Eggshell type catalyst, preparation method thereof and application of eggshell type catalyst in preparation of tetravalent uranium through catalytic reduction of hexavalent uranium

Abstract

The invention discloses an eggshell type catalyst, a preparation method of the eggshell type catalyst and application of the eggshell type catalyst to preparation of tetravalent uranium through catalytic reduction of hexavalent uranium. The eggshell type catalyst comprises a carrier and an active component loaded on the carrier, the eggshell type catalyst has a core-shell structure, the core is a carrier, and the shell is an active component; the carrier is a pretreated metal oxide; the active component is a noble metal active component; the pretreatment comprises at least one of alkali treatment, salt treatment and acid treatment, and is applied to a reaction for preparing tetravalent uranium through catalytic reduction of hexavalent uranium. By adopting the eggshell type catalyst, in the reaction process of preparing the tetravalent uranium, the metal utilization rate is high, the reaction condition is mild, and the yield of the tetravalent uranium is high.

Description

Technical Field

[0001] This application relates to an eggshell-type catalyst, a preparation method thereof, and an application thereof in catalytic reduction of hexavalent uranium to prepare tetravalent uranium, belonging to the technical field of nuclear fuel reprocessing. Background Art

[0002] Nuclear fuel reprocessing refers to the process of recovering uranium, plutonium, and other valuable elements from used nuclear fuel. Currently, the main nuclear fuel reprocessing plants at home and abroad mainly adopt the Purex process. The Purex process mainly utilizes the different extraction capabilities of tributyl phosphate (TBP) for uranium, plutonium, and fission products to achieve their separation. Among them, in the uranium-plutonium separation stage, the extraction ability of TBP for trivalent plutonium is very small, realizing the separation of uranium and plutonium. However, plutonium is tetravalent in the front-end process, so a reducing agent needs to be added to reduce it to trivalent. Tetravalent uranium (U(IV)) is a good reducing stripping agent for tetravalent plutonium (Pu(IV)), and it has been widely used due to the advantages of complete plutonium reduction stripping, no introduction of impurities into the system, and fast reaction rate. Therefore, the efficient preparation of tetravalent uranium is of great significance in the Purex process.

[0003] Currently, the main methods for preparing tetravalent uranium are direct electrolysis method and catalytic hydrogenation method. The electrolysis method is to electrolytically reduce UO2(NO3)2 in a nitric acid solution to U(NO3)4 on the electrode. This method has the advantages of simple process and no increase in waste liquid. However, in the actual application process in reprocessing plants, the conversion rate of hexavalent uranium (U(VI)) is relatively low, generally only 50%-60%, resulting in a relatively low concentration of tetravalent uranium (U(IV)). When using it as a reducing agent, it will not only cause dilution of the plutonium product but also increase the recovery burden of the uranium product. The catalytic hydrogenation method is to reduce the UO2(NO3)2 solution to U(NO3)4 using high-pressure hydrogen under the action of a catalyst. The catalytic hydrogenation method has the characteristics of large production capacity and high U(IV) yield, but its equipment is complex, and the pressure of the high-pressure hydrogen used reaches 10 MPa, which is a huge safety hazard for the nuclear fuel reprocessing workshop with extremely strong radioactivity. Therefore, it is crucial to develop a simple, safe, and efficient method for preparing U(IV).

[0004] Using organic reducing agents such as hydrazine, formic acid, and formaldehyde, U(VI) can be reduced to U(IV) under the action of a catalyst. It has the advantages of mild reaction conditions and simple preparation process, and is a promising method for preparing U(VI). Li Bin et al. used platinum black as a catalyst to study the process conditions for the reduction of U(VI) to U(IV) by hydrazine in a nitric acid system. When the uranium concentration was 0.9 mol / L, the HNO3 concentration was 0.8 mol / L, and the hydrazine concentration was 1 mol / L, at 60 °C, the conversion rate of U(VI) could reach over 90% (Journal of Nuclear Chemistry and Radiochemistry, 2013, 35, (1): 24 - 28). Boltoeva et al. investigated the particle size effect of Pt in the reduction of U(VI) to U(IV) by hydrazine in H2SO4, HClO4, and HNO3 systems using Pt / SiO2 catalysts. It was found that as the Pt particle size increased, the U(VI) conversion rate increased (Radiochemistry, 2007, 49, 603 - 606). Anan’ev et al. investigated the reaction performance of the reduction of U(VI) to U(IV) by hydrazine and formic acid in a nitric acid system (Radiochemistry, 2001, 43, 39 - 43). When hydrazine was used as the reducing agent, the reaction for the reduction of U(Ⅵ) was as shown in Equation (1). However, in this reaction system, the self - decomposition of hydrazine nitrate to produce NH4 + might also occur, as shown in Equation (2). Since the reduction reaction of U(Ⅵ) is more likely to occur on the outer surface of the catalyst, while inside the catalyst pores, the decomposition reaction of hydrazine is more likely to occur. Eggshell - type catalysts have the advantages of avoiding the influence of internal diffusion and facilitating the rapid removal of products. Therefore, by constructing eggshell - type catalysts, the U(Ⅵ) conversion rate can be effectively increased, and the spontaneous decomposition of products can be inhibited, which is beneficial to improving the reaction rate of U(Ⅵ) and the selectivity of hydrazine decomposition. Treating the carrier with salts, acids, or bases to change the surface layer structure of the carrier can change the impregnation depth of metal ions on the carrier surface, thereby constructing a catalyst with non - uniform distribution of active components.

[0005] N2H5 + +3H + +2UO2 2+ →2U 4+ +N2+4H2O (1)

[0006] 3N2H5 + +H + →N2+4NH4 + (2)

[0007] Currently, there have been some patent applications for the preparation of uranous solutions. The following lists several reported patents for detailed description:

[0008] Chinese Patent CN201110097474 has a publication title of: A Method for Preparing a Tetravalent Uranium Solution. This patent reports that an organic reducing agent (hydrazine or carboxylic acid and its derivatives) is used to reduce U(VI) to prepare a U(IV) solution under the action of a Pt catalyst. The catalyst involved in this patent is platinum black, and directly using it will result in very high costs and low catalytic efficiency.

[0009] Chinese Patent CN201310743451 has a publication title of: An Apparatus for Electrochemically Reducing and Preparing Tetravalent Uranium. This patent reports an improved apparatus for electrochemically reducing and preparing tetravalent uranium, which mainly solves the problem of low U(VI) conversion rate in a diaphragm-free electrolysis apparatus. In actual production, the effectiveness of this apparatus still needs to be further tested.

[0010] Chinese Patent CN201910865308 has a publication title of: A Catalyst, Method, and Application for Visible Light Catalytic Reduction of Hexavalent Uranium Removal. This patent reports that using a ZnFe2O4 catalyst, under light irradiation conditions, U(VI) in wastewater can be reduced to U(IV) and form a precipitate, thereby removing U(VI) ions from the waste liquid. The catalyst involved in this patent is ZnFe2O4, and the photocatalytic reduction method is used to remove U(VI) ions.

[0011] Currently, due to problems such as high catalyst costs and complex preparation processes, it is difficult to achieve industrial applications. There is an urgent need to develop an efficient catalyst to improve the U(VI) conversion rate and the utilization rate of hydrazine. Summary of the Invention

[0012] Aiming at the problems of harsh reaction conditions and low yield in the above process of preparing tetravalent uranium, a shell-type catalyst, its preparation method, and application are provided. Tetravalent uranium is prepared by catalytically reducing hexavalent uranium in an acidic system using an efficient catalyst.

[0013] In one aspect of the present application, a shell-type catalyst is provided. The shell-type catalyst includes a carrier and an active component supported on the carrier;

[0014] The shell-type catalyst has a core-shell structure, with the core being the carrier and the shell being the active component;

[0015] The carrier is a metal oxide after pretreatment;

[0016] The active component is a noble metal active component;

[0017] The pretreatment is at least one of alkali treatment, salt treatment, and acid treatment.

[0018] Optionally, the noble metal active component exists in an atomic state.

[0019] Optionally, the metal oxide is selected from at least one of SiO2, Al2O3, and TiO2.

[0020] Optionally, the noble metal active component is at least one of Pt, Ir, Rh, Ru, and Pd.

[0021] Optionally, in the eggshell-type catalyst, the mass percentage of the noble metal active component in the eggshell-type catalyst is 0.01-10%.

[0022] Optionally, in the eggshell-type catalyst, the mass percentage of the noble metal active component in the eggshell-type catalyst is 0.1-5%.

[0023] Optionally, in the eggshell-type catalyst, the mass percentage of the noble metal active component in the eggshell-type catalyst independently selects any value of 0.01%, 0.1%, 3%, 5%, 10% or the range value between any two of the above values.

[0024] In another aspect of the present application, a preparation method of the above-mentioned eggshell-type catalyst is provided, including the following steps:

[0025] a: Placing the metal oxide carrier in a pretreatment solution, impregnating I, and drying I to obtain a pretreated carrier;

[0026] The pretreatment solution is selected from at least one of an acid solution, an alkali solution, and a salt solution;

[0027] b: Placing the pretreated carrier prepared in step a in a noble metal salt solution, impregnating II, drying II, calcining, and reducing to obtain the eggshell-type catalyst.

[0028] Optionally, in step a,

[0029] The alkali in the alkali solution is at least one of sodium hydroxide and potassium hydroxide;

[0030] The concentration of the alkali is 0.05-1 mol / L.

[0031] Optionally, the concentration of the alkali independently selects any value of 0.05 mol / L, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1 mol / L or the range value between any two of the above values.

[0032] Optionally, the salt in the salt solution is at least one of sodium carbonate, sodium bicarbonate, potassium carbonate, and potassium bicarbonate;

[0033] The concentration of the salt is 0.05 - 2 mol / L.

[0034] Optionally, the concentration of the salt is independently selected from any value of 0.05 mol / L, 0.1 mol / L, 0.2 mol / L, 0.4 mol / L, 0.6 mol / L, 0.8 mol / L, 1 mol / L, 1.2 mol / L, 1.4 mol / L, 1.6 mol / L, 1.8 mol / L, 2 mol / L or the range value between any two of the above values.

[0035] Optionally, the acid in the acid solution is at least one of nitric acid and citric acid;

[0036] The concentration of the acid is 0.05 - 1 mol / L.

[0037] Optionally, the concentration of the acid is independently selected from any value of 0.05 mol / L, 0.1 mol / L, 0.2 mol / L, 0.4 mol / L, 0.6 mol / L, 0.8 mol / L, 1 mol / L or the range value between any two of the above values.

[0038] Optionally, the solid-liquid ratio of the metal oxide carrier to the pretreatment solution is 0.1 g / ml - 1 g / ml.

[0039] Optionally, the solid-liquid ratio of the metal oxide carrier to the pretreatment solution is independently selected from any value of 0.1 g / ml, 0.2 g / ml, 0.5 g / ml, 0.6 g / ml, 0.8 g / ml, 1 g / ml or the range value between any two of the above values.

[0040] Optionally, in step b,

[0041] In the noble metal salt solution, the noble metal salt is a nitrate and / or chloride containing a noble metal element, and the noble metal element is selected from at least one of Pt, Ir, Rh, Ru, and Pd.

[0042] Optionally, in the noble metal salt solution, the concentration of the noble metal salt is 0.1 wt% - 5 wt%, based on the concentration of the noble metal element.

[0043] Optionally, in the noble metal salt solution, the concentration of the noble metal salt is independently selected from any value of 0.1 wt%, 0.5 wt%, 1 wt%, 2 wt%, 2.78 wt%, 3 wt%, 4 wt%, 5 wt% or the range value between any two of the above values.

[0044] Optionally, the solid-liquid ratio of the pretreated carrier to the noble metal salt solution is 0.5 g / ml - 1.0 g / ml.

[0045] Optionally, the solid-liquid ratio of the pretreated carrier to the noble metal salt solution is independently selected from any value of 0.5 g / ml, 0.6 g / ml, 0.7 g / ml, 0.8 g / ml, 0.9 g / ml, 1.0 g / ml or a range value between any two of the above values.

[0046] Optionally, in step a, the temperature of the first impregnation is 20 to 60 °C, and the time of the first impregnation is 1 to 12 h.

[0047] Optionally, the temperature of the first impregnation is independently selected from any value of 20 °C, 25 °C, 30 °C, 35 °C, 40 °C, 50 °C, 60 °C or a range value between any two of the above values.

[0048] Optionally, the time of the first impregnation is independently selected from any value of 1 h, 4 h, 8 h, 12 h or a range value between any two of the above values.

[0049] Optionally, the temperature of the first drying is 50 to 120 °C, and the time of the first drying is 1 to 12 h.

[0050] Optionally, the temperature of the first drying is independently selected from any value of 50 °C, 80 °C, 100 °C, 120 °C or a range value between any two of the above values.

[0051] Optionally, the time of the first drying is independently selected from any value of 1 h, 4 h, 8 h, 12 h or a range value between any two of the above values.

[0052] Optionally, in step b, the temperature of the second impregnation is 25 to 60 °C, and the time of the second impregnation is 0.5 to 48 h.

[0053] Optionally, the temperature of the second impregnation is independently selected from any value of 25 °C, 35 °C, 50 °C, 60 °C or a range value between any two of the above values.

[0054] Optionally, the time of the second impregnation is independently selected from any value of 0.5 h, 6 h, 12 h, 18 h, 24 h, 36 h, 48 h or a range value between any two of the above values.

[0055] Optionally, the temperature of the second drying is 60 to 120 °C, and the drying time is 1 to 24 h;

[0056] Optionally, the temperature of the second drying is independently selected from any value of 60 °C, 80 °C, 100 °C, 120 °C or a range value between any two of the above values.

[0057] Optionally, the time of the second drying is independently selected from any value among 1 h, 4 h, 8 h, 12 h, 18 h, 24 h or the range value between any two of the above values.

[0058] Optionally, the temperature of the roasting is 200 - 500 °C, and the time of the roasting is 2 - 8 h;

[0059] Optionally, the temperature of the roasting is independently selected from any value among 200 °C, 300 °C, 400 °C, 500 °C or the range value between any two of the above values.

[0060] Optionally, the time of the roasting is independently selected from any value among 2 h, 4 h, 6 h, 8 h or the range value between any two of the above values.

[0061] Optionally, in step b, the reduction is the high-temperature hydrogen reduction method, and the atmosphere of the reduction is hydrogen, which acts as a reducing agent.

[0062] Optionally, the temperature of the reduction is 200 - 500 °C, and the time of the reduction is 4 - 8 h.

[0063] Optionally, the temperature of the reduction is independently selected from any value among 200 °C, 300 °C, 400 °C, 500 °C or the range value between any two of the above values.

[0064] Optionally, the time of the reduction is independently selected from any value among 4 h, 6 h, 8 h or the range value between any two of the above values.

[0065] According to another aspect of the present application, a method for catalytically reducing hexavalent uranium to prepare tetravalent uranium in an acidic system is provided. A mixed solution containing an acidic solution, a catalyst, a reducing agent and uranyl ions reacts to obtain tetravalent uranium.

[0066] Wherein, the uranium in the uranyl ions is hexavalent uranium;

[0067] The catalyst is the above-mentioned eggshell-type catalyst.

[0068] Optionally, the reaction of reducing hexavalent uranium to prepare tetravalent uranium is carried out in a batch reactor or a fixed-bed reactor.

[0069] Optionally, the acidic solution is selected from at least one of nitric acid, sulfuric acid, and perchloric acid.

[0070] Optionally, the reducing agent is selected from at least one of hydrazine, formic acid, and formaldehyde.

[0071] Optionally, the uranyl ions are selected from at least one of uranyl nitrate, uranyl sulfate, and uranyl perchlorate.

[0072] Optionally, in the mixed solution, the concentration of the acidic solution is 0.5 - 1.0 mol / L in terms of hydrogen ion concentration.

[0073] Optionally, in the mixed solution, the concentration of the acidic solution independently selects any value from 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1.0 mol / L or a range value between any two of the above values.

[0074] Optionally, in the mixed solution, the concentration of the reducing agent is 0.5 - 2.0 mol / L;

[0075] Optionally, in the mixed solution, the concentration of the reducing agent independently selects any value from 0.5 mol / L, 0.75 mol / L, 1.0 mol / L, 1.5 mol / L, 2.0 mol / L or a range value between any two of the above values.

[0076] Optionally, in the mixed solution, the concentration of the uranyl ion is 100 - 300 g / L in terms of the mass of uranium ions.

[0077] Optionally, in the mixed solution, the concentration of the uranyl ion independently selects any value from 100 g / L, 150 g / L, 200 g / L, 210 g / L, 250 g / L, 300 g / L or a range value between any two of the above values.

[0078] Optionally, in the mixed solution, the molar ratio of the acidic solution, reducing agent, and uranyl ion is 1:0.3 - 3.0:0.5 - 1.2.

[0079] Optionally, in the mixed solution, the molar ratio of the acidic solution, reducing agent, and uranyl ion independently selects any value from 1:0.3:0.5, 1:1.0:0.7, 1:1.5:0.9, 1:2.0:1.0, 1:3.0:1.2 or a range value between any two of the above values.

[0080] Optionally, the temperature of the reaction is 15 - 60 °C, and the time of the reaction is 0.5 - 6 h;

[0081] Optionally, the temperature of the reaction independently selects any value from 15 °C, 25 °C, 35 °C, 45 °C, 60 °C or a range value between any two of the above values.

[0082] Optionally, the time of the reaction independently selects any value from 0.5 h, 2 h, 3 h, 5 h, 6 h or a range value between the above two values.

[0083] Optionally, the mass ratio of the uranyl ion to the catalyst is 100 to 300, where the mass of the uranyl ion is calculated based on the mass of uranium ions.

[0084] Optionally, the reaction pressure is normal pressure.

[0085] The beneficial effects that can be produced by this application include:

[0086] 1) The eggshell-type catalyst provided by this application can promote the reduction of hexavalent uranium and inhibit the spontaneous decomposition reaction of hydrazine, which is beneficial to improving the conversion rate of tetravalent uranium and the utilization rate of hydrazine;

[0087] 2) The eggshell-type catalyst provided by this application can reduce hexavalent uranium to tetravalent uranium solution using a liquid-phase reducing agent under acidic conditions. In a stirred tank reactor, the conversion rate of hexavalent uranium can reach 99% within 1 h, and the reaction has high stability in a fixed reactor.

[0088] 3) The method for catalytically reducing hexavalent uranium to prepare tetravalent uranium using the eggshell-type catalyst provided by this application has high metal utilization rate, mild reaction conditions, and high yield of tetravalent uranium. Description of the Drawings

[0089] Figure 1 It is a schematic diagram for preparing the eggshell-type catalyst in Example 1 of this application. Detailed Embodiments

[0090] The following describes this application in detail with reference to the embodiments, but this application is not limited to these embodiments.

[0091] Unless otherwise specified, the raw materials in the embodiments of this application are all purchased through commercial channels.

[0092] The conversion rate in the embodiments of this application is calculated as follows:

[0093]

[0094] Example 1

[0095] According to Figure 1 the process shown, prepare the eggshell-type catalyst, and the specific preparation process is as follows:

[0096] 1) Pretreat the metal oxide support.

[0097] Weigh 5 g of the SiO2 support, weigh 0.042 g of NaHCO3, dissolve it in 5 ml of deionized water to form a mixed solution with a molar concentration of NaHCO3 of 0.1 mol / L, and drop it into the SiO2 support and mix evenly. Immerse it at 25 °C for 4 h and dry it at 50 °C for 4 h to obtain the pretreated support SiO2-NaHCO3.

[0098] 2) Prepare an eggshell-type catalyst.

[0099] Weigh 5.0 g of the prepared SiO2-NaHCO3 support. Weigh 0.399 g of H2PtCl6·6H2O, dissolve it in 5 ml of deionized water to form a mixed solution with a Pt mass concentration of 2.78 wt%, and add it dropwise to the SiO2-NaHCO3 support and mix them evenly. Impregnate at room temperature for 12 h, dry at 80 °C for 12 h, and calcine in an air atmosphere at 300 °C for 4 h, with a Pt mass content of 3%. Then, perform hydrogen reduction on it at a temperature of 300 °C, with a heating rate from room temperature to the reduction temperature of 5 °C / min, a hydrogen volumetric space velocity of 100 h -1 , at atmospheric pressure, and a reduction time of 4 h. Obtain the catalyst, denoted as 3Pt / SiO2-NaHCO3.

[0100] Reaction conditions for the catalyst in a stirred tank: nitric acid concentration (in terms of hydrogen ion concentration) is 0.8 mol / L, uranyl (uranyl nitrate) ion concentration is 210 g / L, hydrazine nitrate concentration is 1.0 mol / L, temperature is 60 °C, stirring rate is 800 rpm, pressure is atmospheric pressure, catalyst dosage is 1.0 g, and reaction feed liquid is 25 mL. The reaction results are shown in Table 1.

[0101] Example 2

[0102] 1) Pretreat the metal oxide support.

[0103] Weigh 5 g of the TiO2 support. Weigh 0.048 g of 65 wt% HNO3, dissolve it in 5 ml of deionized water to form a mixed solution with an HNO3 molar concentration of 0.1 mol / L, and add it dropwise to the TiO2 support and mix them evenly. Impregnate at 30 °C for 1 h, dry at 50 °C for 1 h, to obtain the pretreated support TiO2-HNO3.

[0104] 2) Prepare an eggshell-type catalyst.

[0105] Weigh 5.0 g of the prepared TiO2-HNO3 support. Weigh 0.399 g of H2PtCl6·6H2O, dissolve it in 5 ml of deionized water to form a mixed solution with a Pt mass concentration of 2.78 wt%, and add it dropwise to the TiO2-HNO3 support and mix them evenly. Impregnate at room temperature for 48 h, dry at 120 °C for 1 h, and calcine in an air atmosphere at 500 °C for 2 h, with a Pt mass content of 3%. Then, perform hydrogen reduction on it at a temperature of 500 °C, with a heating rate from room temperature to the reduction temperature of 5 °C / min, a hydrogen volumetric space velocity of 100 h -1 , at atmospheric pressure, and a reduction time of 4 h. Obtain the catalyst, denoted as 3Pt / TiO2-HNO3.

[0106] Reaction conditions for the catalyst in a stirred tank: nitric acid concentration (in terms of hydrogen ion concentration) is 0.5 mol / L, uranyl (uranyl nitrate) ion concentration is 100 g / L, hydrazine nitrate concentration is 0.5 mol / L, temperature is 60 °C, stirring rate is 800 rpm, pressure is atmospheric pressure, catalyst dosage is 1.0 g, and reaction feed liquid is 25 mL. The reaction results are shown in Table 1.

[0107] Example 3

[0108] 1) Pretreat the metal oxide support.

[0109] Weigh 5 g of TiO2 support, weigh 0.096 g of citric acid, dissolve it in 5 ml of deionized water to form a mixed solution with a citric acid molar concentration of 0.1 mol / L, and add it dropwise to the TiO2 support and mix evenly. Immerse it at 20 °C for 12 h, dry it at 120 °C for 1 h to obtain the pretreated support TiO2-citric acid.

[0110] 2) Prepare the eggshell-type catalyst.

[0111] Weigh 5.0 g of the prepared TiO2-citric acid support, weigh 0.4019 g of H2IrCl6·6H2O, dissolve it in 5 ml of deionized water to form a mixed solution with an Ir mass concentration of 2.78 wt%, add it dropwise to the TiO2-citric acid support and mix evenly. Immerse it at room temperature for 0.5 h, dry it at 60 °C for 24 h, and calcine it in an air atmosphere at 200 °C for 8 h, with an Ir mass content of 3%. Then, carry out hydrogen reduction on it, with a temperature of 200 °C, a heating rate from room temperature to the reduction temperature of 5 °C / min, and a hydrogen volumetric space velocity of 100 h -1 , pressure is atmospheric pressure, and reduction time is 8 h. Obtain the catalyst, denoted as 3Ir / TiO2-citric acid.

[0112] Reaction conditions for the catalyst in a stirred tank: nitric acid concentration (in terms of hydrogen ion concentration) is 1.0 mol / L, uranyl (uranyl nitrate) ion concentration is 300 g / L, hydrazine nitrate concentration is 2.0 mol / L, temperature is 60 °C, stirring rate is 800 rpm, pressure is atmospheric pressure, catalyst dosage is 1.0 g, and reaction feed liquid is 25 mL. The reaction results are shown in Table 1.

[0113] Example 4

[0114] 1) Pretreat the metal oxide support.

[0115] Weigh 5 g of the Al2O3 support, weigh 0.02 g of NaOH, dissolve it in 5 ml of deionized water to prepare a mixed solution with a molar concentration of NaOH of 0.1 mol / L, and add it dropwise to the Al2O3 support and mix evenly. Impregnate at 60 °C for 1 h, dry at 120 °C for 1 h to obtain the pretreated support Al2O3-NaOH.

[0116] 2) Prepare the eggshell-type catalyst.

[0117] Weigh 5.0 g of the prepared Al2O3-NaOH support, weigh 0.399 g of H2PtCl6·6H2O, dissolve it in 5 ml of deionized water to prepare a mixed solution with a mass concentration of Pt of 2.78 wt%, add it dropwise to the Al2O3-NaOH support and mix evenly. Impregnate at room temperature for 0.5 h, dry at 60 °C for 24 h, and calcine in air atmosphere at 400 °C for 4 h, with the Pt mass content being 3%. Then, carry out hydrogen reduction at a temperature of 400 °C, with a heating rate from room temperature to the reduction temperature of 5 °C / min, a hydrogen volumetric space velocity of 100 h -1 , at atmospheric pressure, and a reduction time of 4 h. Obtain the catalyst, denoted as 3Pt / Al2O3-NaOH.

[0118] The reaction conditions of the catalyst in the stirred tank are as follows: the concentration of nitric acid (in terms of hydrogen ion concentration) is 0.8 mol / L, the concentration of uranyl (uranyl nitrate) ions is 210 g / L, the concentration of hydrazine nitrate is 1.0 mol / L, the temperature is 60 °C, the stirring rate is 800 rpm, the pressure is atmospheric pressure, the catalyst dosage is 1.0 g, and the reaction feed liquid is 25 mL. The reaction results are shown in Table 1.

[0119] Example 5

[0120] 1) Pretreat the metal oxide support.

[0121] Weigh 5 g of the Al2O3 support, weigh 0.028 g of KOH, dissolve it in 5 ml of deionized water to prepare a mixed solution with a molar concentration of KOH of 0.1 mol / L, and add it dropwise to the Al2O3 support and mix evenly. Impregnate at 30 °C for 2 h, dry at 80 °C for 4 h to obtain the pretreated support Al2O3-KOH.

[0122] 2) Prepare the eggshell-type catalyst.

[0123] Weigh 5.0 g of the prepared Al2O3-KOH support. Weigh 0.3079 g of RuCl3·3H2O and dissolve it in 5 ml of hydrochloric acid with a concentration of 10 wt% to prepare a mixed solution. The mass concentration of Ru is 2.78 wt%. Drop it into the Al2O3-KOH support and mix them evenly. Impregnate at room temperature for 12 h, dry at 80 °C for 12 h, and calcine in an air atmosphere at 400 °C for 4 h, with the Ru mass content being 3%. Then, conduct hydrogen reduction on it at a temperature of 400 °C, with a heating rate from room temperature to the reduction temperature of 5 °C / min and a hydrogen volumetric space velocity of 100 h -1 , at atmospheric pressure, and the reduction time is 4 h. Obtain the catalyst, denoted as 3Ru / Al2O3-KOH.

[0124] Reaction conditions for the catalyst in a stirred tank: The nitric acid concentration (in terms of hydrogen ion concentration) is 0.8 mol / L, the uranyl (uranyl nitrate) ion concentration is 210 g / L, the hydrazine nitrate concentration is 1.0 mol / L, the temperature is 60 °C, the stirring rate is 800 rpm, the pressure is atmospheric pressure, the catalyst dosage is 1.0 g, and the reaction feed liquid is 25 mL. The reaction results are shown in Table 1.

[0125] Example 6

[0126] Weigh 5.0 g of the SiO2-NaHCO3 support prepared in Example 1. Weigh 0.0133 g of H2PtCl6·6H2O and dissolve it in 5 ml of deionized water to prepare a mixed solution. The mass concentration of Pt is 0.1 wt%. Drop it into the SiO2-NaHCO3 support and mix them evenly. Impregnate at room temperature for 12 h, dry at 80 °C for 12 h, and calcine in an air atmosphere at 400 °C for 4 h, with the Pt mass content being 3%. Then, conduct hydrogen reduction on it at a temperature of 400 °C, with a heating rate from room temperature to the reduction temperature of 5 °C / min and a hydrogen volumetric space velocity of 100 h -1 , at atmospheric pressure, and the reduction time is 4 h. Obtain the catalyst, denoted as 0.1Pt / SiO2-NaHCO3.

[0127] Reaction conditions for the catalyst in a stirred tank: The nitric acid concentration (in terms of hydrogen ion concentration) is 0.8 mol / L, the uranyl (uranyl nitrate) ion concentration is 210 g / L, the hydrazine nitrate concentration is 1.0 mol / L, the temperature is 60 °C, the stirring rate is 800 rpm, the pressure is atmospheric pressure, the catalyst dosage is 1.0 g, and the reaction feed liquid is 25 mL. The reaction results are shown in Table 1.

[0128] Example 7

[0129] Weigh 5.0 g of the SiO2-NaHCO3 support prepared in Example 1. Weigh 0.6638 g of H2PtCl6·6H2O and dissolve it in 5 ml of deionized water to form a mixed solution. The mass concentration of Pt is 4.41 wt%. Drop it into the SiO2-NaHCO3 support and mix them evenly. Impregnate at room temperature for 12 h, dry at 80 °C for 12 h, and calcine in an air atmosphere at 400 °C for 4 h, with the Pt mass content being 3%. Then, carry out hydrogen reduction on it at a temperature of 400 °C, with a heating rate from room temperature to the reduction temperature of 5 °C / min and a hydrogen volumetric space velocity of 100 h -1 , at atmospheric pressure, and the reduction time is 4 h. A catalyst is obtained, denoted as 5Pt / SiO2-NaHCO3.

[0130] The reaction conditions of the catalyst in the stirred tank are as follows: the nitric acid concentration (in terms of hydrogen ion concentration) is 0.8 mol / L, the uranyl (uranyl nitrate) ion concentration is 210 g / L, the hydrazine nitrate concentration is 1.0 mol / L, the temperature is 60 °C, the stirring rate is 800 rpm, the pressure is atmospheric pressure, the catalyst dosage is 1.0 g, and the reaction feed liquid is 25 mL. The reaction results are shown in Table 1.

[0131] Example 8

[0132] 1) Pretreat the metal oxide support.

[0133] Weigh 5 g of the SiO2 support. Weigh 0.053 g of Na2CO3 and dissolve it in 5 ml of deionized water to form a mixed solution. The molar concentration of Na2CO3 is 0.1 mol / L. Drop it into the SiO2 support and mix them evenly. Impregnate at 30 °C for 2 h and dry at 80 °C for 4 h to obtain the pretreated support SiO2-Na2CO3.

[0134] 2) Prepare the eggshell-type catalyst.

[0135] Weigh 5.0 g of the prepared SiO2-Na2CO3 support. Weigh 0.3797 g of RhCl3·3H2O and dissolve it in 5 ml of hydrochloric acid with a concentration of 10 wt% to form a mixed solution. The mass concentration of Pt is 2.78 wt%. Drop it into the SiO2-Na2CO3 support and mix them evenly. Impregnate at room temperature for 12 h, dry at 80 °C for 12 h, and calcine in an air atmosphere at 400 °C for 4 h, with the Pt mass content being 3%. Then, carry out hydrogen reduction on it at a temperature of 400 °C, with a heating rate from room temperature to the reduction temperature of 5 °C / min and a hydrogen volumetric space velocity of 100 h -1 , at atmospheric pressure, and the reduction time is 4 h. A catalyst is obtained, denoted as 3Rh / SiO2-Na2CO3.

[0136] Reaction conditions of the catalyst in a stirred tank: nitric acid concentration (in terms of hydrogen ion concentration) is 0.8 mol / L, uranyl (uranyl nitrate) ion concentration is 210 g / L, hydrazine nitrate concentration is 1.0 mol / L, temperature is 60 °C, stirring rate is 800 rpm, pressure is atmospheric pressure, catalyst dosage is 1.0 g, and reaction feed liquid is 25 mL. The reaction results are shown in Table 1.

[0137] Example 9

[0138] 1) Pretreat the metal oxide support.

[0139] Weigh 5 g of SiO2 support, weigh 0.069 g of K2CO3, dissolve it in 5 ml of deionized water to form a mixed solution with a molar concentration of K2CO3 of 0.1 mol / L. Drop it into the SiO2 support and mix evenly. Impregnate at 30 °C for 2 h, dry at 80 °C for 4 h to obtain the pretreated support SiO2-K2CO3.

[0140] 2) Prepare the eggshell-type catalyst.

[0141] Weigh 5.0 g of the prepared SiO2-K2CO3 support, weigh 0.32499 g of PdCl2, dissolve it in 5 ml of hydrochloric acid with a concentration of 10 wt%, the mass concentration of Pt is 2.78 wt%, drop it into the SiO2-K2CO3 support and mix evenly. Impregnate at room temperature for 12 h, dry at 80 °C for 12 h, calcine in an air atmosphere at 400 °C for 4 h, with the Pt mass content being 3%. Then, carry out hydrogen reduction, the temperature is 400 °C, the heating rate from room temperature to the reduction temperature is 5 °C / min, the volume space velocity of hydrogen is 100 h -1 , the pressure is atmospheric pressure, and the reduction time is 4 h. Obtain the catalyst, denoted as 3Pd / SiO2-K2CO3.

[0142] Reaction conditions of the catalyst in a stirred tank: nitric acid concentration (in terms of hydrogen ion concentration) is 0.8 mol / L, uranyl (uranyl nitrate) ion concentration is 210 g / L, hydrazine nitrate concentration is 1.0 mol / L, temperature is 60 °C, stirring rate is 800 rpm, pressure is atmospheric pressure, catalyst dosage is 1.0 g, and reaction feed liquid is 25 mL. The reaction results are shown in Table 1.

[0143] Example 10

[0144] Take the 3Pt / SiO2-NaHCO3 catalyst that has been reduced and activated in Example 1. Reaction conditions in the stirred tank: sulfuric acid concentration (in terms of hydrogen ion concentration) is 1.0 mol / L, uranyl (uranyl sulfate) ion concentration is 210 g / L, hydrazine nitrate concentration is 1.0 mol / L, temperature is 60 °C, stirring rate is 800 rpm, pressure is atmospheric pressure, catalyst dosage is 1.0 g, and the reaction feed liquid is 25 mL. The reaction results are shown in Table 1.

[0145] Example 11

[0146] Take the 3Pt / SiO2-NaHCO3 catalyst that has been reduced and activated in Example 1. Reaction conditions in the stirred tank: perchloric acid concentration (in terms of hydrogen ion concentration) is 1.0 mol / L, uranyl (uranyl perchlorate) ion concentration is 210 g / L, hydrazine nitrate concentration is 1.0 mol / L, temperature is 60 °C, stirring rate is 800 rpm, pressure is atmospheric pressure, catalyst dosage is 1.0 g, and the reaction feed liquid is 25 mL. The reaction results are shown in Table 1.

[0147] Example 12

[0148] Take the 3Pt / SiO2-NaHCO3 catalyst that has been reduced and activated in Example 1. Reaction conditions in the stirred tank: nitric acid concentration (in terms of hydrogen ion concentration) is 1.0 mol / L, uranyl (uranyl nitrate) ion concentration is 210 g / L, formic acid concentration is 1.0 mol / L, temperature is 60 °C, stirring rate is 800 rpm, pressure is atmospheric pressure, catalyst dosage is 1.0 g, and the reaction feed liquid is 25 mL. The reaction results are shown in Table 1.

[0149] Example 13

[0150] Take the 3Pt / SiO2-NaHCO3 catalyst that has been reduced and activated in Example 1. Reaction conditions in the stirred tank: nitric acid concentration (in terms of hydrogen ion concentration) is 1.0 mol / L, uranyl (uranyl nitrate) ion concentration is 210 g / L, formaldehyde concentration is 1.0 mol / L, temperature is 60 °C, stirring rate is 800 rpm, pressure is atmospheric pressure, catalyst dosage is 1.0 g, and the reaction feed liquid is 25 mL. The reaction results are shown in Table 1.

[0151] Example 14

[0152] Take the 3Pt / SiO2-NaHCO3 catalyst that has been reduced and activated in Example 1. Reaction conditions in the fixed bed: nitric acid concentration (in terms of hydrogen ion concentration) is 0.8 mol / L, uranyl (uranyl nitrate) ion concentration is 210 g / L, hydrazine nitrate concentration is 1.0 mol / L, temperature is 15 °C, liquid hourly space velocity is 0.3 h -1, the pressure is normal pressure, the catalyst dosage is 20 g, and the reaction feed liquid flow rate is 0.2 ml / min. The reaction results are shown in Table 1.

[0153] Comparative Example 1

[0154] Take 5.0 g of ordinary spherical SiO2 support, weigh 0.399 g of H2PtCl6·6H2O, dissolve it in 5 ml of deionized water to form a mixed solution with a Pt molar concentration of 0.154 mol / L, drop it into the SiO2 support and mix it evenly. Impregnate at room temperature for 12 h, dry at 80 °C for 12 h, and calcine in air atmosphere at 300 °C for 4 h, with a Pt mass content of 3%, denoted as 3Pt / SiO2 catalyst. Take 40 ml of hydrazine hydrate with a concentration of 0.5 mol / L, pour the calcined catalyst into it, and maintain at 30 °C for 4 h. After filtering and washing the reduced catalyst, dry it at 60 °C for standby. Denote it as 3Pt / SiO2-uniform type.

[0155] Reaction conditions of the catalyst in the stirred tank: the concentration of nitric acid (in terms of hydrogen ion concentration) is 0.8 mol / L, the concentration of uranyl (uranyl nitrate) ions is 210 g / L, the concentration of hydrazine nitrate is 1.0 mol / L, the temperature is 60 °C, the stirring rate is 800 rpm, the pressure is normal pressure, the catalyst dosage is 1.0 g, and the reaction feed liquid is 25 mL. The reaction results are shown in Table 1.

[0156] Analysis of the results of the examples:

[0157] It can be seen from the data analysis in Table 1 that when using the pretreated support to prepare the eggshell-type noble metal Pt or Ir or Rh or Ru or Pd-based catalyst, when using hydrazine nitrate, formic acid, formaldehyde, etc. as reducing agents under acidic conditions, the conversion rate of U(VI) in the U(VI) to U(IV) reaction is higher than 90%. In the 3Pt / SiO2-NaHCO3 catalyst, when using hydrazine nitrate as the reducing agent, the conversion rate of U(VI) can reach 99%. And in the fixed-bed reactor, the 3Pt / SiO2-NaHCO3 catalyst also shows good reaction performance in the reduction of U(VI) to U(IV) with hydrazine nitrate, and the conversion rate of U(VI) can reach 99%. While for the uniformly distributed catalyst obtained with ordinary SiO2, the conversion rate of U(Ⅵ) is only 60% at 180 min

[0158] It shows that using the pretreated support to prepare the eggshell-type noble metal catalyst has good performance for the U(VI) to U(IV) reaction when using hydrazine nitrate, formic acid, formaldehyde, etc. as reducing agents under acidic conditions.

[0159] Table 1 Reaction performance of catalytic reduction of hexavalent uranium to tetravalent uranium on different catalysts

[0160]

[0161]

[0162] As described above, these are only several embodiments of the present application and do not impose any form of limitation on the present application. Although the present application is disclosed above with preferred embodiments, it is not intended to limit the present application. Any person skilled in the art, without departing from the technical solution of the present application, making some changes or modifications using the disclosed technical content is equivalent to equivalent embodiments and all fall within the scope of the technical solution.

Claims

1. An eggshell-type catalyst, characterized in that, The eggshell-type catalyst includes a carrier and an active component supported on the carrier; The eggshell-type catalyst has a core-shell structure, with the core being the carrier and the shell being the active component; The carrier is a pretreated metal oxide; The active component is a noble metal active component; The pretreatment is at least one of alkali treatment, salt treatment, and acid treatment.

2. The eggshell-type catalyst according to claim 1, characterized in that, The metal oxide is selected from at least one of SiO2, Al2O3, and TiO2; Preferably, the noble metal active component is at least one of Pt, Ir, Rh, Ru, and Pd; Preferably, in the eggshell-type catalyst, the mass percentage of the noble metal active component in the eggshell-type catalyst is 0.01-10%; Preferably, in the eggshell-type catalyst, the mass percentage of the noble metal active component in the eggshell-type catalyst is 0.1-5%.

3. A method for preparing the eggshell-type catalyst according to any one of claims 1 to 2, characterized in that, It includes the following steps: a: Place the metal oxide carrier in a pretreatment solution, impregnate I, and obtain the pretreated carrier after drying I; The pretreatment solution is selected from at least one of an acid solution, an alkali solution, and a salt solution; b: Place the pretreated carrier prepared in step a in a noble metal salt solution, impregnate II, dry II, calcine, and reduce to obtain the eggshell-type catalyst.

4. The preparation method according to claim 3, characterized in that, In step a, The alkali in the alkali solution is at least one of sodium hydroxide and potassium hydroxide; The concentration of the alkali is 0.05-1 mol / L; Preferably, the salt in the salt solution is at least one of sodium carbonate, sodium bicarbonate, potassium carbonate, and potassium bicarbonate; The concentration of the salt is 0.05-2 mol / L; Preferably, the acid in the acid solution is at least one of nitric acid and citric acid; The concentration of the acid is 0.05-1 mol / L; Preferably, the solid-liquid ratio of the metal oxide carrier to the pretreatment solution is 0.1 g / ml-1 g / ml.

5. The preparation method according to claim 3, characterized in that, In step b, In the noble metal salt solution, the noble metal salt is a nitrate and / or chloride containing a noble metal element, where the noble metal element is selected from at least one of Pt, Ir, Rh, Ru, and Pd; Preferably, in the noble metal salt solution, the concentration of the noble metal salt is 0.1 wt%-5 wt%, calculated based on the concentration of the noble metal element; Preferably, the solid-liquid ratio of the pretreated carrier to the noble metal salt solution is 0.5 g / ml-1.0 g / ml.

6. The preparation method according to claim 3, characterized in that, In step a, the temperature of impregnation I is 20-60 °C, and the time of impregnation I is 1-12 h; Preferably, the temperature of drying I is 50-120 °C, and the time of drying I is 1-12 h; In step b, the temperature of impregnation II is 25-60 °C, and the time of impregnation II is 0.5-48 h; Preferably, the temperature of drying II is 60-120 °C, and the time of drying II is 1-24 h; Preferably, the temperature of calcination is 200-500 °C, and the time of calcination is 2-8 h; Preferably, in step b, the reducing atmosphere is hydrogen, the temperature of reduction is 200-500 °C, and the time of reduction is 4-8 h.

7. A method for catalytically reducing hexavalent uranium to tetravalent uranium in an acidic system, characterized in that, A mixed solution containing an acidic solution, a catalyst, a reducing agent and uranyl ions reacts to obtain tetravalent uranium; Among them, the uranium in the uranyl ions is hexavalent uranium; The catalyst is the eggshell-type catalyst described in any one of claims 1 to 2.

8. The method according to claim 7, characterized in that, The reaction is carried out in a batch reactor or a fixed-bed reactor.

9. The method according to claim 7, characterized in that, The acidic solution is selected from at least one of nitric acid, sulfuric acid, and perchloric acid; Preferably, the reducing agent is selected from at least one of hydrazine, formic acid, and formaldehyde; Preferably, the uranyl ions are selected from at least one of uranyl nitrate, uranyl sulfate, and uranyl perchlorate.

10. The method according to claim 7, characterized in that, In the mixed solution, the concentration of the acidic solution is 0.5 to 1.0 mol / L in terms of hydrogen ion concentration; Preferably, in the mixed solution, the concentration of the reducing agent is 0.5 to 2.0 mol / L; Preferably, in the mixed solution, the concentration of the uranyl ions is 100 to 300 g / L in terms of the mass of uranium ions; Preferably, in the mixed solution, the molar ratio of the acidic solution, the reducing agent, and the uranyl ions is 1:0.3 to 3.0:0.5 to 1.2; Preferably, the temperature of the reaction is 15 to 60 °C, and the time of the reaction is 0.5 to 6 h; The mass ratio of the uranyl ions to the catalyst is 100 to 300, where the mass of the uranyl ions is in terms of the mass of uranium ions.

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