Body defect titanium dioxide nano-particles loaded with metal single atoms on surfaces as well as preparation method and application of body defect titanium dioxide nano-particles

Abstract

The invention discloses a body defect titanium dioxide nano-particle loaded with metal single atoms on the surface as well as a preparation method and application of the body defect titanium dioxide nano-particle. According to the method, pure anatase phase titanium dioxide is taken as a matrix, and modification is carried out through high-temperature annealing in air so as to introduce bulk defects into titanium dioxide nano-particles. Then, the surface is coated with a metal ligand through a hydrothermal method, annealing is carried out in air again, the ligand is removed, and the body defect TiO2 nano-particles with the surface loaded with metal single atoms are obtained. The method is simple, required materials are easy to obtain, and the prepared titanium dioxide semiconductor material can be used as a high-efficiency catalyst in the field of photocatalysis, and particularly has excellent catalytic activity and good cycling stability in the field of preparing methanol and formaldehyde through photocatalytic partial oxidation of methane; the method also has good application prospects in the fields of photocatalytic water decomposition hydrogen production, photocatalytic hydrogen peroxide production and the like.

Description

Technical Field

[0001] The present invention belongs to the field of semiconductor photocatalyst preparation, and specifically relates to a bulk-defect titanium dioxide (TiO 2 ) nanoparticle loaded with single metal atoms, its preparation method and use. This semiconductor photocatalyst can be used for the photocatalytic partial oxidation of methane to methanol and formaldehyde. Background Art

[0002] With the intensification of energy crisis and environmental problems, the development and efficient utilization of clean energy have become the focus of global attention. Methane, as the main component of natural gas, is a hydrocarbon with rich reserves and relatively clean. Converting methane into high-value-added chemicals such as methanol and formaldehyde through partial oxidation can not only significantly improve the economic value of methane, but also reduce greenhouse gas emissions caused by complete oxidation. However, the symmetric molecular structure of methane and the high bond energy of C-H bond (439 kJ / mol) make its activation extremely challenging.

[0003] Traditional methane conversion technologies usually adopt indirect processes at high temperature and high pressure, such as steam reforming to syngas and then further synthesizing methanol. These methods have complex processes, high energy consumption, and are accompanied by a large amount of carbon dioxide emissions, which do not meet the requirements of modern green chemistry development. Photocatalytic technology provides a sustainable solution for the partial oxidation of methane with its characteristics of using solar energy as the energy drive, mild reaction conditions, and environmental friendliness. Titanium dioxide (TiO 2 ) as an important semiconductor material has been widely studied in the field of photocatalysis. It has the advantages of high chemical stability, non-toxicity, and low cost. However, pure-phase TiO 2 limits the absorption of visible light due to its wide bandgap (about 3.2 eV in anatase phase), and the recombination efficiency of photo-generated electrons and holes is relatively high, which severely restricts its photocatalytic performance. However, due to the high selectivity requirements for methane activation, as well as problems such as fast recombination of photo-generated carriers and low activity of traditional photocatalysts, the practical application of photocatalytic partial oxidation of methane still faces many bottlenecks. For example:

[0004] Low liquid-phase product yield: Traditional TiO 2 catalysts are limited by their wide bandgap (~3.2 eV) and can only absorb ultraviolet light, and the recombination rate of photo-generated electron-hole pairs is fast, resulting in low methane conversion yield.

[0005] Complex method for introducing bulk defects: Existing methods for introducing bulk defects (such as high-temperature hydrogenation, plasma treatment) require harsh conditions and it is difficult to precisely control the defect concentration and distribution, which is not conducive to large-scale preparation.

[0006] Insufficient cycle stability: Surface defects are easily oxidized or annihilated during the reaction, resulting in rapid decay of catalyst activity.

[0007] Therefore, the present invention develops a bulk-defect TiO with single metal atoms loaded on its surface 2 nanoparticle photocatalyst, which can be particularly used for photocatalytic partial oxidation of methane. It can not only significantly improve the efficiency of photocatalytic partial oxidation of methane to methanol and formaldehyde, but also has excellent cyclic stability. This provides a green and feasible solution for realizing solar energy-driven clean energy conversion, and has important scientific significance and industrial application prospects. Summary of the Invention

[0008] The object of the present invention is to provide a bulk-defect titanium dioxide (TiO 2 ) nanoparticle with single metal atoms loaded on its surface and a preparation method thereof. This photocatalyst can efficiently realize photocatalytic partial oxidation of methane to methanol and formaldehyde, and particularly solves the key problems of fast recombination rate of photo-generated carriers and poor cyclic stability of the catalyst in the prior art.

[0009] To achieve the above object, the present invention adopts the following technical solutions:

[0010] An embodiment of the present invention provides a preparation method of a bulk-defect titanium dioxide nanoparticle with single metal atoms loaded thereon, including: Step 1, subjecting anatase TiO 2 nanoparticles to high-temperature annealing treatment in an air atmosphere to obtain TiO 2 nanoparticles rich in bulk defects;

[0011] Step 2, taking the TiO 2 nanoparticles rich in bulk defects obtained in Step 1, adding them to deionized water, ultrasonically vibrating to fully disperse them, adding a metal ion solution, heating and stirring the mixture at a constant temperature, separating the product, and drying to obtain TiO 2 nanoparticles loaded with metal ligands;

[0012] Step 3, subjecting the TiO 2 nanoparticles loaded with metal ligands obtained in Step 2 to annealing treatment in an air atmosphere to obtain bulk-defect TiO 2 nanoparticles with single metal atoms loaded on their surfaces. According to the embodiment of the present invention, the preparation method is preferably as follows:

[0013] Step 1, introducing bulk defects by high-temperature annealing

[0014] Heating pure anatase TiO 2 nanoparticles to 750 - 850 °C in an air atmosphere and holding for at least 2 hours. Then naturally cooling to obtain TiO 2 nanoparticles rich in bulk defects. The air atmosphere is crucial for introducing bulk defects, which can effectively avoid the introduction of surface defects. The introduction of bulk defects is beneficial to reducing TiO 2with a band gap and effectively promoting the separation of photo-generated electrons and holes in the photocatalytic process.

[0015] Step 2. Hydrothermal loading of metal

[0016] Take the TiO with bulk defects obtained in Step 1 2 nanoparticles and add them to deionized water, and ultrasonically oscillate to fully disperse them. Accurately add a certain amount of metal ion solution according to the calculation (the metal is selected from one or more of Fe, Co, Ni, Zn, Pt, or Pd). React the obtained mixture at a constant temperature of 80 °C for 3 - 4 hours, then separate the product by centrifugation or filtration, and dry it at 60 - 80 °C for 8 - 24 hours to obtain TiO loaded with metal ligands 2 nanoparticles.

[0017] Step 3. Annealing to remove ligands

[0018] Heat the TiO nanoparticles with bulk defects loaded with metal ligands 2 in an air atmosphere to 325 - 375 °C, keep warm for at least 2 hours and then cool naturally to finally obtain TiO nanoparticles with single metal atoms loaded 2 nanoparticles.

[0019] In the above technical solution, further, in Step 1, the pure anatase phase TiO 2 should be nano-TiO 2 (CAS: 13463 - 67 - 7).

[0020] Further, in Step 2, the metal ion solution is prepared by dissolving a metal salt in deionized water to form a metal ion solution with a metal ion concentration of 0.01 - 0.1 M, and the metal salt is selected from one of the chlorides of Fe, Co, Ni, Zn, Pt, Pd. When preparing the metal ion solution of Fe, dropwise add concentrated hydrochloric acid to make the solution acidic to prevent precipitation. When preparing the metal ion solution of Pt or Pd, since the metal salts of Pd and Pt are insoluble in water, and NH 3 is a ligand that can form stable complexes with transition metals Pd 2+ and Pt 2+ to gradually replace Cl - to generate colorless soluble complexes [Pd(NH 3 ) 4 2+ and [Pt(NH 3 ) 4 2+ while [Pd(NH 3 ) 2 Cl 2 and [Pt(NH 3 )​​4 Cl 2 also has a relatively high solubility to promote the dissolution of PdCl 2 . Therefore, concentrated ammonia water is added dropwise to fully dissolve the metal ions by forming ligands.

[0021] Further, in the second step, the metal loading of the surface-loaded single-atom metal on the bulk-defect titanium dioxide nanoparticles is 0.1-0.5 wt%.

[0022] Further, in the third step, the particle size of the surface-loaded single-atom metal on the bulk-defect titanium dioxide nanoparticles is generally 40-70 nm.

[0023] Further, in the third step, the specific surface area of the surface-loaded single-atom metal on the bulk-defect titanium dioxide nanoparticles is generally 10-20 m 2 / g.

[0024] Further, the surface-loaded single-atom metal on the bulk-defect titanium dioxide nanoparticles is used for the photocatalytic partial oxidation of methane to methanol and formaldehyde.

[0025] The beneficial effects of the present invention are at least as follows:

[0026] Based on pure anatase titanium dioxide nanoparticles, the present invention modifies and prepares a bulk-defect titanium dioxide (TiO 2 ) nanoparticle with a surface-loaded single-atom metal. By introducing bulk defects through high-temperature annealing, the band gap of TiO 2 can be effectively reduced, and its absorption ability for visible light can be enhanced, thereby utilizing more solar energy. This modification measure makes up for the deficiency of traditional TiO 2 photocatalysts in the visible light response ability. Due to the single-atom dispersion characteristics of single-atom metal catalysts, the agglomeration and deactivation problems of traditional metal nanoparticle catalysts during the reaction process can be avoided. In addition, the synergistic effect between bulk defects and single-atom metals further promotes the separation of photo-generated carriers during the photocatalytic process, and the catalytic activity is three times higher than that of untreated pure anatase titanium dioxide. The catalyst preparation process designed by the present invention is simple, the conditions are mild, and it is convenient for large-scale production. Its high photocatalytic performance can provide an economically feasible technical route for the high-value utilization of methane resources and has broad industrial application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 It is a schematic diagram of the preparation process of the bulk-defect titanium dioxide nanoparticles with a surface-loaded single-atom metal in the present invention.

[0028] Figure 2are the Transmission Electron Microscope (TEM) photos and the statistical graph of measured particle sizes. Among them, a-b are the TEM photos of the untreated pure anatase TiO 2 nanoparticles and Sample 1. c-d are the statistical graphs of the measured particle sizes of the untreated pure anatase TiO 2 nanoparticles and Sample 1.

[0029] Figure 3 are the untreated pure anatase TiO 2 nanoparticles, TiO 2 nanoparticles rich in bulk defects (D-TiO 2 ) and Sample 1 (Pd@D-TiO 2 ) X-Ray diffraction (XRD) patterns.

[0030] Figure 4 is the X-ray absorption near-edge structure spectrum of the Pd element in Sample 1.

[0031] Figure 5 is the Fourier transform extended X-ray fine structure fitting spectrum of the Pd element in Sample 1.

[0032] Figure 6 are the Electron Paramagnetic Resonance (EPR) spectra of the untreated pure anatase TiO 2 nanoparticles, TiO 2 nanoparticles rich in bulk defects and Sample 1.

[0033] Figure 7 are the energy band structure diagrams of the untreated pure anatase TiO 2 nanoparticles and Sample 1.

[0034] Figure 8 are the photocatalytic activity test results of the samples obtained by annealing titanium dioxide in air at different temperatures and in different atmospheres (hydrogen-argon mixture, oxygen, argon) at 800 °C.

[0035] Figure 9 are the photocatalytic activity test results of the untreated pure anatase TiO 2 nanoparticles, TiO 2 nanoparticles rich in bulk defects, Sample 1, Sample 2, and Sample 3.

[0036] Figure 10 is the stability cycle test result of Sample 2. Specific implementation manners

[0037] The technical solution of the present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments, but the protection scope of the present invention is not limited thereto.

[0038] According to a specific embodiment of the present invention, a method for preparing bulk-defect titanium dioxide (TiO 2 ) nanoparticles with surface-loaded single metal atoms, as Figure 1 shown, may include the following steps:

[0039] (1) Introducing bulk defects by high-temperature annealing

[0040] Take pure anatase TiO 2 nanoparticles (CAS: 13463-67-7), and place them in a muffle furnace. Under an air atmosphere, heat them at a heating rate of 5 °C / minute to 750-850 °C. After holding for 2 hours, cool naturally to obtain bulk-defect TiO 2 nanoparticles with a particle size of 40-70 nm and a specific surface area of 10-20 m 2 / g. It should be noted here that the annealing temperature and annealing atmosphere should be strictly restricted.

[0041] Preferably, when annealing, the titanium dioxide particle powder should be paved flat, and the thickness after being evenly spread out should not exceed 2 cm to ensure sufficient calcination.

[0042] (2) Loading metals by hydrothermal method

[0043] Weigh 200 mg of the prepared bulk-defect TiO 2 nanoparticles, add them to 150 ml of deionized water, and ultrasonically oscillate to disperse them fully.

[0044] Prepare metal ion solutions as needed: Dissolve the chlorides of Fe, Co, Ni, Zn, Pt or Pd in deionized water to prepare solutions with a metal ion concentration of 0.01-0.1 M. When preparing the Fe metal ion solution, an appropriate amount of concentrated hydrochloric acid needs to be added dropwise to adjust the solution to be acidic; when preparing the Pt or Pd metal ion solution, concentrated ammonia water needs to be added dropwise to form stable ligands and promote the dissolution of metal ions. Accurately calculate and add an appropriate amount of metal ion solution to make the final metal loading 0.1-0.5 wt%. It should be noted here that a higher metal loading may cause metal atoms to aggregate to form clusters or nanoparticles during subsequent further annealing to remove ligands. Place the above mixture in a constant-temperature reaction at 80 °C for 3-4 hours to ensure that the metal ions are evenly loaded on the surface of the defective TiO 2 nanoparticles. Centrifuge or filter the product to separate it, and dry it at 60-80 °C for 8-24 hours to obtain bulk-defect TiO 2 nanoparticles loaded with metal ligands.

[0045] (3) Annealing to remove the ligands will load the metal ligands

[0046] The obtained bulk defect TiO 2 The nanoparticles were placed in a muffle furnace and heated to 325-375°C at a heating rate of 5°C / min in an air atmosphere. After 2 hours of heating, they were naturally cooled to obtain a volume defect TiO with metal single atoms loaded on the surface. 2 Nanoparticles.

[0047] Example 1

[0048] Preparation of a bulk-defective TiO with single palladium atoms loaded on its surface 2 The nanoparticles contain 0.1 wt% palladium.

[0049] Weigh 1g of pure anatase phase TiO 2 Nanoparticles (CAS: 13463-67-7) were placed in a magnetic boat and heated to 800°C in a muffle furnace under air atmosphere at a heating rate of 5°C / min. After being kept at this temperature for 2 hours, they were naturally cooled to obtain TiO 2 Nanoparticles.

[0050] Weigh 200 mg of prepared bulk defect TiO 2 Nanoparticles were added to 150 ml of deionized water and ultrasonically dispersed. 4+ Metal ion solution: H 2 PdCl 6 Dissolve in deionized water, add appropriate amount of concentrated ammonia to form a stable ammonia complex. Add an accurate amount of Pd metal ion solution to control the metal loading to 0.1wt%. Stir the mixture in a water bath at 80℃ for 4 hours, centrifuge and dry the product at 80℃ for 24 hours to obtain a bulk defect TiO loaded with Pd metal ligands. 2 The above product was placed in a muffle furnace and heated to 350°C at a heating rate of 5°C / min. After being kept at this temperature for 2 hours, it was naturally cooled to obtain a bulk defect TiO with a single Pd atom on the surface. 2 Nanoparticles, denoted as sample 1.

[0051] Untreated pure anatase phase TiO 2 Transmission electron microscope (TEM) photos of nanoparticles and sample 1 and the statistical results of particle size measurement are shown in Figure 2 shown.

[0052] Untreated pure anatase phase TiO 2 Nanoparticles, bulk defect-rich TiO 2The X-ray diffraction patterns (X-Ray diffraction, XRD) of the nanoparticles and Sample 1 are as Figure 3 shown.

[0053] The X-ray absorption near-edge structure spectrum of the Pd element in Sample 1 is as Figure 4 shown.

[0054] The Fourier transform extended X-ray fine structure fitting spectrum of the Pd element in Sample 1 is as Figure 5 shown. Among them, Figure 4 and Figure 5 both well prove that single metal atoms are successfully loaded on the surface of the characterized sample.

[0055] For the untreated pure anatase TiO 2 nanoparticles, TiO 2 nanoparticles rich in bulk defects, and Sample 1, low-temperature electron paramagnetic resonance (Electron Paramagnetic Resonance, EPR) characterization was carried out, and the results are as Figure 6 shown, and the results prove that bulk defects are successfully introduced into the prepared catalyst.

[0056] The energy band structures of the untreated pure anatase TiO 2 nanoparticles and Sample 1 are as Figure 7 shown, and the results well prove that the modification of pure rutile titanium dioxide narrows the band gap to a certain extent.

[0057] Photocatalytic partial oxidation performance tests were carried out on samples of titanium dioxide annealed in air at different temperatures and in different atmospheres (hydrogen-argon mixture, oxygen, argon) at 800 °C. The results are as Figure 8 shown. It can be seen that the results well prove the importance of strictly controlling the annealing atmosphere and temperature, and also prove the importance of introducing bulk defects in improving the yield of catalytic products.

[0058] Example 2

[0059] Prepare a bulk-defect TiO 2 nanoparticle with nickel single atoms loaded on the surface, where the mass fraction of nickel metal is 0.1 wt%.

[0060] Weigh 1 g of pure anatase TiO 2 nanoparticles (CAS: 13463-67-7) in a magnetic boat, and heat them in a muffle furnace under an air atmosphere at a heating rate of 5 °C per minute to 800 °C. After holding for 2 hours, cool naturally to obtain TiO 2 nanoparticles rich in bulk defects. Weigh 200 mg of the prepared bulk-defect TiO 2Nanoparticles were added to 150 ml of deionized water and ultrasonically oscillated to disperse them fully. A 0.05 M Ni 2+ metal ion solution was prepared: NiCl 2 was dissolved in deionized water, and an appropriate amount of concentrated hydrochloric acid was added dropwise to adjust the solution to be acidic to ensure complete dissolution of the metal ions. After precise calculation, an appropriate amount of Ni2+ metal ion solution was added to the dispersion, and the metal loading was controlled at 0.3 wt%. The mixture was stirred at a constant temperature of 80 °C in a water bath for 3 hours, and then the product was centrifuged. The separated solid was dried at 60 °C for 12 hours to obtain body-defect TiO 2 nanoparticles loaded with Ni metal ligands. The above mixture was stirred at a constant temperature of 80 °C in a water bath for 4 hours, centrifuged, and the product was dried at 80 °C for 24 hours to obtain body-defect TiO 2 nanoparticles loaded with Ni metal ligands. The above product was placed in a muffle furnace and heated to 350 °C at a heating rate of 5 °C / min, held for 2 hours, and then naturally cooled to obtain body-defect TiO 2 nanoparticles with Ni single atoms loaded on the surface, denoted as Sample 2.

[0061] Example 3

[0062] To prepare body-defect TiO 2 nanoparticles with cobalt metal single atoms loaded on the surface, where the mass fraction of cobalt metal is 0.1 wt%.

[0063] Weigh 1 g of pure anatase TiO 2 nanoparticles (CAS: 13463-67-7) in a magnetic boat, and heat them to 800 °C at a heating rate of 5 °C / min in a muffle furnace under an air atmosphere. After holding for 2 hours, naturally cool to obtain body-defect-rich TiO 2 nanoparticles. Weigh 200 mg of the prepared body-defect TiO 2 nanoparticles, add them to 150 ml of deionized water, and ultrasonically oscillate to disperse them fully. Prepare a 0.05 M Co 2+ metal ion solution: CoCl 2 was dissolved in deionized water, and an appropriate amount of concentrated hydrochloric acid was added dropwise to adjust the solution to be acidic to ensure complete dissolution of the metal ions. After precise calculation, an appropriate amount of Co 2+ metal ion solution was added to the dispersion, and the metal loading was controlled at 0.1 wt%. The mixture was stirred at a constant temperature of 80 °C in a water bath for 3 hours, and then the product was centrifuged. The separated solid was dried at 60 °C for 12 hours to obtain body-defect TiO 2 nanoparticles loaded with Co metal ligands. The above mixture was stirred at a constant temperature of 80 °C in a water bath for 4 hours, centrifuged, and the product was dried at 80 °C for 24 hours to obtain body-defect TiO 2Nanoparticles. The above product was placed in a muffle furnace and heated to 350 °C at a heating rate of 5 °C per minute, held for 2 hours, and then naturally cooled to obtain bulk-defect TiO with Co single atoms supported on the surface. 2 nanoparticles, denoted as Sample 3.

[0064] Photocatalytic partial oxidation performance test of methane

[0065] The photocatalytic partial oxidation performance of the bulk-defect titanium dioxide nanoparticles with different metal single atoms supported on the surface, the bulk-defect titanium dioxide nanoparticles without metal support, and the pure anatase titanium dioxide nanoparticles prepared in Examples 1-3 above was tested as follows:

[0066] Weigh 10 mg of the prepared bulk-defect titanium dioxide nanoparticles with different metal single atoms supported on the surface, and ultrasonically disperse them in 500 ml of deionized water. Place the above mixture in a photocatalytic high-pressure reactor, and place a magnetic stirrer in it to stir at 800 r / min. Subsequently, 0.1 MPa of high-purity oxygen and 2 MPa of high-purity methane were respectively introduced. Under the irradiation of a 300 W xenon lamp with a full spectrum (controlling the light intensity at 60 W / cm -2 ), a photocatalytic partial oxidation experiment of methane was carried out. During the reaction, a condensation device was turned on to ensure that the experimental temperature was controlled at 20-25 °C. After 4 hours of reaction, the reaction gas containing the gas-phase product was collected with a Teflon gas bag, and the supernatant after centrifuging the reaction solution was collected as the reaction solution containing the liquid-phase product. Quantitative analysis of the gas-phase product carbon dioxide was carried out by a gas chromatograph (Gas Chromatography, GC) using a flame ionization detector (Flame Ionization Detector, FID). A formaldehyde color-developing solution was prepared, and quantitative analysis of the liquid-phase product formaldehyde was carried out by ultraviolet-visible absorption spectroscopy (UV-Visible Spectrum). Quantitative analysis of liquid-phase products such as methanol and peroxymethanol was carried out by nuclear magnetic resonance spectroscopy (Nuclear Magnetic Resonance, NMR).

[0067] Untreated pure anatase TiO 2 nanoparticles, bulk-defect TiO-rich 2 nanoparticles, the activity test results of Sample 1, Sample 2, and Sample 3 are as Figure 9 shown. It can be seen that the samples prepared by the present invention have excellent photocatalytic partial oxidation performance of methane.

[0068] The stability cycle test results of Sample 2 are as Figure 10 shown.

[0069] The embodiments described above are only some of the better solutions of the present invention, but they are not intended to limit the present invention. Those of ordinary skill in the relevant technical field can still make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, all technical solutions obtained by means of equivalent replacement or equivalent transformation fall within the protection scope of the present invention.

Claims

1. A method for preparing bulk-defective titanium dioxide nanoparticles with metal single atoms loaded on the surface, characterized in that: include: Step 1, subjecting anatase phase TiO2 nanoparticles to a high temperature annealing treatment in an air atmosphere to obtain TiO2 nanoparticles rich in body defects; Step 2: Take the TiO2 nanoparticles rich in body defects obtained in step 1, add them into deionized water, perform ultrasonic vibration to fully disperse them, add metal ion solution, heat the mixed solution and stir it at a constant temperature, separate the product, and dry it to obtain TiO2 nanoparticles loaded with metal ligands; Step 3: After annealing treatment in air atmosphere, the TiO2 nanoparticles loaded with metal ligands prepared in step 2 are subjected to body defect TiO2 nanoparticles loaded with metal single atoms on the surface.

2. The method for preparing the surface-loaded metal single atom-loaded bulk-defective titanium dioxide nanoparticles according to claim 1, characterized in that: The high temperature annealing treatment is carried out at 750-850° C. for at least 2 hours.

3. The method for preparing the surface-loaded metal single atom-loaded bulk-defective titanium dioxide nanoparticles according to claim 1, characterized in that: The metal ion solution is prepared by dissolving a soluble metal salt in deionized water, and the metal is selected from one or more of Fe, Co, Ni, Zn, Pt, and Pd. When preparing the Fe metal ion solution, concentrated hydrochloric acid is added dropwise to make the solution acidic, and when preparing the Pt or Pd metal ion solution, concentrated ammonia water is added dropwise to make the metal ions form ligands and fully dissolve.

4. The method for preparing the volume-defective titanium dioxide nanoparticles with metal single atoms loaded on the surface according to claim 1, characterized in that: The concentration of metal ions in the metal ion solution is 0.01-0.1M.

5. The method for preparing the bulk-defective titanium dioxide nanoparticles with metal single atoms loaded on the surface according to claim 1, characterized in that: The heating and constant temperature stirring treatment is a constant temperature reaction at 80° C. for 3-4 hours.

6. The method for preparing the volume-defective titanium dioxide nanoparticles with metal single atoms loaded on the surface according to claim 1, characterized in that: The annealing treatment in step 3 is carried out at 325-375° C. for at least 2 hours.

7. Bulk-defective titanium dioxide nanoparticles with metal single atoms loaded on the surface, characterized in that: The method is prepared by the method according to any one of claims 1 to 6.

8. The bulk-defective titanium dioxide nanoparticles loaded with metal single atoms on the surface according to claim 7, characterized in that: The metal loading in the nanoparticles is 0.1-0.5 wt %.

9. The bulk-defective titanium dioxide nanoparticles with metal single atoms loaded on the surface according to claim 7, characterized in that: The particle size of the nanoparticles is 40-70nm and the specific surface area is 10-20m 2 / g.

10. The bulk-defective titanium dioxide nanoparticles with metal single atoms loaded on the surface as claimed in claim 7 are used for photocatalytic partial oxidation of methane to produce methanol and formaldehyde.

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