Method for coating titanium dioxide on surface of polyoxometalate and application thereof

Abstract

The application belongs to the technical field of material preparation and application, and specifically discloses a method for coating titanium dioxide on the surface of polyoxometalate and application, which comprises the following steps: providing polyoxometalate; taking titanium source injection, inert gas purging, reaction gas injection and inert gas purging as one ALD cycle, and performing multiple cycles under the same condition to complete the deposition of a titanium dioxide coating layer on the surface of the polyoxometalate; and the material obtained by using the above method has obviously improved stability and dispersity, the obtained titanium dioxide coating layer is dense and uniform, and when the material is applied to a photocatalytic carbon dioxide reduction reaction, the polyoxometalate and the titanium dioxide coating layer cooperate with each other, and the catalytic activity is obviously improved, and the defects of poor stability and easy agglomeration of the existing polyoxometalate are solved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of material preparation and application, and particularly relates to a method for coating titanium dioxide on the surface of a polyoxometalate and application thereof. BACKGROUND

[0002] Polyoxometalates (POMs for short) are a class of inorganic metal oxo-cluster compounds with a certain structure formed by transition metal ions connected by oxygen, which have excellent redox activity, catalytic performance and photoelectric performance, and show great application potential in the fields of catalysis, energy, environment and biological medicine.

[0003] However, pure polyoxometalates have many limitations in practical application: first, the stability is poor, and they are easy to dissolve or decompose in aqueous solution and other environments, and the performance decays; second, the specific surface area is small, and they are easy to agglomerate, reducing the exposure degree of active sites and affecting the catalytic performance; third, it is difficult to recover, especially the nanoscale polyoxometalate particles, which are difficult to separate from the treatment system after being applied to water treatment and other applications, and it is difficult to recover, which can easily cause secondary pollution and resource waste. Modifying polyoxometalates to improve their stability and practicality has become a research hotspot in the field.

[0004] Currently, the performance of polyoxometalates is generally improved by functional compounding, such as patent application file 202110505036.5, which discloses a method for preparing a doped mixed oxide photocatalyst by uniformly dispersing polyoxometalates on an oxide heterojunction with excellent semiconductor properties to improve its reaction activity. The metal oxoacid salt-based oxide photocatalyst disclosed has high photocatalytic oxidation efficiency, can exist stably in the environment, is easy to separate and recover, and has good cycle stability. Patent application file 202011201654.2 discloses a method for preparing a polyoxometalate-titanium dioxide nanocomposite by one-pot method. The MnW 12 / TiO2 nanocomposite has two micro-morphologies of micrometer flower and nanorod, and has obviously enhanced photocatalytic hydrogen production activity and cycle stability. However, the above functional compounding methods generally have high process difficulty and harsh reaction conditions, and still cannot improve the problem of easy agglomeration of polyoxometalates. Stability and easy agglomeration are the key factors affecting the practical application of polyoxometalates, and providing a process that can specifically solve the poor stability and easy agglomeration of polyoxometalates is one of the important ways to solve the inherent defects of polyoxometalates and expand their application range. SUMMARY

[0005] The technical problem solved by the present application is to provide a method for coating titanium dioxide on the surface of polyoxometalate and application, which has significantly improved stability and dispersibility, and the titanium dioxide coating layer is dense and uniform, and the polyoxometalate and the titanium dioxide coating layer cooperate in the photocatalytic carbon dioxide reduction reaction, thereby significantly improving the catalytic activity, and solving the defects of poor stability and easy agglomeration of the existing polyoxometalate.

[0006] To solve the above technical problems, the technical scheme adopted by the present application is:

[0007] On the one hand, a method for coating titanium dioxide on the surface of polyoxometalate is provided, comprising:

[0008] Providing polyoxometalate;

[0009] Titanium source injection, inert gas purging, reaction gas injection and inert gas purging are taken as one ALD cycle, and multiple cycles are performed under the same conditions to complete the deposition of the titanium dioxide coating layer on the surface of the polyoxometalate.

[0010] On the other hand, the application of the polyoxometalate with the deposited titanium dioxide coating layer prepared by the above method for photocatalytic reduction of carbon dioxide is provided.

[0011] Compared with the prior art, the present application has the following advantages:

[0012] 1. The present application provides a method for depositing a titanium dioxide coating layer on the surface of polyoxometalate by atomic layer deposition, and the obtained titanium dioxide coating layer is uniform, dense and strongly combined with the polyoxometalate matrix.

[0013] 2. The method for depositing a titanium dioxide coating layer on the surface of polyoxometalate can obtain a titanium dioxide coating layer with a predetermined thickness by controlling the number of cycles, and realizes atomic-level accurate regulation of the thickness of the coating layer.

[0014] 3. The polyoxometalate with the deposited titanium dioxide coating layer prepared by the method of the present application has high catalytic activity, redox performance of polyoxometalate and high chemical stability and optical performance of titanium dioxide, and cooperates to have the characteristics of efficient reduction of carbon dioxide, high reaction stability and resistance to poisoning.

[0015] 4. The method of the present application is simple, controllable and reproducible, suitable for large-scale production, and the prepared composite material has a wide application range and important practical application value and market prospect in the field of photocatalysis.

[0016] 5. The application provides the application of the polyoxometalate with a titanium dioxide coating layer prepared by the method in the above in the photocatalytic reduction of carbon dioxide, and the yield of the obtained products CO and H2 is close to the standard synthesis gas.

[0017] The technical solutions of the application will be further described in detail below with reference to the accompanying drawings and examples. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 Scanning electron microscope images of various materials;

[0019] Figure 2 Transmission electron microscope images of the titanium dioxide coated polyoxometalate composite material of Example 1;

[0020] Figure 3 XRD images of the titanium dioxide coated polyoxometalate composite material (Anderson Co@TiO2) of Example 1 and Anderson Co;

[0021] Figure 4 A schematic diagram of the photocatalytic carbon dioxide reduction performance test results of the titanium dioxide coated polyoxometalate composite material (Anderson Co@TiO2) of Example 1, the polyoxometalate (Anderson Co) or pure titanium dioxide as a catalyst. DETAILED DESCRIPTION

[0022] The technical solutions will be described in detail below with reference to the examples of the application. Obviously, the described examples are part of the examples of the application, rather than all the examples of the application. Based on the examples in the application, all other examples obtained by those skilled in the art without creative labor fall within the scope of protection of the application.

[0023] In the following description, the term "and / or" is used to describe the association relationship of the associated objects, which means that there can be three relationships, for example, A and / or B, which can represent the cases of A alone, B alone and A and B together. Wherein A and B can be singular or plural.

[0024] In the following description, the terms "include", "contain", "have" and "contain" and the like are all open terms, that is, they mean to include but not limited to.

[0025] Those skilled in the art should understand that in the following description of the examples of the application, the order of the serial numbers does not mean the order of execution, and some or all steps can be executed in parallel or in sequence, and the execution order of each process should be determined according to its function and inherent logic, and should not constitute any limitation on the implementation process of the examples of the application.

[0026] Those skilled in the art will appreciate that numerical ranges recited in the embodiments herein are to be understood as specifically encompassing each and every integer value within the range. In this application, ranges include the beginning and end points of the ranges. In this application, any open-ended range in a claim, should be construed to be encompassing at least the lower limit of the range. In this application, any range of "about" a value or values can be construed as meaning at least the lower limit of the range.

[0027] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application, preferred methods and materials are described. All publications mentioned in this specification are herein incorporated by reference to disclose and describe the methods and / or materials in connection with which the publications are cited. The citation of any reference in this specification is not an admission that it is prior art.

[0028] The technical principle adopted by the present application is based on atomic layer deposition technology, and titanium dioxide is deposited on the surface of metal oxyacid salt by alternately introducing titanium source and reaction gas, and a titanium dioxide coating layer is realized on the surface of metal oxyacid salt through layer-by-layer deposition.

[0029] In one aspect, a method for coating titanium dioxide on the surface of a polyoxometalate is provided, comprising:

[0030] providing a polyoxometalate;

[0031] Titanium dioxide is deposited on the surface of the polyoxometalate by taking titanium source injection, inert gas purging, reaction gas injection and inert gas purging as one ALD cycle, and performing multiple cycles under the same conditions to complete the deposition of the titanium dioxide coating layer on the surface of the polyoxometalate.

[0032] The present application creatively provides a method for depositing a titanium dioxide coating layer on the surface of a polyoxometalate based on atomic layer deposition technology, which can fully combine the characteristics of high chemical stability, acid and alkali corrosion resistance, non-toxicity and excellent optical performance of the semiconductor material titanium dioxide, realize the protection of the surface of the polyoxometalate, and synergize with the function of the polyoxometalate to optimize the photocatalytic performance of the polyoxometalate.

[0033] The method for coating titanium dioxide on the surface of polyoxometalate includes the following steps: first, injecting a titanium source, chemical adsorption of the titanium source on the surface of the polyoxometalate, then, removing the titanium source not adsorbed in the reaction cavity by blowing inert gas, then, chemical reaction of the reaction gas with the titanium source adsorbed on the surface of the polyoxometalate to generate titanium dioxide, and finally, removing the unreacted reaction gas and the remaining impurities by blowing inert gas. The deposition of the titanium dioxide coating layer on the surface of the polyoxometalate is completed by repeatedly performing the above-mentioned cycle. The method can effectively increase the number of active sites on the surface of the polyoxometalate, promote the chemical adsorption of the titanium source on the surface of the polyoxometalate matrix, enhance the stability of the coating layer and the matrix, and obtain a thin and uniform coating layer without cracks or holes, a strong bonding force between the coating layer and the matrix, a higher stability of the product, and less agglomeration.

[0034] In some embodiments, the thickness of the titanium dioxide coating layer is 2-10 nm.

[0035] The thickness of the titanium dioxide coating layer in the polyoxometalate deposited with the titanium dioxide coating layer obtained by the method for coating titanium dioxide on the surface of polyoxometalate is 2-10 nm. The thickness of the coating product can not only help to protect the polyoxometalate with the titanium dioxide coating layer, but also can avoid excessive active sites being covered due to the over-thickness of the titanium dioxide coating layer. It is found in the research process that when the thickness of the titanium dioxide coating layer exceeds 10 nm, the amount of hydrogen generated in the photocatalytic reduction of carbon dioxide is reduced, and the generation of hydrogen is inhibited. When the thickness of the titanium dioxide coating layer is less than 2 nm, the amount of carbon monoxide generated in the photocatalytic reduction of carbon dioxide is reduced, and the generation of carbon monoxide is inhibited. The reason is that the thickness of the coating layer affects the specific surface area of the catalyst. The thicker the coating layer, the larger the specific surface area, and the more CO is generated. The thinner the coating layer, the smaller the specific surface area, and the more hydrogen is generated. When the thickness of the titanium dioxide coating layer is 2-10 nm, carbon monoxide and hydrogen can be generated in a ratio close to that of standard synthesis gas.

[0036] In some embodiments, the temperature in the cycle process is 180-200℃, and / or the cycle number is 20-50 times.

[0037] The preparation method of the present application can obtain a titanium dioxide coating layer with a thickness linearly related to the cycle number by depositing at a temperature of 180-200℃ according to the above-mentioned ALD cycle. The thickness of the titanium dioxide coating layer obtained by each ALD cycle under the above-mentioned conditions is 0.1-0.2 nm. By controlling the cycle number to be 20-50 times, the thickness of the coating layer can be accurately controlled to be 2-10 nm. At the same time, by limiting the ALD cycle to be performed at a temperature of 180-200℃, the decomposition of the polyoxometalate and the titanium source can be effectively avoided, and the titanium source and the reaction gas can be fully adsorbed on the surface of the polyoxometalate to form a stable saturated monolayer.

[0038] In some embodiments, the titanium source injection time is 0.1-0.5 s, the inert gas purging and re-purging time is 5-10 s, and the reaction gas injection time is 0.2-0.6 s.

[0039] In some embodiments, the titanium source is titanium tetrachloride, titanium tetraisopropoxide or tetrabutyl titanate; in some preferred embodiments, the titanium source is titanium tetrachloride.

[0040] The titanium source is preferably titanium tetrachloride, which has low toxicity, moderate volatility and stable reactivity.

[0041] In some embodiments, the inert gas is nitrogen or argon.

[0042] In some embodiments, the reaction gas is water vapor, ozone or oxygen plasma; in some preferred embodiments, the reaction gas is ozone.

[0043] The reaction gas is preferably ozone, which is more likely to react with the titanium source, has high reactivity, and the by-products generated are more easily removed by inert gas purging.

[0044] In some embodiments, the ALD reaction cavity vacuum degree in the ALD cycle is 0.001-0.1 Pa.

[0045] In some embodiments, the polyoxometalate topological skeleton is Anderson type, and specifically, the polyoxometalate is Anderson Co{(NH4)3[CoMo6O 24 H6]·7H2O]}.

[0046] The polyoxometalate for titanium dioxide coating layer deposition is preferably Anderson Co, which has redox properties and catalytic carbon dioxide reduction activity, and is stable as a deposition matrix.

[0047] In some embodiments, the polyoxometalate is prepared by: under boiling conditions, adding a hydrogen peroxide solution containing cobalt sulfate to a molybdate aqueous solution, filtering while hot, to obtain polyoxometalate Anderson Co{(NH4)3[CoMo6O 24 H6]·7H2O]}.

[0048] In another aspect, the application provides a use of the polyoxometalate with a titanium dioxide coating layer prepared by the above method for photocatalytic reduction of carbon dioxide.

[0049] In some specific embodiments, the application is to use the polyoxometalate with a titanium dioxide coating layer as a photocatalyst to catalyze the reduction of carbon dioxide to obtain CO and H2, and the yield ratio of the CO and H2 is close to the standard synthesis gas, and the application specifically comprises: using the polyoxometalate with a titanium dioxide coating layer as a photocatalyst, and contacting with carbon dioxide under visible light or ultraviolet light irradiation in a reaction system composed of a photosensitizer, acetonitrile, triethanolamine and deionized water; the mass-volume ratio of the photocatalyst, deionized water, photosensitizer, triethanolamine and acetonitrile is 1 mg: 2 mL: 8 mg: 2 mL: 8 mL.

[0050] The polyoxometalate with a titanium dioxide coating layer of the application is used as a photocatalyst in the catalytic reduction reaction of carbon dioxide, and the yield ratio of the products CO and H2 is close to 1.3:1 of the standard synthesis gas.

[0051] The application has undergone a series of experiments before the application, and now some test results are listed to further describe the application in detail, which will be described in detail below in combination with the embodiments.

[0052] Embodiment 1

[0053] The embodiment provides a method for coating titanium dioxide on the surface of a polyoxometalate, which specifically comprises the following steps:

[0054] Step one, synthesizing a polyoxometalate, specifically comprising:

[0055] Step 101, placing 30.9 g of ammonium molybdate tetrahydrate in 260 mL of deionized water, heating to boiling, and stirring until completely dissolved under boiling conditions to obtain a molybdate solution;

[0056] Step 102, dissolving 4.2 g of cobalt sulfate heptahydrate and 4.4 g of hydrogen peroxide solution with a mass percentage of 30% in 40 mL of deionized water to obtain a cobalt salt solution;

[0057] Step 103, keeping the boiling state, adding the cobalt salt solution to the molybdate solution, stirring for 2 min under boiling conditions, filtering while hot, cooling to room temperature, and standing for 1 day to obtain a dark green crystal, i.e. a polyoxometalate Anderson Co{(NH4)3[CoMo6O 24 H6]·7H2O]}, labeled as Anderson Co;

[0058] Step two, ALD deposition preparation, specifically comprising:

[0059] Step 201: The polyoxometalate is evenly spread in a 316L stainless steel chamber with a volume of about 0.33L, placed in the reaction chamber of the tubular ALD reaction device, the reaction chamber is closed and the airtightness is checked, the vacuum pump is started and the vacuum is evacuated to 0.01Pa; the tubular ALD reaction device is model GM100 and the manufacturer is YUNMAO.

[0060] Step 3: Deposit the TiO2 coating layer, which specifically includes:

[0061] Step 301: Set the reaction chamber temperature to 180℃. After the temperature stabilizes, perform the deposition cycle according to the following procedure: introduce titanium tetrachloride for 0.3s, then purge with nitrogen for 8s, then introduce ozone for 0.4s, and finally purge with nitrogen again for 8s; repeat the above cycle 30 times to obtain the coated product. After cooling to room temperature, remove the product to complete the coating of titanium dioxide on the surface of the polyoxometalate.

[0062] Performance Evaluation

[0063] Figure 1 Here are scanning electron microscope (SEM) images of each material, among which... Figure 1 Image a is a scanning electron microscope (SEM) image of the titanium dioxide-coated polyoxometalate composite material from Example 1 at the 1 μm scale. Figure 1 Image b is a scanning electron microscope (SEM) image of the titanium dioxide-coated polyoxometalate composite material from Example 1 at a scale of 500 nm. Figure 1 c is a scanning electron microscope image of the titanium dioxide-coated polyoxometalate composite material of Example 1 at the 1 μm scale (and...). Figure 1 (where 'a' represents different perspectives) According to Figure 1 a~ Figure 1 As can be seen from c, the composite material exhibits a distinct layered structure. Figure 1 (a), with spherical particles distributed on the surface ( Figure 1 b and Figure 1 (c) indicates that the present invention successfully obtained Anderson Co coated with titanium dioxide, with spherical titanium dioxide adhering to the surface of the polyoxometalate. In Example 1, the thickness of the titanium dioxide coating layer in the titanium dioxide-coated polyoxometalate composite material is 2-3 nm. Figure 1 The image d is a SEM image of the polyoxometalate Anderson Co prepared in Example 1, which clearly shows that its morphology is a blocky structure with layers stacked together. Figure 1 The image shown in Figure 'e' is a SEM image of commercially available titanium dioxide powder, which clearly shows that its morphology is spherical. Figure 1 f is the element surface scan. Figure 1 The g~k diagram shows the corresponding elemental distribution, and a uniform distribution of titanium species can be observed on the surface.

[0064] Figure 2Transmission electron microscope (TEM) image of the titanium dioxide coated polyoxometalate composite material of Example 1, wherein Figure 2 a is a transmission electron microscope image at a scale of 10 nm, it can be seen that a crystal phase with a lattice spacing of 0.34 nm appears in the composite material, which matches the (101) lattice spacing of titanium dioxide, Figure 2 b is a transmission electron microscope image at a scale of 20 nm, there is obvious interface layering, indicating that the titanium dioxide layer is uniformly coated on the surface of the polyoxometalate Anderson Co, and the titanium dioxide coating layer is tightly combined with the core Anderson Co.

[0065] Figure 3 XRD pattern of the titanium dioxide coated polyoxometalate composite material (Anderson Co@TiO2) of Example 1 and Anderson Co, wherein the Anderson Co is the Anderson Co obtained in step one of Example 1, and the titanium dioxide coated polyoxometalate composite material appears characteristic peaks attributed to Anderson Co and titanium dioxide (standard card), indicating the successful combination of titanium dioxide and polyoxometalate.

[0066] Figure 4 Schematic diagram of the photocatalytic carbon dioxide reduction performance test results of using the titanium dioxide coated polyoxometalate composite material (Anderson Co@TiO2) of Example 1, polyoxometalate (Anderson Co) or pure titanium dioxide as a catalyst, the test method includes: in a 80 mL quartz reactor with a cover, using MC-PF30-1 xenon lamp (300 W, wavelength range 320-780 nm) as a light source, 1 mg of the catalyst, 2 mL of deionized water, 8 mg of photosensitizer, 2 mL of triethanolamine and 8 mL of acetonitrile are added into the quartz reactor, before light irradiation, ultrahigh purity carbon dioxide (99.999%) is introduced into the quartz reactor for 15 min to ensure that the reaction system is saturated and air is discharged, the temperature of the reaction system is maintained at 15°C by using circulating water, carbon dioxide is introduced, the quartz reactor is kept at one standard atmosphere, and the reaction is carried out under light irradiation, the reaction is carried out for 1 h, and the products are analyzed by gas chromatography (Agilent GC-8860); wherein the pure titanium dioxide is purchased from Macklin CAS: 13463-67-7, the photosensitizer is tris (2, 2'-bipyridine) ruthenium chloride, hexahydrate, CAS: 50525-27-4. According to Figure 4As can be seen, the titanium dioxide-coated polyoxometalate composite material of Example 1 exhibits significantly improved catalytic performance. In the catalytic carbon dioxide reduction reaction, the yields of CO and H2 are significantly higher than those of the pure titanium dioxide catalyst and the Anderson Co catalyst. Moreover, in the corresponding catalytic reaction, the yields of CO and H2 of the titanium dioxide-coated polyoxometalate composite material of Example 1 are 11.1 mmol / g / h and 8.5 mmol / g / h, respectively, with production rates of 3689 ppm and 2876 ppm, respectively, which are close to the production rates of standard syngas at a ratio of 1.3:1. This indicates that the titanium dioxide-coated polyoxometalate composite material of the present invention has a synergistic effect in promoting photocatalysis.

Claims

1. The application of a polyoxometalate coated with titanium dioxide, prepared by coating the surface of a polyoxometalate with titanium dioxide, in the photocatalytic reduction of carbon dioxide, characterized in that, The photocatalytic reduction of carbon dioxide yields CO and H2; the method of coating titanium dioxide onto the surface of polyoxometalates includes: Provide a polyoxometalate; said polyoxometalate is Anderson Co{(NH4)3[CoMo6O 24 H6]·7H2O]}; An ALD cycle consists of titanium source injection, inert gas purging, reactive gas injection, and inert gas re-purging. Multiple cycles are performed under the same conditions to deposit a titanium dioxide coating layer on the surface of the polyoxometalate. The thickness of the titanium dioxide coating layer is 2~10 nm. The application includes using the polyoxometalate coated with titanium dioxide as a photocatalyst, forming a reaction system with a photosensitizer, acetonitrile, triethanolamine and deionized water, and contacting carbon dioxide under visible or ultraviolet light irradiation.

2. The application according to claim 1, characterized in that, During the cycle, the temperature is 180~200℃; and / or the number of cycles is 20~50.

3. The application according to claim 1, characterized in that, The titanium source injection time is 0.1~0.5s, the inert gas purging and re-purging time is 5~10s, and the reaction gas injection time is 0.2~0.6s.

4. The application according to claim 1, characterized in that, The titanium source is titanium tetrachloride, titanium tetraisopropoxide, or tetrabutyl titanate; the inert gas is nitrogen or argon; and the reaction gas is water vapor, ozone, or oxygen plasma.

5. The application according to claim 4, characterized in that, The titanium source is titanium tetrachloride; the reaction gas is ozone.

6. The application according to claim 1, characterized in that, The vacuum level of the ALD reaction chamber in the ALD cycle is 0.001~0.1 Pa.

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