Metallic molybdenum trioxide photocatalyst as well as preparation method and application thereof

Abstract

The invention relates to the technical field of catalysts, in particular to a metallic molybdenum trioxide photocatalyst and a preparation method and application thereof. The preparation method comprises the step of carrying out more than two times of reduction reaction on molybdenum oxide to obtain the metallic molybdenum trioxide photocatalyst. According to the preparation method of the metallic molybdenum trioxide photocatalyst, the reducing agent is introduced into the molybdenum oxide solution, and the absorption peak position of localized surface plasmon resonance of the molybdenum trioxide photocatalyst obtained by the reaction and the quantity of low-valence Mo can be regulated and controlled through more than two times of reduction reaction; the molybdenum trioxide photocatalyst is in a metal state and can provide more catalytic active sites in a photocatalytic reaction, so that the efficiency of the photocatalytic reaction is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of catalysts, and in particular to a metallic molybdenum trioxide photocatalyst, a preparation method thereof, and an application thereof. Background Art

[0002] Ammonia (NH 3 ) is an essential key raw material for synthesizing nitrogen-rich compounds in modern industry. In addition, NH 3 is also used in the manufacture of plastics, synthetic fibers, explosives, and many compounds, and is also the key and core for increasing the yield of modern agricultural crops. 76% of the industrially synthesized NH 3 globally is used for the production of nitrogen fertilizers every year. However, the production of NH 3 still relies on the industrial Haber-Bosch process. Currently, the amount of NH 3 produced globally through this process is approximately 150 million tons per year. As the world's population and demand continue to grow, it is expected to increase by 2-3% annually. However, due to the relatively high bond energy of N 2 molecules (dissociation energy is 941 kJ mol -1 ), the Haber-Bosch process is a high-temperature (300 - 600 °C), high-pressure (150 - 300 atm) process, which accounts for 1-2% of the world's total energy consumption. In addition, the raw material gas for NH 3 synthesis is purified N 2 and H 2 . Therefore, the Haber-Bosch process consumes approximately 50% of the global hydrogen production every year. It is estimated that about 2 tons of carbon dioxide are generated for every ton of NH 3 produced, accounting for about 3% of the total greenhouse gas emissions. With the increasing depletion of oil and natural gas resources, the environmental pressure is increasing. Given the increasing global demand for energy, for the sustainable development of humanity, properly addressing environmental pollution and energy shortages is an urgent problem to be solved. Therefore, in the context of "dual carbon", it is imperative to develop sustainable and green alternative production methods for NH 3 with a neutral carbon footprint.

[0003] Converting solar energy into chemical energy using photocatalytic technology (e.g., NH 3 ), due to its clean, green, and sustainable advantages, has been considered one of the best ways to solve future renewable energy. However, so far, artificial photocatalytic nitrogen fixation technology still faces many challenges, especially the photocatalytic reduction of N 2 to NH 3 . Improving the rate of photocatalytic N 2 reduction and the energy conversion efficiency from solar energy to ammonia, as well as the adsorption and activation of N 2 have always been extremely challenging. However, due to N 2The bond energy of the molecule is relatively high. Therefore, if we want to improve the adsorption and activation efficiency of N 2 and enhance the wide-spectrum capture ability of the photocatalyst for sunlight, it is the key core point.

[0004] Among the numerous reported photocatalytic materials, molybdenum trioxide has become the most widely used semiconductor photocatalyst at present due to its stable chemical properties, corrosion resistance, non-toxicity and low cost. Moreover, defective molybdenum trioxide has local surface plasmon resonance effect and abundant low-valent Mo sites. These special properties enable defective molybdenum trioxide to not only have a relatively wide sunlight capture spectrum, but also have abundant N 2 active sites. Therefore, molybdenum trioxide rich in defects has great potential in improving photo-generated high-energy electrons and converting N 2 molecules to NH 3 . However, there is currently no literature reporting a visible-light-capturing solar photocatalytic N 2 reduction metal molybdenum trioxide photocatalyst, especially the visible-light absorption photocatalytic synthesis of ammonia by metal molybdenum trioxide. Summary of the Invention

[0005] The present invention provides a metal molybdenum trioxide photocatalyst, a preparation method thereof and an application thereof to solve the above technical problems existing in the prior art.

[0006] According to the first aspect of the present invention, a preparation method of a metal molybdenum trioxide photocatalyst is provided, which includes performing two or more reduction reactions on molybdenum oxide to obtain a metal molybdenum trioxide photocatalyst.

[0007] In the above solution, in the preparation method of a metal molybdenum trioxide photocatalyst of the present invention, a reducing agent is introduced into the molybdenum oxide solution, and through two or more reduction reactions, the absorption peak position of the local surface plasmon resonance of the obtained molybdenum trioxide photocatalyst and the number of low-valent Mo can be regulated, so that the obtained molybdenum trioxide photocatalyst is in a metal state, and more catalytic active sites can be provided in the photocatalytic reaction, thereby improving the efficiency of the photocatalytic reaction.

[0008] Further, it includes the following steps:

[0009] Step 1): Uniformly disperse molybdenum oxide in an aqueous solution to obtain solution A;

[0010] Step 2): Add a reducing agent solution to the solution A to perform a first reduction reaction to obtain solution B;

[0011] Step 3): Centrifuge the obtained solution B, disperse the centrifuged filter cake in an aqueous solution, and add the reducing agent solution again to perform a second reduction reaction to obtain a solid precipitate C;

[0012] Step 4): Wash and dry the obtained solid precipitate C to obtain a metal-state molybdenum trioxide photocatalyst.

[0013] Furthermore, the preparation process of the present invention requires that the whole process be carried out under deionized water or ultrapure water conditions. Deionized water or ultrapure water does not contain impurity ions that affect the reduction reaction, enabling the molybdenum oxide to be fully reduced under the reducing agent and improving the reduction efficiency.

[0014] Furthermore, the molar ratio of the reducing agent in the reducing agent solution to the molybdenum oxide is (1 - 5):1.

[0015] Optionally, the molar ratio of the reducing agent in the reducing agent solution to the molybdenum oxide can be 1:1, 1.5:1, 2:1, 2.5:1, 3:1, 3.5:1, 4:1, 4.5:1 or 5:1, etc. Of course, it can also be other values within the above range, which are not limited herein. Preferably, the molar ratio of the reducing agent in the reducing agent solution to the molybdenum oxide is (2.5 - 5):1.

[0016] In the above solution, by limiting the molar ratio of the reducing agent in the reducing agent solution to the molybdenum oxide within a reasonable range value, it is beneficial to improve the efficiency of the reduction reaction, and thus obtain a metal-state molybdenum trioxide photocatalyst with an ideal target.

[0017] Furthermore, the temperature of the first reduction reaction and the second reduction reaction are each independently 5 - 25 °C, and the time is each independently 10 - 24 h. By controlling the reduction temperature and time, the reduction efficiency can be improved.

[0018] Furthermore, in the solution A, the concentration of molybdenum oxide is 100 - 1000 mmol·L -1 .

[0019] Optionally, in the solution A, the concentration of molybdenum oxide can be 100 mmol·L -1 , 200 mmol·L -1 , 300 mmol·L -1 , 400 mmol·L -1 , 500 mmol·L -1 , 600 mmol·L -1 , 700 mmol·L -1 , 800 mmol·L -1 , 900 mmol·L -1 or 1000 mmol·L -1 etc. Of course, it can also be other values within the above range, which are not limited herein. Preferably, in the solution A, the concentration of molybdenum oxide is 800 - 900 mmol·L -1Preferably, in the solution A, the concentration of molybdenum oxide is 834 mmol·L -1 .

[0020] Furthermore, in the reducing agent solution, the concentration of the reducing agent is 50 - 1000 mmol·L -1 . Optionally, the concentration of the reducing agent in the reducing agent solution can be 50 mmol·L -1 , 100 mmol·L -1 , 200 mmol·L -1 , 300 mmol·L -1 , 400 mmol·L -1 , 500 mmol·L -1 , 600 mmol·L -1 , 700 mmol·L -1 , 800 mmol·L -1 , 900 mmol·L -1 or 1000 mmol·L -1 etc. Of course, it can also be other values within the above range, which are not limited here. Preferably, the concentration of the reducing agent in the reducing agent solution is 200 - 800 mmol·L -1 . More preferably, the concentration of the reducing agent in the reducing agent solution is 176 - 705 mmol·L -1 . Even more preferably, the concentration of the reducing agent in the reducing agent solution is 440 mmol·L -1 .

[0021] Furthermore, the reducing agent in the reducing agent solution is sodium borohydride.

[0022] Furthermore, in step 3), the rotation speed of the centrifugation is 10000 - 12000 r / min; the time of the centrifugation is 0.1 - 0.5 h.

[0023] Optionally, the rotation speed of the centrifugation can be 10000 r / min, 10500 r / min, 11000 r / min, 11500 r / min or 12000 r / min etc. Of course, it can also be other values within the above range, which are not limited here. The time of the centrifugation can be 0.1 h, 0.2 h, 0.3 h, 0.4 h or 0.5 h etc. Of course, it can also be other values within the above range, which are not limited here.

[0024] Furthermore, in step 4), the temperature of the drying is -52 to -47 °C; the time of the drying is 12 to 24 h.

[0025] Optionally, the drying temperature can be -52°C, -51°C, -50°C, -49°C, -48°C or -47°C, etc. Of course, it can also be other values within the above range, which are not limited herein. The drying time can be 12h, 14h, 16h, 18h, 20h, 22h or 24h, etc. Of course, it can also be other values within the above range, which are not limited herein.

[0026] In the above solution, freeze-drying can ensure the structural properties of the reduction product and avoid the damage to the structure of the reduction product caused by high-temperature drying.

[0027] Further, in step 4), the washing method is to wash with deionized water 3 to 5 times.

[0028] Within the given washing times, drying temperatures and drying time ranges of the present invention, the washing and drying parameters have little influence on the catalytic performance of the product.

[0029] Further, the molybdenum oxide is molybdenum trioxide.

[0030] According to the second aspect of the present invention, the present invention also provides a metal-state molybdenum trioxide photocatalyst prepared by using the above preparation method.

[0031] Further, the position of the LSPR absorption peak of the metal-state molybdenum trioxide photocatalyst is at 640 - 915 nm.

[0032] According to the third aspect of the present invention, the present invention also provides the application of the metal-state molybdenum trioxide photocatalyst prepared by the above preparation method or the above metal-state molybdenum trioxide photocatalyst in photocatalytic ammonia synthesis.

[0033] In the prior art, the main route for preparing ammonia from hydrogen and nitrogen is to use noble metals Ru and Fe oxides as catalysts and carry out the reaction in a high-temperature and high-pressure system; compared with the prior art, the present invention first uses a photo-driven ammonia synthesis reaction, which is not only more environmentally friendly and energy-saving than the prior art system, but also first uses a cheap metal-state molybdenum trioxide photocatalyst for photocatalytic ammonia synthesis reaction, and the ammonia yield is high. The present invention is expected to be scaled up in industry and applied practically.

[0034] Preferably, the application is specifically the application of the metal-state molybdenum trioxide photocatalyst in photocatalytic synthesis of ammonia from nitrogen and water.

[0035] Preferably, the process of photocatalytic ammonia synthesis specifically includes the following steps:

[0036] In a light-transmissive reaction device, the metal-state molybdenum trioxide photocatalyst and ultrapure water are mixed to obtain a mixed solution. High-purity nitrogen is introduced under light-shielded conditions, the reaction device is sealed, the condensing water device is turned on, and visible light illumination is carried out.

[0037] Preferably, the time of visible light illumination is 10 min - 2 h.

[0038] Preferably, in the mixed solution, the concentration of the metal molybdenum trioxide photocatalyst is 0.2 - 5 g·L -1 .

[0039] Preferably, the duration of passing the high-purity nitrogen is 0.5 - 5 h.

[0040] In the present invention, the ultrapure water is water with a resistivity reaching 18 MΩ*cm (25 °C); the high-purity nitrogen is nitrogen with a volume fraction of not less than 99.999 vol%.

[0041] Unless otherwise specified, any range described in the present invention includes the end values and any values between the end values, as well as any sub-ranges formed by any values between the end values or the end values.

[0042] The beneficial effects of the present invention are as follows:

[0043] (1) In the preparation method of a metal molybdenum trioxide photocatalyst of the present invention, a reducing agent is introduced into the molybdenum oxide solution, and the reducing agent reduces the molybdenum oxide, so that more catalytic active sites of low-valent Mo are generated in the obtained molybdenum trioxide photocatalyst.

[0044] (2) The preparation method of a metal molybdenum trioxide photocatalyst of the present invention prepares a metal molybdenum trioxide photocatalyst with higher activity by controlling the content of the reducing agent solution and the reduction times and time of the reaction.

[0045] (3) The metal molybdenum trioxide photocatalyst of the present invention is metal molybdenum trioxide, which has excellent performance in the reaction of photocatalytic ammonia synthesis. It realizes for the first time the preparation of high-yield ammonia using a metal molybdenum trioxide photocatalyst, and the ammonia synthesis yield can reach 1260 μmol·g -1 ·h -1 , while the yield of the traditional photocatalyst Fe 2 O 3 under the same reaction conditions is only 6 μmol·g -1 ·h -1 .

[0046] (4) The metal molybdenum trioxide photocatalyst for photocatalytic preparation of ammonia from nitrogen and water of the present invention has low cost, is easy to prepare, has a simple process, is easy to scale up production, and for the first time uses a metal molybdenum trioxide photocatalyst in the photocatalytic ammonia synthesis reaction, with a high ammonia yield, and is expected to be applied in industrial applications. Description of the Drawings

[0047] To more clearly illustrate the technical solutions in the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0048] Figure 1 It is the UV-vis DRS spectrogram of the products obtained in Examples 1-5 and Comparative Example 1 of the present invention;

[0049] Figure 2 It is the photocatalytic ammonia synthesis reaction performance diagram of the products obtained in Examples 1-5 and Comparative Example 1 of the present invention under the same reaction conditions. Detailed implementation manners

[0050] To make the objectives, technical solutions and advantages of the present invention clearer, the following will clearly and completely describe the technical solutions in the present invention in conjunction with the drawings in the present invention. Obviously, the described embodiments are some embodiments of the present invention, rather than all embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

[0051] The commercial molybdenum oxides used in the following examples and comparative examples are from Shanghai Aladdin Biochemical Technology Co., Ltd.

[0052] Example 1

[0053] A preparation method of a metallic molybdenum trioxide photocatalyst includes the following steps:

[0054] 1) Prepare a commercial molybdenum oxide precursor dispersion solution: Disperse 83.4 mmol of commercial molybdenum trioxide in 100 ml of deionized water, and fully disperse and mix evenly to obtain solution A.

[0055] 2) Place the solution A obtained in step 1) on a magnetic stirrer, and dropwise add 30 mL of sodium borohydride reducing agent solution (concentration of 440 mmol L -1 ) while stirring at 25 °C. After the addition is completed, continue stirring for 12 h to obtain solution B.

[0056] 3) Centrifuge the solution B obtained in step 2), disperse the centrifuged filter cake in 100 ml of deionized water, and fully disperse and mix evenly to obtain a uniform solution; subject the obtained uniform solution to reduction again through step 2) to obtain a solid precipitate C.

[0057] 4) The solid precipitate C obtained in step 3) was washed 4 times with deionized water and then freeze-dried at -50 °C for 24 h to obtain metallic molybdenum trioxide, which is the metallic molybdenum trioxide photocatalyst.

[0058] The metallic molybdenum trioxide photocatalyst prepared above was used for photocatalytic ammonia synthesis, including the following steps:

[0059] 10 mg of the metallic molybdenum trioxide photocatalyst and 600 mL of ultrapure water (i.e., water with a resistivity of 18 MΩ·cm (25 °C)) were added to a light-transmissive reaction device. High-purity nitrogen (i.e., nitrogen with a volume fraction of 99.999 vol%) was introduced for 0.5 h under dark conditions. After the nitrogen introduction was completed, the reaction device was sealed. Then, the condensing water device was turned on, and visible light illumination was carried out for 2 h. The reaction solution after the light illumination reaction was centrifuged, and 1 mL was taken for detecting the ammonia content by cation chromatography.

[0060] The UV-vis DRS spectrum of the metallic molybdenum trioxide photocatalyst obtained in this example is as Figure 1 shown. As can be seen from the curve in Figure 1 , under these conditions, the synthesized metallic molybdenum trioxide photocatalyst has a strong local surface plasmon resonance (LSPR) absorption spectrum in the visible to near-infrared light region, and its local surface plasmon resonance characteristic peak is very obvious. The position of the LSPR absorption peak of the metallic molybdenum trioxide synthesized in this example is about 650 nm. Under visible light illumination for 2 h in this example, the ammonia production is as Figure 2 shown, and its ammonia synthesis catalytic performance can reach 1260 μmol·g -1 ·h -1 .

[0061] The proof of the metallicity of the metallic molybdenum trioxide photocatalyst obtained in this example is shown in Table 1. The prepared metallic molybdenum trioxide photocatalyst was tested for transient absorption spectra under different power conditions, and the electroacoustic coupling lifetimes of the metallic molybdenum trioxide photocatalyst under different power conditions were obtained by fitting. The results are shown in Table 1.

[0062] Table 1 Electroacoustic coupling lifetimes of metallic molybdenum trioxide under different pump light excitations

[0063] Power (nJ) 107 214 339 427 538 Lifetime (ps) 1.24 1.45 1.86 2.24 2.54

[0064] Conclusion: As can be seen from Table 1, under the excitation of pump light with different powers, there is a certain relationship between the electroacoustic coupling lifetime of the metallic molybdenum trioxide photocatalyst and the power of the pump light. Within a certain range, the higher the pump laser power, the longer the obtained electroacoustic coupling lifetime. When the pump laser power is 107 nJ, its electroacoustic coupling lifetime is 1.24 ps. This pump light power dependence of the metallic molybdenum trioxide photocatalyst can confirm the optical metallicity characteristics of metallic molybdenum trioxide; when the pump laser power is too high, the obtained electroacoustic coupling lifetime of metallic molybdenum trioxide gradually increases, but the electroacoustic coupling lifetime will increase somewhat. When the pump laser power exceeds 538 nJ, the electroacoustic coupling lifetime shows an obvious non-linear change. This is mainly because when the pump laser power is too high, a photolithography phenomenon will occur on the surface structure of metallic molybdenum trioxide, resulting in the non-linear change of the electroacoustic coupling lifetime.

[0065] Example 2

[0066] A preparation method of a metallic molybdenum trioxide photocatalyst includes the following steps:

[0067] 1) Prepare a commercial molybdenum oxide precursor dispersion solution: Disperse 83.4 mmol of commercial molybdenum trioxide in 100 ml of deionized water and disperse it evenly to obtain solution A.

[0068] 2) Place the solution A obtained in step 1) on a magnetic stirrer and dropwise add 30 mL of sodium borohydride reducing agent (176 mmol L -1 ) solution while stirring at 25 °C. After the addition is completed, continue stirring for 12 h to obtain solution B.

[0069] 3) Centrifuge the solution B obtained in step 2), disperse the centrifuged filter cake in 100 ml of deionized water, and disperse it evenly to obtain a homogeneous solution; subject the obtained homogeneous solution to reduction again through step 2) to obtain a solid precipitate C.

[0070] 4) Wash the solid precipitate C obtained in step 3) 4 times with deionized water and freeze-dry it at -50 °C for 24 h to obtain metallic molybdenum trioxide, which is the metallic molybdenum trioxide photocatalyst.

[0071] Use the above-prepared metallic molybdenum trioxide photocatalyst for photocatalytic ammonia synthesis, including the following steps: Add 10 mg of the metallic molybdenum trioxide photocatalyst and 600 mL of ultrapure water (i.e., water with a resistivity reaching 18 MΩ·cm (25 °C)) to a light-transmitting reaction device. Purge with high-purity nitrogen (i.e., nitrogen volume fraction of 99.999 vol%) for 0.5 h under dark conditions. After the nitrogen purging is completed, seal the reaction device; then turn on the condensing water device and perform visible light illumination for 2 h. Centrifuge the reaction solution after the light irradiation reaction is completed, and take out 1 mL for detecting the ammonia content by cation chromatography.

[0072] The UV-vis DRS spectrum of the molybdenum trioxide photocatalyst in the metallic state obtained in this example is as Figure 1 shown. As can be seen from the curve in Figure 1 , under these conditions, the synthesized molybdenum trioxide photocatalyst in the metallic state has a strong local surface plasmon resonance absorption spectrum in the visible to near-infrared light region, and its local surface plasmon resonance characteristic peak is very obvious. The position of the LSPR absorption peak of the molybdenum trioxide in the metallic state synthesized in this example is around 915 nm. Under visible light irradiation for 2 h in this example, the ammonia production is as Figure 2 shown, and its ammonia synthesis performance can reach 473 μmol·g -1 ·h -1 .

[0073] Example 3

[0074] A preparation method of a molybdenum trioxide photocatalyst in the metallic state, comprising the following steps:

[0075] 1) Prepare a commercial molybdenum oxide precursor dispersion solution: Disperse 83.4 mmol of commercial molybdenum trioxide in 100 ml of deionized water, and fully disperse to obtain solution A.

[0076] 2) Place the solution A obtained in step 1) on a magnetic stirrer, and dropwise add 30 mL of sodium borohydride reducing agent (264 mmol L -1 ) solution while stirring at 25 °C. After the addition is completed, continue stirring for 12 h to obtain solution B.

[0077] 3) Centrifuge the solution B obtained in step 2), disperse the centrifuged filter cake in 100 ml of deionized water, and fully disperse to obtain a homogeneous solution; subject the obtained homogeneous solution to reduction again through step 2) to obtain a solid precipitate C.

[0078] 4) Wash the solid precipitate C obtained in step 3) 4 times with deionized water, and then freeze-dry at -50 °C for 24 h to obtain molybdenum trioxide in the metallic state, which is the molybdenum trioxide photocatalyst in the metallic state.

[0079] Use the above-prepared molybdenum trioxide photocatalyst in the metallic state for photocatalytic ammonia synthesis, comprising the following steps:

[0080] Add 10 mg of molybdenum trioxide photocatalyst in the metallic state and 600 mL of ultrapure water (i.e., water with a resistivity of 18 MΩ·cm (25 °C)) to a light-transmissive reaction device. Under dark conditions, introduce high-purity nitrogen (i.e., nitrogen with a volume fraction of 99.999 vol%) for 0.5 h. After the nitrogen introduction is complete, seal the reaction device. Then, turn on the condensing water device and perform visible light illumination for 2 h. Centrifuge the reaction solution after the light illumination reaction is completed, and take out 1 mL for detecting the ammonia content by cation chromatography.

[0081] The UV-vis DRS spectrum of the molybdenum trioxide photocatalyst in the metallic state obtained in this example is as Figure 1 shown. As can be seen from the curve in Figure 1 , under these conditions, the synthesized molybdenum trioxide photocatalyst in the metallic state has a strong local surface plasmon resonance absorption spectrum in the visible to near-infrared light region, and its local surface plasmon resonance characteristic peak is very obvious. The position of the LSPR absorption peak of the molybdenum trioxide in the metallic state synthesized in this example is around 896 nm. Under visible light illumination for 2 h in this example, the ammonia production is as Figure 2 shown, and its ammonia synthesis performance can reach 556 μmol·g -1 ·h -1 .

[0082] Example 4

[0083] A preparation method of a molybdenum trioxide photocatalyst in the metallic state, comprising the following steps:

[0084] 1) Prepare a commercial molybdenum oxide precursor dispersion solution: Disperse 83.4 mmol of commercial molybdenum trioxide in 100 ml of deionized water, and fully disperse and mix evenly to obtain solution A.

[0085] 2) Place the solution A obtained in step 1) on a magnetic stirrer, and dropwise add 30 mL of sodium borohydride reducing agent (352 mmol L -1 ) solution while stirring at 25 °C. After the dropping is completed, continue stirring for 12 h to obtain solution B.

[0086] 3) Centrifuge the solution B obtained in step 2), disperse the centrifuged filter cake in 100 ml of deionized water, and fully disperse and mix evenly to obtain a uniform solution; subject the obtained uniform solution to reduction again through step 2) to obtain a solid precipitate C.

[0087] 4) Wash the solid precipitate C obtained in step 3) 4 times with deionized water, and then freeze-dry at -50 °C for 24 h to obtain molybdenum trioxide in the metallic state, which is the molybdenum trioxide photocatalyst in the metallic state.

[0088] The obtained metallic molybdenum trioxide photocatalyst is used for photocatalytic ammonia synthesis, including the following steps: Add 10 mg of the metallic molybdenum trioxide photocatalyst and 600 mL of ultrapure water (i.e., water with a resistivity of 18 MΩ·cm (25 °C)) into a light-transmissive reaction device. Under dark conditions, introduce high-purity nitrogen (i.e., nitrogen with a volume fraction of 99.999 vol%) for 0.5 h. After the nitrogen introduction is completed, seal the reaction device; then turn on the condensing water device and perform visible light illumination for 2 h. Centrifuge the reaction solution after the light illumination reaction is completed, and take out 1 mL for detecting the ammonia content by cation chromatography.

[0089] The UV-vis DRS spectrum of the obtained metallic molybdenum trioxide photocatalyst in this example is as Figure 1 shown. As can be seen from the curve in Figure 1 , under these conditions, the synthesized metallic molybdenum trioxide photocatalyst has a strong local surface plasmon resonance absorption spectrum in the visible to near-infrared light region, and its local surface plasmon resonance characteristic peak is very obvious. The position of the LSPR absorption peak of the synthesized metallic molybdenum trioxide in this example is about 842 nm. Under visible light illumination for 2 h in this example, the ammonia production is as Figure 2 shown, and its performance for catalytic ammonia synthesis can reach 733 μmol·g -1 ·h -1 .

[0090] Example 5

[0091] A preparation method of a metallic molybdenum trioxide photocatalyst includes the following steps:

[0092] 1) Prepare a commercial molybdenum oxide precursor dispersion solution: Disperse 83.4 mmol of commercial molybdenum trioxide in 100 ml of deionized water, and fully disperse and mix evenly to obtain solution A.

[0093] 2) Place the solution A obtained in step 1) on a magnetic stirrer, and while stirring at 25 °C, dropwise add 30 mL of sodium borohydride reducing agent solution (705 mmol L -1 ). After the dropping is completed, continue stirring for 12 h to obtain solution B.

[0094] 3) Centrifuge the solution B obtained in step 2), disperse the centrifuged filter cake in 100 ml of deionized water, and fully disperse and mix evenly to obtain a homogeneous solution; subject the obtained homogeneous solution to reduction again through step 2) to obtain a solid precipitate C.

[0095] 4) Wash the solid precipitate C obtained in step 3) 4 times with deionized water, and then freeze-dry at -50 °C for 24 h to obtain metallic molybdenum trioxide, which is the metallic molybdenum trioxide photocatalyst.

[0096] The prepared molybdenum trioxide photocatalyst in the metallic state is used for photocatalytic ammonia synthesis, which includes the following steps: Add 10 mg of the molybdenum trioxide photocatalyst in the metallic state and 600 mL of ultrapure water (i.e., water with a resistivity of 18 MΩ·cm (25 °C)) into a light-transmissive reaction device. Purge with high-purity nitrogen (i.e., nitrogen with a volume fraction of 99.999 vol%) for 0.5 h under dark conditions. After the nitrogen purging is completed, seal the reaction device. Then turn on the condensing water device and conduct visible light irradiation for 2 h. Centrifuge the reaction solution after the light irradiation reaction is completed, and take out 1 mL for detecting the ammonia content by cation chromatography.

[0097] The UV-vis DRS spectrum of the obtained molybdenum trioxide photocatalyst in the metallic state in this example is as Figure 1 shown. As can be seen from the curve in Figure 1 , under these conditions, the synthesized molybdenum trioxide photocatalyst in the metallic state has a strong local surface plasmon resonance absorption spectrum in the visible to near-infrared light region, and its local surface plasmon resonance characteristic peak is very obvious. The position of the LSPR absorption peak of the synthesized molybdenum trioxide in the metallic state in this example is around 640 nm. Under visible light irradiation for 2 h in this example, the ammonia production is as Figure 2 shown, and its ammonia synthesis performance can reach 879 μmol·g -1 ·h -1 .

[0098] Comparative Example 1

[0099] A preparation method of a molybdenum trioxide photocatalyst is different from that of Example 1 only in that the amount of sodium borohydride in step 2) is 0 mmol.

[0100] The prepared molybdenum trioxide photocatalyst is used for photocatalytic ammonia synthesis.

[0101] The UV-vis DRS spectrum of the obtained MoO 3 photocatalyst is as Figure 1 shown. As can be seen from the curve in Figure 1 , under these conditions, the synthesized MoO 3 photocatalyst has an obvious local surface plasmon resonance characteristic peak (945 nm). Under visible light irradiation for 2 h in this comparative example, the ammonia production is as Figure 2 shown, and its ammonia synthesis performance is 0 μmol·g -1 ·h -1 .

[0102] Comparative Example 2

[0103] A preparation method of a molybdenum trioxide photocatalyst is different from that of Example 1 only in that molybdenum trioxide is only reduced once by sodium borohydride with the same concentration, and step 3) is not included. It specifically includes the following steps:

[0104] 1) Prepare a commercial molybdenum oxide precursor dispersion solution: Disperse 83.4 mmol of commercial molybdenum trioxide in 100 ml of deionized water and disperse it evenly to obtain a homogeneous solution.

[0105] 2) Place the homogeneous solution obtained in step 1) on a magnetic stirrer and dropwise add 30 mL of sodium borohydride reducing agent solution (concentration: 440 mmol / L -1 ) while stirring at 25 °C. After the addition is complete, continue stirring for 12 h to obtain the final product.

[0106] 3) Wash the final product obtained in step 2) 4 times with deionized water and then freeze-dry it at -50 °C for 24 h to obtain defective molybdenum trioxide, which is the defective molybdenum trioxide photocatalyst, and name it defective-rich molybdenum trioxide.

[0107] Use the above-prepared molybdenum trioxide photocatalyst for photocatalytic ammonia synthesis.

[0108] Under visible light irradiation for 2 h in this comparative example, its ammonia synthesis performance is 0.213 mol·g -1 ·h -1

[0109] Use the molybdenum trioxide photocatalysts obtained in Examples 1-5 and Comparative Examples 1-2 for photocatalytic ammonia synthesis, and the results are shown in Table 2.

[0110] Table 2 Ammonia production obtained by different molybdenum trioxide photocatalysts for photocatalytic ammonia synthesis

[0111] Number <![CDATA[Reducing agent (mmol L -1 )]]> <![CDATA[Ammonia production (mmol·g -1 ·h -1 )]]> Comparative Example 1 0 0 Comparative Example 2 440 0.213 Example 1 440 1.260 Example 2 176 0.473 Example 3 264 0.556 Example 4 352 0.733 Example 5 705 0.879

[0112] Conclusion: As can be seen from Table 2, under visible light irradiation for 2 h, the ammonia production is greatly related to the amount of reducing agent in the precursor. Within a certain range, the higher the amount of reducing agent, the higher the ammonia production. When the reducing agent concentration reaches 440 mmol·L -1 , its ammonia synthesis performance is the best, and it can reach up to 1260 μmol·g -1 ·h -1 ; while when the amount of reducing agent is too high, the performance of the metallic molybdenum trioxide photocatalyst decreases. When the reducing agent exceeds 440 mmol·L -1When the catalytic performance of ammonia synthesis significantly decreases, this is mainly because when there is an excessive amount of reducing agent in the precursor, it will lead to a decrease in the hydrophilicity of the surface of metallic molybdenum trioxide, resulting in a reduction in catalytic performance. In addition, the ammonia production is also greatly related to the number of reduction times. The longitudinal depth of the defects induced by a single reduction of commercial molybdenum trioxide is limited, which in turn makes its free carrier concentration relatively low, and thus metallic molybdenum trioxide cannot be obtained. Multiple reductions can achieve a defect layer with a higher longitudinal depth, enabling the defect layer of molybdenum trioxide to have the free electron density of a metallic material (10 22 cm -3 ), and then metallic molybdenum trioxide catalyst can be obtained, and its high free electron concentration makes its LSPR absorption peak located in the visible light region.

[0113] Example 6 - 13

[0114] The metallic molybdenum trioxide photocatalyst obtained in Example 1 was used for photocatalytic ammonia synthesis. The steps were the same as in Example 1, except that the visible light irradiation time was changed. The results are shown in Table 3.

[0115] Table 3 Ammonia production obtained by photocatalytic ammonia synthesis using metallic molybdenum trioxide photocatalyst at different times

[0116] Number Illumination (min) <![CDATA[Ammonia production (μmol·g -1 )]]> Comparative Example 3 0 0 Example 6 15 334 Example 7 30 479 Example 8 45 817 Example 9 60 991 Example 10 75 1198 Example 11 90 1631 Example 12 105 1941 Example 13 120 2512

[0117] As can be seen from Table 3, under visible light irradiation, the ammonia production is greatly related to the light irradiation time. Within a certain range, as the irradiation time increases, the obtained ammonia production is higher, and the catalytic ammonia synthesis performance is correspondingly improved.

[0118] Comparative Example 4 - 26

[0119] Different types of photocatalysts were used for photocatalytic ammonia synthesis. The steps were the same as in Example 1. The results are shown in Table 4.

[0120] Table 4 Ammonia production obtained by photocatalytic ammonia synthesis using different types of photocatalysts

[0121]

[0122]

[0123] As can be seen from Table 4, under 2 h of visible light irradiation, the ammonia production is greatly related to the catalyst type. Compared with the reported catalytic materials, the metallic molybdenum trioxide photocatalyst synthesized in the present invention has the best catalytic ammonia synthesis performance, and can reach up to 1260 μmol·g -1 ·h -1 . Compared with traditional photocatalysts, the metallic molybdenum trioxide photocatalyst synthesized in the present invention has more exposed catalytic active sites and stronger light capture ability, resulting in its excellent performance.

[0124] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features. These modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A preparation method of a metallic molybdenum trioxide photocatalyst, characterized in that, it includes performing two or more reduction reactions on molybdenum oxide to obtain a metallic molybdenum trioxide photocatalyst.

2. The preparation method according to claim 1, characterized in that, it includes the following steps: Step 1): Uniformly disperse molybdenum oxide in an aqueous solution to obtain solution A; Step 2): Add a reducing agent solution to the solution A to perform a first reduction reaction to obtain solution B; Step 3): Centrifuge the obtained solution B, disperse the centrifuged filter cake in an aqueous solution, and add the reducing agent solution again to perform a second reduction reaction to obtain a solid precipitate C; Step 4): Wash and dry the obtained solid precipitate C to obtain a metallic molybdenum trioxide photocatalyst.

3. The preparation method according to claim 2, characterized in that, the molar ratio of the reducing agent in the reducing agent solution to the molybdenum oxide is (1 - 5):

1.

4. The preparation method according to claim 2 or 3, characterized in that, In the solution A, the concentration of molybdenum oxide is 100 - 1000 mmol·L -1 , preferably 800 - 900 mmol·L -1 ; And / or, the concentration of the reducing agent in the reducing agent solution is 50 - 1000 mmol·L -1 , preferably 176 - 705 mmol·L -1 .

5. The preparation method according to any one of claims 2 - 4, characterized in that, the reducing agent in the reducing agent solution is sodium borohydride.

6. The preparation method according to any one of claims 2 - 5, characterized in that, in step 3), the rotation speed of the centrifugation is 10000 - 12000 r / min; the time of the centrifugation is 0.1 - 0.5 h; and / or, in step 4), the drying temperature is -52 to -47 °C; the drying time is 12 - 24 h.

7. The preparation method according to claim 1, characterized in that, the molybdenum oxide is molybdenum trioxide.

8. A metallic molybdenum trioxide photocatalyst, characterized in that, it is prepared by using the preparation method according to any one of claims 1 - 7.

9. The metallic molybdenum trioxide photocatalyst according to claim 8, characterized in that, the position of the LSPR absorption peak of the metallic molybdenum trioxide photocatalyst is at 640 - 915 nm.

10. Application of the metallic molybdenum trioxide photocatalyst prepared by using the preparation method according to any one of claims 1 - 7 or the metallic molybdenum trioxide photocatalyst according to claim 8 or 9 in photocatalytic ammonia synthesis.

Images