Metallic monatomic photocatalyst, preparation method and application thereof

Abstract

The application provides a metal monatomic photocatalyst, a preparation method and application thereof. The preparation method of the metal monatomic photocatalyst comprises the following steps: (a) adding metal chloride, polyvinylpyrrolidone and metal oxide powder into water, uniformly stirring continuously to obtain a precursor suspension, and centrifuging the obtained precipitate to obtain a precursor material; (b) washing the precursor material in step (a), and performing high-temperature calcination under a hydrogen atmosphere after drying to obtain the metal monatomic photocatalyst. The metal monatomic photocatalyst can rely on the synergistic regulation of energy band structure of double active sites of metal monatomic and oxygen vacancies, improve the light excitation capacity of the photocatalyst, and synergistically adsorb nitrate substrates, so that the activity and selectivity of the reaction of photocatalytic reduction of nitrate to synthesize ammonia are improved.

Description

Technical Field

[0001] This invention relates to the field of photocatalysts, and more particularly to an oxygen-deficient semiconductor-supported metal single-atom photocatalyst, its preparation method, and its application. Background Technology

[0002] Ammonia is one of the world's largest-produced chemical products, primarily manufactured through the traditional, extremely energy-intensive Haber-Bosch process. This process requires temperatures of 400–600°C and pressures of 20 MPa, consuming up to 1% of the world's annual energy consumption and generating up to 450 million tons of CO2 annually, exacerbating global warming and triggering natural disasters. Nitrate-containing wastewater is widespread globally, leading to water quality degradation, reduced biodiversity, and contributing to diseases such as methemoglobinemia and cancer in humans. Current commercial technologies for treating nitrates in wastewater suffer from numerous drawbacks, including complex operation and high costs, such as ion exchange and microbial transformation. Nitrate-containing wastewater often contains alcohols and aldehydes, which can be used to eliminate vacancies in catalyst materials during photocatalysis, improving photocatalytic efficiency. As an important component of energy conversion and environmental protection, utilizing solar photocatalysis to convert nitrates and organic pollutants in wastewater to synthesize the high-value-added product ammonia under mild environmental conditions is a highly meaningful and valuable research endeavor.

[0003] CN 109292899A relates to a catalyst for photocatalytic reduction of nitrate nitrogen in water and its application method. This method involves preparing an ion-induced oxygen-deficient perovskite-like catalyst via a hydrothermal process for highly selective nitrogen synthesis. The catalyst prepared by this method exhibits good stability and can be used to remove nitrate nitrogen from water to generate nitrogen gas. However, its drawbacks include high preparation cost, unsafe preparation method, low metal atom utilization, and low added value of the nitrogen product.

[0004] The photocatalytic reduction of nitrate to ammonia exhibits low kinetic rates, unclear intermediate products and mechanisms, complex byproducts, and low selectivity. Selecting single-atom active sites with the highest atom utilization as active centers can reduce metal usage and lower costs. Furthermore, highly unsaturated single-atom sites facilitate substrate adsorption, adjust the reaction pathway, and improve catalyst activity. Currently, designing fine structures for photocatalyst materials, improving active site utilization, enhancing photocatalyst photoexcitation capabilities, and increasing the synthesis efficiency and selectivity of high-value-added products remain areas of focus for those skilled in the art. Summary of the Invention

[0005] This invention proposes a metal single-atom photocatalyst, its preparation method, and its application, including the following aspects.

[0006] In a first aspect, the present invention proposes a method for preparing a metal single-atom photocatalyst.

[0007] The preparation method of the metal single-atom photocatalyst of the present invention includes the following steps:

[0008] (a) Add metal chloride, polyvinylpyrrolidone, and metal oxide powder to water, stir continuously until homogeneous to obtain a precursor suspension, and centrifuge it to obtain the precipitate as the precursor material.

[0009] (b) The precursor material described in step (a) is cleaned, dried, and then calcined at high temperature in a hydrogen atmosphere to obtain a metal single-atom photocatalyst.

[0010] According to the present invention, in step (a), the metal chloride may be one or more of nickel chloride hexahydrate, copper chloride dihydrate and palladium chloride, preferably nickel chloride hexahydrate; the metal oxide is preferably a semiconductor metal oxide, which may be one or more of tungsten trioxide, molybdenum trioxide, titanium dioxide and bismuth oxide, more preferably tungsten trioxide.

[0011] According to the present invention, in step (a), the mass concentration of the metal oxide in the precursor suspension can be 5-10 g / L, preferably 8-10 g / L; the molar ratio of the metal chloride to the metal oxide can be 1:200-1:400, preferably 1:200-1:300; and the mass ratio of the metal chloride to polyvinylpyrrolidone can be 1:2-1:10, preferably 1:2-1:8.

[0012] According to the present invention, in step (a), the molecular weight of the polyvinylpyrrolidone is preferably 30,000 to 80,000, more preferably 50,000 to 60,000; the particle size distribution of the metal oxide powder is preferably 50 nm to 10 μm, more preferably 500 nm to 5 μm.

[0013] According to the present invention, in step (a), the continuous stirring time can be 5 to 15 hours, preferably 6 to 10 hours. The continuous stirring can be carried out at room temperature.

[0014] According to the present invention, in step (b), the method for cleaning the precursor material can be one or more of centrifugation, alcohol washing, water washing, and ultrasonic methods, preferably using centrifugation and alcohol washing together. In the centrifugation method, the centrifuge speed is preferably 5000–50000 r / min, and the centrifugation time is preferably 1–10 min. The alcohol used in the alcohol washing method is preferably a C1–C5 alcohol, such as methanol, ethanol, propanol, butanol, or pentanol. The combined centrifugation and alcohol washing method can include multiple centrifugations and multiple alcohol washings, for example, one, two, three, four, or five or more times. In the combined centrifugation and alcohol washing method, the alcohol after washing can be separated from the precursor material before centrifuging the precursor material, or the alcohol can be separated directly after centrifugation without separating it from the precursor material.

[0015] According to the present invention, in step (b), the drying temperature of the precursor material can be 50–120°C, for example, 50°C, 60°C, 70°C, 80°C, 90°C, 100°C, 110°C, or 120°C, preferably 70–100°C, and the drying time of the precursor material can be 1–30 h, preferably 5–20 h. Preferably, the precursor material is vacuum dried, and the vacuum degree of the vacuum drying is preferably less than 5 Pa.

[0016] According to the present invention, in step (b), the temperature for high-temperature calcination is preferably 300-600°C, more preferably 500-600°C, and the time for high-temperature calcination is preferably 1-10 hours, more preferably 2-4 hours.

[0017] The metal single-atom photocatalyst prepared in this invention is an oxygen-deficient semiconductor-supported metal single-atom photocatalyst. This invention selects a semiconductor metal oxide as the photocatalytic material substrate, introduces oxygen vacancies to induce localized surface plasmon resonance effects, enhances the material's ability to be excited by light to improve the catalytic activity of the photocatalytic material, and introduces metal single atoms on the substrate surface to improve the selectivity of ammonia synthesis.

[0018] Secondly, the present invention provides a metal single-atom photocatalyst, which is prepared according to the preparation method of the first aspect.

[0019] The oxygen-deficient semiconductor-supported metal single-atom photocatalyst substrate prepared according to the method of the first aspect is a metal oxide, wherein the metal single atoms are uniformly loaded on the surface of the metal oxide. In the metal single-atom photocatalyst, the presence of oxygen vacancies induces a localized surface plasmon resonance effect, which enhances the photoexcitation ability of the metal oxide in the visible light region, thus exhibiting good photoexcitation ability and enabling it to be used for photocatalytic reduction reactions.

[0020] Thirdly, this invention proposes the application of the aforementioned metal single-atom photocatalyst in the photocatalytic reduction of nitrate to ammonia.

[0021] The application of the metal single-atom photocatalyst of the present invention in the photocatalytic reduction of nitrate to ammonia synthesis includes the following steps: adding the metal single-atom photocatalyst to an aqueous solution containing nitrate nitrogen, then adding ethylene glycol hole sacrificial agent, and then carrying out a photocatalytic reaction under light source irradiation.

[0022] According to the present invention, the mass concentration of the metal single-atom photocatalyst in the aqueous solution containing nitrate nitrogen is 0.1-0.5 g / L, preferably 0.1-0.2 g / L; the concentration of nitrate nitrogen in the aqueous solution containing nitrate nitrogen, based on nitrogen element, is 10-100 mg / L, preferably 10-20 mg / L; the reaction time is 1-4 h, preferably 1-2 h; and the amount of ethylene glycol hole sacrificial agent added is 5%-20% of the volume of the aqueous solution containing nitrate nitrogen, preferably 8%-15%.

[0023] According to the present invention, the light source can be a xenon lamp or a sodium lamp, preferably a xenon lamp with a power of 50W to 500W.

[0024] The metal single-atom photocatalyst of this invention can synergistically regulate the band structure through the dual active sites of metal single atoms and oxygen vacancies, thereby enhancing the photoexcitation ability of the photocatalyst and synergistically adsorbing nitrate substrates, thus improving the activity and selectivity of the photocatalytic reduction of nitrate to ammonia. The metal single-atom photocatalyst of this invention exhibits good stability and strong adaptability, and can be used to treat water bodies containing different concentrations of nitrate, which is beneficial for preventing water pollution and efficiently producing the high-value-added product ammonia. Attached Figure Description

[0025] Figure 1 This is a high-angle annular dark-field image of the nickel single-atom photocatalyst prepared in Example 1 using scanning transmission electron microscopy. The nickel single-atom photocatalyst is named Ni / H. x WO 3-y The photocatalyst has a structure in which Ni metal within the white circle is uniformly dispersed in the monoclinic H phase in the form of single atoms. x WO 3-y Crystal substrate surface;

[0026] Figure 2 Ni / H prepared in Example 1 x WO 3-y High-resolution transmission electron microscopy dark-field image of the photocatalyst shows a disordered layer about 1 nm thick on the surface due to the presence of oxygen vacancies.

[0027] Figure 3 Ni / H prepared in Example 1 xWO 3-y X-ray diffraction (XRD) pattern of photocatalyst;

[0028] Figure 4 Ni / H prepared in Example 1 x WO 3-y Electron paramagnetic resonance (ESR) spectrum of photocatalyst;

[0029] Figure 5 Ni / H prepared in Example 1 x WO 3-y UV-Vis absorption spectrum of photocatalyst;

[0030] Figure 6 Raman spectra of the photocatalysts prepared in Examples 1-3 and Comparative Examples 1-2;

[0031] Figure 7 Comparison of ammonia production rates of photocatalysts prepared in Examples 1-3 and Comparative Examples 1-2;

[0032] Figure 8 Ni / H in Example 1 x WO 3-y Results of photocatalytic reduction of nitrate to ammonia synthesis under different species concentrations and time periods;

[0033] Figure 9 Ni / H in Example 1 x WO 3-y Figure showing the results of the cycle stability test of the photocatalyst. Detailed Implementation

[0034] The technical solutions of the present invention will be clearly and completely described below with reference to specific embodiments. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention and should not be construed as limiting the scope of protection of the present invention.

[0035] The main raw materials used are as follows: tungsten trioxide (particle size approximately 2μm, 99.99%), polyvinylpyrrolidone (K29-32, molecular weight 58000), nickel chloride hexahydrate (99.9%), copper chloride dihydrate (99.99%), and palladium chloride (99.99%), all purchased from Shanghai Maclean Biochemical Technology Co., Ltd.

[0036] Example 1

[0037] This embodiment provides an oxygen-deficient tungsten trioxide-supported nickel single-atom (Ni / H) metal. x WO 3-y The photocatalyst is prepared by the following steps:

[0038] (a) 2.05 mg nickel chloride hexahydrate, 9.57 mg polyvinylpyrrolidone and 400 mg tungsten trioxide powder were added to 50 mL of water and stirred continuously for 6 h to mix evenly to obtain a precursor suspension. After centrifugation at 10000 r / min for 4 min, the precipitate was obtained, which is the precursor material.

[0039] (b) The precursor material described in step (a) was cleaned by centrifugation with ethanol at 10000 r / min for 4 min each time, repeated 3 times. The cleaned precursor material was then dried in a vacuum chamber at 60 °C (vacuum degree <5 Pa) for 12 h, and then calcined in a tube furnace at 500 °C for 3 h under H2 atmosphere to obtain a nickel single-atom photocatalyst, named Ni / H. x WO 3-y Photocatalyst, meaning Ni metal supported in single-atom form on a monoclinic H2O phase. x WO 3-y Photocatalyst formed on the surface of a crystal substrate, wherein the H x WO 3-y The crystal substrate represents the monoclinic phase, where x is between 0 and 0.3 and y is between 0 and 0.15.

[0040] For the Ni / H x WO 3-y The photocatalyst was analyzed by scanning transmission electron microscopy using high-angle annular dark-field images, such as... Figure 1 As shown. By Figure 1 It can be seen that the obtained Ni / H x WO 3-y In the photocatalyst, Ni metal is uniformly dispersed in the monoclinic H phase in the form of single atoms. x WO 3-y No aggregation occurred on the surface of the crystal substrate.

[0041] For the Ni / H x WO 3-y Photocatalysts were captured in high-resolution transmission electron microscopy dark-field images, such as... Figure 2 As shown. By Figure 2 It can be seen that the presence of abundant oxygen vacancies causes a disordered layer with a width of less than 1 nm to be generated on the catalyst surface.

[0042] For the Ni / H x WO 3-y The photocatalyst was subjected to X-ray diffraction experiments to analyze its phase structure. The obtained X-ray diffraction (XRD) pattern is shown below. Figure 3 .Depend on Figure 3 As can be seen from the corresponding PDF card, the WO3 substrate is a monoclinic phase and there are no other crystals present.

[0043] For the Ni / H x WO 3-y Electron paramagnetic resonance (ESR) experiments were conducted on the photocatalyst to analyze the concentration of oxygen vacancies in the catalyst. The obtained ESR spectra are shown below. Figure 4 .Depend on Figure 4 It can be seen that a significant signal peak appeared at g = 2.003, indicating that there are abundant oxygen vacancies in the catalyst.

[0044] For the Ni / H x WO 3-y The photocatalyst was subjected to ultraviolet-visible (UV-Vis) absorption spectroscopy, and the obtained spectra are shown below. Figure 5 The presence of oxygen vacancies causes a localized surface plasmon resonance effect in the tungsten trioxide catalyst, resulting in... Figure 5 The spectrum shows obvious surface plasmon resonance peaks, indicating that the catalyst has a strong absorption capacity for visible light, which can improve the photocatalytic activity of the catalyst.

[0045] Example 2

[0046] Except for replacing 2.05 mg of nickel chloride hexahydrate in Example 1 with 1.47 mg of copper chloride dihydrate, the preparation method in this example is the same as in Example 1, yielding a single-atom copper photocatalyst, which is named Cu / H x WO 3-y Photocatalyst.

[0047] Example 3

[0048] Except for replacing 2.05 mg of nickel chloride hexahydrate in Example 1 with 1.53 mg of palladium chloride, the preparation method in this example is the same as in Example 1, yielding a palladium single-atom photocatalyst, which is named Pd / H x WO 3-y Photocatalyst.

[0049] Comparative Example 1

[0050] This comparative example presents a comparative photocatalyst, the preparation method of which includes the following steps:

[0051] (a) Add 9.57 mg of polyvinylpyrrolidone and 400 mg of tungsten trioxide powder to 50 mL of water, stir continuously for 6 h to mix evenly to obtain a precursor suspension, centrifuge at 10000 r / min for 4 min to obtain the precipitate, which is the precursor material.

[0052] (b) The precursor material described in step (a) was cleaned by centrifugation at 10,000 r / min with ethanol for 4 min each time, and repeated 3 times. The cleaned precursor material was then placed in a vacuum chamber at 60 °C (vacuum degree < 5 Pa) and dried for 12 h to obtain the comparative WO3 photocatalyst.

[0053] Comparative Example 2

[0054] Except for the absence of metal chlorides, the preparation method of this comparative example is the same as that of Example 1, yielding comparative H. x WO 3-y Photocatalyst.

[0055] Raman spectroscopy experiments were conducted on the photocatalysts prepared in Examples 1-3 and Comparative Examples 1-2. Their Raman spectra are shown below. Figure 6 .Depend on Figure 6 It can be seen that the peak shape of the catalyst after hydrogen calcination becomes flatter than that before hydrogen calcination, but the peak position remains unchanged. This indicates that the crystallinity of the catalyst decreases after hydrogen calcination, but the crystal form does not change.

[0056] Example 4: Application of photocatalyst in the catalytic reduction of nitrate.

[0057] Photocatalytic ammonia synthesis experiments were conducted using the catalysts described in Examples 1-3 and Comparative Examples 1-2, respectively, including the following steps: At room temperature, 0.2 g / L of photocatalyst was added to 45 mL of an aqueous solution of potassium nitrate with a concentration of 10 mg / L, and then 5 mL of ethylene glycol hole sacrificial agent was added. The photocatalytic reaction was then carried out under irradiation with a 300 W xenon lamp for 1 h.

[0058] The ammonia production rate of the photocatalysts used in Examples 1-3 and Comparative Examples 1-2 for the reduction of nitrate to ammonia is shown in the figure. Figure 7 ,Depend on Figure 7 It can be seen that the metal single-atom photocatalyst of the present invention has a higher ammonia synthesis efficiency compared with the comparative photocatalyst; the Ni / H in Example 1 x WO 3-y The photocatalyst enhances the adsorption and activation of nitrates through the synergistic effect of oxygen vacancies and Ni single atoms, exhibiting the highest ammonia synthesis efficiency of 2.94 mmol g. -1 h -1 .

[0059] Ni / H in Example 1 x WO 3-yThe photocatalytic reduction of nitrate to ammonia was tested using the photocatalyst to determine the content of each product in the system at different time points. The results are shown in Table 8. Table 8 shows that the catalyst converts nitrate at a rate of 3.24 mmol g / L within 1 hour. -1 h -1 The rate of ammonia formation was 2.94 mmol g. -1 h -1 The rate of nitrite formation was 0.06 mmol / g. -1 h -1 The rate of nitrogen generation was 0.12 mmol g. -1 h -1 The total amount of nitrogen species remains constant during the reaction, and the selectivity for ammonia formation reaches 98.3%, achieving high-efficiency and high-selectivity ammonia formation. The formula for calculating the ammonia formation selectivity is (8*2.94) / [(5 / 2)*0.12+8*2.94+2*0.06]×100%≈98.3%.

[0060] Ni / H in Example 1 x WO 3-y The photocatalyst underwent cycle stability testing; the test results are shown below. Figure 9 .Depend on Figure 9 It can be seen that after fifteen rounds of cyclic testing, the rate of ammonia production from nitrate reduction by the photocatalyst did not decrease significantly, indicating that it has good stability.

[0061] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable those skilled in the art to understand the content of the present invention and implement it accordingly. They should not be construed as limiting the scope of protection of the present invention. All equivalent changes or modifications made in accordance with the spirit and essence of the present invention are within the scope of protection of the present invention.

Claims

1. The application of a metal single-atom photocatalyst in the photocatalytic reduction of nitrate to ammonia, wherein the preparation method of the metal single-atom photocatalyst includes the following steps: (a) Add metal chloride, polyvinylpyrrolidone, and metal oxide powder to water, stir continuously until homogeneous to obtain a precursor suspension, and centrifuge the precipitate to obtain the precursor material; the metal chloride is selected from one or more of nickel chloride hexahydrate, copper chloride dihydrate, and palladium chloride; the metal oxide is selected from tungsten trioxide. (b) The precursor material described in step (a) is cleaned, dried, and then calcined at high temperature in a hydrogen atmosphere to obtain a metal single-atom photocatalyst.

2. The application according to claim 1, characterized in that, In step (a), the mass concentration of the metal oxide in the precursor suspension is 5-10 g / L; the molar ratio of the metal chloride to the metal oxide is 1:200-1:400; and the mass ratio of the metal chloride to polyvinylpyrrolidone is 1:2-1:

10.

3. The application according to claim 1, characterized in that, In step (a), the mass concentration of the metal oxide in the precursor suspension is 8-10 g / L; the molar ratio of the metal chloride to the metal oxide is 1:200-1:300; and the mass ratio of the metal chloride to polyvinylpyrrolidone is 1:2-1:

8.

4. The application according to claim 1, characterized in that, In step (a), the molecular weight of the polyvinylpyrrolidone is 30,000 to 80,000; the particle size distribution of the metal oxide powder is 50 nm to 10 μm.

5. The application according to claim 1, characterized in that, In step (a), the molecular weight of the polyvinylpyrrolidone is 50,000 to 60,000; the particle size distribution of the metal oxide powder is 500 nm to 5 μm.

6. The application according to claim 1, characterized in that, In step (b), the method for cleaning the precursor material is one or more of centrifugation, alcohol washing, water washing, and ultrasonic methods.

7. The application according to claim 1, characterized in that, In step (b), the method for cleaning the precursor material is a combination of centrifugation and alcohol washing.

8. The application according to claim 1, characterized in that, The precursor material is dried at a temperature of 50~120℃ for a time of 1~30 h.

9. The application according to claim 1, characterized in that, The precursor material is dried at a temperature of 70~100℃ for a time of 5~20 h.

10. The application according to claim 1, characterized in that, The precursor material is subjected to vacuum drying, wherein the vacuum degree of the vacuum drying is less than 5 Pa.

11. The application according to claim 1, characterized in that, In step (b), the high-temperature calcination temperature is 300~600℃ and the high-temperature calcination time is 1~10 h.

12. The application according to claim 1, characterized in that, In step (b), the high-temperature calcination temperature is 500~600℃ and the high-temperature calcination time is 2~4 h.

13. The application according to any one of claims 1 to 12 further includes the following steps: The metal single-atom photocatalyst was added to an aqueous solution containing nitrate nitrogen, followed by the addition of ethylene glycol hole sacrificial agent, and then a photocatalytic reaction was carried out under light source irradiation.

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