Method for preparing high-dispersion monatomic catalyst through laser ice phase reduction

Abstract

The invention provides a method for preparing a high-dispersion monatomic catalyst through laser ice-phase reduction, and belongs to the technical field of monatomic catalysts. The preparation method comprises the following steps: mixing a carrier aqueous dispersion and a soluble active metal precursor aqueous solution to obtain a carrier adsorption metal ion dispersion; freezing the carrier adsorption metal ion dispersion liquid by using liquid nitrogen to obtain carrier adsorption metal ion ice blocks; in liquid nitrogen, carrying out laser reduction on the carrier adsorbed metal ion ice block to obtain a carrier loaded metal monatomic ice block; and sequentially melting, washing and drying the carrier-loaded metal monatomic ice block to obtain the high-dispersion monatomic catalyst. According to the method for preparing the high-dispersion monatomic catalyst through laser ice-phase reduction, only water is used as a solvent, aggregation of metal monatomic can be effectively inhibited through water molecule crystallization under the freezing condition, and therefore the dispersity of the monatomic is improved, the cost is low, and the environment is not polluted.

Description

Technical Field

[0001] The present invention relates to the technical field of single-atom catalysts, and particularly relates to a method for preparing highly dispersed single-atom catalysts by laser ice-phase reduction. Background Art

[0002] In modern catalytic science, single-atom catalysts have attracted much attention due to their excellent performance. Compared with traditional catalysts, the metal atoms of single-atom catalysts are individually dispersed on the surface of the support, and each metal single atom can independently participate in the catalytic reaction, significantly improving the catalytic efficiency and selectivity.

[0003] Pulsed laser technology provides new possibilities for the processing of nanomaterials with its high energy density, short action time, and strong controllability, and is considered a new generation of nanosynthesis technology that may replace traditional methods. In the single-atom synthesis method, laser liquid-phase synthesis can effectively improve the dispersion of single atoms, including the following steps: (1) dissolving a metal precursor salt and an organic ligand in a solvent, and then drop-coating and drying on a substrate; (2) transferring the substrate in step (1) to a container and performing laser treatment in a liquid-phase environment. However, during the synthesis process, a large amount of organic substances are required both when dissolving the metal precursor salt and during laser treatment, resulting in high costs and environmental pollution. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to provide a method for preparing highly dispersed single-atom catalysts by laser ice-phase reduction. The present invention only uses water as a solvent, and the crystallization of water molecules under freezing conditions can effectively inhibit the aggregation of metal single atoms, thereby obtaining highly dispersed single-atom catalysts with low cost and no environmental pollution.

[0005] In order to achieve the above invention purpose, the present invention provides the following technical solutions:

[0006] The present invention provides a method for preparing highly dispersed single-atom catalysts by laser ice-phase reduction, including the following steps:

[0007] Mixing a carrier aqueous dispersion and an aqueous solution of a soluble active metal precursor to obtain a carrier-adsorbed metal ion dispersion;

[0008] Freezing the carrier-adsorbed metal ion dispersion with liquid nitrogen to obtain a carrier-adsorbed metal ion ice block;

[0009] Performing laser reduction on the carrier-adsorbed metal ion ice block in liquid nitrogen to obtain a carrier-supported metal single atom ice block;

[0010] Successively melting, washing, and drying the carrier-supported metal single atom ice block to obtain the highly dispersed single-atom catalyst.

[0011] Preferably, the mass percentage of the carrier in the carrier aqueous dispersion is not more than 5%.

[0012] Preferably, the metal elements in the aqueous solution of the soluble active metal precursor include noble metal elements and / or transition metal elements.

[0013] Preferably, the soluble active metal precursor in the aqueous solution of the soluble active metal precursor includes one or more of chlorides, hydrochlorates, nitrates, sulfates and organic salts.

[0014] Preferably, the mass percentage of the soluble active metal precursor in the carrier adsorbed metal ion dispersion is not more than 0.5%.

[0015] Preferably, the mass ratio of the soluble active metal precursor in the aqueous solution of the soluble active metal precursor to the carrier in the carrier dispersion is not more than 10%, and the mass of the soluble active metal precursor is not zero.

[0016] Preferably, when freezing, the volume ratio of liquid nitrogen to the carrier adsorbed metal ion dispersion is not less than 10:1.

[0017] Preferably, the diameter of the laser spot for laser reduction is 1 - 50 μm, and the spot energy density is 10 4 ~10 6 W / cm 2 .

[0018] Preferably, the wavelength of the laser reduction is 355, 1064 or 10640 nm.

[0019] Preferably, the scanning mode of the laser reduction is three-dimensional scanning. The scanning speed of the three-dimensional scanning in the horizontal direction is 100 - 10000 mm / s, and the scanning speed in the vertical direction is 0.1 - 3 mm / min.

[0020] The present invention provides a method for preparing a highly dispersed single-atom catalyst by laser ice-phase reduction, comprising the following steps: mixing a carrier aqueous dispersion and an aqueous solution of a soluble active metal precursor to obtain a carrier adsorbed metal ion dispersion; freezing the carrier adsorbed metal ion dispersion with liquid nitrogen to obtain a carrier adsorbed metal ion ice block; performing laser reduction on the carrier adsorbed metal ion ice block in liquid nitrogen to obtain a carrier loaded with metal single-atom ice block; and sequentially melting, washing and drying the carrier loaded with metal single-atom ice block to obtain the highly dispersed single-atom catalyst.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0022] The method for preparing a highly dispersed single-atom catalyst by laser ice-phase reduction of the present invention only uses water as a solvent. The crystallization of water molecules under freezing conditions can effectively inhibit the aggregation of metal single atoms, thereby improving the dispersion of single atoms. It has low cost and does not pollute the environment. At the same time, the present invention uses liquid nitrogen for freezing to maintain good dispersion of the support in the ice block adsorbed with metal ions on the support. In the frozen state, the metal ions adsorbed on the surface of the support are fixed in the ice lattice, forming a relatively restricted spatial environment. This spatial confinement effect can prevent the redistribution and aggregation of metal ions during the laser reduction process, helping to maintain their monodispersed state. Laser reduction has a high photon energy density. The high photon energy density can excite the electrons in the support to promote the reduction of metal ions and introduce defects in the support, enhancing the anchoring of metal single atoms, reducing random distribution, and enhancing the stability of the single-atom catalyst. The combination of laser reduction and liquid nitrogen freezing reduces the single-atom formation time and accelerates the high-temperature dissipation process, reducing the diffusion distance of thermal energy inside the support, helping to maintain the dispersed state of single atoms, and inhibiting the aggregation of single atoms. The method for preparing a highly dispersed single-atom catalyst by laser ice-phase reduction of the present invention changes the surface energy state of single atoms through the synergistic effects of rapid cooling by liquid nitrogen freezing, spatial confinement by liquid nitrogen freezing, and laser reduction, jointly improving the dispersion of single atoms, thereby obtaining better catalytic activity and stability.

[0023] Furthermore, the present invention only needs to adjust the parameters of the metal precursor aqueous solution and laser reduction (including the laser spot diameter, spot energy density, and laser reduction wavelength) to prepare various metal single atoms, which has universality. Brief Description of the Drawings

[0024] Figure 1 is a flow chart of the method for preparing a highly dispersed single-atom catalyst by laser ice-phase reduction in the embodiment of the present invention;

[0025] Figure 2 is the XPS spectrum of the highly dispersed single-atom catalysts prepared in Examples 1 to 3;

[0026] Figure 3 is the HAADF-STEM image of the highly dispersed single-atom catalysts prepared in Examples 1 to 3;

[0027] Figure 4 is the XAFS spectrum of the highly dispersed single-atom catalysts prepared in Examples 1 to 3;

[0028] Figure 5 is the photocatalytic hydrogen production effect diagram of the highly dispersed single-atom catalysts prepared in Examples 1 and 3;

[0029] Figure 6 is the photocatalytic degradation of antibiotics effect diagram of the highly dispersed single-atom catalyst prepared in Example 2. Detailed implementation mode

[0030] The present invention provides a method for preparing a highly dispersed single-atom catalyst by laser ice-phase reduction, comprising the following steps:

[0031] Mix a carrier aqueous dispersion and an aqueous solution of a soluble active metal precursor to obtain a carrier-adsorbed metal ion dispersion;

[0032] Freeze the carrier-adsorbed metal ion dispersion with liquid nitrogen to obtain a carrier-adsorbed metal ion ice block;

[0033] In liquid nitrogen, laser-reduce the carrier-adsorbed metal ion ice block to obtain a carrier-supported metal single-atom ice block;

[0034] Melt, wash, and dry the carrier-supported metal single-atom ice block in sequence to obtain the highly dispersed single-atom catalyst.

[0035] In the present invention, unless otherwise specified, the raw materials used are all commercially available products in the art.

[0036] The present invention mixes a carrier aqueous dispersion and an aqueous solution of a soluble active metal precursor to obtain a carrier-adsorbed metal ion dispersion.

[0037] The present invention preferably disperses the carrier in water to obtain the carrier dispersion.

[0038] In the present invention, the carrier preferably includes metal oxides, metal nitrides, carbides, and two-dimensional layered nanomaterials. The metal oxides preferably include FeO x , ZnO, NiO, and TiO 2 One or more of them. The metal nitrides preferably include TiN and / or VN. The carbides preferably include Mxene and / or g-C 3 N 4 , and the two-dimensional layered nanomaterials preferably include MoS 2 , WS 2 And one or more of MOF.

[0039] In the present invention, the mass percentage of the carrier in the carrier dispersion is preferably not more than 5%. If the carrier concentration in the carrier dispersion is too high, it will absorb laser light and affect the light illumination of the bottom layer in the subsequent carrier-adsorbed metal ion ice block.

[0040] The present invention preferably puts the carrier into water and ultrasonically disperses it to obtain a carrier dispersion.

[0041] In the present invention, the ultrasonic frequency for ultrasonic dispersion is preferably 20 - 120 kHz, specifically it can be 20, 24, 40, 60, 84 or 120 kHz, the power is preferably 50 - 2000 W, specifically it can be 50, 100, 500, 1000, 1500 or 2000 W, and the time is preferably 0.5 - 5 h, specifically it can be 0.5, 1, 2, 3, 4 or 5 h. The reason for the time preferably not exceeding 5 h is to avoid turning the nanosheets in the carrier into quantum dots due to too long time.

[0042] In the present invention, the types of metal elements in the aqueous solution of soluble active metal precursor preferably include noble metal elements and / or transition metal elements. The noble metal elements more preferably include one or more of gold, silver, platinum and palladium, and the transition metal elements more preferably include one or more of copper, nickel and cobalt.

[0043] In the present invention, the soluble active metal precursor in the aqueous solution of soluble active metal precursor preferably includes one or more of chlorides, hydrochlorides, nitrates, sulfates and organic salts. There are no special limitations on the specific types of the chlorides, hydrochlorides, nitrates, sulfates and organic salts in the present invention, and the types well-known to those skilled in the art can be used.

[0044] In the present invention, the mass percentage of the soluble active metal precursor in the carrier adsorbed metal ion dispersion is preferably not more than 0.5%, to prevent the concentration of metal single atoms from being too high and causing agglomeration.

[0045] In the present invention, the mass ratio of the soluble active metal precursor in the aqueous solution of soluble active metal precursor to the carrier in the carrier dispersion is preferably not more than 10%, and the mass of the soluble active metal precursor is preferably not zero. The function is to prevent the density of adsorbed metal ions on the carrier surface from being too high and causing agglomeration.

[0046] There are no special limitations on the preparation method of the aqueous solution of soluble active metal precursor in the present invention, and it can be prepared by using the solution preparation methods well-known to those skilled in the art.

[0047] In the present invention, it is preferred to add the aqueous solution of soluble active metal precursor to the carrier dispersion, and then stir to form stable adsorption of metal ions on the carrier surface, obtaining the carrier adsorbed metal ion dispersion. There are no special limitations on the specific parameters of the stirring in the present invention, and the methods well-known to those skilled in the art can be used.

[0048] After obtaining the carrier adsorbed metal ion dispersion, the present invention freezes the carrier adsorbed metal ion dispersion with liquid nitrogen to obtain carrier adsorbed metal ion ice cubes.

[0049] In the present invention, the volume ratio of the liquid nitrogen to the carrier adsorbed with metal ions dispersion liquid is preferably not less than 10:1, to avoid that if the amount of liquid nitrogen is too small, the center of the carrier adsorbed with metal ions dispersion liquid will not freeze.

[0050] In the present invention, it is preferred to transfer the carrier adsorbed with metal ions dispersion liquid to a stainless steel cup, and immerse the stainless steel cup into the liquid nitrogen, such that the liquid nitrogen level is higher than the dispersion liquid level and lower than the cup mouth, so that the dispersion liquid is quickly frozen into ice cubes.

[0051] After obtaining the carrier adsorbed with metal ions ice cubes, in the present invention, the carrier adsorbed with metal ions ice cubes are subjected to laser reduction to obtain carrier-supported metal single-atom ice cubes.

[0052] In the present invention, the diameter of the laser spot for the laser reduction is preferably 1 - 50 μm, specifically it can be 1, 5, 10, 20, 30, 40 or 50 μm, and the spot energy density is preferably 10 4 ~10 6 W / cm 2 Specifically it can be 10 4 、10 5 or 10 6 W / cm 2 .

[0053] In the present invention, the wavelength of the laser reduction is preferably 355, 1064 or 10640 nm. Specifically, such as using a 355 nm ultraviolet laser, a 1064 nm infrared laser or a 10640 nm CO 2 laser.

[0054] In the present invention, the scanning mode of the laser reduction is preferably three-dimensional scanning. The scanning speed of the three-dimensional scanning in the horizontal direction is preferably 100 - 10000 mm / s, specifically it can be 100, 500, 1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, 9000 or 10000 mm / s, and the scanning speed in the vertical direction is preferably 0.1 - 3 mm / min, specifically it can be 0.1, 0.5, 1, 2 or 3 mm / min.

[0055] In the present invention, the three-dimensional scanning preferably includes two methods. The first one is to control the laser beam by a galvanometer to perform horizontal two-dimensional scanning, and fix the laser on the Z-axis displacement stage to achieve vertical scanning; the second one is to fix the stainless steel cup on the three-dimensional displacement stage, and move the displacement stage to achieve three-dimensional scanning to obtain metal single atoms confined by the ice cubes.

[0056] In the present invention, during the three-dimensional scanning process, it can be scanned layer by layer from the bottom layer to the top layer, or scanned layer by layer from the top layer to the bottom layer.

[0057] In the present invention, the number of times of the three-dimensional scanning is preferably once or repeated multiple times.

[0058] In the present invention, the laser for laser reduction is preferably a pulsed laser, more preferably a nanosecond laser, a picosecond laser or a femtosecond laser.

[0059] In the present invention, the parameters of the pulsed laser preferably include laser power, pulse frequency and pulse width, which can be adjusted according to the actual situation to reduce as many metal ions as possible to metal single atoms without agglomeration. During the laser reduction process, one pulse frequency refers to one laser / excitation process. Within one pulse frequency, laser is emitted in a very short time, which is the excitation process, and then the emission stops for a long time, which is the de-excitation process. The role of the excitation process is to reduce metal ions to metal single atoms, and the side effect is heating, which may cause atomic agglomeration. The role of the de-excitation process is to cool the metal single atoms. Within one pulse frequency, the shorter the pulse width, the longer the cooling time, and the better the effect of synthesizing metal single atoms. In the liquid nitrogen environment, during the cooling process, the temperature can be quickly transferred outwards, and the effect is better.

[0060] In a specific embodiment of the present invention, the laser power is preferably 1-5 W, specifically it can be 1, 2, 3, 4 or 5 W, the pulse frequency is preferably 10-100 kHz, specifically it can be 10, 20, 30, 40, 45, 50, 60, 70, 80, 90 or 100 kHz, and the pulse width is preferably 8-50 ns, specifically it can be 8, 10, 20, 30, 40 or 50 ns. Controlling the laser power, pulse frequency and pulse width within the above ranges is to control the single-pulse energy, that is, the number of instantaneous photons generated during the laser reduction process, control the generation rate of metal single atoms, and at the same time be able to control the cooling process after laser heating to avoid agglomeration of metal single atoms.

[0061] The present invention preferably controls the laser spot and scanning speed of the laser reduction through an optical light path. The optical light path preferably includes at least one laser galvanometer and a lens group. The relationship between the focal length of the lens group and the laser spot diameter is described by the formula d = 2.44λf / D, where λ is the laser wavelength, f is the focal length of the lens group, and d and D are the spot diameters after and before focusing respectively.

[0062] The present invention preferably transfers the stainless steel cup containing the ice cubes adsorbed with metal ions by the carrier under the pulsed laser. The laser beam is focused on the upper surface of the ice cubes through the optical light path. The laser is turned on, and three-dimensional laser scanning of the ice cubes adsorbed with metal ions by the carrier is started. During the scanning, liquid nitrogen is continuously added outside the stainless steel cup to ensure that the ice cubes are in a frozen state and will not melt during the scanning process.

[0063] After obtaining the carrier-supported metal single-atom ice cubes, the present invention melts, washes, and dries the carrier-supported metal single-atom ice cubes in sequence to obtain the highly dispersed single-atom catalyst.

[0064] In the present invention, the melting is preferably natural melting at room temperature.

[0065] In the present invention, the washing is preferably washing with deionized water, and the number of times of washing with deionized water is preferably 3 to 5 times. The function of the washing is to remove unbound metal single atoms and metal ions.

[0066] In the present invention, the mass ratio of deionized water to the powder after melting during washing is preferably not less than 1000:1.

[0067] In the present invention, the drying is preferably freeze-drying, and the freeze-drying is preferably carried out in a freeze dryer. The time of freeze-drying can be appropriately adjusted according to different materials.

[0068] In a specific embodiment of the present invention, the temperature of the freeze-drying is -60 °C and the time is 24 h.

[0069] Next, the technical solutions in the present invention will be clearly and completely described in conjunction with the embodiments in the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0070] Figure 1 It is a flow chart of the method for preparing a highly dispersed single-atom catalyst by laser ice-phase reduction in the embodiments of the present invention.

[0071] Example 1

[0072] A method for preparing a highly dispersed Pt single-atom catalyst by laser ice-phase reduction, comprising the steps of:

[0073] (1) Disperse 100 mg of g-C 3 N 4 nanosheets in 100 mL of deionized water, and ultrasonically treat (frequency: 20 kHz, power: 2000 W) for 30 min to form a uniformly dispersed solution. The mass concentration of g-C 3 N 4 nanosheets in the dispersed solution is 1 mg / mL. Add 150 μL of a chloroplatinic acid solution with a concentration of 10 mg / mL to the uniformly dispersed solution of g-C 3 N 4 nanosheets and stir for 1 h to obtain a carrier-adsorbed metal ion dispersed solution. Calculated in terms of platinum, the chloroplatinic acid in the carrier-adsorbed metal ion dispersed solution and g-C3 N 4 The mass ratio of the nanosheets is 5%. Transfer the carrier adsorbed metal ion dispersion to a stainless steel cup, and then immerse it in liquid nitrogen, making the liquid nitrogen level higher than the dispersion level and lower than the cup mouth for rapid freezing to obtain carrier adsorbed metal ion ice cubes.

[0074] (2) Use a 355 nm ultraviolet nanosecond laser with a power of 4 W and a pulse frequency of 45 kHz to emit laser light. After passing through the optical path, irradiate the ice cubes and perform a three-dimensional scan of the entire ice cube for one cycle. Continuously add liquid nitrogen to the outside of the ice cubes during the scan to ensure that the ice cubes are in a frozen state and do not melt during the scan. The scanning speed of the three-dimensional scan in the horizontal direction is 100 mm / s, and the scanning speed in the vertical direction is 0.1 mm / min.

[0075] (3) After the ice cubes irradiated by the laser melt at room temperature, wash them three times with deionized water, and then dry them at -60 °C for 24 h by freeze-drying to obtain the final product g-C 3 N 4 supported Pt single-atom catalyst, denoted as Pt / g-C 3 N 4 .

[0076] Example 2

[0077] A method for preparing highly dispersed Ag single-atom catalyst by laser ice-phase reduction, comprising the steps:

[0078] (1) Disperse 100 mg of g-C 3 N 4 nanosheets in 100 mL of deionized water, and ultrasonically treat (frequency 20 kHz, power 2000 W) for 30 min to form a homogeneous dispersion. The mass concentration of g-C 3 N 4 nanosheets in the dispersion is 1 mg / mL. Weigh 0.05 g of silver nitrate and dissolve it in 50 mL of deionized water, stir until completely dissolved to obtain a silver nitrate solution. Dropwise add 5 mL of the silver nitrate solution into the g-C 3 N 4 dispersion, and continue to stir for 30 min to obtain a carrier adsorbed metal ion dispersion. Transfer the carrier adsorbed metal ion dispersion to a stainless steel cup, and then immerse it in liquid nitrogen, making the liquid nitrogen level higher than the dispersion level and lower than the cup mouth for rapid freezing to obtain carrier adsorbed metal ion ice cubes.

[0079] (2) Use a 355 nm ultraviolet nanosecond laser to emit laser light. After passing through the optical path, irradiate the ice cubes and perform a three-dimensional scan of the entire ice cube. The laser power is 5 W, the pulse frequency is 3 kHz, the pulse width is 20 ns, and the power density of the focused light spot is 10 5 mW / cm2 During three-dimensional scanning, the scanning rate in the horizontal direction is 500 mm / s, and the vertical scanning rate in the vertical direction is 0.5 mm / min. During the scanning process, liquid nitrogen is continuously added to the outside of the ice block to ensure that the ice block remains in a frozen state and does not melt during the scanning process.

[0080] (3) After the ice block irradiated by the laser melts at room temperature, it is washed with ethanol and deionized water in turn. The washing times of ethanol and deionized water are three times respectively, and then dried at -60 °C for 24 h by freeze-drying to obtain the final product g-C 3 N 4 single-atom Ag-loaded catalyst, denoted as Ag / g-C 3 N 4 .

[0081] Example 3

[0082] A method for preparing a highly dispersed Co single-atom catalyst by laser ice-phase reduction, comprising the steps of:

[0083] (1) Disperse 100 mg of g-C 3 N 4 nanosheets in 100 mL of deionized water, and ultrasonically treat (frequency: 20 kHz, power: 2000 W) for 30 min to form a homogeneous dispersion. The mass concentration of g-C 3 N 4 nanosheets in the dispersion is 1 mg / mL. Weigh 0.05 g of cobalt nitrate and dissolve it in 50 mL of deionized water, stir until completely dissolved to obtain a cobalt nitrate solution. Gradually add 5 mL of the cobalt nitrate solution dropwise into the g-C 3 N 4 dispersion, continue to stir for 30 min to obtain a carrier-adsorbed metal ion dispersion. Transfer the carrier-adsorbed metal ion dispersion to a stainless steel cup, and then immerse it in liquid nitrogen so that the liquid nitrogen level is higher than the dispersion level and lower than the cup mouth for rapid freezing to obtain a carrier-adsorbed metal ion ice block.

[0084] (2) Place the ice block of the carrier-adsorbed metal ion ice block on the sample stage under a 1064 nm infrared pulsed laser. Set the laser parameters as follows: laser power 3 W, pulse frequency 10 kHz, pulse width 20 ns. Focus the laser into a spot with a diameter of 30 m, and perform laser scanning from the upper surface to the lower surface of the ice block until the entire ice block is covered. The horizontal scanning rate is 200 mm / s, the vertical scanning rate is 0.3 mm / min, and the duration is 40 min. During the scanning process, liquid nitrogen is continuously added to the outside of the ice block to ensure that the ice block remains in a frozen state and does not melt during the scanning process.

[0085] (3) After the ice cubes irradiated by laser melted at room temperature, they were washed successively with ethanol and deionized water, and the washing times of ethanol and deionized water were three times respectively. Then they were dried at -60 °C for 24 h by freeze-drying to obtain the final product g-C 3 N 4 supported Co single-atom catalyst, denoted as Co / g-C 3 N 4 .

[0086] Figure 2 are the XPS spectra of the single-atom catalysts prepared in Examples 1-3, Figure 3 are the HAADF-STEM images of the single-atom catalysts prepared in Examples 1-3. It can be seen that it is proved that well-dispersed metal single-atom catalysts were synthesized in Examples 1, 2, and 3.

[0087] Figure 4 are the XAFS spectra of the single-atom catalysts prepared in Examples 1-3. It can be seen that well-dispersed metal single-atom catalysts were synthesized in Examples 1, 2, and 3, and there are no metal atom clusters or nanoparticles.

[0088] Example 4

[0089] 50 mg of the single-atom catalysts prepared in Examples 1 and 3 were respectively dispersed in 80 mL of deionized water and treated with ultrasonic waves for 10 min to obtain catalyst solutions. 20 mL of triethanolamine was added to the catalyst solutions as a sacrificial agent to scavenge holes to obtain a mixed solution. The mixed solution was placed in the reactor of a closed gas circulation test system. Vacuum grease was evenly applied to the joints of all glass components to ensure the tightness of the system and prevent gas leakage. The switch of the magnetic stirrer was turned on to keep the catalyst suspended in the solution to maximize the contact area between the photocatalyst and light. The vacuum pump was started to completely evacuate the air in the system to ensure an anaerobic state in the reaction environment and avoid the interference of oxygen. A 300 W xenon lamp was used as the light source in the experiment to excite the catalyst for photocatalytic reaction. The distance between the light source and the liquid surface was maintained at 10 cm to ensure the effective transmission of light energy. The temperature of the reactor was controlled at 2 °C by water-cooled circulation. Using N 2 as the carrier and TCD as the thermal conductivity detector, an online gas chromatograph was used to monitor the generated hydrogen in real time. The reaction time was set to 30 min to collect sufficient data for analysis. After the experiment, the data of the gas chromatograph were collected and the hydrogen production was analyzed. See Figure 5 , it can be seen that the photocatalytic hydrogen production effect of the single-atom catalysts synthesized in Examples 1 and 3 was greatly improved compared with the materials without coated single atoms.

[0090] Example 5

[0091] The piezophotocatalytic degradation experiment of levofloxacin was carried out in a parallel light reaction system. An ultraviolet LED lamp with a wavelength of 365 nm and a power of 50 W was used as the light source to provide illumination, and a 100 W probe-type ultrasonic processor was used as the piezoelectric excitation source to enhance the catalytic activity through mechanical vibration. The reaction temperature of the photoreactor was maintained at a constant 20 °C through a cooling water circulation system. 50 mg of levofloxacin was accurately weighed, dissolved in water and transferred to a 500 mL brown volumetric flask. After constant volume, it was fully stirred to prepare a 100 mg / L levofloxacin stock solution, which was stored refrigerated in the refrigerator. 4 mL of the levofloxacin stock solution and 1.6 mg of the g-C 3 N 4 single-atom catalyst loaded with Ag prepared in Example 2 were mixed in a quartz tube containing 36 mL of deionized water. After covering with an anti-condensation plate, it was stirred at a speed of 600 rpm for 60 min to achieve adsorption-desorption equilibrium. During the 12-min reaction process, 2.2 mL of the solution was taken out every 2 min and placed in a centrifugal filtration tube equipped with a 0.22-μm MCE filter membrane, and centrifuged at a speed of 5000 rpm for 2 min using a high-speed centrifuge to completely separate the catalyst. The remaining concentration of levofloxacin in the filtrate was determined by a high-performance liquid chromatograph at a detection wavelength of 294 nm. The mobile phase was composed of a 0.05 mol / L phosphoric acid solution (the pH value was adjusted to 2.3 with triethylamine) and acetonitrile in a volume ratio of 8:2. See Figure 6 , it can be seen that the photocatalytic degradation effect of the single-atom catalyst synthesized in Example 2 on antibiotics has been greatly improved compared with the material without single-atom coating.

[0092] In summary, in the present invention, liquid nitrogen is used to freeze the dispersion containing the metal precursor and the carrier into ice cubes, and a single-atom catalyst with low loading, high dispersion and strong stability is obtained by laser reduction.

[0093] The above are only the preferred embodiments of the present invention, and do not impose any formal limitations on the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A method for preparing highly dispersed single-atom catalysts by laser ice phase reduction, characterized in that: The following steps are involved: Mixing a carrier aqueous dispersion with a soluble active metal precursor aqueous solution to obtain a carrier adsorbed metal ion dispersion; Freezing the carrier-adsorbed metal ion dispersion liquid by using liquid nitrogen to obtain carrier-adsorbed metal ion ice cubes; In liquid nitrogen, the carrier adsorbs metal ion ice cubes and performs laser reduction to obtain carrier-loaded metal single-atom ice cubes; The carrier-loaded metal single-atom ice cubes are melted, washed and dried in sequence to obtain the highly dispersed single-atom catalyst.

2. The method according to claim 1, characterized in that The mass percentage of the carrier in the carrier aqueous dispersion is not greater than 5%.

3. The method according to claim 1, characterized in that The metal elements in the soluble active metal precursor aqueous solution include noble metal elements and / or transition metal elements.

4. The method according to claim 1 or 3, characterized in that: The soluble active metal precursor in the aqueous solution of the soluble active metal precursor includes one or more of chloride, hydrochloride, nitrate, sulfate and organic salt.

5. The method according to claim 1, characterized in that: The mass percentage of the soluble active metal precursor in the carrier-adsorbed metal ion dispersion is not greater than 0.5%.

6. The method according to claim 1, characterized in that The mass ratio of the soluble active metal precursor in the soluble active metal precursor aqueous solution to the carrier in the carrier dispersion is not greater than 10%, and the mass of the soluble active metal precursor is not zero.

7. The method according to claim 1, characterized in that During the freezing, the volume ratio of liquid nitrogen to the dispersion of the metal ions adsorbed by the carrier is not less than 10:

1.

8. The method according to claim 1, characterized in that: The laser spot diameter of the laser reduction is 1 to 50 μm, and the spot energy density is 10 4 ~10 6 W / cm 2 .

9. The method according to claim 1 or 8, characterized in that: The wavelength of the laser reduction is 355, 1064 or 10640 nm.

10. The method according to claim 1 or 8, characterized in that: The scanning mode of the laser restoration is three-dimensional scanning, and the scanning speed of the three-dimensional scanning in the horizontal direction is 100-10000 mm / s, and the scanning speed in the vertical direction is 0.1-3 mm / min.

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