Titanium dioxide-coated mil-53(fe) catalyst and preparation method and application thereof

Abstract

The application discloses a preparation method of a titanium dioxide coated MIL-53(Fe) electrocatalyst. First, a catalyst MIL-53(Fe) is obtained through a hydrothermal method, then the catalyst is heated, stirred and dried under mild conditions, and titanium dioxide obtained by hydrolysis of tetra-n-butyl titanate is uniformly coated on the MIL-53(Fe) under the action of dodecyl sulfate, finally, the catalyst is improved in contact with nitrogen through heat treatment, so that the titanium dioxide coated MIL-53(Fe) with good stability is prepared.

Description

TECHNICAL FIELD

[0001] The application specifically relates to a titanium dioxide-coated MIL-53 (Fe) catalyst, a preparation method and application thereof, and belongs to the technical field of material preparation, electrocatalysis and fine chemical industry. BACKGROUND

[0002] In recent years, researchers have explored various ammonia synthesis methods to achieve nitrogen gas synthesis of ammonia under mild conditions. The electrocatalytic nitrogen reduction reaction (NRR) for synthesizing ammonia has obvious advantages, and it can be directly generated from water and nitrogen in the air under environmental conditions. However, it still faces many challenges. The solid-liquid-gas three-phase reaction leads to poor adsorption of nitrogen on the catalyst, and the presence of water inevitably leads to a hydrogen evolution competition reaction. The existence of nitrogen-nitrogen triple bond leads to a large energy barrier for the reaction. It is these reasons that lead to poor activity and selectivity of the NRR reaction. Therefore, designing and synthesizing new efficient and stable electrocatalysts has important research significance for improving the performance of electrocatalytic synthesis of ammonia.

[0003] So far, researchers have screened a lot of catalysts with NRR activity. Metal-organic framework (MOF) materials have high porosity, structures conducive to exposing active sites, and flexible and controllable metal sites. Metal-organic framework (MOF) materials can also be combined with other functional materials, and have been widely used in the fields of catalysis, hydrogen storage, photoelectricity and magnetism, adsorption, etc. However, the research on the electrocatalytic nitrogen reduction reaction for synthesizing ammonia is still in the early stage, and has great potential. As is known to all, the biggest defect of metal-organic framework (MOF) materials is poor stability. Therefore, the metal-organic framework (MOF) material can be used as a "core", and a layer of "shell" can be wrapped on the surface to improve the stability of the metal-organic framework (MOF) material.

[0004] Titanium dioxide is widely used in the field of catalysis due to its easy preparation, low cost, non-toxicity and good stability, and is also a common encapsulating shell of core-shell structure. Currently, the common method for titanium dioxide encapsulating MIL-53(Fe) is to use "one-pot method", that is, to introduce titanium source in the process of synthesizing MIL-53(Fe), which inevitably causes the competition between titanium ions and iron ions in the generation process of metal organic framework (MOF), affecting the generation of pure phase MIL-53(Fe), and the generated titanium dioxide is more in the form of doping rather than forming a core-shell structure of titanium dioxide encapsulating MIL-53(Fe). In addition, using MIL-53(Fe) as a precursor and introducing TiO2 shell by sol method is also a possible strategy. However, there is no effective method to synthesize MIL-53(Fe)@TiO2 core-shell structure material at present, which is mainly due to the difficulty of MIL-53(Fe) to effectively interact with Ti precursor, resulting in two-phase separation, titanium dioxide is difficult to encapsulate and is not uniform. It is necessary to invent a new method to realize the effective preparation of the material. SUMMARY

[0005] In order to solve the problems existing in the prior art, the present application provides a titanium dioxide encapsulating MIL-53(Fe) catalyst and a preparation method thereof, and the prepared titanium dioxide encapsulating MIL-53(Fe) catalyst is applied in the electrocatalytic synthesis of ammonia reaction.

[0006] The technical scheme of the present application is as follows:

[0007] One of the purposes of the present application is to provide a preparation method of titanium dioxide encapsulating MIL-53(Fe) catalyst. First, the catalyst MIL-53(Fe) is obtained by hydrothermal method, and then it is heated, stirred and dried under mild conditions. The titanium dioxide obtained by hydrolysis of tetrabutyl titanate is uniformly coated on MIL-53(Fe) under the action of dodecyl sulfate, and finally the contact between the catalyst and nitrogen is improved by heat treatment, so that the titanium dioxide encapsulating MIL-53(Fe) with good stability is prepared.

[0008] Further, the preparation method of the titanium dioxide encapsulating MIL-53(Fe) catalyst specifically comprises the following steps:

[0009] (1) Disperse terephthalic acid and ferric chloride hexahydrate in N, N-dimethylformamide, and after stirring vigorously for 1-2 h, ultrasonic for 0.5-1 h to obtain a clear solution. Put the solution in a polytetrafluoroethylene reaction kettle for high-temperature hydrothermal reaction for 16-24 h, then soak the hydrothermal reaction product in methanol and centrifugal clean, and dry to obtain a MIL-53(Fe) sample;

[0010] (2) The MIL-53(Fe) sample obtained in step (1) was ultrasonically dispersed in a methanol solution, tetrabutyl titanate and dodecyl sulfate were added, and the mixture was stirred and reacted at a temperature of 50~80 ℃ for 6~8 h. After standing and precipitating, the precipitate was washed by centrifugation with methanol and dried to obtain titanium dioxide-coated MIL-53(Fe) sample.

[0011] (3) Heat treatment is performed on the titanium dioxide-coated MIL-53(Fe) sample obtained in step (2).

[0012] Furthermore, in step (1), the mass ratio of terephthalic acid to ferric chloride hexahydrate is 1:3~3.5.

[0013] Furthermore, the hydrothermal temperature in step (1) is 150~170 ℃.

[0014] Furthermore, in step (2), the mass ratio of MIL-53(Fe) sample to tetrabutyl titanate is 1:10~20.

[0015] Furthermore, in step (2), the molar ratio of dodecyl sulfate to tetrabutyl titanate is 1:50~100.

[0016] Furthermore, in step (3), the heat treatment is carried out under a nitrogen atmosphere, with the temperature increased to 100-300 ℃ at a rate of 5-7 ℃ / min, and the heating time is 2-4 h.

[0017] The second objective of this invention is to provide a titanium dioxide-encapsulated MIL-53(Fe) catalyst prepared by the method described above.

[0018] The third objective of this invention is to provide an application of titanium dioxide-encapsulated MIL-53(Fe) catalyst in the electrocatalytic synthesis of ammonia.

[0019] Furthermore, the titanium dioxide-encapsulated MIL-53(Fe) catalyst is dispersed in a mixture of ethanol and water and ultrasonically dispersed for 1-2 hours. The uniformly dispersed mixture is then drop-coated onto hydrophobic carbon paper to assemble a working electrode, and an electrocatalytic ammonia synthesis reaction is carried out using a three-electrode system.

[0020] Compared with existing technologies, the advantages of this invention are:

[0021] (1) This invention provides a novel method for encapsulating MIL-53(Fe) with titanium dioxide. The method involves hydrolyzing tetrabutyl titanate under mild conditions to generate titanium dioxide-encapsulated MIL-53(Fe). Compared with the "one-pot method" in the prior art, this method avoids the competition between titanium ions and iron ions and does not affect the generation of pure phase MIL-53(Fe). Moreover, the titanium dioxide generated by hydrolysis can uniformly encapsulate MIL-53(Fe) in the form of a "shell". The preparation method is simple and easy to control, the production process is green and environmentally friendly, with low energy consumption, high yield and low cost.

[0022] (2) This invention introduces dodecyl sulfate as a surfactant for the first time in the process of synthesizing the MIL-53(Fe)@TiO2 core-shell structure, which can overcome the problems of difficulty in encapsulation, uneven and weak encapsulation, and easy exposure of catalyst in the prior art;

[0023] (3) This invention is the first to apply the titanium dioxide with high activity and stability to the electrocatalytic reduction of nitrogen to synthesize ammonia. After titanium dioxide is coated, the porous structure, high specific surface area and open structure of the MIL-53(Fe) catalyst are retained, which is conducive to the exposure of active sites. While the catalytic activity of MIL-53(Fe) remains basically unchanged, the electrochemical stability is improved at the same time. Attached Figure Description

[0024] Figure 1 Field emission transmission electron microscopy images of titanium dioxide-encapsulated MIL-53(Fe) catalysts prepared according to Examples 1 to 3 of the present invention;

[0025] Figure 2 The X-ray powder diffraction patterns are those of titanium dioxide-encapsulated MIL-53(Fe) catalysts prepared according to Example 1 and Comparative Examples 1 and 2 of the present invention.

[0026] Figure 3 The stability diagram shows the electrocatalytic nitrogen reduction synthesis of ammonia using the titanium dioxide-encapsulated MIL-53(Fe) catalyst prepared according to Comparative Example 1.

[0027] Figure 4 The stability diagram shows the electrocatalytic nitrogen reduction synthesis of ammonia using the titanium dioxide-encapsulated MIL-53(Fe) catalyst prepared according to Comparative Example 2.

[0028] Figure 5 This is a stability diagram of the electrocatalytic nitrogen reduction synthesis of ammonia using the titanium dioxide-encapsulated MIL-53(Fe) catalyst prepared according to Example 1 of the present invention. Detailed Implementation

[0029] The present invention will be further described below with reference to preferred embodiments. The endpoints and any values ​​of the ranges disclosed in the present invention are not limited to the precise ranges or values. These ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be regarded as specifically disclosed herein.

[0030] Unless otherwise specified, the experimental methods used in the following examples are conventional methods, performed according to the techniques or conditions described in the literature in this field or according to the product instructions. Unless otherwise specified, the materials and reagents used in the following examples are commercially available.

[0031] Example 1

[0032] A method for preparing a titanium dioxide-coated MIL-53(Fe) catalyst specifically includes the following steps:

[0033] (1) Take 1.236 g of terephthalic acid and 4.05 g of ferric chloride hexahydrate and disperse them in N,N-dimethylformamide. After stirring vigorously for 1 h, sonicate for 30 min to obtain a clear solution. Place the solution in a polytetrafluoroethylene reactor and perform a high-temperature hydrothermal reaction at 170℃ for 24 h. When the temperature drops to room temperature, soak and wash the collected product with methanol. Centrifuge the product and treat it in an oven at 80℃ and a vacuum drying oven at 100℃ for 6 h respectively to finally obtain MOF material MIL-53(Fe);

[0034] (2) The 100 mg MIL-53(Fe) sample obtained in step (1) was ultrasonically dispersed in a methanol solution, 1 mL tetrabutyl titanate and 10 mg dodecyl sulfate were added, and the mixture was stirred at 80 °C for 8 h. After hydrolysis and condensation, the mixture was washed by centrifugation with methanol several times, and dried in an oven at 80 °C to obtain the MIL-53(Fe)@TiO2 precursor.

[0035] (3) Place the MIL-53(Fe)@TiO2 precursor obtained in step (2) in a ceramic boat, heat it to 200℃ for 4 h under nitrogen at a rate of 5℃ / min, and remove the solvent of excess reactants to obtain titanium dioxide-encapsulated MIL-53(Fe) electrocatalyst.

[0036] Example 2

[0037] A method for preparing a titanium dioxide-coated MIL-53(Fe) catalyst specifically includes the following steps:

[0038] (1) Take 1.000 g terephthalic acid and 3.000 g iron chloride hexahydrate and disperse them in N, N-dimethylformamide, stir vigorously for 2 h, then ultrasonic for 1 h to obtain a clear solution, place the solution in a polytetrafluoroethylene reaction kettle and react at 150℃ for 16 h, when the temperature decreases to room temperature, soak and wash the collected product with methanol, centrifuge the product, and then treat it in an oven at 80℃ and a vacuum drying box at 100℃ for 6 h, to obtain MOF material MIL-53(Fe);

[0039] (2) Ultrasonically disperse 200 mg of the MIL-53(Fe) sample obtained in step (1) in a methanol solution, add 4 mL of titanium tetrabutoxide and 25 mg of dodecyl sulfate, stir at a temperature of 60℃ for 6 h, after hydrolysis and condensation, wash the mixture with methanol several times, and dry it in an oven at 80℃ to obtain a MIL-53(Fe)@TiO2 precursor;

[0040] (3) Place the MIL-53(Fe)@TiO2 precursor obtained in step (2) in a porcelain boat, heat it to 300℃ at a rate of 7℃ / min under nitrogen, heat for 2 h to perform heat treatment, and remove the solvent of the excess reactant, to obtain a titanium dioxide-coated MIL-53(Fe) electrocatalyst.

[0041] Example 3

[0042] A method for preparing a titanium dioxide-coated MIL-53(Fe) catalyst, specifically comprising the following steps:

[0043] (1) Take 1.000 g terephthalic acid and 3.500 g iron chloride hexahydrate and disperse them in N, N-dimethylformamide, stir vigorously for 1.5 h, then ultrasonic for 40 min to obtain a clear solution, place the solution in a polytetrafluoroethylene reaction kettle and react at 160℃ for 20 h, when the temperature decreases to room temperature, soak and wash the collected product with methanol, centrifuge the product, and then treat it in an oven at 80℃ and a vacuum drying box at 100℃ for 6 h, to obtain MOF material MIL-53(Fe);

[0044] (2) Ultrasonically disperse 100 mg of the MIL-53(Fe) sample obtained in step (1) in a methanol solution, add 1.5 mL of titanium tetrabutoxide and 18.75 mg of dodecyl sulfate, stir at a temperature of 50℃ for 7 h, after hydrolysis and condensation, wash the mixture with methanol several times, and dry it in an oven at 80℃ to obtain a MIL-53(Fe)@TiO2 precursor;

[0045] (3) The MIL-53(Fe)@TiO2 precursor obtained in step (2) is placed in a porcelain boat, heated at a rate of 6 ℃ / min to 100 ℃ under nitrogen for 3 h for heat treatment, and the solvent of the excess reactants is removed, so that the titanium dioxide coated MIL-53(Fe) electrocatalyst is obtained.

[0046] Example 4

[0047] The titanium dioxide coated MIL-53(Fe) prepared according to the above Example 1 is applied in the electrocatalytic synthesis of ammonia reaction, 10 mg of the titanium dioxide coated MIL-53(Fe) electrocatalyst is dispersed in a mixed solution comprising 480 μL of ethanol, 480 μL of water and 40 μL of Nafion, and ultrasonic dispersion is performed for 1-2 h, the uniformly dispersed mixed solution is drop-coated on a hydrophobic carbon paper to assemble a working electrode, and a three-electrode system is used to perform the electrocatalytic synthesis of ammonia reaction.

[0048] Comparative Example 1

[0049] A method for preparing a titanium dioxide coated MIL-53(Fe) catalyst, using terephthalic acid, iron chloride hexahydrate and titanium tetrabutoxide as raw materials, obtaining the catalyst MIL-53(Fe)@TiO2 by a hydrothermal method, and then obtaining the titanium dioxide coated MIL-53(Fe) catalyst by heat treatment at 200 ℃, specifically comprising the following steps:

[0050] (1) 1.236 g of terephthalic acid, 4.05 g of iron chloride hexahydrate and 1 mL of titanium tetrabutoxide are dissolved in an N, N-dimethylformamide solution, ultrasonic treatment is performed for 30 min to obtain a clear solution, which is moved to a reaction kettle for treatment at 170 ℃ for 24 h, when the temperature decreases to room temperature, the collected product is soaked and washed with methanol, and the obtained sample is treated in an oven at 80 ℃ and a vacuum drying box at 100 ℃ for 6 h, respectively, to finally obtain a MIL-53(Fe)@TiO2 precursor;

[0051] (2) The MIL-53(Fe)@TiO2 precursor obtained in step (1) is placed in a porcelain boat, heated at a rate of 5 ℃ / min to 200 ℃ under nitrogen for 4 h for heat treatment, and the solvent of the excess reactants is removed, so that the titanium dioxide coated MIL-53(Fe) catalyst is obtained.

[0052] Comparative Example 2

[0053] The preparation method of the titanium dioxide coated MIL-53(Fe) catalyst includes the following steps:

[0054] (1) 1.236 g of terephthalic acid and 4.05 g of iron chloride hexahydrate are dissolved in an N, N-dimethylformamide solution, and a clear solution is obtained by ultrasonic treatment for 30 min, and then the solution is moved to a reaction kettle and treated at 170℃ for 24 h. When the temperature decreases to room temperature, the collected product is soaked and washed with methanol, and the obtained sample is centrifuged and treated in an oven at 80℃ and a vacuum drying box at 100℃ for 6 h, and finally the MOF material MIL-53(Fe) is obtained;

[0055] (2) 100 mg of the MIL-53(Fe) obtained in step (1) is ultrasonically dispersed in 200 mL of a methanol solution, and then 1 mL of tetrabutyl titanate is added, and the mixture is heated and stirred at 80℃. After the hydrolysis and condensation process, the mixture is washed with methanol by centrifugation for multiple times, and the MIL-53(Fe)@TiO2 precursor is obtained after drying in an oven at 80℃.

[0056] (3) The MIL-53(Fe)@TiO2 precursor obtained in step (2) is placed in a porcelain boat, heated to 200℃ at a rate of 5℃ / min under nitrogen, and heated for 4 h for heat treatment, and the solvent of the excess reactant is removed, and the titanium dioxide coated MIL-53(Fe) catalyst is obtained.

[0057] The titanium dioxide coated MIL-53(Fe) obtained according to Example 1 and Comparative Examples 1 and 2 is analyzed and tested, and the specific results are as follows:

[0058] Figure 1 The field emission transmission electron microscope images of the titanium dioxide coated MIL-53(Fe) catalyst samples prepared by different preparation methods according to Example 1 and Comparative Examples 1 and 2 are shown in the following table: Figure 1From the figure, it can be observed that MIL-53(Fe)@TiO2(sample 1) prepared by using the "one-pot method" in Comparative Example 1 does not form a core-shell structure (as shown in figure a), MIL-53(Fe)@TiO2(sample 2) prepared in Comparative Example 2 without adding a surfactant is not uniformly coated with titanium dioxide (as shown in figure b); and MIL-53(Fe)@TiO2(sample 3) prepared in Example 1 by adding dodecyl sulfate as a surfactant is uniformly coated with titanium dioxide on MIL-53(Fe), forming a typical core-shell structure.

[0059] Figure 2 The X-ray powder diffraction patterns of the titanium dioxide-coated MIL-53(Fe) catalyst samples prepared by different preparation methods according to Example 1, Comparative Examples 1 and 2 of the present application are provided in Table 1. Figure 2 From Table 1, it can be observed that MIL-53(Fe)@TiO2 prepared by using the "one-pot method" in Comparative Example 1 has impurity peaks, and the prepared MIL-53(Fe) is not pure (sample 1), and the diffraction peak patterns of the MIL-53(Fe) catalysts prepared in Comparative Example 2 (sample 2) and Example 1 (sample 3) are consistent with the literature reports, indicating that the MIL-53(Fe) catalyst is successfully synthesized, and the diffraction peaks of MIL-53(Fe) before and after being coated with titanium dioxide are basically unchanged.

[0060] Table 1 is the statistical data of the best reduction activity of nitrogen and Faraday efficiency of the titanium dioxide-coated MIL-53(Fe) catalyst prepared by different preparation methods according to Example 1, Comparative Examples 1 and 2 of the present application. The performance test of the electrocatalytic reduction of nitrogen is carried out in an H-type reactor, and a three-electrode system is used in the electrocatalytic process. The catalyst prepared by Example 1, Comparative Examples 1 and 2 of the present application is assembled into a working electrode, a platinum sheet is used as a counter electrode, and a saturated silver / silver chloride is used as a reference electrode. Before the performance test, purified nitrogen is blown into the electrolyte (potassium sulfate with a pH value of 6), and when the nitrogen in the electrolyte is saturated, different bias voltages can be applied for reaction. All the potentials are relative to the standard hydrogen electrode.

[0061] Table 1 Statistical data of the best reduction activity of nitrogen and Faraday efficiency of the titanium dioxide-coated MIL-53(Fe) catalyst

[0062]

[0063] From Table 1, it can be observed that the ammonia synthesis yield of sample 3 (Example 1) with added surfactant can reach 30.1 μg h - 1 mg cat. -1The faraday efficiency reaches 29.2%, which is higher than that of sample 1 (comparative example 1) and sample 2 (comparative example 2).

[0064] From Figure 3 It can be seen that the catalyst (sample 1) prepared according to comparative example 1 has poor activity and cycle stability due to the existence of impurities and the poor wrapping of titanium dioxide. Figure 4 It can be seen that the catalyst (sample 2) prepared according to comparative example 1 has poor activity after being used for the second time due to the uneven and unstable wrapping of titanium dioxide, but is still higher than that of sample 1 without wrapping. Figure 5 It can be seen from

[0065] The above-mentioned examples are preferred embodiments of the present application, but the embodiments of the present application are not limited to the above-mentioned examples, and any changes, modifications, substitutions, combinations and simplifications made without departing from the spirit and principles of the present patent should be equivalent replacement methods and should be included in the protection scope of the present patent.

Claims

1. A method for preparing a titanium dioxide-encapsulated MIL-53(Fe) electrocatalyst, characterized in that: First, catalyst MIL-53(Fe) was obtained via a hydrothermal method. Then, it was heated and dried under mild conditions with stirring. Titanium dioxide obtained from the hydrolysis of tetrabutyl titanate was uniformly coated onto MIL-53(Fe) with dodecyl sulfate. Finally, heat treatment was used to improve the contact between the catalyst and nitrogen gas, resulting in a stable titanium dioxide-coated MIL-53(Fe) product. The process includes the following steps: (1) Terephthalic acid and ferric chloride hexahydrate were dispersed in N,N-dimethylformamide, stirred vigorously for 1-2 h, and then sonicated for 0.5-1 h to obtain a clear solution. The solution was placed in a polytetrafluoroethylene reactor for high-temperature hydrothermal reaction for 16-24 h. The hydrothermal reaction product was then washed by soaking in methanol and centrifugation, and dried to obtain the MIL-53(Fe) sample. (2) The MIL-53(Fe) sample obtained in step (1) was ultrasonically dispersed in a methanol solution, tetrabutyl titanate and dodecyl sulfate were added, and the mixture was stirred and reacted at a temperature of 50~80 ℃ for 6~8 h. After standing and precipitating, the precipitate was washed by centrifugation with methanol and dried to obtain titanium dioxide-coated MIL-53(Fe) sample. (3) Heat treatment is performed on the titanium dioxide-coated MIL-53(Fe) sample obtained in step (2).

2. The preparation method of the titanium dioxide-encapsulated MIL-53(Fe) electrocatalyst as described in claim 1, characterized in that, In step (1), the mass ratio of terephthalic acid to ferric chloride hexahydrate is 1:3~3.

5.

3. The preparation method of the titanium dioxide-encapsulated MIL-53(Fe) electrocatalyst as described in claim 1, characterized in that, The hydrothermal temperature in step (1) is 150~170 ℃.

4. The preparation method of a titanium dioxide-encapsulated MIL-53(Fe) catalyst as described in claim 1, characterized in that, In step (2), the mass ratio of MIL-53(Fe) sample to tetrabutyl titanate is 1:10~20.

5. The preparation method of a titanium dioxide-encapsulated MIL-53(Fe) catalyst as described in claim 1, characterized in that, In step (2), the molar ratio of dodecyl sulfate to tetrabutyl titanate is 1:5~20.

6. The preparation method of a titanium dioxide-encapsulated MIL-53(Fe) catalyst as described in claim 1, characterized in that, In step (3), the heat treatment is carried out in a nitrogen atmosphere, with the temperature increased to 100-300 ℃ at a rate of 5-7 ℃ / min, and the heating time is 2-4 h.

7. The titanium dioxide-encapsulated MIL-53(Fe) electrocatalyst prepared by the preparation method according to any one of claims 1 to 6.

8. The application of the titanium dioxide-encapsulated MIL-53(Fe) electrocatalyst as described in claim 7 in the electrocatalytic synthesis of ammonia.

9. The application of the titanium dioxide-encapsulated MIL-53(Fe) electrocatalyst as described in claim 8 in the electrocatalytic synthesis of ammonia, characterized in that: The titanium dioxide-encapsulated MIL-53(Fe) electrocatalyst was dispersed in a mixture of ethanol and water and ultrasonically dispersed for 1-2 hours. The uniformly dispersed mixture was then drop-coated onto hydrophobic carbon paper to assemble a working electrode, and an electrocatalytic ammonia synthesis reaction was carried out using a three-electrode system.

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