Preparation method of Ag / P25 composite catalyst and application of Ag / P25 composite catalyst in photocatalytic conversion of CH4 into CH3OH

Abstract

The invention discloses a preparation method of an Ag / P25 composite catalyst and application of the Ag / P25 composite catalyst in photocatalytic conversion of CH4 into CH3OH, and belongs to the technical field of CH4 photocatalytic conversion. The preparation method comprises the following steps: firstly, ultrasonically dispersing commercial P25 in an AgNO3 aqueous solution, then slowly dropwise adding a NaBH4 aqueous solution, continuously stirring for reaction after dropwise adding, and finally performing centrifugal separation, washing and drying after the reaction is completed. The Ag / P25 composite catalyst with Ag nanoparticles loaded on the surface and containing a large amount of Ov is prepared through a simple wet impregnation method, the performance of CH4 photocatalytic conversion into CH3OH is greatly improved, and the yield and selectivity of CH3OH within 2 h reach 5.5 mmol g <-1 > h <-1 > and 94% respectively under the conditions of the room temperature and the total pressure of CH4 + O2 (19: 1) of 2 MPa.

Description

Technical Field

[0001] This invention belongs to the field of CH4 photocatalytic conversion technology, specifically relating to a method for preparing an Ag / P25 composite catalyst and its application in the photocatalytic conversion of CH4 to CH3OH. Background Technology

[0002] Methane (CH4) is a high-quality clean energy source widely used in power generation and heating. In recent years, with the exploration and development of CH4 hydrates, shale gas, and coalbed methane, global CH4 reserves have surged. However, due to the flammability and explosiveness of CH4 and its remote and dispersed production sites, the high cost of CH4 energy utilization stems from the challenging storage and transportation requirements. Furthermore, as CH4 is a potent greenhouse gas (25 times more potent than CO2), the leakage and combustion of residual gas at production sites cause significant environmental pressure and energy waste. Therefore, exploring efficient methods for CH4 utilization and achieving its resource-based utilization is an urgent and crucial strategy for achieving a win-win situation for economic, environmental, and energy benefits.

[0003] Converting CH4 into liquid fuels such as methanol (CH3OH), which are easier to transport and store, and then directly using them in industrial manufacturing, can not only alleviate the increasingly scarce petroleum resources but also promote the green development of the chemical industry. However, due to the stable tetrahedral structure, extremely low polarizability, and extremely high CH bond energy (439 KJ / mol) of the CH4 molecule, its activation is extremely challenging and has become a global problem. Efficient activation of CH4 typically requires harsh reaction conditions such as high temperature (>600 °C), strong oxidants (e.g., fuming sulfuric acid), or external fields (e.g., plasma). For example, traditional industrial conversion of CH4 requires first performing CH4 steam reforming under high temperature and high pressure conditions (800~1000 °C, 2~7 MPa) before chemical synthesis. These harsh CH4 steam reforming conditions not only lead to energy waste and product over-oxidation but also significantly shorten catalyst life due to carbon deposition. Furthermore, the target product CH3OH is easily oxidized again to form HCHO and CO. x The selective photocatalytic synthesis of CH3OH from CH4, along with other byproducts, was once considered the "holy grail" of catalysis.

[0004] Skipping the energy-intensive steps of syngas generation and directly driving the conversion of CH4 to CH3OH using green and sustainable light energy under mild conditions (temperature <150 °C in a non-strong acidic medium) is undoubtedly an ideal way to achieve efficient CH4 utilization. Photocatalysis is an effective way to achieve CH4→CH3OH using light energy. It utilizes photogenerated carriers to pre-activate stable CH bonds, thereby lowering the activation energy barrier to achieve CH4 conversion under mild conditions. Among these, the photooxidation of CH4 using green and low-cost O2 as an oxidant is currently a research hotspot. Under illumination, when the photon energy (hν) is greater than the band gap E... g When a semiconductor material absorbs an incident photon, it is excited to generate an electron (ep). - ) and holes (h + ), where e - O2 molecules can be reduced to ·OOH, while h + It can oxidize CH4 and H2O molecules to form ·CH3 and ·OH, respectively. These ·OH groups further act as important reactive oxygen species to activate the formation of ·CH3 from CH4. The formed ·CH3 can combine with ·OOH / ·OH to obtain the target product CH3OH.

[0005] Currently, the representative research progress on the photocatalytic synthesis of CH3OH from CH4 at room temperature (25~30 ℃) is as follows: In 2021, Professor Ye Jinhua's research group at Tianjin University used Ag to modify TiO2 nanosheets with mainly exposed (001) crystal planes, and used the oxygen vacancies (Ov) generated by this crystal plane under light to regulate the CH4 conversion route (Ov→Ti-O2·→Ti-OO-Ti→Ti-OCH3HOTi→Ti-O-Ti+CH3OH). Under a total pressure of 2.1 MPa CH4+O2 (20:1), the yield and selectivity of CH3OH reached 4.8 mmol g within 2 h. -1 h -1 In 2022, Professor Sun Yongfu's research group at the University of Science and Technology of China utilized Fe2O3 and ZnO oxides to polarize CH4 molecules and enhance the OH bond strength of the product CH3OH to prevent over-oxidation. Under a CH4 pressure of 1 atm, they achieved a CH3OH yield and selectivity of 178.3 μmol g / L. -1 And 100%; In 2023, Professor Tang Zhiyong's research group at the National Center for Nanoscience and Technology used an In2O3 catalyst loaded with Au nanoparticles to regulate the generation of ·OH. Under a total pressure of 3 MPa CH4+O2 (2:1), the yield and selectivity of CH3OH reached 5.95 mmol g within 3 h. -1 h -1And 89.42%; In 2024, Professor Wu Wenting's research group at China University of Petroleum used AuPd alloy to modify ZnO to regulate the adsorption configuration and transformation pathway of O2 molecules (O2→*OOH→·OH). Under a total pressure of 2.1 MPa CH4+O2 (20:1), the yield and selectivity of CH3OH were increased to 4.04 mmol g. -1 h -1 In 2025, the research group further utilized a ZnO catalyst co-modified with Au and Ov to regulate the O2→·OH conversion rate to improve the CH3OH selectivity. Under a total pressure of 3 MPa CH4+O2 (29:1), the CH3OH yield reached 5.5 mmol g within 1 h. -1 h -1 Furthermore, the selectivity was 98% (Q. Zhou, Y. Wan, X. Wang, Q. Zhang, H. Zhang, C. Qi, Z. Liu, H. Yang, T. Xing, M. Wang, M. Wu, W. Wu. Regulation of O2activation pathway boosts efficient photocatalytic methane oxidation tomethanol. Sep. Purif. Technol. 2025, 354, 129064.). In the same year, Professor Zhang Zizhong's research group at Fuzhou University used a ZnO catalyst co-modified with AuPd and interstitial Zn atoms to regulate the selective conversion of O2 to ·OOH. Under a total pressure of 2.1 MPa CH4+O2 (20:1), the yield of CH3OH reached 8.8 mmol g within 1 h. -1 h -1 Selectivity 98.3% (Z. Xiao, J. Shen, J. Jiang, J. Zhang, S. Liang, S. Han, J. Long, W. Dai, Y. Li, X. Wang, H. Xi, 2422726.).

[0006] As can be seen from the above, research on the direct photocatalytic conversion of CH4 to CH3OH has made rapid progress and achieved encouraging results in recent years. However, current research on the photocatalytic conversion of CH4 still faces challenges such as complex and costly catalyst preparation processes, and low yield and selectivity of CH3OH. Summary of the Invention

[0007] To overcome the numerous technical shortcomings of existing CH4 photocatalytic conversion methods, this invention aims to propose a method for preparing an Ag / P25 composite catalyst and its application in the photocatalytic conversion of CH4 to CH3OH. An Ag / P25 composite catalyst with Ag nanoparticles supported on its surface and containing a large amount of Ov is prepared by a simple wet impregnation method, which significantly improves its performance in the photocatalytic conversion of CH4 to CH3OH. Under room temperature and a total pressure of 2 MPa CH4+O2 (19:1), the yield and selectivity of CH3OH reached 5.5 mmol g within 2 h. -1 h -1 and 94%.

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

[0009] The Ag / P25 composite catalyst was prepared by wet impregnation. The surface was loaded with Ag nanoparticles and contained a large amount of Ov. The specific steps were as follows: First, commercial P25 was ultrasonically dispersed in AgNO3 aqueous solution. Then, NaBH4 aqueous solution was slowly added dropwise. After the addition was completed, the reaction was stirred. Finally, after the reaction was completed, the catalyst was centrifuged, washed, and dried.

[0010] As a preferred embodiment of the present invention, 200 mg of commercial P25 is ultrasonically dispersed in 10 mL of AgNO3 aqueous solution, and 10 mL of 100 mM NaBH4 aqueous solution is slowly added dropwise to the above solution under stirring. The resulting mixed solution is reacted for another 5 h under stirring. After centrifugation, the product is washed three times with water and dried under vacuum at 60 °C for 24 h.

[0011] As a preferred embodiment of the present invention, the amount of AgNO3 added in the reaction system relative to the mass ratio of P25 is preferably 0.3 wt%, 0.6 wt%, 1.2 wt%, 1.8 wt%, 2.4 wt%, 3 wt%, 3.6 wt%, 4.2 wt%, 4.8 wt%, 6.6 wt%, 8.4 wt%, and most preferably 4.8 wt%.

[0012] As a preferred technical solution of the present invention, the amount of NaBH4 added in the reaction system is preferably 3, 4, or 5 times the amount of AgNO3 added; the reaction temperature is room temperature; and the reaction time is preferably 1 h, 2 h, 3 h, 4 h, 5 h, 8 h, or 10 h, with the most preferred being 5 h.

[0013] The Ag / P25 composite catalyst prepared by this invention is mainly composed of particles with a size of 1-50 nm, and the Ag nanocrystal size on its surface is 1-5 nm; the percentage content of Ag in the composite catalyst is 1%-6%, and the Ov content is 10%-30%.

[0014] In addition, this invention also provides the application of the Ag / P25 composite catalyst in the photocatalytic conversion of CH4, the steps of which are as follows:

[0015] 1) Take 5 mg of Ag / P25 composite catalyst and disperse it in 80 mL of H2O. Place it in a high-pressure photocatalytic reaction vessel. Before irradiation, purge the air in the vessel with O2 gas. Then, introduce CH4+O2 with different partial pressure ratios and pressurize to 2 MPa.

[0016] 2) Using a 300 W xenon lamp as the light source, set the xenon lamp operating current, maintain the liquid phase system at 800 rpm stirring, and set the circulating water temperature to 30 ℃;

[0017] 3) After the photocatalytic reaction has been going on for a period of time, turn off the light source, set the circulating water temperature to 5 ℃, and after the temperature inside the reactor drops below 10 ℃, analyze the gas phase and liquid phase components and contents.

[0018] As a preferred technical solution of the present invention, the partial pressure ratio of CH4 and O2 under a total pressure of 2 MPa is preferably 1.95:0.05, 1.9:0.1, 1.8:0.2, 1.7:0.3 or 1.6:0.4, and most preferably 1.9:0.1.

[0019] As a preferred embodiment of the present invention, the xenon lamp operating current is preferably set to 13 A, 16 A, or 21 A, with 16 A being the most preferred.

[0020] As a preferred technical solution of the present invention, the photocatalytic reaction time is preferably 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h or 8 h, and most preferably 2 h.

[0021] As a further preferred technical solution of the present invention, the Ag / P25 composite catalyst prepared by using AgNO3 with a relative mass ratio of 4.8 wt% to P25 exhibits the best performance in the photocatalytic conversion of CH4 to CH3OH. Under the optimal experimental conditions (5 mg catalyst, 2 MPa CH4 + O2 (19:1), 30 ℃, and reaction time of 2 h), the total amount of CH3OH product obtained is 55.4 μmol, and the CH3OH selectivity is 94% (CH3OH + HCHO selectivity is 100%).

[0022] This invention employs a wet impregnation method to prepare an Ag / P25 composite catalyst with Ag nanoparticles supported on its surface and containing a large amount of Ov. By controlling experimental conditions, the content and distribution of Ag nanoparticles and Ov on the catalyst surface were regulated. The catalyst was then used in the photocatalytic conversion of CH4 to CH3OH. In this part of the experiment, the effects of catalyst preparation conditions and photocatalytic experimental conditions on the yield and selectivity of the target product CH3OH were investigated. Compared with the P25 catalyst, the introduction of Ag and Ov significantly improved its yield and selectivity for the photocatalytic conversion of CH4 to CH3OH. Compared with the prior art, the beneficial effects of this invention are as follows:

[0023] 1) The Ag / P25 composite catalyst prepared in this invention has a large number of Ag nanoparticles and Ov loaded on its surface, and their content and distribution are controlled by the experimental conditions of catalyst preparation. Studies have shown that under light irradiation, more Ov can be formed on the TiO2 surface, and "hot electrons" are generated on the Ag nanoparticle surface; the "hot electrons" are transferred from Ag to TiO2, accepted by O2 molecules adsorbed at the Ov and form Ti-OO-Ti; Ti-OO-Ti forms Ti-O··O-Ti under light irradiation and activates CH4 to form CH3OH molecules (Ti-O··O-Ti→Ti-OCH3HO-Ti→Ti-O-Ti→CH3OH).

[0024] 2) This invention demonstrates through experiments that the size and content of Ag nanoparticles on the surface of the Ag / P25 composite catalyst, as well as the content and distribution of Ov, can be effectively controlled by altering the wet chemical reduction experimental conditions, thereby achieving regulation of its photocatalytic performance in the production of CH3OH from CH4. The catalyst prepared by loading 3.05 wt.% Ag onto a commercial P25 surface exhibits the best photocatalytic performance in the production of CH3OH from CH4. Under optimal experimental conditions (5 mg catalyst, 2 MPa CH4 + O2 (19:1), 30 ℃, reaction time 2 h): the total amount of CH3OH product obtained was 55 μmol, with a selectivity of 94% (CH3OH + HCHO selectivity 100%), which is superior to previous reports.

[0025] 3) The preparation method of this Ag / P25 composite catalyst is simple, reproducible, and yields high output, which facilitates mass production and lays the foundation for the commercial production of direct photocatalytic CH4 to CH3OH catalyst. This is of great significance for the rational utilization of global fossil resources and the solution of environmental problems. Attached Figure Description

[0026] Figure 1 a) SEM image, b, c) TEM image, d) SAED image, eh) EDS mapping image, i) XRD image, j) UV-vis absorption spectrum and k) solid-state electron spin resonance (ESR) spectrum of Ag / P25 composite catalyst prepared with AgNO3 at a mass ratio of 4.8 wt% to P25.

[0027] Figure 2 The bar graphs show the effects of a) AgNO3 mass percentage, b) O2:CH4 partial pressure ratio, c) CH4 partial pressure, and d) xenon lamp operating current (illuminance) on the yield and selectivity of CH3OH. Under optimal experimental conditions (4.8 wt% AgNO3, O2:CH4 = 1:19 (total pressure 2 MPa), xenon lamp operating current 16 A), the effects of e) illumination time on the yield and selectivity of CH3OH and f) catalyst cycle stability tests (single 2 h) are also shown.

[0028] Figure 3 The preparation of Ag / P25 composite catalysts under optimal experimental conditions (4.8 wt% AgNO3, O2:CH4 = 1:19 (total pressure 2 MPa), xenon lamp operating current 16 A) includes: a) high-resolution Ag 3d XPS spectra of the catalyst under illumination and non-illumination conditions; b) and c) in-situ ESR spectra of the catalytic system measured in water and methanol solutions, respectively; and d) spectra obtained by reacting H2O and H2O. 18 GC-MS spectra of gaseous and liquid products obtained from the photocatalytic CH4 to CH3OH experiment, e) solid-state ESR spectra of the catalyst before and after the reaction, and f) in-situ FTIR spectra of the reaction system. Detailed Implementation

[0029] The present invention will be further described in detail below with reference to the embodiments and accompanying drawings.

[0030] Example 1

[0031] 200 mg of commercial P25 (purchased from Shandong Keyuan Biochemical Co., Ltd.) was ultrasonically dispersed in 10 mL of AgNO3 aqueous solution (containing 9.6 mg AgNO3, with a relative mass ratio of 4.8 wt% to P25). 10 mL of 100 mM NaBH4 aqueous solution (containing 37.91 mg NaBH4) was slowly added dropwise to the above solution under stirring. The resulting mixture was reacted for another 5 h under stirring. The product was then centrifuged, washed three times with water, and vacuum dried at 60 ℃ for 24 h. The final Ag / P25 composite catalyst had a surface loading of 3.05 wt.% Ag.

[0032] Figure 1 Images of the Ag / P25 composite catalyst prepared at a mass ratio of 4.8 wt% AgNO3 to P25 are shown in the following formats: a) SEM, b, c) TEM, d) SAED, e) EDS mapping, i) XRD, j) UV-vis absorption spectrum, and k) solid-state electron spin resonance (ESR) spectrum. The SEM and TEM images reveal that the catalyst is primarily composed of particles with a size of 1-50 nm. Figure 1 As shown in a), the size of the Ag nanocrystals on its surface is 1-5 nm. Figure 1 (As shown in b, c, and d); EDS mapping images show that the elements are uniformly distributed on the catalyst surface ( Figure 1 (As shown in eh); Due to the high dispersion of Ag and its extremely small particle size, only the diffraction pattern of the anatase (A) phase TiO2 can be observed in the XRD pattern ( Figure 1 As shown in i); compared to P25, due to the localized surface plasmon resonance effect of Ag, the catalyst exhibits additional absorption peaks in the visible region ( Figure 1 (as shown in j); furthermore, solid-state ESR spectroscopy shows that the introduction of Ag significantly increases the content of catalyst Ov (g=2.003 signal peak) (as shown in j). Figure 1 (as shown in k).

[0033] Example 2

[0034] First, the content of AgNO3 in 10 mL of AgNO3 aqueous solution prepared in Example 1 was set to be 0.6 mg, 1.2 mg, 2.4 mg, 3.6 mg, 4.8 mg, 6 mg, 7.2 mg, 8.4 mg, 9.6 mg, 13.2 mg, and 16.8 mg, respectively. The relative mass ratios to P25 were 0.3 wt%, 0.6 wt%, 1.2 wt%, 1.8 wt%, 2.4 wt%, 3 wt%, 3.6 wt%, 4.2 wt%, 4.8 wt%, 6.6 wt%, and 8.4 wt%, respectively. Multiple Ag / P25 composite catalysts were ultimately prepared.

[0035] Next, the prepared Ag / P25 composite catalyst was applied to the photocatalytic conversion of CH4, and the steps are as follows:

[0036] 1) Take 5 mg of Ag / P25 composite catalyst and disperse it in 80 mL of H2O (σ<0.5 μS / cm) to form a dispersion solution. Place it in a quartz liner and put it in a high-pressure photocatalytic reactor (CEL-HPR100T, Zhongjiao Jinyuan). Before irradiation, purge the air in the reactor with O2 gas. Then, introduce CH4+O2 with different partial pressure ratios and pressurize to 2 MPa. The partial pressure ratios of CH4 and O2 at a total pressure of 2 MPa are set to 1.95:0.05, 1.9:0.1, 1.8:0.2, 1.7:0.3 or 1.6:0.4, respectively.

[0037] 2) Using a 300 W xenon lamp (CEL-HXF300-T3, Zhongjiao Jinyuan) as the light source, the xenon lamp operating current was set to 13 A, 16 A, and 21 A respectively. The liquid phase system was kept in a stirring state of 800 rpm, and the circulating water temperature was set to 30 ℃.

[0038] 3) After the photocatalytic reaction has proceeded for 1 h, 2 h, 3 h, 4 h, 5 h, 6 h, 7 h, and 8 h, the light source is turned off, the circulating water temperature is set to 5 ℃, and after the temperature inside the reactor drops below 10 ℃, the gas phase and liquid phase components and contents are analyzed:

[0039] After the reaction, the collected liquid products were stored at low temperature and analyzed by 1H NMR spectroscopy. In the 1H NMR test, 3-trimethylsilyl-1-propanesulfonate (DSS) was used as an internal standard, and different products were quantitatively analyzed by calculating the peak area and combining it with a pre-plotted standard curve.

[0040] The details of HNMR sample preparation are as follows: 100 μL of 0.02 wt% deuterium aqueous solution and 700 μL of reaction solution were respectively added to the NMR tube and ultrasonically dispersed for 3 min before testing. The hydrogen nuclei of trimethylsilyl (-Si(CH3)3) are in a highly shielded chemical environment, resulting in a stable resonance signal and a chemical shift internationally standardized as 0 ppm. 1 The standard for HNMR chemical shift, with deuterium water as the solvent, and its deuterium nuclei ( 2 H) and hydrogen nucleus ( 1 The resonant frequencies of H) are significantly different and will not interfere with each other. 1 HNMR signal, through 2 The resonant stable magnetic field of H can provide a field-locking signal, ensuring the stability of the spectrum. 1 H signal in 1In HNMR, the intensity is usually extremely high. The water peak suppression technique is used to selectively excite the resonance frequency corresponding to the water peak to eliminate the interference of strong water peak. Due to different chemical environments, the hydrogen nuclei of CH3OH, HCOOH, and CH3OOH will produce characteristic peaks at specific chemical shifts. The product content can be quantitatively analyzed by the peak area. The chemical shift of CH3OH is 3.34 ppm, and the chemical shift of CH3OOH is 3.85 ppm. Under weakly acidic conditions, the chemical shift of HCHO peak is similar to that of water peak, and the water peak suppression range is wide, thus suppressing the HCHO signal.

[0041] Therefore, the HCHO content analysis in the experiment adopted spectrophotometry. The principle is as follows: In an acetate-ammonium acetate buffer solution with a pH of 6.0~6.5, HCHO and acetylacetone (CH3COCH2COCH3) undergo a condensation reaction under heating conditions to generate 3,5-diacetyl-1,4-dihydrorutidine (a yellow compound). This compound has a maximum absorption peak at 412 nm, and its absorbance is linearly related to the formaldehyde concentration within a certain range. The formaldehyde content in the sample can be calculated through a standard curve. Therefore, quantitative analysis of HCHO can be achieved by measuring the absorbance intensity using a UV-Vis near-infrared spectrophotometer (UV-vis, Lambda 1050).

[0042] Buffer solution preparation steps: Weigh 15 g ammonium acetate, measure 0.3 mL glacial acetic acid and 0.2 mL acetylacetone solution into 100 mL deionized water, and sonicate for 2 min to mix thoroughly. For the HCHO assay, measure 500 μL of the liquid phase product solution and 500 μL of the buffer solution into 2 mL of water, incubate at 35 ℃ for 40 min, transfer to a quartz cuvette, and measure the absorption spectrum after leveling the instrument with deionized water as the baseline.

[0043] Figure 2 The following are bar charts showing the effects of a) AgNO3 mass percentage, b) O2:CH4 partial pressure ratio, c) CH4 partial pressure, and d) xenon lamp operating current (illuminance) on the yield and selectivity of CH3OH. It should be noted that the experimental conditions for evaluating the effect of AgNO3 mass percentage on the yield and selectivity of CH3OH were: O2:CH4 = 1:19 (total pressure 2 MPa), xenon lamp operating current 16 A, and reaction time 2 h; the experimental conditions for evaluating the effect of O2:CH4 partial pressure ratio and CH4 partial pressure on the yield and selectivity of CH3OH were: 4.8 wt% AgNO3, total pressure 2 MPa, xenon lamp operating current 16 A, and reaction time 2 h; and the experimental conditions for evaluating the effect of xenon lamp operating current (illuminance) on the yield and selectivity of CH3OH were: 4.8 wt% AgNO3, O2:CH4 = 1:19 (total pressure 2 MPa), and reaction time 2 h.

[0044] Under optimal experimental conditions (4.8 wt% AgNO3, O2:CH4=1:19 (total pressure 2 MPa), xenon lamp operating current 16A), the effects of e) illumination time on CH3OH yield and selectivity, and f) catalyst cycle stability (single 2 h) were tested.

[0045] Studies have shown that when the amount of precursor AgNO3 is 4.8 wt% (corresponding to an Ag content of 3.05 wt% in the catalyst), O2:CH4 = 1:19 (total pressure 2 MPa), and the xenon lamp operating current is 16 A, the catalyst exhibits the best performance in the photocatalytic production of CH3OH from CH4. Figure 2 (as shown in ad), at this point, the yield of CH3OH within 2 h was 5.5 mmol g. -1 h -1 The selectivity was 94%; within an 8-hour reaction period, the yield of CH3OH continued to increase with reaction time, while the selectivity decreased slightly (73%). Figure 2 (as shown in e); furthermore, the catalyst exhibited good reusability in 5 cycles, with the CH3OH yield and selectivity reaching 4.4 mmol g in the 5th experiment. -1 h -1 and 81% ( Figure 2 (as shown in f).

[0046] Figure 3 The preparation of Ag / P25 composite catalysts under optimal experimental conditions (4.8 wt% AgNO3, O2:CH4 = 1:19 (total pressure 2 MPa), xenon lamp operating current 16 A) includes: a) high-resolution Ag 3d XPS spectra of the catalyst under illumination and non-illumination conditions; b) and c) in-situ ESR spectra of the catalytic system measured in water and methanol solutions, respectively; and d) spectra obtained by reacting H2O and H2O. 18 GC-MS spectra of gaseous and liquid products obtained from the photocatalytic CH4 to CH3OH experiment, e) solid-state ESR spectra of the catalyst before and after the reaction, and f) in-situ FTIR spectra of the reaction system.

[0047] Studies have shown that the high-resolution Ag 3d XPS spectra of the catalyst under both light and non-light conditions reveal a shift of the Ag 3d peak towards lower angles under light illumination, indicating that Ag loses electrons under light conditions, i.e., "hot electrons" are transferred from Ag to TiO2. Figure 3 (As shown in a). Subsequently, the O2 molecules adsorbed at Ov on the TiO2 surface gain electrons and are reduced to form Ti-O2·(Ti-O··O-Ti), which in turn activates CH4 molecules (Ti-OCH3HO-Ti), ultimately yielding Ti-O-Ti and CH3OH molecules. The in-situ ESR spectrum of the reaction system shows the presence of free radicals such as ·OH, ·CH3, and ·O2 in the system. Figure 3 (as shown in b and c), H2 18 O isotope experiments showed that the oxygen in the product CH3OH originated from both O2 and H2O, with the main source being O2 ( Figure 3 As shown in d); solid-state ESR spectroscopy shows that after the photocatalytic reaction, the Ov content in the catalyst decreases, indicating that O in O2 fills the Ov (as shown in d). Figure 3 (as shown in e). In the in-situ FTIR spectrum, 830 cm⁻¹ -1 The peak corresponds to the Ti-OO-Ti vibration, confirming that O2 molecules are adsorbed near Ov; 940 cm⁻¹ -1 The nearby negative peak corresponds to the surface Ti-OO vibration, indicating that this material in the system is rapidly consumed; 1158 cm⁻¹ -1 The peaks correspond to the CO vibration of the CH3O group; 1322, 1380, and 1460 cm⁻¹ -1 These vibrational modes belong to CH3OH, CH3, and CH2, respectively, confirming the process of CH4 molecules breaking bonds to form CH3OH on the catalyst surface. Figure 3 (as shown in f).

Claims

1. A method for preparing an Ag / P25 composite catalyst, characterized in that, First, commercial P25 is ultrasonically dispersed in an aqueous solution of AgNO3. Then, an aqueous solution of NaBH4 is slowly added dropwise. After the addition is complete, the mixture is stirred and reacted. Finally, after the reaction is complete, the mixture is centrifuged, washed, and dried.

2. The preparation method according to claim 1, characterized in that, The amount of AgNO3 added relative to the mass ratio of P25 is 0.3wt%-8.4wt%.

3. The preparation method according to claim 1, characterized in that, The amount of NaBH4 added should be 3-5 times that of AgNO3 added.

4. The preparation method according to claim 1, characterized in that, The reaction temperature is room temperature, and the reaction time is 1-10 h.

5. The Ag / P25 composite catalyst prepared by the method according to any one of claims 1-4, characterized in that, It is mainly composed of particles with a size of 1-50 nm, and the Ag nanocrystals on its surface have a size of 1-5 nm; the Ag content in the composite catalyst is 1%-6%, and the Ov content is 10%-30%.

6. The application of the Ag / P25 composite catalyst as described in claim 5 in the photocatalytic conversion of CH4 to CH3OH, characterized in that, The steps are as follows: 1) Take 5 mg of Ag / P25 composite catalyst and disperse it in 80 mL of H2O. Place it in a high-pressure photocatalytic reaction vessel. Before irradiation, purge the air in the vessel with O2 gas. Then, introduce CH4+O2 with different partial pressure ratios and pressurize to 2 MPa. 2) Using a 300 W xenon lamp as the light source, set the xenon lamp operating current, maintain the liquid phase system at 800 rpm stirring, and set the circulating water temperature to 30 ℃; 3) After the photocatalytic reaction has been going on for a period of time, turn off the light source, set the circulating water temperature to 5 ℃, and after the temperature inside the reactor drops below 10 ℃, analyze the gas phase and liquid phase components and contents.

7. The application as described in claim 6, characterized in that, When a total pressure of 2 MPa is introduced, the partial pressure ratio of CH4 to O2 is 1.9:0.

1.

8. The application as described in claim 6, characterized in that, The xenon lamp operating current is set to 16 A.

9. The application as described in claim 6, characterized in that, The photocatalytic reaction time was 2 h.

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