps:

[0050] S1. Place Fe3O4 in a container, then add an alcohol solvent and acetonitrile, disperse, add ammonia water, and stir to obtain a first mixed material;

[0051] S2, adding the cerium source and the titanium source to the alcohol solvent, and stirring to obtain a second mixed material;

[0052] S3. Add the second mixed material dropwise into the first mixed material, stir at 40-50° C., and obtain a cerium-doped core-shell catalyst after solid-liquid separation.

[0053] It should be noted that the catalyst prepared by the method of the present invention is a cerium-doped core-shell catalyst. Specifically, the catalyst comprises a Fe3O4 core layer and a TiO2 shell layer coated on the Fe3O4 core layer, and the TiO2 shell layer is a Ce-doped TiO2 shell layer. Ce doping can effectively improve the pore structure, increase the specific surface area and improve the pore structure; Ce doping can introduce Ce 4+ / Ce 3+ Redox pairing increases Fe on the catalyst surface 2+ 、Ti 3+ and oxygen vacancy concentration; the saturation magnetization of the catalyst decreases with the increase of Ce doping amount, but it can still be separated by an external magnetic field; H2-TPR analysis shows that the redox performance of the catalyst is enhanced after Ce doping. At the same time, the cerium-doped core-shell catalyst of the present invention, Ce 4+ and Ce 3+ The conversion of Ce leads to an imbalance in surface coordination atoms, causing lattice oxygen migration and the generation of oxygen vacancies. In the redox reaction, the introduction of Ce can promote the decomposition of H2O2 into OH, achieving higher denitrification efficiency, while also enhancing the catalyst's resistance to water and sulfur.

[0054] In some embodiments, the cerium source includes at least one of cerium chloride, cerium sulfate, and cerium nitrate.

[0055] In some embodiments, the titanium source includes at least one of ethyl titanate, isopropyl titanate, and tetrabutyl titanate.

[0056] 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 5-30 hours to obtain a Ce-doped Fe3O4@TiO2 catalyst, that is, a cerium-doped core-shell catalyst.

[0057] In some embodiments, the method for preparing Fe3O4 comprises the following steps:

[0058] S1, Fe 2+ The salt is added to the acid solution to obtain a metal ion solution;

[0059] 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.

[0060] Specifically, in the above embodiments, the inert gas includes but is not limited to nitrogen, argon, helium, neon, etc.

[0061] 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.

[0062] 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.

[0063] 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).

[0064] 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%.

[0065] In some embodiments, the molar ratio of cerium in the cerium source to titanium in the titanium source is (1-10):100.

[0066] In some embodiments, the molar ratio of titanium in the titanium source to Fe in Fe 3 O 4 is (1-3):1, and the mass or volume of the titanium source can be calculated based on the mass of Fe 3 O 4 .

[0067] In some embodiments, the amount of Ce source added is adjusted according to the Ce content during preparation. Specifically, the molar ratio of cerium to titanium in the cerium-doped core-shell catalyst is (1-10):100.

[0068] In some embodiments, the alcohol solvent includes ethanol and / or methanol.

[0069] Based on the same inventive concept, the present invention also provides a cerium-doped core-shell catalyst, which is prepared using the above-mentioned preparation method.

[0070] Based on the same inventive concept, the present invention also provides a cerium-doped core-shell catalyst prepared by the above-mentioned preparation method or the use of the above-mentioned cerium-doped core-shell catalyst in catalytic oxidation denitrification of flue gas.

[0071] In some embodiments, the above application includes the following steps:

[0072] S1. providing a catalytic reactor, and placing a cerium-doped core-shell catalyst in the catalytic reactor;

[0073] S2. Providing a mixing tube, wherein the mixing tube is connected to the catalytic reactor and is provided with a heating device;

[0074] 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.

[0075] In some embodiments, in the above application, the flow rate of the flue gas is 0.25 to 2 L / min, the concentration of the H2O2 solution is 1 to 5 mol / L, the injection rate of the syringe is 10 to 50 μL / min, the temperature of the catalytic reactor is 100 to 240°C, and the second alkali solution includes NaOH solution and / or KOH solution.

[0076] 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.

[0077] Specifically, the H2O2 solution in the present invention is an H2O2 aqueous solution.

[0078] 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. 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 gases. NO, SO2, O2 and N2 enter the flue gas mixing tube 16 and mix to obtain flue gas. Flue gas with different compositions can be 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. The H2O2 solution is injected into the mixing tube 18 through an injection pump 20. The mixing tube 18 is a quartz tube. The mixing tube 18 contains 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 (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. The mixed gas enters the catalytic reactor 21 and reacts under the action of the cerium-doped core-shell catalyst. The mixed gas after the reaction enters the The gas enters the washing bottle 22, which is provided with a second alkaline solution. 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. 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. 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.

[0079] Specifically, the removal efficiency of NO and SO2 under different working conditions after the reaction is obtained by the following formula:

[0080]

[0081] Where η is the removal efficiency (%), C in and C out are the inlet and outlet concentrations of NO or SO2, respectively.

[0082] The following further illustrates the cerium-doped core-shell catalyst of the present application, its preparation method, and its application using specific examples. This section further illustrates the present invention in conjunction with specific examples, but should not be construed as limiting the present invention. Unless otherwise specified, the technical means used in the examples are conventional means well known to those skilled in the art. Unless otherwise specified, the reagents, methods, and equipment used in the present invention are conventional reagents, methods, and equipment in the art.

[0083] Example 1

[0084] The present invention provides a method for preparing a Fe3O4 catalyst, comprising the following steps:

[0085] 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;

[0086] S2, 5g NaNO3 and 10g NaOH were dissolved in 75mL ultrasonic deionized water with stirring to form an alkaline solution;

[0087] 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.

[0088] Example 2

[0089] The present invention provides a method for preparing a cerium-doped core-shell catalyst, comprising the following steps:

[0090] S1. According to the method in Example 1, Fe3O4 particles were prepared; 0.4 g of Fe3O4 was placed in a three-necked flask, 150 ml of anhydrous ethanol, 50 ml of acetonitrile and 3 ml of 28 wt% ammonia water were added, and ultrasonicated for 15 min with stirring to obtain a first mixture;

[0091] S2. A certain amount of cerium chloride (the mass of cerium chloride can be calculated by the molar ratios of Ce and Ti in the cerium-doped core-shell catalyst being 1:100, 3:100, 5:100, and 10:100, respectively, and 1.2 ml of tetrabutyl titanate) and 1.2 ml of tetrabutyl titanate are stirred and dissolved in 50 ml of anhydrous ethanol to obtain a second mixture;

[0092] 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 cerium-doped core-shell catalyst (Fe3O4@TiO2-Ce); by controlling the amount of cerium chloride added, the molar ratios of Ce and Ti in the finally prepared cerium-doped core-shell catalyst are 1:100, 3:100, 5:100, and 10:100, respectively, and the corresponding catalysts are recorded as FTC1, FTC3, FTC5, and FTC10, respectively.

[0093] Comparative Example 1

[0094] This comparative example provides a method for preparing a core-shell catalyst, comprising the following steps:

[0095] 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;

[0096] S2. Dissolving a certain amount of tetrabutyl titanate in 50 ml of anhydrous ethanol with stirring to obtain a second mixture;

[0097] 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 ratio of Fe3O4 and TiO2 in the core-shell catalyst Fe3O4@TiO2 finally prepared is 1:2, and the obtained core-shell catalyst Fe3O4@TiO2 is recorded as FT2.

[0098] 1. Catalyst Characterization (Characterization of the Different Catalysts Prepared in Examples 1-2 and Comparative Example 1)

[0099] 1.1 TEM analysis

[0100] TEM image (left) and HRTEM image (right) of FTC5 catalyst, as shown in Figure 2 shown.

[0101] from Figure 2It can be seen that the TiO2 shell envelops the Fe3O4 core, maintaining a distinct core-shell structure. The HRTEM image reveals lattice fringes of amorphous TiO2 and crystals, with interplanar spacings of 0.25 nm and 0.31 nm corresponding to the (311) plane of Fe3O4 (JCPDS PDF#88-0866) and the (111) plane of CeO2 (JCPDS PDF#34-0394), respectively, indicating successful Ce doping onto the catalyst surface. The ICP-OES results of the FTC5 catalyst are shown in Table 1.

[0102] Table 1 - ICP-OES test results of FTC5 catalyst

[0103] Detection elements Mass fraction (%) Mole fraction (%) (excluding oxygen elements) Fe 25.42 57.64 Ti 15.23 40.40 Ce 2.17 1.97

[0104] As can be seen from Table 1, the molar ratio of Fe:Ti:Ce is about 29.2:20.5:1, which is close to the 30:20:1 during preparation. Combined with the TEM and HRTEM images, it can be seen that TiO2 and CeO2 are successfully loaded on the surface of the Fe3O4 core.

[0105] 1.2XRD analysis

[0106] Figure 3 The XRD patterns of FT2, FTC3, FTC5 and FTC10 are shown. Figure 3 It can be seen that the spectrum of the catalyst after doping with Ce is highly similar to the FT2 spectrum, and no diffraction peak of TiO2 is observed, indicating that TiO2 also exists in an amorphous form. The XRD diffraction peak is very consistent with the standard cubic spinel structure (JCPDS 88-0866). The characteristic diffraction peak of CeO2 was not observed in the XRD spectrum. This is because the mass fraction of Ce in FTC10 with the largest doping amount is still less than 5wt%, and the Ce loading is lower than the detection limit of XRD. At the same time, CeO2 is evenly dispersed in the shell, and the diffraction peak is not easy to detect. The diffraction peak angle of the catalyst did not change after doping with Ce, and no diffraction peak of the Fe / Ce solid solution was found in the XRD spectrum, indicating that Ce was not doped into the Fe3O4 lattice, causing a change in the lattice constant. In addition, the mixing of TiO2 and CeO2 precursors will inhibit their crystallization during the co-precipitation process, thereby generating smaller grains. While Ti 4+ The diffusion of ions is easily affected by the larger radius Ce 3+ / Ce 4+ The restraint of the catalyst will also inhibit the growth of grains. These are all conducive to the increase of the specific surface area and pore volume of the catalyst.

[0107] 1.3BET Analysis

[0108] The specific surface area and pore size distribution of different catalysts were determined by N2 adsorption-desorption isotherms. Figures 4-5 As shown ( Figure 4 is the N2 adsorption-desorption curve, Figure 5 is the pore size distribution).

[0109] from Figure 4 It can be seen that the Ce-doped catalyst has a similar pore structure to the FT2 catalyst. The adsorption-desorption isotherms are both typical IV type, and the hysteresis loop is a typical H3 type, indicating that there are mesopores on the surface of the catalyst. In the relative pressure (P / P0) range of 0 to 0.4, the adsorption amount of the isotherm increases significantly, indicating that there are a large number of narrow mesopores in the catalyst. Figure 5 The mesopore size distribution shows that the pore size of the Ce-doped catalyst increases compared to the FT2 catalyst. However, as the doping level increases, the catalyst pore size gradually increases. After 10% Ce doping, the bimodal pore size distribution disappears, becoming similar to the pore size distribution of the FT2 catalyst. This may be because at high doping levels, the uniform distribution of amorphous TiO2 is disrupted, causing some micropores and small pores (pore size <3nm) to collapse and connect together, forming larger mesopores. Table 2 shows the specific surface area, average pore size, and pore volume of different catalysts.

[0110] Table 2 Specific surface area, average pore diameter and pore volume of different catalysts

[0111] FT2 FTC3 FTC5 FT10 <![CDATA[Specific surface area (m 2 / g)]]> 201.52 220.52 256.35 232.97 Pore ​​diameter (nm) 3.62 3.72 3.91 4.45 <![CDATA[Pore volume (cm 3 / g)]]> 0.186 0.205 0.247 0.263

[0112] As can be seen in Table 2, with increasing Ce doping levels, the specific surface area of ​​the catalyst first increases and then decreases, while both the pore volume and pore diameter increase. The increase in the specific surface area and pore volume of the sample provides more channels and more active sites for reactants to enter the core-shell interface. Compared with the increase in the specific surface area and pore volume of the catalyst, the slight increase in the catalyst pore diameter is acceptable. It can be inferred that Ce doping can effectively improve the pore structure. Among them, the FTC5 catalyst has the best pore structure, with a high surface area and loose surface features. This allows reactants and intermediates to fully undergo catalytic reactions at the two-phase interface, which is conducive to improving catalytic performance.

[0113] 1.4XPS analysis

[0114] XPS analysis of different catalysts was performed. The binding energy and relative content of Fe 2p3 / 2 are shown in Table 3.

[0115] Table 3 - Binding energy and relative content of Fe and O elements on the surface of different catalysts

[0116]

[0117] From Table 3, we can see that Fe 2+ The relative content of Ce first increases and then decreases, and is higher than that of FT2 catalyst, indicating that Ce doping is beneficial to improve the Fe 2+ This may be attributed to the interaction between the core and shell. The electronegativity of Ce is 1.12, which is much smaller than 1.54 of Ti and 1.83 of Fe. Therefore, the presence of CeO2 allows Fe to attract more electrons, increasing the amount of charge transfer between the two phases. The redox cycles between Fe ions and Ce ions, and Ti ions and Ce ions are shown in Equations 1-1 to 1-2. 3+ and Ti 4+ Ce 3+ Reduction to Fe 2+ and Ti 3+ , which is beneficial to increase the Fe 2+ concentration and promote the Fenton reaction.

[0118] Fe 3+ +Ce 3+ →Fe 2+ +Ce 4+ (1-1)

[0119] Ti 4+ +Ce 3+ →Ti 3+ +Ce 4+ (1-2)

[0120] The O1s XPS analysis of the catalyst was performed. The O 1s spectrum can be mainly divided into lattice oxygen (O lat , 530.1eV), chemically adsorbed oxygen (O ads , 531.7eV) and water molecular oxygen (O H2O , 533.5eV) three characteristic peaks. Among them, O lat is the lattice oxygen of the catalyst, O ads is surface active oxygen, O H2O It is the oxygen contained in the water molecules adsorbed on the catalyst surface. lat and O ads The relative contents of oxygen ions are shown in Table 3.

[0121] O ads It has higher activity and mobility and is easy to desorb from the catalyst surface to form oxygen vacancies, so O ads The concentration is positively correlated with the concentration of oxygen vacancies on the catalyst surface. As can be seen from Table 3, with the increase of Ce doping content, O ads The relative content of Ce first increases and then decreases. The oxygen vacancy content is the highest when 5% Ce is doped. The doping of Ce on the surface can promote the formation of oxygen vacancies, and the FTC5 catalyst may have higher catalytic activity.

[0122] Ce 3d spectra of different catalyst surfaces such as Figure 6 As shown in Figure 2, the Ce 3d spectrum can be divided into eight peaks, which are composed of four pairs of spin-orbit doublets. The u, u′, u″, u″′ and v, v′, v″, v″′ peaks correspond to the Ce 3d5 / 2 and Ce 3d3 / 2 energy level peaks respectively. The u′ and v′ peaks are attributed to the Ce 3+ , the other peaks are attributed to Ce 4+ 。 Ce 3+ Easy to lose electrons to form Ce 4+ , so active oxygen can be 4+ and Ce 3+ Flow and storage between. 3+ and Ce 4+ The presence of redox pairs is beneficial to the cyclic redox reaction. The Ce of each catalyst was calculated based on the area of ​​the Ce characteristic peak. 3+ The relative contents are shown in Table 4.

[0123] Table 4 - Relative content of Ce element on the catalyst surface

[0124] catalyst <![CDATA[CeO2]]> FTC3 FTC5 FTC10 <![CDATA[Ce 3+ / This 4+ ]]> 0.207 0.378 0.387 0.391

[0125] Compared with pure CeO2 catalyst, the Ce content of FTC catalyst is 3+ The relative content is high. However, after doping 3% Ce, further increase in doping amount does not lead to Ce 3+ The obvious increase in the concentration is due to the synergistic effect between Ce, Fe and Ti, which causes the electron cloud on the catalyst surface to move toward Ti and Fe, generating an electron defect center centered on the Ce atom, causing its lattice to collapse. 3+ The existence of CeO2 may be attributed to the surface defects of the core and shell layers, which lead to the breakage of the CeO2 fluorite structure. At the same time, Ce doping may destroy the isotropy and uniformity of amorphous TiO2, change the short-range order of the amorphous state, increase its instability and reducibility, and also lead to Ce 3+ The resulting charge imbalance can effectively increase the content of chemically adsorbed oxygen on the surface of oxygen vacancies, thereby improving the performance of the catalyst.

[0126] Ti 2p maps of different catalyst surfaces Figure 7 As shown. The XPS spectrum of Ti 2p mainly contains the energy level peaks of Ti 2p 1 / 2 and Ti2p3 / 2. There is a charge transfer between Ti and Fe in the core-shell catalyst, which reduces the electron density around Ti and increases the binding energy of Ti 2p3 / 2. After Ce doping, the binding energy of Ti 2p3 / 2 on the catalyst surface decreases, indicating that some Ti 4+ Reduced to Ti 3+, indicating that there is also an interaction between Ce and Ti. Ce doping makes the charge in the TiO2 lattice unbalanced, which will produce a part of Ti 3+ , which makes the binding energy of Ti2p 3 / 2 smaller. In addition, Ce 4+ Ionic radius Much larger than Ti 4+ Ionic radius Therefore, part of Ce 4+ Doping replaces Ti on the lattice points 4+ It will cause the distortion and deformation of the crystal structure, which is conducive to the increase of oxygen vacancy concentration and the formation of active centers. The diffraction peak intensity of TiO2 increases after Ce doping, indicating that the coating amount of TiO2 has increased. This may be because a part of Ce is doped into TiO2 to form Ce 3+ -TiO2 oligomers, which are then deposited on the surface of Fe3O4 particles. This process promotes the hydrolysis of TBOT, reduces the amount of TiO2 oligomers directly generated into TiO2 particles, and is beneficial to improving the stability and catalytic activity of the catalyst.

[0127] 1.5 VSM Analysis

[0128] The VSM curve of the catalyst is as follows Figure 8 As shown, the saturation magnetizations of catalysts FTC3, FTC5, and FTC10 are 31.25, 24.42, and 15.45 emu / g, respectively, all lower than the 37.50 emu / g of catalyst FT2. This is likely due to the shielding effect of CeO2 on the magnetic properties of Fe3O4. XPS analysis also shows that Ce doping increases the TiO2 coating, resulting in a decrease in the saturation magnetization of the FTC catalysts with increasing Ce doping levels. The saturation magnetization of catalyst FTC10 remains above 15 emu / g and exhibits ferromagnetism with near-zero coercivity and remanence. This allows for the catalyst to be recovered and reused by applying an external magnetic field. Therefore, a cyclic injection arrangement can be used for catalytic oxidation. Instead of a separate reactor, H2O2 vapor and catalyst are injected separately into the low-temperature flue between the air preheater and the electrostatic precipitator for mixing and reaction. The catalyst is then captured and discharged from the electrostatic precipitator and separated by an external magnetic field. The separated catalyst can be regenerated or re-injected into the flue for recycling depending on its catalytic activity.

[0129] 1.6H2-TPR analysis

[0130] H2-TPR spectra of different catalysts are shown in Figure 2. Figure 9 shown.

[0131] The redox performance of different catalysts and the synergistic effect of the core layer and shell layer were studied by H2-TPR. Figure 11 As shown. It can be seen that the low-temperature reduction peak of the catalyst further decreases at a lower temperature after doping with Ce, and its redox ability is enhanced to a certain extent. This may be because the adsorption capacity of the catalyst has been improved to a certain extent after doping with Ce, which is conducive to hydrogen adsorption and reduction. CeO2 has two broad reduction peaks. The reduction peak at around 550°C is attributed to the reduction of surface adsorbed oxygen, and the reduction peak at around 800°C is attributed to the reduction of CeO2 lattice oxygen. The main reduction peak of Fe3O4 is in the range of 400-700°C. Therefore, the reduction peak around 600°C should correspond to the reduction of Fe and Ce, and the fusion of the reduction peaks indicates that there is a strong interaction between Fe and Ce. At the same time, the decrease in the temperature corresponding to the reduction peak also indicates that the redox performance of the catalyst has improved after doping with Ce.

[0132] 2 Analysis of catalyst denitrification and desulfurization performance

[0133] 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.

[0134] 2.1 Denitrification performance of modified catalyst

[0135] According to the above method, the NO conversion of FTC1, FTC3, FTC5, FTC10 and FT2 catalysts was compared under the reaction conditions of reaction temperature of 140℃, flue gas flow rate of 0.5L / min, H2O2 concentration of 2mol / L, H2O2 injection rate of 30μL / min, and initial NO and SO2 concentrations of 500ppm and 1000ppm. x Removal efficiency, the results are as follows Figure 10 shown.

[0136] from Figure 10 It can be seen that with the increase of Ce doping amount, the NO and NO xRemoval efficiency gradually increased, reaching optimal efficiencies of 92.8% and 85.2%. Previous characterization results indicate that the modified catalyst exhibits improved surface structure, oxygen vacancy concentration, and number of active sites, resulting in higher catalytic activity than the FT2 catalyst. The FTC5 and FTC10 catalysts exhibit similar denitrification efficiencies. From a cost perspective, the FTC5 catalyst has greater potential as a commercial catalyst.

[0137] 2.2 Effect of gas hourly space velocity

[0138] The gas hourly space velocity can measure the amount of catalyst required in the reaction system and has an important impact on the investment and operating costs of the system. By changing the volume of quartz sand added in each experiment (specifically, during the experiment, the catalyst and quartz sand were evenly mixed and placed on the quartz wool in the middle of the catalytic reactor. The amount of catalyst added could be changed by changing the volume of quartz sand), the GHSV value was adjusted (the temperature of the catalytic reactor was 140°C, the H2O2 concentration was 2mol / L, the H2O2 injection rate was 30μL / min, and the flow rate of the flue gas into the mixing tube was 0.5L / min). The effect of GHSV on the catalytic efficiency under the condition of a certain and sufficient amount of H2O2 / NO was investigated. The results are as follows: Figure 11 shown.

[0139] from Figure 11 It can be seen that when GHSV is less than 1.5×10 5 h -1 When GHSV exceeds a certain range, the NO removal efficiency of the catalyst gradually decreases with the increase of GHSV. This is because the increase in GHSV reduces the contact time between the catalyst and the oxidant. In contrast, the decrease in the denitrification efficiency of the catalyst after Ce doping is smaller, indicating that Ce doping can improve the adsorption characteristics of the catalyst. This may be because the pore size of the shell is slightly increased after Ce doping, which can reduce the resistance of the reactant molecules to penetrate the shell to a certain extent, thereby increasing their reaction time on the active site. At the same time, CeO2 itself also has good adsorption properties, so doping Ce is beneficial to broaden the GHSV value of the catalyst to obtain higher economic efficiency. Considering the comprehensive catalytic efficiency and cost, the GHSV is taken as 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 FTC5 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.

[0140] 2.3 Effect of H2O concentration

[0141] The concentration of water vapor in the flue gas of actual power plants is high and varies with the load. Therefore, the effect of H2O concentration on the denitrification performance of the catalyst was investigated in a large range. The results are as follows: Figure 12 As shown. When the H2O content (volume content) in the flue gas is between 2.3% and 6.5%, the NO removal efficiency of the FT2 and FTC5 catalysts decreases slightly; when the H2O content in the flue gas further increases to 12.3%, the denitrification efficiency of the FT2 and FTC5 catalysts decreases by 10.1% and 5.7%, respectively. The reason for the decrease in catalyst efficiency is that 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. Doping Ce can improve the catalyst's water resistance, and it can still maintain a denitrification efficiency of more than 87.8% under 12.3% H2O conditions, thus being able to adapt to the changing operating conditions of actual power plants.

[0142] 2.4 Catalyst stability study

[0143] Good reaction stability of the catalyst is a prerequisite for industrial application. Under the same experimental conditions, a 6-hour continuous experiment was conducted on the FT2 catalyst and the FTC5 catalyst. The experimental results are as follows: Figure 13 As shown. It can be seen that 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 FTC5 catalyst decreased by 2.6%. The reason for the decrease in the denitrification efficiency of the catalyst during long-term operation is that the continuous addition of H2O2 during the reaction will remove the Fe in the catalyst. 2+ Converted to Fe with lower catalytic activity 3+ Due to Fe 2+ Converted to Fe 3+ The reaction constant is low, so the H2O2 catalytic efficiency and denitrification efficiency of the catalyst will continue to decline. The FTC5 catalyst provides more redox pairs due to Ce doping, which promotes the continuous progress of the cyclic reaction. The Fe 2+ The concentration can remain stable for a longer time, so it has higher stability.

[0144] 3. Catalyst denitrification reaction mechanism

[0145] Figure 14The following are EPR spectra of DMPO-captured free radicals in systems consisting of various catalysts and H₂O₂. The test results all show a quadruple signal peak corresponding to the DMPO-·OH species, indicating that the primary free radical present in the system is ·OH. The figure shows that the amount of ·OH generated in the system increases with increasing Ce doping. During the catalytic oxidation reaction, ·OH is the primary active oxidizing species. The increased ·OH generation indicates that the catalytic performance of the Ce-doped catalyst is enhanced, favoring the catalytic oxidation of more NO.

[0146] In order to study the mechanism of Ce doping modification on the promotion of FT2 catalyst activity, XPS characterization was performed on the catalyst after reaction. The XPS spectra of each catalyst surface after reaction are as follows: Figure 15 The relative contents of each element are shown in Table 5. It can be seen that the Fe 2+ 、Ti 3+ 、Ce 3+ and O ads / (O ads +O lat ) contents decreased, indicating that the surface Fe 2+ 、Ti 3+ and Ce 3+ It is the main active site for the catalytic decomposition of H2O2. There is a dual redox cycle in the FTC5 catalyst (Fe 3+ +Ti 3+ ←→Fe 2+ +Ti 4+ , Fe 3+ +Ce 3+ ←→Fe 2+ +Ce 4+ ), and the two cycles also have a synergistic promoting effect, which is beneficial to reduce the energy required for electron transfer between Fe, Ce and Ti active sites.

[0147] Table 5 Comparison of relative contents of elements in FTC5 catalyst before and after reaction and relative differences with FT2 catalyst

[0148]

[0149]

[0150] XPS characterization was performed on the FTC5 catalyst after the reaction (i.e., used FTC5) to further analyze the tolerance mechanism of the core-shell structure catalyst to SO2, such as Figure 16As shown. The catalyst before the reaction did not have a peak corresponding to S 2p. However, the two peaks near 168.5eV and 169.5eV in the S 2p spectrum of the catalyst after the reaction were attributed to surface adsorbed SO3 and SO2-4, respectively, indicating that SO2 was oxidized into sulfite and sulfate species during the reaction. Figure 13 It can be seen that the binding energies of both O1s and Ti2p in the FTC5 catalyst increased after the reaction. This is because the sulfate formed on the catalyst surface has a strong electronegativity, which reduces the electron cloud density of O and Ti, indicating that the sulfate species are primarily absorbed by the titanium dioxide shell. This demonstrates that the shell has an adsorption and barrier effect on SO2, reducing the competitive adsorption of SO2 with NO and H2O2, and protecting the catalytic core. Therefore, the core-shell catalyst has excellent sulfur resistance.

[0151] 2SO2 + 2H2O2 → 2H2SO3 + O2 (1-3)

[0152] H2SO3 → SO2 + H2O (1-4)

[0153] SO3 + 2·OH → SO2- 4 + H2O (1-5)

[0154] Combining the above experimental and analytical results, the flue gas denitrification reaction can be divided into three steps: the catalytic decomposition of H2O2 on the catalyst, the oxidation of NO, and the absorption of the oxidation products. Under appropriate operating conditions, H2O2 rapidly decomposes on the catalyst surface to produce a large amount of OH, which efficiently oxidizes NO to high-valent oxides such as NO2, HNO2, and HNO3. During the reaction, the shell's barrier to and adsorption of SO2 ensures that NO and H2O2 can fully adsorb and react at the active sites, achieving high denitrification efficiency.

[0155] 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 cerium-doped core-shell catalyst in catalytic oxidation denitrification of flue gas; The preparation method of the cerium-doped core-shell catalyst comprises the following steps: Put Fe3O4 in a container, then add alcohol solvent and acetonitrile, disperse it, add ammonia water, stir, and obtain a first mixed material; Adding the cerium source and the titanium source into the alcohol solvent and stirring to obtain a second mixed material; The second mixture is added dropwise to the first mixture, stirred at 40-50° C., and solid-liquid separation is performed to obtain a cerium-doped core-shell catalyst; The cerium source is cerium chloride; The titanium source is tetrabutyl titanate; 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 molar ratio of cerium to titanium in the cerium-doped core-shell catalyst is 5:100; The alcohol solvent is ethanol; The application comprises the following steps: Providing a catalytic reactor, placing a cerium-doped 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; The metal ion solution is added to the first alkaline solution, and heated and stirred at 80-100° C. under the protection of inert gas. After solid-liquid separation, Fe3O4 is obtained.

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 preparation method of the first alkali solution comprises: adding NaNO3 and NaOH to water and stirring to obtain the first alkali solution; wherein the mass ratio of NaNO3, NaOH and water is (3-7):(5-15):(70-80).

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