Metal monatomic photocatalyst as well as preparation method and application thereof

Abstract

The invention provides a metal monatomic photocatalyst as well as 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, continuously stirring uniformly to obtain a precursor suspension, and centrifuging the precursor suspension to obtain a precipitate, namely a precursor material; and (b) cleaning the precursor material in the step (a), drying, and carrying out high-temperature calcination in a hydrogen atmosphere to obtain the metal monatomic photocatalyst. The metal monatomic photocatalyst disclosed by the invention can cooperatively regulate and control an energy band structure by virtue of double active sites of metal monatomic and oxygen vacancies, so that the light excitation capability of the photocatalyst is improved, a nitrate substrate is cooperatively adsorbed, and the activity and selectivity of a photocatalytic nitrate reduction synthesis ammonia reaction are improved.

Description

Technical Field

[0001] The present invention relates to the field of photocatalysts, and in particular to an oxygen-deficient semiconductor-loaded metal single atom photocatalyst and a preparation method and application thereof. Background Art

[0002] Ammonia is one of the world's largest chemical products, mainly produced by the traditional Haber-Bosch process, which is extremely energy-intensive. This process requires high temperatures of 400-600°C and high pressures of 20 MPa, and its energy consumption can account for 1% of the world's total annual energy consumption. 2 The annual emission is as high as 450 million tons, which will aggravate global warming and cause natural disasters. Wastewater containing nitrates is widely present all over the world, which will lead to water quality degradation, biodiversity reduction, and cause people to suffer from diseases such as methemoglobinemia and cancer. The current commercial technologies used to treat nitrates in wastewater have many disadvantages such as complex operation and high cost, such as ion exchange method and microbial conversion method. Nitrate-containing wastewater often contains alcohol and aldehyde organic pollutants, which can be used to eliminate holes in catalyst materials during the photocatalytic process and improve the photocatalytic efficiency. As an important part of energy conversion and environmental protection, the use of solar light to catalytically convert nitrates and organic pollutants in wastewater to synthesize high value-added products such as ammonia under mild environmental conditions is a very meaningful and application-worthy work.

[0003] CN 109292899A relates to a catalyst for photocatalytic reduction to remove nitrate nitrogen from water and an application method thereof. The method is to prepare an ion-induced oxygen-deficient perovskite-like catalyst by a hydrothermal method for highly selective synthesis of nitrogen. The catalyst prepared by the method has good stability and can be used to remove nitrate nitrogen from water and generate nitrogen, but its disadvantages are high preparation cost, unsafe preparation method, low metal atom utilization rate, and low added value of product nitrogen.

[0004] The kinetic rate of the photocatalytic reduction of nitrate to synthesize ammonia is low, the intermediate products and mechanism of the ammonia synthesis process are unclear, the by-products are complex, and the selectivity is low. Selecting single-atom active sites with the highest atomic utilization as active centers can reduce the amount of metal used and reduce costs. At the same time, highly unsaturated single-atom sites are more likely to adsorb substrates, adjust the reaction path, and improve catalyst activity. At present, designing the fine structure of photocatalyst materials, improving the utilization of active sites, improving the ability of photocatalysts to be excited by light, and improving the synthesis efficiency and selectivity of high value-added products are still the efforts of technicians in this field. Summary of the invention

[0005] The present invention provides a metal single atom photocatalyst and a preparation method and application thereof, including the following aspects.

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

[0007] The method for preparing the metal single atom photocatalyst of the present invention comprises the following steps:

[0008] (a) adding metal chloride, polyvinyl pyrrolidone and metal oxide powder into water, stirring continuously to obtain a precursor suspension, and centrifuging the suspension to obtain a precipitate which is the precursor material;

[0009] (b) cleaning the precursor material described in step (a), drying it, and then calcining it 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, cupric 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; the mass ratio of the metal chloride to polyvinyl pyrrolidone 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 polyvinyl pyrrolidone 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 may be 5 to 15 hours, preferably 6 to 10 hours. The continuous stirring may be performed 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 method, preferably centrifugation and alcohol washing method are used together, the speed of the centrifuge in the centrifugation method is preferably 5000-50000 r / min, and the centrifugation time is preferably 1-10 min; the alcohol in the alcohol washing method is preferably C 1 ~C 5The alcohol used in the centrifugation and alcohol washing method may include performing multiple centrifugation and multiple alcohol washing methods, for example, once, twice, three times, four times or five times or more. In the centrifugation and alcohol washing method used in the centrifugation and alcohol washing method, the precursor material may be centrifuged after the alcohol after alcohol washing is separated from the precursor material, or the alcohol may be directly centrifuged without being separated from the precursor material and then separated out.

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

[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 h, more preferably 2-4 h.

[0017] The metal single atom photocatalyst prepared by the present invention is an oxygen-deficient semiconductor-supported metal single atom photocatalyst. The present invention selects semiconductor metal oxide as a photocatalytic material substrate, introduces oxygen defects to induce the localized surface plasmon resonance effect, enhances the ability of the material 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 synthetic ammonia.

[0018] In a second aspect, 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 by the preparation method of the first aspect is a metal oxide, wherein the metal single atoms are uniformly supported on the surface of the metal oxide. In the metal single-atom photocatalyst, due to the presence of oxygen vacancies, a localized surface plasmon resonance effect is induced, which enhances the photoexcitation ability of the metal oxide in the visible light region, has good photoexcitation ability, and can be used for photocatalytic reduction reactions.

[0020] In a third aspect, the present invention proposes the use of the above-mentioned metal single-atom photocatalyst in the photocatalytic reduction of nitrate to synthesize ammonia.

[0021] The application of the metal single atom photocatalyst of the present invention in the photocatalytic reduction of nitrate to synthesize ammonia comprises 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 performing a photocatalytic reaction under irradiation of a light source.

[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; in terms of nitrogen element, the concentration of the nitrate nitrogen in the aqueous solution containing nitrate nitrogen is 10-100 mg / L, preferably 10-20 mg / L; the reaction time is 1-4 hours, preferably 1-2 hours; the amount of the 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 may 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 the present invention can rely on the dual active sites of the metal single atom and the oxygen vacancy to coordinately regulate the energy band structure, improve the light excitation ability of the photocatalyst, and coordinately adsorb the nitrate substrate to improve the activity and selectivity of the photocatalytic nitrate reduction reaction to synthesize ammonia. The metal single-atom photocatalyst of the present invention has good stability and strong adaptability, and can be used to treat water bodies containing nitrates of different concentrations, which is beneficial to preventing and controlling water pollution and efficiently producing high-value-added product ammonia. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is a high-angle annular dark field image of a scanning transmission electron microscope of a metal nickel single-atom photocatalyst prepared in Example 1. The metal nickel single-atom photocatalyst is named Ni / H x WO 3-y The photocatalyst has a structure in which the Ni metal in the white circle is uniformly dispersed in the monoclinic H x WO 3-y Crystal basal 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. Due to the presence of oxygen vacancies, a disordered layer of about 1 nm thick was observed on the surface.

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

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

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

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

[0031] Figure 7 A comparison chart of ammonia production rates of the photocatalysts prepared in Example 1-3 and Comparative Example 1-2;

[0032] Figure 8 is the Ni / H of Example 1 x WO 3-y The test results of photocatalytic reduction of nitrate to synthesize ammonia under different species concentrations and different time;

[0033] Fig. 9 is the Ni / H of Example 1 x WO 3-y Figure 2. Cyclic stability test results of photocatalyst. DETAILED DESCRIPTION

[0034] The following will be combined with specific embodiments to clearly and completely describe the technical solutions in the present invention. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention, and cannot be used to limit the protection scope of the present invention.

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

[0036] Example 1

[0037] This embodiment provides an oxygen-deficient tungsten trioxide loaded with nickel metal atoms (Ni / H x WO 3-y ) photocatalyst, the preparation method thereof comprises the following steps:

[0038] (a) Add 2.05 mg of nickel chloride hexahydrate, 9.57 mg of polyvinyl pyrrolidone, and 400 mg of tungsten trioxide powder to 50 mL of water, stir for 6 h to obtain a precursor suspension, and centrifuge at 10,000 r / min for 4 min to obtain a precipitate, which is the precursor material;

[0039] (b) The precursor material described in step (a) was cleaned by centrifugation at 10000 r / min with ethanol, each centrifugation time was 4 min, and the cleaned precursor material was placed in a 60° C. vacuum box (vacuum degree <5 Pa) for 12 h, and then placed in a tube furnace at H 2 The metal nickel single atom photocatalyst was obtained by calcination at 500 °C for 3 h in an atmosphere and named Ni / H x WO 3-y Photocatalyst, which means that Ni metal is supported in the form of single atoms on the monoclinic H x WO 3-y The photocatalyst formed on the surface of the crystal substrate, the H x WO 3-y Represents a crystal base of a 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 photocatalysts were analyzed by scanning transmission electron microscopy high-angle annular dark field imaging, such as Figure 1 As shown. Figure 1 It can be seen that the 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 base.

[0041] For the Ni / H x WO 3-y The photocatalyst was imaged using a high-resolution transmission electron microscope dark field image. Figure 2 As shown. 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 form on the catalyst surface.

[0042] For the Ni / H x WO 3-y The photocatalyst was subjected to an X-ray diffraction test to analyze the phase structure of the catalyst. The obtained X-ray diffraction (XRD) pattern is shown in Figure 3 .Depend on Figure 3 From the corresponding PDF card, we can see that WO 3 The base is a monoclinic phase, and no other crystals exist.

[0043] For the Ni / H x WO 3-y The photocatalyst was subjected to an electron paramagnetic resonance (ESR) test to analyze the concentration of oxygen vacancies in the catalyst. The obtained electron paramagnetic resonance (ESR) spectrum is shown in Figure 4 .Depend on Figure 4 It can be seen that an obvious signal peak appears 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 tested by UV-Vis absorption spectroscopy, and the obtained spectrum is shown in Figure 5 The presence of oxygen vacancies will cause the tungsten trioxide catalyst to produce a localized surface plasmon resonance effect. Figure 5 It can be seen that an obvious domain surface plasmon resonance peak appears in the spectrum, indicating that the catalyst has a strong absorption ability for visible light, thereby improving the photocatalytic activity of the catalyst.

[0045] Example 2

[0046] The preparation method of this embodiment is the same as that of Example 1, except that 1.47 mg of cupric chloride dihydrate is used instead of 2.05 mg of nickel chloride hexahydrate in Example 1, to obtain a metallic copper single-atom photocatalyst, which is named Cu / H x WO 3-y Photocatalyst.

[0047] Example 3

[0048] The preparation method of this embodiment is the same as that of embodiment 1, except that 1.53 mg of palladium chloride is used instead of 2.05 mg of nickel chloride hexahydrate in embodiment 1, to obtain a metal palladium single atom photocatalyst, which is named Pd / H x WO 3-y Photocatalyst.

[0049] Comparative Example 1

[0050] This comparative example proposes a comparative photocatalyst, and its preparation method comprises the following steps:

[0051] (a) Add 9.57 mg of polyvinyl pyrrolidone and 400 mg of tungsten trioxide powder to 50 mL of water, stir for 6 h to mix evenly to obtain a precursor suspension, and centrifuge at 10,000 r / min for 4 min to obtain a precipitate, which is the precursor material;

[0052] (b) The precursor material described in step (a) was cleaned by centrifugation at 10000 r / min using ethanol, and each centrifugation time was 4 min, and the cleaned precursor material was placed in a 60° C. vacuum box (vacuum degree <5 Pa) and dried for 12 h to obtain a comparative WO 3 Photocatalyst.

[0053] Comparative Example 2

[0054] Except that no metal chloride is added, the preparation method of this comparative example is the same as that of Example 1 to obtain comparative H x WO 3-y Photocatalyst.

[0055] The photocatalysts prepared in Examples 1-3 and Comparative Examples 1-2 were subjected to Raman spectroscopy tests, and their Raman spectra are shown in FIG. Figure 6 .Depend on Figure 6 It can be seen that the peak shape of the catalyst after hydrogen calcination becomes flatter than that of the catalyst before hydrogen calcination, but the peak position remains unchanged, indicating that the crystallinity of the catalyst after hydrogen calcination tends to decrease, but the crystal form does not change.

[0056] Example 4 Application of Photocatalyst for Catalytic Reduction of Nitrate

[0057] The photocatalytic ammonia synthesis test was carried out using the catalysts described in Examples 1-3 and Comparative Examples 1-2, respectively, comprising the following steps: at room temperature, 0.2 g / L of the 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, and then a photocatalytic reaction was carried out under irradiation with a 300 W xenon lamp for 1 hour.

[0058] The results of the ammonia production rate of the photocatalysts of Examples 1-3 and Comparative Examples 1-2 for nitrate reduction and ammonia synthesis are shown in 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 than the comparative photocatalyst; the Ni / H x WO 3-y The photocatalyst can enhance the adsorption and activation of nitrate through the synergistic effect of oxygen vacancies and Ni atoms, which is manifested as the highest ammonia synthesis efficiency of 2.94mmol g -1 h -1 .

[0059] For Ni / H in Example 1 x WO 3-yThe photocatalyst was used to perform a photocatalytic reduction of nitrate to synthesize ammonia in order to detect the content of each product in the system at different times. The test results are shown in Table 8. As can be seen from Table 8, the rate of nitrate conversion by the catalyst within 1 h is 3.24 mmol g -1 h -1 The rate of ammonia generation is 2.94 mmol g -1 h -1 The rate of nitrite generation is 0.06mmolg -1 h -1 The rate of nitrogen generation is 0.12 mmol g -1 h -1 During the reaction, the total amount of nitrogen species remains unchanged, and the selectivity of ammonia generation can reach 98.3%, achieving high efficiency and high selectivity in ammonia generation. The calculation formula for ammonia generation selectivity is (8*2.94) / [(5 / 2)*0.12+8*2.94+2*0.06]×100%≈98.3%.

[0060] For the Ni / H of Example 1 x WO 3-y The photocatalyst is undergoing cycle stability testing. The test results are shown in Fig. 9 .Depend on Fig. 9 It can be seen that after fifteen rounds of cycle testing, the rate of ammonia production by reducing nitrate by the photocatalyst did not decrease significantly, indicating good stability.

[0061] The above embodiments are only for illustrating the technical concept and features of the present invention, and are intended to enable people familiar with the technology to understand the content of the present invention and implement it accordingly, and cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention are within the protection scope of the present invention.

Claims

1. A method for preparing a metal single atom photocatalyst, comprising the following steps: (a) adding metal chloride, polyvinyl pyrrolidone and metal oxide powder into water, stirring continuously to obtain a precursor suspension, and centrifuging the suspension to obtain a precipitate which is the precursor material; (b) cleaning the precursor material described in step (a), drying it, and then calcining it at high temperature in a hydrogen atmosphere to obtain a metal single atom photocatalyst.

2. The preparation method according to claim 1, characterized in that: In step (a), the metal chloride is selected from one or more of nickel chloride hexahydrate, cupric chloride dihydrate and palladium chloride; the metal oxide is selected from semiconductor metal oxides (preferably one or more of tungsten trioxide, molybdenum trioxide, titanium dioxide and bismuth oxide).

3. The preparation method according to claim 1, characterized in that: In step (a), the mass concentration of the metal oxide in the precursor suspension is 5 to 10 g / L (preferably 8 to 10 g / L); the molar ratio of the metal chloride to the metal oxide is 1:200 to 1:400 (preferably 1:200 to 1:300); the mass ratio of the metal chloride to polyvinyl pyrrolidone is 1:2 to 1:10 (preferably 1:2 to 1:8).

4. The preparation method according to claim 1, characterized in that: In step (a), the molecular weight of the polyvinyl pyrrolidone is 30,000 to 80,000 (preferably 50,000 to 60,000); the particle size distribution of the metal oxide powder is 50 nm to 10 μm (preferably 500 nm to 5 μm).

5. The preparation method 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 method (preferably, centrifugation and alcohol washing are used together).

6. The preparation method according to claim 1, characterized in that: The temperature for drying the precursor material is 50 to 120° C. (preferably 70 to 100° C.), and the time for drying the precursor material is 1 to 30 hours (preferably 5 to 20 hours).

7. The preparation method according to claim 1, characterized in that: The precursor material is vacuum dried, and the vacuum degree of the vacuum drying is less than 5Pa.

8. The preparation method according to claim 1, characterized in that: In step (b), the high temperature calcination is carried out at a temperature of 300 to 600° C. (preferably 500 to 600° C.) and for a time of 1 to 10 h (preferably 2 to 4 h).

9. A metal single atom photocatalyst, wherein the metal single atom photocatalyst is prepared according to the preparation method according to any one of claims 1 to 8.

10. Use of the metal single atom photocatalyst prepared by the preparation method according to any one of claims 1 to 8 in the photocatalytic reduction of nitrate to synthesize ammonia, comprising the following steps: The metal single atom photocatalyst is added into an aqueous solution containing nitrate nitrogen, and then an ethylene glycol hole sacrificial agent is added, and then a photocatalytic reaction is carried out under the irradiation of a light source.

Images