A phosphate and oxygen vacancy co-modified photocatalyst and a preparation method and application thereof

Abstract

The application discloses a phosphate and oxygen vacancy co-modified photocatalyst, surface oxygen vacancies and phosphate sites are simultaneously constructed on TiO2 nanosheets, water oxidation and oxygen reduction in methane photocatalysis are promoted, O2 reduction and H2O oxidation two ways simultaneously generate ·OH, so that methane photocatalytic conversion into liquid oxidants can be effectively and highly selectively realized, the yield is increased by 2.5-2.8 times compared with that of titanium dioxide with only surface oxygen vacancies or only phosphate modification, and the selectivity can reach 94.2%.

Description

Technical Field

[0001] This invention relates to the field of photocatalyst technology, and in particular to a photocatalyst co-modified with phosphate and oxygen vacancies, its preparation method, and its application. Background Technology

[0002] Currently, with the development of industrial technology, greenhouse gas emissions are constantly increasing, exacerbating the greenhouse effect. Besides carbon dioxide, methane is also a major greenhouse gas. According to the latest statistics from the International Energy Agency, my country's methane emissions reached 55.6761 million tons in 2022, accounting for 15.65% of global emissions.

[0003] Meanwhile, methane is also an important fossil fuel and industrial raw material, widely found in natural gas, shale gas, methane hydrates, and other substances, and can be used to produce a variety of high-value-added chemicals. Therefore, converting methane into other valuable chemical substances is of paramount importance for solving both environmental and energy problems.

[0004] However, the highly symmetrical molecular structure of methane makes the activation of the CH bond extremely difficult. In traditional processes, the conversion of methane typically requires harsh conditions such as high temperature, high pressure, and strong oxidants, leading to enormous energy consumption and carbon dioxide emissions. The efficient conversion of methane under mild conditions is considered the "holy grail" reaction in the field of catalysis.

[0005] Solar-driven methane conversion, powered by renewable energy, holds great potential to address the aforementioned challenges. Numerous studies have reported on photocatalytic methane conversion. To achieve higher product yields, H₂O₂ is often added as an oxidant to the reaction system; however, H₂O₂ is more expensive than the products obtained. Therefore, using inexpensive oxygen or water instead of these costly oxidants can significantly improve the economics of photocatalytic methane conversion. Previous work has also demonstrated that constructing oxygen vacancies on metal oxides facilitates the chemisorption of O₂, thereby enhancing the performance of photocatalytic methane conversion.

[0006] Furthermore, it has been recognized that reactive oxygen species play a crucial role in the aerobic conversion of CH4. In order to regulate the formation of reactive oxygen species, noble metal co-catalysts such as Au, Ag and Pd are usually introduced on semiconductors, which increases the reaction cost. Summary of the Invention

[0007] This invention aims to at least solve one of the aforementioned technical problems existing in the prior art. Therefore, the purpose of this invention is to provide a photocatalyst co-modified with phosphate and oxygen vacancies, its preparation method, and its application. Using a surface co-modification strategy, surface oxygen vacancies and phosphate sites are simultaneously constructed on TiO2 nanosheets, promoting the catalyst's oxidation of water and reduction of oxygen. This achieves the simultaneous generation of ·OH through both O2 reduction and H2O oxidation pathways, thereby effectively and selectively converting methane into liquid oxides via photocatalysis.

[0008] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0009] The first aspect of the present invention provides a method for preparing a photocatalyst.

[0010] A second aspect of the present invention provides a photocatalyst.

[0011] A third aspect of the invention proposes an application of a photocatalyst.

[0012] According to a first aspect of the present invention, a method for preparing a photocatalyst is provided, comprising the following steps:

[0013] Sodium hypophosphite is used as the phosphate source and heated and annealed together with titanium dioxide nanosheets in the absence of oxygen, so that the surface of the titanium dioxide nanosheets is modified with phosphate and oxygen vacancies; wherein the titanium dioxide nanosheets are anatase type.

[0014] In some embodiments of the present invention, the mass ratio of sodium hypophosphite to titanium dioxide nanosheets is 8 to 15:1, preferably 10 to 15:1.

[0015] In some embodiments of the present invention, the preparation is carried out in an inert atmosphere in a tube furnace, and the titanium dioxide nanosheets are placed downstream of the phosphate source with a spacing of 5 cm to 8 cm, preferably 6 to 7 cm, and more preferably 7 cm; the inert atmosphere is selected from nitrogen, helium, and argon, with nitrogen being preferred.

[0016] In this invention, sodium hypophosphite decomposes upon heating to produce PH3. Titanium dioxide is placed downstream of PH3, and PH3 is blown toward the titanium dioxide by passing an inert gas, so that the surface of the titanium dioxide is successfully modified with phosphate and oxygen vacancies. If the distance is too close, the sodium hypophosphite may be blown directly onto the surface of the titanium dioxide by the gas flow, while if the distance is too far, the PH3 produced by the decomposition may not be able to react with the titanium dioxide.

[0017] In some embodiments of the present invention, the heating temperature is 550K to 600K, preferably 570K to 580K.

[0018] In some embodiments of the present invention, the heating time is 0.5h to 1h.

[0019] In some embodiments of the present invention, the annealing time is 1 to 2 hours, preferably 1 to 1.5 hours.

[0020] In some preferred embodiments of the present invention, the annealing process further includes: cooling, washing, and vacuum drying; the washing uses ultrapure water and is performed 3 to 5 times; the vacuum drying temperature is 320K to 335K and the time is 10h to 13h.

[0021] In some preferred embodiments of the present invention, the method for preparing the titanium dioxide nanosheets includes:

[0022] HF is mixed with Ti(OBu)4, reacted, and the precipitate is collected to obtain the product.

[0023] In this invention, TiO2 in the form of nanosheets has a larger surface area than TiO2 in other forms such as nanowires and nanoflowers, making it easier to modify for co-catalysts and adsorb more reaction substrates.

[0024] In some preferred embodiments of the present invention, the concentration of HF is 40 wt%, and the volume ratio of HF to Ti(OBu)4 is 1:(7-9).

[0025] In some preferred embodiments of the present invention, the mixing includes stirring at 25°C to 30°C for 2 to 3 hours.

[0026] In some preferred embodiments of the present invention, the reaction is carried out in an autoclave at a temperature of 450K to 470K for a time of 35h to 40h.

[0027] In some preferred embodiments of the present invention, the collection of the precipitate further includes purification and vacuum drying. The purification includes: dispersing the precipitate in a sodium hydroxide solution, stirring for 12-15 hours, and washing with ultrapure water until neutral.

[0028] In some preferred embodiments of the present invention, the vacuum drying temperature is 320K to 335K and the time is 10h to 13h.

[0029] According to a second aspect of the present invention, a photocatalyst prepared by the preparation method of the first aspect is provided, comprising titanium dioxide nanosheets co-modified with phosphate and oxygen vacancy surfaces.

[0030] In some embodiments of the present invention, the thickness of the titanium dioxide nanosheets is 5 nm to 7 nm.

[0031] In some embodiments of the present invention, the phosphate content of the photocatalyst is 0.8% to 1.3%.

[0032] According to a third aspect of the present invention, an application of a photocatalyst prepared by the method described in the first aspect in the catalytic conversion of methane is proposed.

[0033] In some embodiments of the present invention, the methane is converted into a liquid oxide.

[0034] In some embodiments of the present invention, the liquid oxide includes methanol, peroxymethanol, formaldehyde, and formic acid, preferably formaldehyde.

[0035] The beneficial effects of this invention are:

[0036] This invention simultaneously constructs surface oxygen vacancies and phosphate sites on TiO2 nanosheets, promoting the oxidation of water and reduction of oxygen in methane photocatalysis. This enables the simultaneous generation of ·OH through both O2 reduction and H2O oxidation pathways, thereby effectively and selectively converting methane into liquid oxides via photocatalysis. The yield is 2.5 to 2.8 times higher than that of titanium dioxide modified with only surface oxygen vacancies or only phosphate sites, and the selectivity of total liquid oxides can reach 94.2%.

[0037] The preparation method of this invention is simple, the raw materials are inexpensive and readily available, and it is suitable for large-scale production applications. Attached Figure Description

[0038] Figure 1 The X-ray diffraction patterns are of the photocatalysts prepared in Example 1, Comparative Example 1, and Comparative Example 2 of this invention.

[0039] Figure 2 TEM, HRTEM, FFT, and HAADF-STEM images of the photocatalyst prepared in Example 1 of this invention;

[0040] Figure 3 TEM, HRTEM, and FFT images of the photocatalysts prepared in Comparative Examples 1 and 2 of this invention;

[0041] Figure 4 The FT-IR spectra of the photocatalysts prepared in Example 1, Comparative Example 1, and Comparative Example 2 of this invention are shown below.

[0042] Figure 5 The results show the products, yields, and selectivity of liquid products of the photocatalysts prepared in Examples 1, 1, and 2 of this invention for the photocatalytic conversion of methane.

[0043] Figure 6 The results show the products, yields, and selectivity of liquid products of the photocatalysts prepared in Examples 1, 4, 3, and 4 of this invention for photocatalytic methane conversion.

[0044] Figure 7The transient photocurrent response test results are for the photocatalysts prepared in Example 1, Comparative Example 1, and Comparative Example 2 of this invention.

[0045] Figure 8 The electrochemical impedance spectroscopy spectra of the photocatalysts prepared in Example 1, Comparative Example 1, and Comparative Example 2 of this invention under light irradiation conditions are shown.

[0046] Figure 9 The time-resolved photoluminescence spectra of the photocatalysts prepared in Example 1, Comparative Example 1, and Comparative Example 2 of this invention are shown below.

[0047] Figure 10 The open-circuit voltage decay diagrams and converted OCVD curves of the photocatalysts prepared in Example 1, Comparative Example 1, and Comparative Example 2 of this invention are shown.

[0048] Figure 11 The CV curves of the photocatalysts prepared in Example 1, Comparative Example 1, and Comparative Example 2 of this invention are shown.

[0049] Figure 12 The ·OH in-situ DMPO spin trapping EPR spectra of the photocatalysts prepared in Example 1, Comparative Example 1, and Comparative Example 2 of this invention under an O2 atmosphere;

[0050] Figure 13 The in-situ Raman spectra of TiO2-H2 prepared in Comparative Example 2 and TiO2-P prepared in Example 1 in water under different irradiation times are shown. Detailed Implementation

[0051] The present invention will be further described in detail below through specific embodiments. Unless otherwise specified, the raw materials, reagents, or apparatus used in the embodiments and comparative examples are all available from conventional commercial sources or can be obtained by existing technical methods. Unless otherwise specified, the test or experimental methods are conventional methods in the art.

[0052] Example 1

[0053] This embodiment prepares a photocatalyst (TiO2-P) co-modified with phosphate and oxygen vacancies. The specific process is as follows:

[0054] 3.0 mL of hydrofluoric acid (HF, 40 wt%) was added dropwise to 25.0 mL of tetrabutyl titanate (Ti(OBu)4), and the mixture was stirred at 25 °C for 2 hours. The solution was then transferred to a 50 mL Teflon autoclave and heated at 453 K for 36 hours. After cooling to 25 °C, the mixture was washed repeatedly by centrifugation with ethanol and ultrapure water, and the white precipitate was collected. The white precipitate was then dispersed in 0.1 M NaOH solution and stirred continuously for 12 hours to remove residual fluoride ions. Finally, the mixture was washed repeatedly by centrifugation with ultrapure water until the supernatant became neutral, and then dried under vacuum at 333 K for 12 hours to obtain TiO2 nanosheets.

[0055] Sodium hypophosphite (NaH₂PO₂) was placed in a tube furnace, and 80 mg of the previously obtained TiO₂ nanosheets were placed downstream of NaH₂PO₂, at a distance of approximately 7 cm. The amount of NaH₂PO₂ was controlled at 800 mg, and the mass ratio of NaH₂PO₂ to TiO₂ was 10:1. The mixture was heated to 573 K under a N₂ atmosphere, annealed for 1 h, cooled to room temperature (25 °C), washed three times with ultrapure water by centrifugation, and dried under vacuum at 333 K for 12 h to obtain the photocatalyst co-modified with phosphate and oxygen vacancies.

[0056] Example 2

[0057] This embodiment prepares a photocatalyst co-modified with phosphate and oxygen vacancies, and the specific process is as follows:

[0058] 4.0 mL of hydrofluoric acid (HF, 40 wt%) was added dropwise to 25.0 mL of tetrabutyl titanate (Ti(OBu)4), and the mixture was stirred at 25 °C for 2 hours. The solution was then transferred to a 50 mL Teflon autoclave and heated at 453 K for 40 hours. After cooling to 25 °C, the mixture was washed repeatedly by centrifugation with ethanol and ultrapure water, and the white precipitate was collected. The white precipitate was then dispersed in 0.1 M NaOH solution and stirred continuously for 12 hours to remove residual fluoride ions. Finally, the mixture was washed repeatedly by centrifugation with ultrapure water until the supernatant became neutral, and then dried under vacuum at 333 K for 12 hours to obtain TiO2 nanosheets.

[0059] Sodium hypophosphite (NaH₂PO₂) was placed in a tube furnace, and 80 mg of the previously obtained TiO₂ nanosheets were placed downstream of NaH₂PO₂, with a distance of approximately 7 cm between them. The amount of NaH₂PO₂ was controlled at 640 mg, and the mass ratio of NaH₂PO₂ to TiO₂ was 8:1. The mixture was heated to 573 K under a N₂ atmosphere, annealed for 1 h, cooled to room temperature (25 °C), washed three times with ultrapure water by centrifugation, and dried under vacuum at 333 K for 12 h to obtain the photocatalyst co-modified with phosphate and oxygen vacancies.

[0060] Example 3

[0061] This embodiment prepares a photocatalyst co-modified with phosphate and oxygen vacancies, and the specific process is as follows:

[0062] 3.0 mL of hydrofluoric acid (HF, 40 wt%) was added dropwise to 25.0 mL of tetrabutyl titanate (Ti(OBu)4), and the mixture was stirred at 25 °C for 2 hours. The solution was then transferred to a 50 mL Teflon autoclave and heated at 453 K for 36 hours. After cooling to 25 °C, the mixture was washed repeatedly by centrifugation with ethanol and ultrapure water, and the white precipitate was collected. The white precipitate was then dispersed in 0.1 M NaOH solution and stirred continuously for 12 hours to remove residual fluoride ions. Finally, the mixture was washed repeatedly by centrifugation with ultrapure water until the supernatant became neutral, and then dried under vacuum at 333 K for 12 hours to obtain TiO2 nanosheets.

[0063] Sodium hypophosphite (NaH₂PO₂) was placed in a tube furnace, and 80 mg of the previously obtained TiO₂ nanosheets were placed downstream of NaH₂PO₂, with a distance of approximately 6 cm between them. The amount of NaH₂PO₂ was controlled at 960 mg, and the mass ratio of NaH₂PO₂ to TiO₂ was 12:1. The mixture was heated to 573 K under a N₂ atmosphere, annealed for 1 h, cooled to room temperature (25 °C), washed three times with ultrapure water by centrifugation, and dried under vacuum at 333 K for 12 h to obtain the photocatalyst co-modified with phosphate and oxygen vacancies.

[0064] Example 4

[0065] This embodiment prepares a photocatalyst co-modified with phosphate and oxygen vacancies, and the specific process is as follows:

[0066] 3.0 mL of hydrofluoric acid (HF, 40 wt%) was added dropwise to 25.0 mL of tetrabutyl titanate (Ti(OBu)4), and the mixture was stirred at 25 °C for 2 hours. The solution was then transferred to a 50 mL Teflon autoclave and heated at 453 K for 36 hours. After cooling to 25 °C, the mixture was washed repeatedly by centrifugation with ethanol and ultrapure water, and the white precipitate was collected. The white precipitate was then dispersed in 0.1 M NaOH solution and stirred continuously for 12 hours to remove residual fluoride ions. Finally, the mixture was washed repeatedly by centrifugation with ultrapure water until the supernatant became neutral, and then dried under vacuum at 333 K for 12 hours to obtain TiO2 nanosheets.

[0067] Sodium hypophosphite (NaH₂PO₂) was placed in a tube furnace, and 80 mg of the previously obtained TiO₂ nanosheets were placed downstream of NaH₂PO₂, with a distance of approximately 6 cm between them. The amount of NaH₂PO₂ was controlled at 1200 mg, and the mass ratio of NaH₂PO₂ to TiO₂ was 15:1. The mixture was heated to 573 K under a N₂ atmosphere, annealed for 1 h, cooled to room temperature (25 °C), washed three times with ultrapure water by centrifugation, and dried under vacuum at 333 K for 12 h to obtain the photocatalyst co-modified with phosphate and oxygen vacancies.

[0068] Comparative Example 1

[0069] This comparative example prepared a photocatalyst (TiO2-O2), and the specific process is as follows:

[0070] 3.0 mL of hydrofluoric acid (HF, 40 wt%) was added dropwise to 25.0 mL of tetrabutyl titanate (Ti(OBu)4), and the mixture was stirred at 25 °C for 2 hours. The solution was then transferred to a 50 mL Teflon autoclave and heated at 453 K for 36 hours. After cooling to 25 °C, the mixture was washed repeatedly by centrifugation with ethanol and ultrapure water, and the white precipitate was collected. The white precipitate was then dispersed in 0.1 M NaOH solution and stirred continuously for 12 hours to remove residual fluoride ions. Finally, the mixture was washed repeatedly by centrifugation with ultrapure water until the supernatant became neutral, and then dried under vacuum at 333 K for 12 hours to obtain TiO2 nanosheets.

[0071] The prepared TiO2 nanosheets were annealed at 573 K for 1 h in an O2 atmosphere, cooled to room temperature (25 °C), washed three times by centrifugation with ultrapure water, and dried under vacuum at 333 K for 12 h to obtain the final product.

[0072] Comparative Example 2

[0073] This comparative example prepared a photocatalyst (TiO2-H2), and the specific process is as follows:

[0074] 3.0 mL of hydrofluoric acid (HF, 40 wt%) was added dropwise to 25.0 mL of tetrabutyl titanate (Ti(OBu)4), and the mixture was stirred at 25 °C for 2 hours. The solution was then transferred to a 50 mL Teflon autoclave and heated at 453 K for 36 hours. After cooling to 25 °C, the mixture was washed repeatedly by centrifugation with ethanol and ultrapure water, and the white precipitate was collected. The white precipitate was then dispersed in 0.1 M NaOH solution and stirred continuously for 12 hours to remove residual fluoride ions. Finally, the mixture was washed repeatedly by centrifugation with ultrapure water until the supernatant became neutral, and then dried under vacuum at 333 K for 12 hours to obtain TiO2 nanosheets.

[0075] The prepared TiO2 nanosheets were annealed at 573K for 1 h in an H2 / Ar atmosphere, cooled to room temperature (25℃), washed three times by centrifugation with ultrapure water, and dried under vacuum at 333K for 12 h to obtain the final product.

[0076] Comparative Example 3

[0077] This comparative example prepared a photocatalyst, which differs from Example 1 in that the mass ratio of NaH2PO2 to TiO2 is 2:1, while the rest is the same as in Example 1.

[0078] Comparative Example 4

[0079] This comparative example prepared a photocatalyst, which differs from Example 1 in that the mass ratio of NaH2PO2 to TiO2 is 5:1, while the rest is the same as in Example 1.

[0080] Test case

[0081] I. Material Characterization

[0082] X-ray diffraction was performed on the photocatalysts prepared in Example 1, Comparative Example 1, and Comparative Example 2, respectively. The results are shown in the figure. Figure 1 .like Figure 1 As shown, the X-ray diffraction pattern shows that the prepared catalysts all have a typical TiO2 anatase phase (JCPDS#21-1272), indicating that no phase transition occurred after heat treatment.

[0083] Figure 2 Images a and b are TEM images of the TiO2-P photocatalyst prepared in Example 1 at scales of 50 nm and 10 nm, respectively. Image c is an HRTEM image (scale bar: 2 nm) and an FFT image (scale bar: 5 nm). -1 Figure d shows the HAADF-STEM image of TiO2-P and the corresponding elemental mapping results (scale bar: 50 nm).

[0084] Figure 3 In the image, a and d correspond to TEM images of the photocatalyst TiO2-O2 prepared in Comparative Example 1 and the photocatalyst TiO2-H2 prepared in Comparative Example 2 at a scale of 50 nm, respectively; b and e correspond to HRTEM images of TiO2-O2 and TiO2-H2 at a scale of 2 nm, respectively; c and f correspond to FFT images of TiO2-O2 and TiO2-H2 at a scale of 5 nm, respectively. -1 ).

[0085] High-resolution transmission electron microscope images ( Figure 2 Images a and b) show the nanosheet structure of TiO2-P, with dimensions between 40 and 60 nm and thicknesses between 4 and 7 nm. Figure 2As shown in Figure c, orthogonal lattice fringes with an interplanar spacing of 0.187 nm corresponding to the {100} plane reveal the main exposed surface (001) of the TiO2-P nanosheets, which is also confirmed by the fast Fourier transform image (see Figure c). Figure 2 c). Similar structures were also observed in TiO2-O2 and TiO2-H2. Figure 3 ).like Figure 2 As shown in d, the results of high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive spectroscopy indicate that Ti, O, and P elements are uniformly distributed on TiO2-P.

[0086] Figure 4 In the image, a represents the FT-IR spectra of TiO2-O2, TiO2-H2, and TiO2-P, and b represents the locally magnified FT-IR spectra. The Fourier transform infrared spectroscopy results confirm the presence of phosphate ions on TiO2-P. From... Figure 4 As can be seen, at 3448cm -1 and 1647cm -1 The strong bands appearing at 1127 and 1020 cm⁻¹ are likely due to surface adsorption of water and hydroxyl groups. -1 The bands can be attributed to PO4 respectively. 3- Symmetrical tensile vibrations of OPO and PO.

[0087] The other embodiments have similar effects to Embodiment 1 above, and will not be described in detail here.

[0088] II. Catalytic performance testing

[0089] The photocatalytic methane conversion performance of the photocatalysts prepared in Examples 1, 4, and Comparative Examples 1-4 was tested.

[0090] Photocatalytic methane conversion was carried out in a 50 mL Teflon high-pressure reactor, which allowed for top illumination. A 300 W xenon lamp was fixed directly above the high-pressure reactor, with the UV intensity set to 50 mW cm⁻¹. -2 In a Teflon liner, 5 mg of photocatalyst was dispersed in 10 ml of ultrapure water. The liner was then transferred to the reactor, sealed, and purged with argon for 1 hour, followed by the introduction of 0.1 MPa oxygen and 1.9 MPa methane. After irradiation at room temperature for 1 hour, the xenon lamp was turned off, and the reactor was placed in an ice-water bath for thorough cooling. The gaseous and liquid products were collected separately.

[0091] Liquid products: Qualitative and quantitative analysis of some liquid products (CH3OH, CH3OOH, and HCOOH) was performed using nuclear magnetic resonance spectroscopy. CH3OOH is peroxymethanol, formed by the combination of CH3 and ·OOH. D2O was used as the deuterium source, and DMSO as the internal standard. 1The H-NMR pressure peak was detected.

[0092] The yield of HCHO was determined by colorimetric method. First, ammonium acetate (15 g), acetic acid (0.3 mL), acetylacetone (0.2 mL), and ultrapure water (100 mL) were thoroughly mixed and dissolved to prepare the HCHO detection solution. Then, 2 mL of the detection solution was mixed with 0.5 mL of the product solution, heated in a water bath for 30 min, and the absorbance at 413 nm was recorded using a U-3900 UV-Vis spectrometer to determine the HCHO concentration.

[0093] Gaseous products: Gaseous products were collected using gas bags and then analyzed by gas chromatography. The concentration of the gaseous products was determined using the external standard method. CO2 was detected using an Agilent 7820A gas chromatograph equipped with an HP-PLOT / Q column and a thermal conductivity detector (TCD). Other gaseous products (such as CO, H2, and multi-carbon gaseous products) were detected using a Shimadzu GC-2014 gas chromatograph equipped with Porapak N, Porapak Q, and MolSieve 5A columns, as well as a flame ionization detector (FID) and a TCD.

[0094] like Figure 5 As shown, the main product of methane photocatalysis by each catalyst was HCHO, and CO and H2 were not detected in the gaseous products. Specifically, the HCHO formation rate of methane photocatalyzed by TiO2-O2 prepared in Comparative Example 1 was 819 μmol g. -1 h -1 The liquid product selectivity was 80.1%. Surface oxygen vacancies increased the rate of HCHO formation from TiO2-H2 prepared in Comparative Example 2 to 917 μmol g. - 1 h -1 However, due to the excessive oxidation of CH4 to CO2, the selectivity of the liquid product remained at around 80%. For the TiO2-P prepared in Example 1, the introduction of phosphate groups on the surface significantly improved the yield of HCHO, reaching 2309 μmol g. -1 h -1 The yields of TiO2-O2 and TiO2-H2 were 2.8 and 2.5 times higher, respectively. Meanwhile, the total yield of liquid products reached 3080 μmol g. -1 h -1 The selectivity was 94.2%. This demonstrates that surface-modified phosphate not only promotes the conversion of methane but also inhibits the excessive oxidation of methane to CO2.

[0095] Figure 6The results show the product yield and liquid product selectivity of the photocatalysts prepared in Examples 1, 4, 3, and 4 for the photocatalytic conversion of methane. It can be seen that the catalyst performance increases within a certain range with the increase of the proportion of NaH2PO2 in the NaH2PO2:TiO2 mass ratio. When the NaH2PO2:TiO2 mass ratio reaches 10:1, the catalyst exhibits optimal performance, while when the ratio increases to 15:1, the catalyst performance does not show a significant improvement or decrease.

[0096] The other embodiments have similar effects to the above embodiments 1 and 4, and will not be described in detail here.

[0097] III. Photoelectrochemical Testing

[0098] A series of photoelectrochemical tests were performed on the photocatalysts of Example 1, Comparative Example 1, and Comparative Example 2. All photoelectrochemical tests were performed using a three-electrode system connected to a CHI 760e electrochemical workstation. First, the catalyst was uniformly dispersed in an ethanol solution, and then uniformly coated onto fluorine-doped tin oxide glass (FTO glass) to obtain a loaded 0.5 g cm⁻¹. -2 Fluorine-doped tin oxide glass was used as the working electrode, while an Ag / AgCl electrode and a platinum sheet served as the reference and counter electrodes, respectively. A suitable amount of 0.5M Na₂SO₄ solution was then added to a quartz cell. The following parameters were measured: photocurrent response curve, electrochemical impedance spectroscopy, transient open-circuit photovoltage decay curve, and cyclic voltammetry curve.

[0099] (1) Photoresponse test: Under a bias potential of 0.7V vs. Ag / AgCl, the light was cut off every 10s to obtain the photocurrent It curve. The light source was a 300W xenon lamp equipped with an ultraviolet enhancement film.

[0100] (2) Electrochemical impedance spectroscopy (EIS) test: in the range of 0.1–10 6 Impedance testing was performed within a frequency range of Hz.

[0101] (3) Transient Open-Circuit Photovoltage Degradation (OCVD) Test: After the light source reaches a stable photovoltage after a certain period of illumination, the light source is cut off, and the degradation curve is obtained. Then, the average lifetime (τ) of the photogenerated carriers is obtained. n ).

[0102] (4) Cyclic voltammetry (CV) curves were scanned in a mixture of 0.5 M Na2SO4 and 0.1 M Na3PO4, with a scan range of (-2.0) to 3.0 V vs. Ag / AgCl.

[0103] Figure 7The transient photocurrent response test results are for the photocatalysts of Example 1, Comparative Example 1, and Comparative Example 2. It can be seen that the TiO2-P prepared in Example 1 has the highest photocurrent response, indicating that surface modification with phosphate groups and oxygen vacancies effectively promotes the separation of photogenerated carriers.

[0104] Figure 8 The electrochemical impedance spectra of the photocatalysts in Example 1, Comparative Example 1, and Comparative Example 2 under illumination conditions are shown. The results show that TiO2-P has the smallest arc radius and the lowest charge transfer resistance, indicating that the transfer rate of photogenerated electrons and holes to the interface is the fastest.

[0105] Figure 9 The time-resolved photoluminescence spectra of the photocatalysts in Example 1, Comparative Example 1, and Comparative Example 2 show that the average luminescence lifetime of TiO2-P is 2.3 ns, which is longer than that of TiO2-O2 (2.0 ns) and TiO2-H2 (1.4 ns). This indicates that the modification of the surface phosphate group effectively suppresses the recombination of photogenerated carriers, thereby prolonging the carrier lifetime.

[0106] The mean lifetime (τ) of photogenerated carriers was evaluated using transient OCVD testing. n ), Figure 10 In Figure a, the open-circuit voltage decay (OCVD) curves of the photocatalysts in Example 1, Comparative Example 1, and Comparative Example 2 are shown, and in Figure b, the converted OCVD curves are shown. From the converted OCVD curves, it can be seen that τ... n This trend is similar to that of the aforementioned photoelectric testing.

[0107] Figure 11 The CV curves of the photocatalysts in Example 1, Comparative Example 1, and Comparative Example 2 show that, relative to the reversible hydrogen electrode (RHE), the onset potential of H2O oxidation on TiO2-P is 1.58V, which is 130 and 390 mV lower than that of TiO2-H2 and TiO2-O2, respectively. This indicates that the energy barrier for water activation at the phosphate sites of TiO2-P is reduced.

[0108] The other embodiments have similar effects to Embodiment 1 above, and will not be described in detail here.

[0109] In summary, co-modification with phosphate and oxygen vacancies effectively promotes the separation and migration of photogenerated carriers, thereby improving the utilization efficiency of incident light.

[0110] IV. Free Radical Detection

[0111] (1) Electron paramagnetic resonance (EPR): ·OH radicals were determined using 5,5-dimethyl-1-pyrrolino-N-oxide (DMPO) as a spin trapping agent. A suspension was formed by adding 2 mg of catalyst to 500 μL of H₂O, and the resulting suspension was mixed with 40 μL of DMPO in an O₂ atmosphere. The mixture was then subjected to a light intensity of 50 mW cm⁻¹.-2 After irradiation with a UV lamp for 10 minutes, the EPR spectrum was collected.

[0112] (2) In-situ Raman spectroscopy: In-situ Raman spectroscopy is used to detect the *H2O generated during the reaction. In a hermetically sealed polytetrafluoroethylene electrolytic cell that allows light exposure, a piece coated with 0.5 mg cm... -2 The FTO glass catalyst was placed vertically below the lens. The electrolytic cell was purged with argon for 30 minutes and then filled with H2O. Different irradiations (50 mW cm⁻¹) were recorded. -2 Raman spectra under ultraviolet light for 0 min, 10 min, 20 min, 40 min, and 60 min.

[0113] Figure 12 Under an O2 atmosphere, the in-situ DMPO spin-trapping EPR spectra of the photocatalysts in Example 1, Comparative Example 1, and Comparative Example 2 show that TiO2-P exhibits the strongest ·OH signal, indicating that the catalyst surface is modified with PO4. 3- It can effectively promote the oxidation of water and the reduction of oxygen during the reaction process, which is conducive to the generation of ·OH free radicals.

[0114] Figure 13 The in-situ Raman spectra of TiO2-H2 prepared in Comparative Example 2 and TiO2-P prepared in Example 1 in water under different irradiation times are shown. No peaks were observed for either catalyst under dark conditions. With increasing light irradiation, a peak appeared on the TiO2-P surface at 1640 cm⁻¹. -1 The photocatalytic HOH bending vibration peak was observed at TiO2-H2, and its peak intensity increased with prolonged illumination time. In contrast, a rather weak H2O adsorption peak was observed on TiO2-H2, indicating that in the absence of phosphate modification, the defective TiO2 with oxygen vacancies on its surface has poor adsorption capacity for H2O. Phosphate acts as an effective bridge between the catalyst and adsorbed H2O, promoting the oxidation of H2O to ·OH radicals, thereby enhancing photocatalytic methane conversion.

[0115] The other embodiments have similar effects to Embodiment 1 above, and will not be described in detail here.

[0116] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. Use of a photocatalyst in catalyzing the conversion of methane into liquid oxides, characterized in that, The preparation method of the photocatalyst comprises the following steps: The preparation method of the photocatalyst comprises the following steps: The mass ratio of the sodium hypophosphite to the titanium dioxide nanosheet is 8-15:

1. The preparation is carried out in an inert atmosphere in a tube furnace, the titanium dioxide nanosheet is placed downstream of the phosphate source, and the interval distance is 5-8 cm.

2. Use according to claim 1, characterized in that, The heating temperature is 550-600 K.

3. Use according to claim 1, characterized in that, The annealing time is 1-2 h.

4. Use according to claim 1, characterized in that, The preparation method of the titanium dioxide nanosheet comprises: HF and Ti(OBu)4 are mixed, reacted, and the precipitate is collected to obtain the titanium dioxide nanosheet; the volume ratio of the HF to the Ti(OBu)4 is 1:(7-9); the reaction is carried out in an autoclave, the reaction temperature is 450-470 K, and the reaction time is 35-40 h.

5. The use according to claim 1, characterized in that, The photocatalyst is a titanium dioxide nanosheet co-modified by phosphate and oxygen vacancies on the surface.

6. Use according to claim 5, characterized in that, The thickness of the titanium dioxide nanosheet is 5-7 nm.

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