Preparation method and application of metal-doped vanadium phosphorus oxygen catalyst

Abstract

This invention discloses a method for preparing a metal-doped vanadium-phosphorus-oxygen catalyst and its application. The key feature is the utilization of the photochemical properties of vanadium pyrophosphate to generate photogenerated electrons under illumination, thereby reducing metal ions in the system and achieving in-situ introduction of metal elements. This yields a metal-doped vanadium-phosphorus-oxygen catalyst material, significantly improving catalytic performance. Compared with existing metal-doped vanadium-phosphorus-oxygen catalyst preparation techniques, this method avoids the introduction of impurities during catalyst synthesis, thus preventing damage to the crystal structure and ensuring high crystallinity of the catalyst. Simultaneously, the introduced metal elements have smaller particle sizes, ensuring the utilization rate of metal atoms and providing sufficient active sites. The introduced metal elements can be grafted onto the vanadium-phosphorus-oxygen catalyst through chemical bonds, ensuring the stability of the dopant elements and preventing metal element loss during long-term operation.

Description

Technical Field

[0001] This invention relates to the field of vanadium phosphorus oxide (VPO) catalyst preparation, specifically to the preparation method and application of metal-doped vanadium phosphorus oxide catalysts. Background Technology

[0002] Vanadium phosphorus oxide (VPO) catalysts are currently the only industrially applied selective oxidation catalysts for low-carbon alkanes, widely used in the industrial production of maleic anhydride through the selective oxidation of butane. As a composite metal oxide catalyst, its catalytic performance is influenced by many factors, including the catalyst's bulk electronic structure, surface redox properties, and adsorption. Modulating its structure through various enhancement techniques, such as interface engineering and defect engineering, is currently the main method for improving its catalytic performance. Among these, doping engineering, which introduces metal promoters into the bulk structure of VPO catalysts to modulate their electronic structure and generate new active centers, is considered one of the most effective strategies. The method of introducing the metal promoter determines its form within the VPO catalyst, further affecting its performance.

[0003] CN117920291A discloses a method for preparing a transition metal-doped vanadium-phosphorus oxide catalyst based on a solid-aqueous phase method. This method involves introducing a transition metal dopant into the activated vanadium-phosphorus oxide phase and a metal salt in a ball mill and a three-dimensional mixer. The mixture is then calcined again to prepare a transition metal-doped solid-aqueous vanadium-phosphorus oxide catalyst. The metal-doped vanadium-phosphorus oxide catalyst prepared by this method exhibits excellent sulfur and water resistance.

[0004] CN118356957A discloses a method for preparing vanadium-phosphorus-oxygen catalysts by in-situ co-doping with metals and boron compounds. In the catalyst precursor synthesis stage, metal nitrates are added to the reaction system at a specific mass ratio to obtain metal-doped precursor powder. Subsequently, under in-situ activation conditions, a vanadium-phosphorus-oxygen catalyst co-doped with metals and boron is obtained. The vanadium-phosphorus-oxygen catalyst obtained by this patented method has the advantages of high activity and good stability, achieving highly selective production of maleic anhydride from n-butane under normal pressure and effectively suppressing the deep oxidation of n-butane.

[0005] The aforementioned and existing publicly available methods all demonstrate that metal doping can effectively improve the performance of vanadium-phosphorus-oxygen catalysts. Furthermore, different introduction methods of the same metal element have a significant impact on catalyst performance. Existing literature commonly employs in-situ doping, post-implantation, and mechanochemical methods to introduce metal elements. However, these methods can, to some extent, disrupt the catalyst's crystal structure. Additionally, the introduced metal particles are often large, failing to fully utilize metal atomic sites, resulting in only chemical adsorption between the metal element and the catalyst. This leads to metal element loss during long-term catalyst operation. Summary of the Invention

[0006] To address the shortcomings of existing activation technologies, this invention provides a method for preparing a metal-doped vanadium phosphorus oxygen catalyst and its application.

[0007] The inventors of this invention previously discovered that vanadium oxyphosphophosphate (VO)₂P₂O₇, the active phase of the vanadium oxyphosphophosphate catalyst, possesses photoresponsive properties and is a photoresponsive semiconductor material. It can generate reducing photoelectrons under appropriate wavelength illumination, driving redox reactions in the system. Utilizing this characteristic, this invention proposes a strategy to introduce metal elements onto the surface of the vanadium oxyphosphophosphate catalyst through in-situ photochemical deposition, resulting in a highly efficient metal-doped vanadium oxyphosphophosphate catalyst.

[0008] Therefore, this invention provides a method for preparing metal-doped vanadium phosphorus oxygen catalysts by in-situ photodeposition and its application, the specific preparation of which includes the following steps:

[0009] (1) A certain mass of vanadium phosphorus oxygen catalyst powder and an aqueous solution of metal salt are placed in a circulating water jacket reactor in a specific ratio, 50 mL of solvent is added, and the mixture is sonicated for 30 minutes to form a uniform suspension.

[0010] (2) Turn on the circulating water, place the above system under light of a certain wavelength, and at a constant temperature, stir magnetically for a certain time.

[0011] (3) After the photo-irradiation reaction is completed, the above suspension is centrifuged and washed with anhydrous ethanol 3-5 times. The washed powder is then vacuum dried for 12 hours to finally obtain the metal-doped vanadium-phosphorus-oxygen active catalyst.

[0012] According to some embodiments of the method of the present invention, the active phase of the vanadium phosphorus oxycatalyst can be the active phase obtained when preparing the vanadium phosphorus oxycatalyst by conventional methods in the art. Specifically, the crystal phase structure of the vanadium phosphorus oxycatalyst is vanadium oxypyrophosphate (VO)2P2O7.

[0013] The present invention also provides the application of the metal-doped vanadium phosphorus oxygen catalyst prepared by in-situ photodeposition obtained by the above method in the selective oxidation of n-butane to maleic anhydride.

[0014] Compared with the prior art, the present invention has the following significant technical effects and advantages:

[0015] (1) The in-situ photodeposition method used in this invention introduces metal elements into the structure of vanadium phosphorus oxygen catalyst, avoiding the introduction of impurities during catalyst synthesis and thus preventing damage to the crystal structure, thereby ensuring the high crystallinity of the catalyst.

[0016] (2) Compared with the traditional in-situ doping method, the metal element introduced by this method has a smaller particle size, which can ensure the utilization rate of metal atoms and provide sufficient active sites.

[0017] (3) By using this method, the introduced metal elements can be grafted onto the vanadium-phosphorus-oxygen catalyst in the form of chemical bonds, ensuring the stability of the doped elements and avoiding the loss of metal elements during long-term operation. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0019] This invention provides a method for preparing metal-doped vanadium phosphorus oxygen catalysts by in-situ photodeposition and its application, the specific preparation of which includes the following steps:

[0020] (1) A certain mass of vanadium phosphorus oxygen catalyst powder and a certain concentration and volume of metal salt aqueous solution are placed in a circulating water jacket reactor, 50 mL of solvent is added, and ultrasonication is performed for 30 minutes to form a uniform suspension.

[0021] (2) Turn on the circulating water, place the above system under light of a certain wavelength, and at a constant temperature, stir magnetically for a certain time.

[0022] (3) After the photo-irradiation reaction is completed, the above suspension is centrifuged and washed with anhydrous ethanol 3-5 times. The washed powder is then vacuum dried for 12 hours to finally obtain the metal-doped vanadium-phosphorus-oxygen active catalyst.

[0023] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. The technical objectives and beneficial effects of the present invention can be better achieved and realized through the following preferred solutions.

[0024] Preferably, the mass of the vanadium-phosphorus-oxygen catalyst powder in step (1) is 0.1–5.0 g, for example, 0.1 g, 0.5 g, 1.0 g, 1.5 g, 2.0 g, 2.5 g, 3.0 g, or 5.0 g. More preferably, the mass of the vanadium-phosphorus-oxygen catalyst powder is 2.5 g. When the mass is less than 0.1 g, fewer photogenerated electrons are produced under illumination, making it difficult to provide enough electrons for the reduction of metal ions. When the mass is greater than 5.0 g, the solid content of the photodeposition system is too high, making it difficult to achieve effective light utilization.

[0025] Preferably, the concentration of the aqueous solution of the metal salt in step (1) is 0.5–10 mmol / L, and the added volume is 10–100 μL. For example, 10 μL of a 0.5 mmol / L metal salt solution, 30 μL of a 5 mmol / L metal salt solution, 40 μL of a 5 mmol / L metal salt solution, 10 μL of a 7 mmol / L metal salt solution, 0.5 μL of a 10 mmol / L metal salt solution, and 10 μL of a 10 mmol / L metal salt solution. More preferably, the concentration of the aqueous solution of the metal salt is 3.8 mmol / L, and the added volume is 50 μL.

[0026] Preferably, the metal salt in the aqueous solution of the metal salt in step (1) is one or a combination of two or more of the following: Mo, Ti, Mn, Rh, Zn, Au, La, Ce, Co, Ni, Al, Fe, Ag, Pt, Sm, Pd, and Ru. More preferably, the aqueous solution of the metal salt is a combination of Mo and Au aqueous solutions.

[0027] Preferably, the solvent in step (1) is a mixed solution of water, organic alcohol, and hole sacrificial agent, with a volume ratio of water:organic alcohol:hole sacrificial agent = 50:(10-20):(0-10). The organic alcohol is at least one of methanol, ethanol, isobutanol, and isopropanol, and the hole sacrificial agent is one of triethanolamine, lactic acid, sodium sulfate, and sodium sulfite.

[0028] Preferably, the light source in step (2) is monochromatic light or light in a specific wavelength range of 200–800 nm, such as wavelengths of 200 nm, 300 nm, 400 nm, 700 nm, 400–700 nm, 200–700 nm, etc. More preferably, the light source is light in the wavelength range of 400–760 nm.

[0029] Preferably, the reaction temperature in step (2) is 5–35°C. For example, 5°C, 10°C, 15°C, 20°C, 30°C. More preferably, the reaction temperature is 25°C.

[0030] Preferably, the reaction time in step (2) is 2 to 16 hours. For example, 2 hours, 4 hours, 6 hours, 8 hours, 10 hours, and 16 hours. More preferably, the reaction time is 8 hours.

[0031] As a further preferred embodiment of the method described in this invention, the method includes the following steps:

[0032] (1) Place 0.1-5.0 g of vanadium phosphorus oxygen catalyst powder in 10-100 μL of 0.5-10 mmol / L aqueous solution of one or more of the following metal salts: Mo, Ti, Mn, Rh, Zn, Au, La, Ce, Co, Ni, Al, Fe, Ag, Pt, Sm, Pd, Ru, into a circulating water jacket reactor, add 50 mL of water: organic alcohol: hole sacrificial agent = 50: (10-20): (0-10) solvent, and sonicate for 30 minutes to form a homogeneous suspension;

[0033] (2) Turn on the circulating water, place the above system under light with a wavelength of 200-800 nm, keep the temperature constant at 5-35℃, and stir magnetically for 2-16 hours;

[0034] (3) After the photo-irradiation reaction is completed, the above suspension is centrifuged and washed with anhydrous ethanol 3-5 times. The washed powder is then vacuum dried for 12 hours to finally obtain the metal-doped vanadium-phosphorus-oxygen active catalyst.

[0035] The present invention also includes the application of the above-mentioned metal-doped vanadium phosphorus oxygen catalyst in the selective oxidation of n-butane to maleic anhydride.

[0036] Preferably, the reaction conditions for the selective oxidation of n-butane to prepare maleic anhydride are: a hot spot temperature of 420°C and a space velocity of 2000 h⁻¹ for the n-butane-air mixture. -1 The concentration of n-butane is 1.8% v%.

[0037] The following specific examples illustrate the preparation method of low-pressure-drop vanadium-phosphorus-oxygen catalysts. The compounds in the following examples can be prepared directly using existing methods.

[0038] Example 1

[0039] (1) Place 0.1g of vanadium phosphorus oxygen catalyst powder and 10μL of 0.5mmol / L Mo salt aqueous solution in a circulating water jacket reactor, add 50mL of water:ethanol:triethanolamine mixed solvent with a volume ratio of 50:10:0, and sonicate for 30 minutes to form a uniform suspension.

[0040] (2) Turn on the circulating water, place the above system under light with a wavelength of 200nm, keep the temperature constant at 5℃, and stir magnetically for 2h.

[0041] (3) After the photo-irradiation reaction is completed, the above suspension is centrifuged and washed with anhydrous ethanol 3-5 times. The washed powder is then vacuum dried for 12 hours to finally obtain the metal Mo-doped vanadium phosphorus oxygen active catalyst.

[0042] Detection: At the reaction hotspot temperature of 420℃, the space velocity of the n-butane-air mixture was 2000 h⁻¹. -1Under reaction conditions with a n-butane concentration of 1.8 v%, the n-butane conversion rate was 90.3%, the maleic anhydride selectivity was 55.1%, and the maleic anhydride yield was 84.1%.

[0043] Example 2

[0044] (1) Place 5.0g of vanadium phosphorus oxygen catalyst powder and 100μL of 10mmol / L Fe salt aqueous solution in a circulating water jacket reactor, add 50mL of water:methanol:lactic acid mixed solvent with a volume ratio of 50:20:10, and sonicate for 30 minutes to form a homogeneous suspension.

[0045] (2) Turn on the circulating water, place the above system under light with a wavelength of 800nm, keep the temperature constant at 35℃, and stir magnetically for 16h.

[0046] (3) After the photo-irradiation reaction is completed, the above suspension is centrifuged and washed with anhydrous ethanol 3-5 times. The washed powder is then vacuum dried for 12 hours to finally obtain the Fe-doped vanadium phosphorus oxygen active catalyst.

[0047] Detection: At the reaction hotspot temperature of 420℃, the space velocity of the n-butane-air mixture was 2000 h⁻¹. -1 Under reaction conditions with a n-butane concentration of 1.8 v%, the n-butane conversion rate was 85.6%, the maleic anhydride selectivity was 60.1%, and the maleic anhydride yield was 84.1%.

[0048] Detection: At the reaction hotspot temperature of 420℃, the space velocity of the n-butane-air mixture was 2000 h⁻¹. -1 Under reaction conditions with a n-butane concentration of 1.8 v%, the n-butane conversion rate was 93.6%, the maleic anhydride selectivity was 55.7%, and the maleic anhydride yield was 86.9%.

[0049] Example 3

[0050] (1) Place 2.5g of vanadium phosphorus oxygen catalyst powder and 50μL of 3.8mmol / L Mo salt and Au salt mixed aqueous solution (volume ratio 1:1) in a circulating water jacket reactor, add 50mL of water:ethanol:triethanolamine mixed solvent with a volume ratio of 50:10:5, and sonicate for 30 minutes to form a homogeneous suspension.

[0051] (2) Turn on the circulating water, place the above system under light with a wavelength of 400-760nm, keep the temperature constant at 25℃, and stir magnetically for 8 hours.

[0052] (3) After the photo-irradiation reaction is completed, the above suspension is centrifuged and washed with anhydrous ethanol 3-5 times. The washed powder is then vacuum dried for 12 hours to finally obtain the metal Mo / Au doped vanadium phosphorus oxygen active catalyst.

[0053] Detection: At the reaction hotspot temperature of 420℃, the space velocity of the n-butane-air mixture was 2000 h⁻¹. -1 Under reaction conditions with a n-butane concentration of 1.8 v%, the n-butane conversion rate was 97.3%, the maleic anhydride selectivity was 68.2%, and the maleic anhydride yield was 112.1%.

[0054] Example 4

[0055] (1) Place 3.0g of vanadium phosphorus oxygen catalyst powder and 40μL of 5.0mmol / L Pt salt solution in a circulating water jacket reactor, add 50mL of water:ethanol:lactic acid mixed solvent with a volume ratio of 50:10:5, and sonicate for 30 minutes to form a uniform suspension.

[0056] (2) Turn on the circulating water, place the above system under light with a wavelength of 400-760nm, keep the temperature constant at 25℃, and stir magnetically for 10h.

[0057] (3) After the photo-irradiation reaction is completed, the above suspension is centrifuged and washed with anhydrous ethanol 3-5 times. The washed powder is then vacuum dried for 12 hours to finally obtain the metal Pt-doped vanadium phosphorus oxygen active catalyst.

[0058] Detection: At the reaction hotspot temperature of 420℃, the space velocity of the n-butane-air mixture was 2000 h⁻¹. -1 Under reaction conditions with a n-butane concentration of 1.8 v%, the n-butane conversion rate was 93.9%, the maleic anhydride selectivity was 62.2%, and the maleic anhydride yield was 98.7%.

[0059] Example 5

[0060] (1) Place 2.5g of vanadium phosphorus oxygen catalyst powder and 50μL of 3.8mmol / L Mo salt and Au salt mixed aqueous solution (volume ratio of 1:1) in a circulating water jacket reactor, add 50mL of water:ethanol:triethanolamine mixed solvent with a volume ratio of 50:10:0, and sonicate for 30 minutes to form a uniform suspension.

[0061] (2) Turn on the circulating water, place the above system under light with a wavelength of 400-760nm, keep the temperature constant at 25℃, and stir magnetically for 8 hours.

[0062] (3) After the photo-irradiation reaction is completed, the above suspension is centrifuged and washed with anhydrous ethanol 3-5 times. The washed powder is then vacuum dried for 12 hours to finally obtain the metal Mo / Au doped vanadium phosphorus oxygen active catalyst.

[0063] Detection: At the reaction hotspot temperature of 420℃, the space velocity of the n-butane-air mixture was 2000 h⁻¹.-1 Under reaction conditions with a n-butane concentration of 1.8 v%, the n-butane conversion rate was 87.8%, the maleic anhydride selectivity was 60.4%, and the maleic anhydride yield was 89.6%.

[0064] Example 6

[0065] (1) Place 4.5g of vanadium phosphorus oxygen catalyst powder and 0.5μL of 10mmol / L Co salt aqueous solution in a circulating water jacket reactor, add 50mL of water:ethanol:triethanolamine mixed solvent with a volume ratio of 50:10:5, and sonicate for 30 minutes to form a uniform suspension.

[0066] (2) Turn on the circulating water, place the above system under light with a wavelength of 800 nm, keep the temperature constant at 35℃, and stir magnetically for 10 hours;

[0067] (3) After the photo-irradiation reaction is completed, the above suspension is centrifuged and washed with anhydrous ethanol 3-5 times. The washed powder is then vacuum dried for 12 hours to finally obtain the metal Co-doped vanadium phosphorus oxygen active catalyst.

[0068] Detection: At the reaction hotspot temperature of 420℃, the space velocity of the n-butane-air mixture was 2000 h⁻¹. -1 Under reaction conditions with a n-butane concentration of 1.8 v%, the n-butane conversion rate was 90.2%, the maleic anhydride selectivity was 59.7%, and the maleic anhydride yield was 91.0%.

[0069] Comparative Example 1

[0070] (1) Place 2.5g of vanadium phosphorus oxygen catalyst powder and 50μL of 3.8mmol / L Mo salt and Au salt mixed aqueous solution (volume ratio 1:1) in a circulating water jacket reactor, add 50mL of water:ethanol:triethanolamine mixed solvent with a volume ratio of 50:10:5, and sonicate for 30 minutes to form a homogeneous suspension.

[0071] (2) Turn on the circulating water, do not apply light, keep the temperature constant at 25℃, and stir magnetically for 8 hours;

[0072] (3) After stirring, the above suspension is centrifuged and washed with anhydrous ethanol 3-5 times. The washed powder is then vacuum dried for 12 hours to finally obtain the vanadium-phosphorus-oxygen active catalyst.

[0073] Detection: At the reaction hotspot temperature of 420℃, the space velocity of the n-butane-air mixture was 2000 h⁻¹. -1 Under reaction conditions with a n-butane concentration of 1.8 v%, the n-butane conversion rate was 81.4%, the maleic anhydride selectivity was 56.3%, and the maleic anhydride yield was 77.4%.

[0074] Comparative Example 2

[0075] (1) Place 2.5g of vanadium phosphorus oxygen catalyst powder in a circulating water jacket reactor, add 50mL of a mixed solvent of water: ethanol: triethanolamine with a volume ratio of 50:10:5, and sonicate for 30 minutes to form a uniform suspension.

[0076] (2) Turn on the circulating water, place the above system under light with a wavelength of 400-760nm, keep the temperature constant at 25℃, and stir magnetically for 8 hours.

[0077] (3) After stirring, the above suspension is centrifuged and washed with anhydrous ethanol 3-5 times. The washed powder is then vacuum dried for 12 hours to finally obtain the vanadium-phosphorus-oxygen active catalyst.

[0078] Detection: At the reaction hotspot temperature of 420℃, the space velocity of the n-butane-air mixture was 2000 h⁻¹. -1 Under reaction conditions with a n-butane concentration of 1.8 v%, the n-butane conversion rate was 81.9%, the maleic anhydride selectivity was 54.2%, and the maleic anhydride yield was 75.0%.

[0079] Comparative Example 3

[0080] (1) Place 2.5g of vanadium phosphorus oxygen catalyst powder in a circulating water jacket reactor, add 50mL of a mixed solvent of water: ethanol: triethanolamine with a volume ratio of 50:10:5, and sonicate for 30 minutes to form a uniform suspension.

[0081] (2) Turn on the circulating water, do not apply light, keep the temperature constant at 25℃, and stir magnetically for 8 hours;

[0082] (3) After stirring, the above suspension is centrifuged and washed with anhydrous ethanol 3-5 times. The washed powder is then vacuum dried for 12 hours to finally obtain the vanadium-phosphorus-oxygen active catalyst.

[0083] Detection: At the reaction hotspot temperature of 420℃, the space velocity of the n-butane-air mixture was 2000 h⁻¹. -1 Under reaction conditions with a n-butane concentration of 1.8 v%, the n-butane conversion rate was 80.4%, the maleic anhydride selectivity was 55.6%, and the maleic anhydride yield was 75.5%.

[0084] Comparative Example 4

[0085] During the precursor synthesis process (this process can refer to currently available public methods), 50 μL of a 3.8 mmol / L mixed aqueous solution of Mo salt and Au salt (volume ratio 1:1) is added to the synthesis system, followed by the preparation of vanadium phosphorus oxygen catalyst to obtain in-situ chemically doped vanadium phosphorus oxygen catalyst.

[0086] Detection: At the reaction hotspot temperature of 420℃, the space velocity of the n-butane-air mixture was 2000 h⁻¹. -1 Under reaction conditions with a n-butane concentration of 1.8 v%, the n-butane conversion rate was 85.6%, the maleic anhydride selectivity was 57.6%, and the maleic anhydride yield was 83.3%.

[0087] Comparing Example 3 with Comparative Example 1, it can be seen that when the vanadium phosphorus oxide catalyst powder is treated in the same solution, the yield of maleic anhydride in the Comparative Example without light irradiation is only 77.4%, while the yield of maleic anhydride after light irradiation is 112.1%. This proves that under light irradiation, the photogenerated electrons produced by the vanadium phosphorus oxide catalyst can reduce the metal elements in the solution, realizing the in-situ photodeposition preparation of metal-doped vanadium phosphorus oxide catalyst, thereby significantly improving the photocatalytic performance. Comparing Example 3 with Comparative Examples 2 and 3, it can be seen that in this method, both light irradiation and the metal salt solution are indispensable. The improvement in catalytic efficiency is due to the photodeposited metal elements, rather than light irradiation or other solvents.

[0088] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for in-situ photodeposition of metal-doped vanadium-phosphorus oxygen catalysts, characterized in that, The method includes the following steps: (1) A certain mass of vanadium phosphorus oxygen catalyst powder and a certain concentration and volume of metal salt aqueous solution are placed in a circulating water jacket reactor, 50 mL of solvent is added, and ultrasonication is performed for 30 minutes to form a uniform suspension. (2) Turn on the circulating water, place the above system under light of a certain wavelength, and at a constant temperature, stir magnetically for a certain time. (3) After the photo-irradiation reaction is completed, the above suspension is centrifuged and washed with anhydrous ethanol 3-5 times. The washed powder is then vacuum dried for 12 hours to finally obtain the metal-doped vanadium-phosphorus-oxygen active catalyst.

2. The catalyst according to claim 1, characterized in that: The mass of the vanadium-phosphorus-oxygen catalyst powder is 0.1–5.0 g.

3. The concentration of the aqueous solution of the metal salt according to claim 1 is 0.5 to 10 mmol / L, and the added volume is 10 to 100 μL.

4. The metal salt in the aqueous solution of the metal salt according to claim 1 is one or a combination of two or more of the following: Mo, Ti, Mn, Rh, Zn, Au, La, Ce, Co, Ni, Al, Fe, Ag, Pt, Sm, Pd, and Ru.

5. The solvent according to claim 1 is a mixed solution of water, organic alcohol, and hole sacrificial agent, wherein the volume ratio of the three is water:organic alcohol:hole sacrificial agent = 50:(10-20):(0-10). Wherein, The organic alcohol is at least one of methanol, ethanol, isobutanol, and isopropanol, and the hole sacrificial agent is one of triethanolamine, lactic acid, sodium sulfate, and sodium sulfite.

6. The light source according to claim 1 is monochromatic light or light in a specific wavelength band with a wavelength of 200-800 nm.

7. The reaction temperature according to claim 1 is 5 to 35°C.

8. The reaction time according to claim 1 is 2 to 16 hours.