SiO2 / Au nanostar / TiO2 composite catalyst, preparation method and application in synthesis of ammonia

Abstract

The application relates to the field of photocatalytic materials, in particular to a SiO2 / Au nanostar / TiO2 composite catalyst, a preparation method and application in ammonia synthesis. The preparation method comprises the following steps: preparing monodisperse SiO2 microspheres; preparing functionalized modified SiO2 microspheres; preparing Au nanostar loaded SiO2 microspheres; and preparing the SiO2 / Au nanostar / TiO2 composite catalyst. The catalyst has a unique core-shell structure, the core diameter of the Au nanostar is 30-40 nm, the length of the sharp spike is 6-10 nm, and the tip part penetrates into the TiO2 shell layer. In the deionized water system, the catalyst can efficiently and stably catalyze nitrogen reduction to synthesize ammonia under visible light irradiation without adding any sacrificial agent or electrolyte, solves the problems that the existing photocatalytic ammonia synthesis technology depends on an organic sacrificial agent, is easy to corrode equipment, and the catalyst stability is poor, and provides a green, efficient and stable new scheme for synthesizing ammonia.

Description

Technical Field

[0001] This invention relates to the field of photocatalytic materials, specifically to a SiO2 / Au nanostar / TiO2 composite catalyst, its preparation method, and its application in ammonia synthesis. Background Technology

[0002] Ammonia (NH3) is an important chemical raw material and a highly promising clean energy carrier, with huge global annual demand. Currently, the Haber-Bosch process is mainly used in industry to synthesize ammonia. This process requires high temperature (400-500℃) and high pressure (15-25MPa) conditions, which not only consumes a lot of energy but also produces a large amount of carbon dioxide emissions.

[0003] Photocatalytic ammonia synthesis technology, capable of converting nitrogen (N2) and water (H2O) into ammonia under mild conditions using solar energy, is considered a green and sustainable alternative. However, its development is severely constrained by catalyst performance. First, traditional photocatalysts such as titanium dioxide (TiO2) have large band gaps, responding only to ultraviolet light, resulting in low solar energy utilization and rapid recombination rates of photogenerated electron-hole pairs. Second, to suppress carrier recombination, existing technologies often require the addition of hole sacrificial agents such as methanol and ethanol, or electrolytes such as potassium sulfate, to the reaction system. These additives not only increase costs but also introduce a series of problems, such as organic solvent poisoning of catalyst active sites, electrolyte corrosion of reaction equipment, and potential side reactions causing secondary pollution. Furthermore, many highly efficient noble metal catalysts exhibit poor stability in complex solutions containing additives or specific ions, easily undergoing dissolution or aggregation, leading to rapid decline in catalyst activity and short cycle life.

[0004] Therefore, developing a catalyst system that can efficiently and stably drive photocatalytic ammonia synthesis in a deionized water (sacrificial agent / electrolyte-free) environment is of great significance for promoting the practical application of this technology. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies and provide a SiO2 / Au nanostar / TiO2 composite catalyst, its preparation method, and its application in ammonia synthesis. This catalyst possesses a unique multi-layered core-shell structure. Through the tip-localized surface plasmon resonance effect of the Au nanostar and its synergistic effect with the TiO2 shell, it can efficiently and stably achieve visible light-driven ammonia synthesis in a deionized water system.

[0006] The technical solution adopted by this invention to solve its technical problem is: A method for preparing a SiO2 / Au nanostar / TiO2 composite catalyst includes the following steps: Step 1: Preparation of monodisperse SiO2 microspheres: Anhydrous ethanol, ammonia, and deionized water were mixed and stirred to obtain a mixed solution. Tetraethyl orthosilicate (TEOS) was added dropwise to the mixed solution and reacted. After the reaction was completed, the mixture was centrifuged, washed, and dried to obtain monodisperse SiO2 microspheres. Step 2: Preparation of functionalized modified SiO2 microspheres: Monodisperse SiO2 microspheres were added to a polydiallyldimethylammonium chloride (PDDA) solution and stirred. After stirring, the mixture was centrifuged, washed, and dried to obtain functionalized modified SiO2 microspheres. Step 3: Preparation of Au nanostar-supported SiO2 microspheres: S1. PVP-10 (polyvinylpyrrolidone, average molecular weight 10,000) was added to the dispersion containing Au nanospheres, and the mixture was aged. After aging, the mixture was centrifuged and dispersed to obtain a PVP-modified Au seed solution. S2. After mixing HAuCl4 aqueous solution with PVP-10 and deionized water, add PVP-modified Au seed solution, stir, and after stirring, centrifuge and purify, disperse in ethanol to obtain Au nanostar dispersion. S3. Add the functionalized SiO2 microspheres to the dispersion of Au nanostars, stir to allow the Au nanostars to electrostatically self-assemble on the surface of the functionalized SiO2 microspheres. After electrostatic self-assembly, centrifuge, wash and disperse in deionized water to obtain a dispersion of Au nanostars loaded with SiO2 microspheres. Step 4: Preparation of SiO2 / Au nanostars / TiO2 composite catalyst: TiO2 nanoparticles were dispersed in an aqueous solution of sodium citrate and ultrasonically treated. Then, a dispersion containing Au nanostars loaded with SiO2 microspheres was added for heterogeneous deposition. After the heterogeneous deposition was completed, the particles were centrifuged and dried to obtain the SiO2 / Au nanostars / TiO2 composite catalyst.

[0007] Preferably, in step one, the volume ratio of tetraethyl orthosilicate, anhydrous ethanol, ammonia, and deionized water is (1.3-1.5):(16-18):(4-6):(1-2), and the reaction is carried out under the condition of stirring at room temperature for 1.5-2.5 hours.

[0008] Preferably, the concentration of the ammonia water is ≥25wt%, that is, the mass percentage of ammonia (NH3) in the ammonia water is ≥25%.

[0009] Preferably, in step two, the ratio of monodisperse SiO2 microspheres to polydiallyldimethylammonium chloride solution is (15-25) mg / (20-30) mL, and the stirring conditions are stirring at room temperature for 15-25 min.

[0010] Preferably, the polydiallyldimethylammonium chloride solution is prepared by mixing polydiallyldimethylammonium chloride with an aqueous sodium chloride solution; The concentration of the sodium chloride aqueous solution is (0.4-0.6) mol / L; The polydiallyl dimethyl ammonium chloride solution contains (0.5-1.5) mg / mL. The pH value of the polydiallyl dimethyl ammonium chloride solution is controlled at 5±0.2.

[0011] Preferably, in step three, when preparing the PVP-modified Au seed solution, the ratio of PVP-10 to the dispersion containing Au nanospheres is (250-350) mg / 100 mL, and the aging conditions are aging at room temperature for 20-30 h.

[0012] Preferably, the dispersion containing Au nanospheres is prepared by the following steps: Chloroauric acid (HAuCl4·3H2O) was dissolved in deionized water to obtain a chloroauric acid solution. After heating the chloroauric acid solution to boiling, add the trisodium citrate aqueous solution and continue heating to maintain boiling. After heating is complete, cool and dilute to a final volume to obtain a dispersion containing Au nanospheres.

[0013] Preferably, the trisodium citrate aqueous solution is a 1wt% trisodium citrate aqueous solution, and the concentration of the dispersion containing Au nanospheres is 0.5mmol / L.

[0014] Preferably, in step three, when preparing the dispersion of Au nanostars, the ratio of chloroauric acid aqueous solution, PVP-10, deionized water, and PVP-modified Au seed solution is (30-40) mL:(1.2-1.8) g:(12-18) mL:(180-200) μL, and the stirring conditions are stirring at 4℃ for 0.5-1.5 h; The concentration of chloroauric acid in the aqueous chloroauric acid solution is (0.1-0.15) mol / L.

[0015] Preferably, in step three, when preparing the dispersion of Au nanostar-supported SiO2 microspheres, the ratio of functionalized modified SiO2 microspheres to Au nanostar dispersion is (15-25) mg / (2-2.5) mL, and the electrostatic self-assembly condition is stirring at room temperature for 25-35 min.

[0016] Preferably, the concentration of the dispersion containing Au nanostars supported on SiO2 microspheres is (1.5-2.5) mg / mL.

[0017] Preferably, in step four, the ratio of TiO2 nanoparticles, sodium citrate aqueous solution, and dispersion of Au nanostar-loaded SiO2 microspheres is (20-30) mg: (80-120) mL: (8-12) mL, and the heterogeneous deposition condition is stirring at room temperature for 80-100 min.

[0018] Preferably, the concentration of the sodium citrate aqueous solution is (0.5-1.5) mmol / L.

[0019] The present invention also discloses a SiO2 / Au nanostar / TiO2 composite catalyst, which is prepared by the method described above for preparing the SiO2 / Au nanostar / TiO2 composite catalyst.

[0020] An application of the SiO2 / Au nanostar / TiO2 composite catalyst described above in ammonia synthesis.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: The SiO2 / Au nanostar / TiO2 composite catalyst provided by this invention can efficiently drive the visible-light catalytic reduction of nitrogen to ammonia in a deionized water system without the addition of any hole sacrificial agent or electrolyte. Tests showed that the ammonia production rate under visible light irradiation can reach 252.8 μmol·g⁻¹. -1 ·h -1 This avoids the problems of active site poisoning and side reaction pollution caused by the addition of organic sacrificial agents in the traditional photocatalytic ammonia synthesis process, as well as the corrosion of reaction equipment by electrolytes, and truly realizes a green and environmentally friendly new route for ammonia synthesis. This invention uses SiO2 microspheres as a carrier to achieve uniform dispersion of Au nanostars. The unique star-shaped multi-branched structure of Au nanostars generates a strong local surface plasmon resonance effect in the visible-near infrared region, extending the photoresponse range of the catalyst to 420-850 nm and significantly improving the utilization rate of sunlight. More importantly, the tip of the Au nanostar penetrates into the TiO2 shell, forming a direct gold-semiconductor heterojunction contact interface, providing an efficient transport channel for photogenerated electrons, effectively suppressing the recombination of photogenerated electron-hole pairs in TiO2, and enabling more photogenerated electrons to effectively participate in the nitrogen reduction reaction, thereby increasing the ammonia production rate. This invention constructs a TiO2 shell to physically encapsulate and chemically protect Au nanostars, effectively inhibiting the dissolution, shedding, and aggregation of noble metal nanoparticles during the reaction process. After six cycles, the catalyst activity retention rate is still over 90%, demonstrating excellent cycle stability. This gives it significant cost advantages and application potential in long-term operation, overcoming the technical bottlenecks of poor stability and easy deactivation of existing noble metal-based photocatalysts. The catalyst preparation process of this invention is carried out entirely in the aqueous phase, avoiding the use of organic solvents. It has the advantages of simple process, environmental friendliness, and ease of large-scale production, which is in line with the development concept of green chemistry. Attached Figure Description

[0022] Figure 1 This is a TEM image of the Au nanostars prepared in Example 2; Figure 2 This is a TEM image of the SiO2 / Au nanostar / TiO2 composite catalyst prepared in Example 2; Figure 3 The image shows the visible light catalytic nitrogen synthesis of ammonia using the SiO2 / Au nanostar / TiO2 composite catalyst prepared in Example 2. Detailed Implementation

[0023] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0024] Example 1

[0025] This embodiment provides a method for preparing a dispersion containing Au nanospheres, comprising the following steps: Dissolve 19.69 mg (0.05 mmol) of chloroauric acid in 95 mL of deionized water to obtain a chloroauric acid solution; After heating the chloroauric acid solution to boiling, add 10 mL of 1 wt% trisodium citrate aqueous solution and continue heating to boiling for 30 min. When the solution color turns red, stop heating, let it cool naturally to room temperature, and make up to 100 mL to obtain a dispersion containing Au nanospheres. Store at 4℃ in the dark for later use. The concentration of Au nanospheres in the dispersion containing Au nanospheres is 0.5 mmol / L, and the average particle size of the Au nanospheres is 14 ± 1 nm.

[0026] Example 2

[0027] This embodiment provides a method for preparing a SiO2 / Au nanostar / TiO2 composite catalyst, including the following steps: Step 1: Preparation of monodisperse SiO2 microspheres: 17.27 mL of anhydrous ethanol (commercially available reagent, specification: ≥99.9%), 4.87 mL of ammonia water (commercially available reagent, specification: 25%-28%), and 1.44 mL of deionized water were mixed and stirred at room temperature for 15 min to obtain a mixed solution. 1.42 mL of tetraethyl orthosilicate (commercially available reagent, specification: 98%) was added dropwise to the mixed solution, and the mixture was stirred at room temperature for 2.5 h. After the reaction was completed, the mixture was centrifuged at 4500 rpm for 10 min, washed three times with ethanol, and dried under vacuum at 50 °C to constant weight to obtain monodisperse SiO2 microspheres with an average particle size of 480±20 nm. Step 2: Preparation of functionalized modified SiO2 microspheres: 20 mg of monodisperse SiO2 microspheres were added to 25 mL of polydiallyldimethylammonium chloride solution and stirred at room temperature for 20 min. After stirring, the mixture was centrifuged at 6000 rpm for 20 min, washed three times with deionized water, and dried under vacuum at 50 °C to constant weight to obtain functionalized modified SiO2 microspheres. The polydiallyldimethylammonium chloride solution is prepared by mixing polydiallyldimethylammonium chloride with a 0.5 mol / L sodium chloride aqueous solution. The content of polydiallyldimethylammonium chloride in the polydiallyldimethylammonium chloride solution is 1 mg / mL, and the pH value of the polydiallyldimethylammonium chloride solution is controlled at 5 ± 0.2. Step 3: Preparation of Au nanostar-supported SiO2 microspheres: S1. Take 100 mL of the dispersion containing Au nanospheres (0.5 mmol / L) prepared in Example 1, add 300 mg of PVP-10 to the dispersion containing Au nanospheres, age at room temperature for 24 h, centrifuge at 9000 rpm for 15 min, wash the centrifuged sediment three times with anhydrous ethanol, and then disperse it in anhydrous ethanol to obtain the PVP-modified Au seed solution. The amount of anhydrous ethanol used in the dispersion and centrifugation sediment is such that the volume of the PVP-modified Au seed solution is 1 / 10 of the volume of the dispersion containing Au nanospheres, that is, the concentration of the PVP-modified Au seed solution is 5 mmol / L. S2. Mix 35 mL of 0.1192 mol / L (0.004172 mol) HAuCl4 aqueous solution with 1.5 g PVP-10 and 14.78 mL of deionized water, then add 189 μL of PVP-modified Au seed solution (0.945 × 10⁻⁶). -6 The mixture was stirred at 4℃ for 1 hour. After stirring, it was centrifuged at 9000 rpm for 15 minutes. The precipitate was washed three times with anhydrous ethanol and then dispersed in anhydrous ethanol to obtain a dispersion of Au nanostars. The amount of anhydrous ethanol used to disperse the centrifuged sediment is such that the concentration of the Au nanostar dispersion is 5 mmol / L. S3. Add 20 mg of functionalized SiO2 microspheres to 2.25 mL of Au nanostar dispersion and stir at room temperature for 30 min to allow Au nanostars to electrostatically self-assemble on the surface of functionalized SiO2 microspheres. After electrostatic self-assembly, centrifuge at 4200 rpm for 10 min, wash with water 3 times, and disperse in deionized water to obtain a dispersion of Au nanostar-loaded SiO2 microspheres. The amount of deionized water used is such that the concentration of the dispersion containing Au nanostar-loaded SiO2 microspheres is 2 mg / mL. Step 4: Preparation of SiO2 / Au nanostars / TiO2 composite catalyst: 25 mg TiO2 nanoparticles (5 nm) were dispersed in 100 mL of 1 mmol / L sodium citrate aqueous solution. After ultrasonic treatment for 1 h, 10 mL of dispersion containing Au nanostars supported on SiO2 microspheres was added. The mixture was stirred at room temperature for 90 min to carry out heterogeneous deposition. After heterogeneous deposition, the mixture was centrifuged at 4200 rpm for 10 min, washed with water 3 times, and dried under vacuum at 50 °C to constant weight to obtain the SiO2 / Au nanostars / TiO2 composite catalyst.

[0028] Test example: (1) The microstructure of the Au nanostars prepared in Example 2 was observed by transmission electron microscopy (TEM), and the results are as follows: Figure 1 As shown. By Figure 1 It is known that the core diameter of Au nanostars is 30-40 nm, and the length of the spikes is 6-10 nm.

[0029] (2) The microstructure of the SiO2 / Au nanostar / TiO2 composite catalyst prepared in Example 2 was observed by transmission electron microscopy. The results are as follows: Figure 2 As shown. By Figure 2 It is known that in the SiO2 / Au nanostar / TiO2 composite catalyst, the Au tip can penetrate 2-5 nm into the TiO2 shell.

[0030] (3) Visible light photocatalytic nitrogen-to-ammonia synthesis test: Take 1 mg of the SiO2 / Au nanostar / TiO2 composite catalyst prepared in Example 2, add 3 mL of chromatographic grade water, and bubble with N2 for 15 min; irradiate with a xenon lamp for 4 h, centrifuge and collect the supernatant; the ammonia production rate (μmol·g) is detected by the indophenol blue method (Nessler's reagent, 625 nm). -1 ·h -1After 4 hours of reaction, the catalyst was recovered by centrifugation, redispersed with 1 mL of fresh chromatographic grade water, and the photocatalytic nitrogen-to-ammonia reaction was repeated. The ammonia production rate was measured, and the photocatalytic reaction-ammonia production rate measurement process was repeated 5 times. The ammonia production rate measurement results of the 6 photocatalytic reactions are shown in Table 1. Table 1

[0031] As shown in Table 1, the SiO2 / Au nanostar / TiO2 composite catalyst prepared in this invention can efficiently drive the visible light catalytic reduction of nitrogen to ammonia and has excellent cycle stability. After 6 cycles, the catalyst activity retention rate is still over 90% (the ammonia production rate of the second photocatalytic reaction is higher than that of the first reaction because the catalyst usually has low activity in the first reaction due to the activation process on the surface. After the first photocatalytic reaction, more active sites are exposed, resulting in higher activity in the second photocatalytic reaction).

[0032] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a SiO2 / Au nanostar / TiO2 composite catalyst, characterized in that, Includes the following steps: Step 1: Preparation of monodisperse SiO2 microspheres: Anhydrous ethanol, ammonia, and deionized water were mixed and stirred to obtain a mixed solution. Tetraethyl orthosilicate was added dropwise to the mixed solution, and the reaction was carried out. After the reaction was completed, the mixture was centrifuged, washed, and dried to obtain monodisperse SiO2 microspheres. Step 2: Preparation of functionalized modified SiO2 microspheres: Monodisperse SiO2 microspheres were added to a polydiallyldimethylammonium chloride solution and stirred. After stirring, the mixture was centrifuged, washed, and dried to obtain functionalized modified SiO2 microspheres. Step 3: Preparation of Au nanostar-supported SiO2 microspheres: S1. PVP-10 was added to the dispersion containing Au nanospheres and aged. After aging, the mixture was centrifuged and dispersed to obtain a PVP-modified Au seed solution. S2. After mixing HAuCl4 aqueous solution with PVP-10 and deionized water, add PVP-modified Au seed solution, stir, and after stirring, centrifuge and purify, disperse in ethanol to obtain Au nanostar dispersion. S3. Add the functionalized SiO2 microspheres to the dispersion of Au nanostars, stir to allow the Au nanostars to electrostatically self-assemble on the surface of the functionalized SiO2 microspheres. After electrostatic self-assembly, centrifuge, wash and disperse in deionized water to obtain a dispersion of Au nanostars loaded with SiO2 microspheres. Step 4: Preparation of SiO2 / Au nanostars / TiO2 composite catalyst: TiO2 nanoparticles were dispersed in an aqueous solution of sodium citrate and ultrasonically treated. Then, a dispersion containing Au nanostars loaded with SiO2 microspheres was added for heterogeneous deposition. After the heterogeneous deposition was completed, the particles were centrifuged and dried to obtain the SiO2 / Au nanostars / TiO2 composite catalyst.

2. The preparation method of the SiO2 / Au nanostar / TiO2 composite catalyst according to claim 1, characterized in that, In step one, the volume ratio of tetraethyl orthosilicate, anhydrous ethanol, ammonia, and deionized water is (1.3-1.5):(16-18):(4-6):(1-2), and the reaction is carried out under the condition of stirring at room temperature for 1.5-2.5 hours.

3. The method for preparing a SiO2 / Au nanostar / TiO2 composite catalyst according to claim 1, characterized in that, In step two, the ratio of monodisperse SiO2 microspheres to polydiallyldimethylammonium chloride solution is (15-25) mg / (20-30) mL, and the stirring conditions are stirring at room temperature for 15-25 min.

4. The method for preparing a SiO2 / Au nanostar / TiO2 composite catalyst according to claim 3, characterized in that, The polydiallyl dimethyl ammonium chloride solution is prepared by mixing polydiallyl dimethyl ammonium chloride with an aqueous sodium chloride solution; The concentration of the sodium chloride aqueous solution is (0.4-0.6) mol / L; The polydiallyl dimethyl ammonium chloride solution contains (0.5-1.5) mg / mL. The pH value of the polydiallyl dimethyl ammonium chloride solution is controlled at 5±0.

2.

5. The method for preparing a SiO2 / Au nanostar / TiO2 composite catalyst according to claim 1, characterized in that, In step three, when preparing the PVP-modified Au seed solution, the ratio of PVP-10 to the dispersion containing Au nanospheres is (250-350) mg / 100 mL, and the aging conditions are aging at room temperature for 20-30 h.

6. The method for preparing a SiO2 / Au nanostar / TiO2 composite catalyst according to claim 1, characterized in that, In step three, when preparing the dispersion of Au nanostars, the ratio of chloroauric acid aqueous solution, PVP-10, deionized water, and PVP-modified Au seed solution is (30-40) mL:(1.2-1.8) g:(12-18) mL:(180-200) μL, and the stirring conditions are stirring at 4℃ for 0.5-1.5 h. The concentration of chloroauric acid in the aqueous chloroauric acid solution is (0.1-0.15) mol / L.

7. The method for preparing a SiO2 / Au nanostar / TiO2 composite catalyst according to claim 1, characterized in that, In step three, when preparing the dispersion containing Au nanostars supported on SiO2 microspheres, the ratio of functionalized modified SiO2 microspheres to Au nanostar dispersion is (15-25) mg / (2-2.5) mL, and the electrostatic self-assembly condition is stirring at room temperature for 25-35 min. The concentration of the dispersion containing Au nanostar-supported SiO2 microspheres is (1.5-2.5) mg / mL.

8. The method for preparing a SiO2 / Au nanostar / TiO2 composite catalyst according to claim 1, characterized in that, In step four, the ratio of TiO2 nanoparticles, sodium citrate aqueous solution, and dispersion of Au nanostar-loaded SiO2 microspheres is (20-30) mg: (80-120) mL: (8-12) mL, and the heterogeneous deposition condition is stirring at room temperature for 80-100 min. The concentration of the sodium citrate aqueous solution is (0.5-1.5) mmol / L.

9. A SiO2 / Au nanostar / TiO2 composite catalyst, characterized in that, The SiO2 / Au nanostar / TiO2 composite catalyst was prepared using the preparation method described in any one of claims 1-8.

10. The application of the SiO2 / Au nanostar / TiO2 composite catalyst as described in claim 9 in the synthesis of ammonia.

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