Zinc oxide heterojunction photocatalyst as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of photocatalytic methane, and relates to a zinc oxide heterojunction photocatalyst and a preparation method and application thereof. Zinc acetate is dispersed in deionized water, and the solution is marked as a solution A; dispersing sodium hydroxide in deionized water to obtain a solution B; dropwise adding the solution B into the solution A, reacting, standing, washing with deionized water and ethanol respectively, centrifuging, drying in a drying oven overnight, transferring into a muffle furnace, and obtaining ZnO at constant temperature; dispersing ZnO in deionized water to obtain a solution C; adding the Pd precursor solution into the aqueous solution, then adding polyvinylpyrrolidone into the solution until the polyvinylpyrrolidone is uniformly dispersed, and recording the solution as a solution D; dropwise adding the solution D into the solution C, adding an LED array for illumination, and standing after reaction; and respectively washing with deionized water and ethanol, centrifuging, drying with supercritical CO2, transferring a sample into a muffle furnace, and carrying out stepped heating roasting to obtain Pd-PdO / ZnO. According to the preparation method, a reducing agent does not need to be reinforced, side reactions are reduced, the positioning accuracy of metal deposition is improved, and the photocatalytic methane conversion activity is high.

Description

Technical Field

[0001] The present invention belongs to the technical field of photocatalytic methane, and particularly relates to a zinc oxide heterojunction photocatalyst, a preparation method thereof, and an application thereof. Background Art

[0002] In the past 20 years, the global natural gas production has increased from 252.39 billion cubic meters to 403.69 billion cubic meters, with an increase of 60%. Compared with petroleum and coal, natural gas is recognized as one of the cleanest fossil fuels because the sulfur oxides and nitrogen oxides released during the combustion process are significantly reduced, and it has the characteristic of higher calorific value. With the continuous growth of the proven reserves of natural gas, the application of its main component methane (CH4) has expanded from the traditional fuel field to the chemical raw material field, attracting extensive attention from the academic and industrial circles. If the efficient utilization of methane can be achieved, it will effectively alleviate the current resource shortage dilemma. The photocatalytic technology can generate high-energy carriers at room temperature, which can effectively activate methane molecules and reduce the reaction activation energy, breaking through the traditional thermodynamic limitations, and showing unique advantages in reducing fossil energy consumption and CO2 emissions. Among them, the technical route of directly photocatalytically oxidizing methane to prepare high-value-added products has important application value for promoting the transformation of the energy structure due to its environmental friendliness and energy efficiency.

[0003] As a common semiconductor photocatalyst material, ZnO has limitations in the generation of reactive oxygen species due to its low separation efficiency of photo-generated carriers and relatively wide bandgap. Researchers have implemented various strategies to improve the performance of ZnO, including loading noble metals, doping, and constructing heterojunctions, etc. Chinese Patent CN114618469 A discloses a supported ZnO photocatalyst, and a series of photocatalysts with ZnO as the carrier are prepared by the impregnation method, but its reaction temperature is 50 °C, heating is required during the reaction, and the product yield is relatively low. Zhang Zhiguo et al. (Chinese Journal of Catalysis, 2024, 67: 61-70) photocatalytically oxidized methane to prepare C1 oxygenates by using an Ag-AgBr / ZnO heterojunction catalyst under mild reaction conditions, but the catalytic efficiency is low, and the C1 product yield is low, only 14.99 μmol h -1 。 Summary of the Invention

[0004] The purpose of the present invention is to provide a zinc oxide heterojunction photocatalyst, a preparation method thereof, and an application thereof, to solve the problems of low yield and poor selectivity of existing photocatalysts in the direct oxidation reaction of methane.

[0005] To achieve the above purpose, the present invention adopts the following technical solutions: A zinc oxide heterojunction photocatalyst, named Pd-PdO / ZnO, in which the loading amount of Pd is 1% - 7%, and ZnO has an irregular rod-like structure.

[0006] The preparation method of the above zinc oxide heterojunction photocatalyst is as follows: S1: First, disperse 0.9 - 4.5 g of zinc acetate (Zn(AC)2) in 50 - 200 mL of deionized water, and stir until Zn(AC)2 is completely dissolved, denoted as solution A; weigh 0.6 - 3.0 g of sodium hydroxide (NaOH) and disperse it in 10 - 50 mL of deionized water, and stir until NaOH is completely dissolved, denoted as solution B; dropwise add solution B to solution A, stir and react fully for 2 h, after stirring, let it stand for 1 h, wash with deionized water and ethanol respectively, then centrifuge, transfer it to an oven and dry overnight at 60 °C, and then transfer it to a muffle furnace, and heat it up to 500 °C at a heating rate of 5 °C / min, and keep it at a constant temperature for 4 h to obtain ZnO; S2: Disperse 1 g of ZnO in 50 - 200 mL of deionized water, denoted as solution C; take 4.7 - 32.9 mL of Pd precursor solution and add it to 50 - 200 mL of aqueous solution, then add 0.0025 - 0.01 g of polyvinylpyrrolidone (PVP) to the solution, stir for 1 h until it is uniformly dispersed, denoted as solution D; dropwise add solution D to solution C, and simultaneously apply LED array illumination, use the ultraviolet light response characteristics of ZnO to generate electron-hole pairs, and drive the 2+ selective reduction deposition of Pd, stir and react fully for 2 h, after stirring, let it stand for 1 h; S3: Wash with deionized water and ethanol respectively, then centrifuge, use supercritical CO2 drying, transfer the sample to a muffle furnace, and use a stepped heating program for heating to calcine to obtain Pd-PdO / ZnO.

[0007] Further, the molar ratio of zinc acetate to sodium hydroxide in step S1 is (1 - 1.5):3.

[0008] Further, the Pd precursor in step S2 is any one of Pd chloride, nitrate, and sulfate, and the concentration of the Pd precursor solution is 0.005 - 0.01 mg / mL.

[0009] Further, the wavelength of the LED light in step S2 is 350 - 480 nm.

[0010] Further, the drying temperature of supercritical CO2 in step S3 is 40 - 60 °C, and the stepped heating program in the muffle furnace is to heat up to 150 °C at 1 °C / min, and then heat up to 300 - 500 °C at 5 °C / min, and the continuous heating time is 2 - 5 h.

[0011] Principle of catalyst preparation: After Zn(AC)2 is dissolved, it dissociates into Zn 2+ and AC - , after NaOH is dissolved, it produces Na + and OH - , when NaOH is dropped into the solution, the pH increases, causing Zn 2+ to react with OH - to form Zn(OH)2. After the cleaning and drying steps, the excess ions and organic substances are removed. Finally, calcination is carried out to convert Zn(OH)2 into ZnO. Utilizing the ultraviolet light response characteristics of ZnO, electron-hole pairs are generated by LED light illumination to achieve the selective reduction of Pd 2+ . Compared with the traditional chemical reduction method, there is no need to additionally add a strong reducing agent (such as NaBH4), reducing side reactions and improving the positioning accuracy of metal deposition.

[0012] The zinc oxide heterojunction photocatalyst shows great application potential in the photocatalytic methane oxidation reaction, effectively promoting the separation of photo-generated carriers, inhibiting the over-oxidation of products while improving the photocatalytic efficiency, and can be used for photocatalytic conversion of methane into C1 oxygenates. The specific application steps are as follows: Take 5 - 30 mg of the catalyst and dissolve it in 10 - 30 mL of deionized water by ultrasonic method. The methane pressure is 0.1 - 3.0 MPa, oxygen is used as the oxidant, the oxygen pressure is 0 - 1.0 MPa, the total pressure is 0.1 - 3.0 MPa, the light source is a xenon lamp with a wavelength range of 350 - 780 nm, the reaction is carried out at room temperature, and the reaction is carried out for 1 - 6 h under the irradiation of the xenon lamp to detect the contents of gas-phase and liquid-phase products.

[0013] The detection methods for gas-phase and liquid-phase products include: Gas chromatography (GC): used to detect CO, CO2, CH3OH, CH3CH2OH, CH3COOH, etc.; Ion chromatography (IC): used to detect HCOOH, etc.; Acetylacetone colorimetric method: used to detect HCHO.

[0014] Furthermore, the volume ratio of methane to oxygen is 21:9.

[0015] Reaction mechanism of the catalyst for oxidizing methane to prepare C1 oxygenates: When irradiated with light, the e - of ZnO is excited to the conduction band (CB), leaving h + in the valence band (VB); the e - at the CB of ZnO reacts with the h +The composite leaves h with stronger oxidation ability on ZnO + and e with stronger reduction ability on PdO - ; Pd, as an electron acceptor, receives e from the CB of PdO - and reduces O2 to •O2 - ; •O2 - then reacts with H + and e - in the solution to generate •OOH; •OOH is reduced by e - to •OH, and the reaction formula is O2 → •O2 - → •OOH → •OH. CH4 is oxidized by h + remaining at the VB of ZnO to •CH3, and the reaction formula is CH4 + h + → •CH3; there is also a small amount of H2O oxidized by h + at the VB of ZnO to •OH, and the reaction formula is H2O + h + → •OH. The dual •OH generation paths are (O2 → •O2 - → •OOH → •OH and H2O + h + → •OH). Methane conversion follows a stepwise oxidation mechanism: CH4 → •CH3 → CH3OH → HCHO → HCOOH → CO2; the energy band structure design of the heterojunction can, while enhancing the oxidation ability, effectively inhibit the deep oxidation side reaction by regulating the generation of reactive oxygen species, thereby achieving the highly selective accumulation of C1 oxygenates.

[0016] Compared with the prior art, the beneficial effects achieved by the present invention are as follows: (1) Through the LED array photoreduction deposition method, the active components Pd, PdO and the carrier ZnO can be uniformly distributed in the catalyst, avoiding local concentration unevenness or phase separation; (2) By adding polyvinylpyrrolidone (PVP) and LED array illumination, the particle growth can be controlled, the agglomeration of active components can be reduced, and the active sites can be uniformly anchored on the surface of the carrier, improving the catalytic efficiency; (3) The photocatalyst prepared by this method can carry out the reaction of photocatalytic methane oxidation to C1 products at room temperature without heating, and the conditions are mild; secondly, under light illumination, using oxygen as the oxidant is more economical and environmentally friendly; (4) The preparation method of this photocatalyst is simple, easy to operate, does not require a strong reducing agent, reduces side reactions, and the process parameters are easy to control, suitable for large-scale production, and has broad application prospects in the reaction of photocatalytic methane to C1 products. Description of the Drawings

[0017] Figure 1XRD patterns of the photocatalysts of Example 1, Example 2, Example 3, Example 4 and Comparative Example 1; Figure 2 SEM images of the photocatalysts of Example 3 and Comparative Example 1; Figure 3 TEM images of the photocatalysts of Example 3 and Comparative Example 1; Figure 4 Comparison chart of the yields and selectivities of the photocatalytic oxidation of methane to C1 products over the photocatalysts of Example 1, Example 2, Example 3, Example 4 and Comparative Example 1; Figure 5 Comparison chart of the yields and selectivities of the photocatalytic oxidation of methane to C1 products with different gas ratios in Example 5, Example 6, Example 7 and Example 8; Figure 6 Comparison chart of the yields and selectivities of the photocatalytic oxidation of methane to C1 products with different water amounts in Example 9, Example 10, Example 11 and Example 12; Figure 7 Comparison chart of the yields and selectivities of the photocatalytic oxidation of methane to C1 products with different total pressures in Example 13, Example 14, Example 15 and Example 16; Figure 8 Comparison chart of the yields and selectivities of the photocatalytic oxidation of methane to C1 products with different catalyst masses in Example 17, Example 18, Example 19 and Example 20; Figure 9 Comparison chart of the yields and selectivities of the photocatalytic oxidation of methane to C1 products with different reaction times in Example 21, Example 22, Example 23, Example 24, Example 25, Example 26 and Example 27. Detailed implementation manners

[0018] The technical solutions of the present invention will be clearly and completely described below in conjunction with the accompanying drawings and examples. Example 1

[0019] First, 0.9 g of zinc acetate (Zn(AC)2) was dispersed in 50 - 200 mL of deionized water and stirred until all Zn(AC)2 was dissolved, denoted as solution A; 0.6 - 3.0 g of sodium hydroxide (NaOH) was weighed and dispersed in 10 - 50 mL of deionized water and stirred until all NaOH was dissolved, denoted as solution B; solution B was added dropwise to solution A, and the mixture was stirred for 2 h to react fully. After stirring, it was left standing for 1 h, washed with deionized water and ethanol respectively, then centrifuged, dried overnight at 60 °C in an oven, and then transferred to a muffle furnace. It was heated to 500 °C at a heating rate of 5 °C / min and held at a constant temperature for 4 h to obtain ZnO. 1 g of ZnO was dispersed in 50 - 200 mL of deionized water, denoted as solution C; 4.7 mL of PdCl2 solution was added to 50 - 200 mL of aqueous solution, and then 0.0025 - 0.01 g of polyvinylpyrrolidone (PVP) was added to the solution. The mixture was stirred for 1 h until uniformly dispersed, denoted as solution D; solution D was added dropwise to solution C, and at the same time, an LED array light was applied. Using the ultraviolet light response characteristics of ZnO, electron-hole pairs were generated to drive the selective reduction deposition of Pd 2+ for 2 h. After stirring, it was left standing for 1 h. It was washed with deionized water and ethanol respectively, then centrifuged, dried by supercritical CO2, the sample was transferred to a muffle furnace, and the temperature was raised to 300 °C using a stepped heating program and held at a constant temperature for 2 h to obtain 1% Pd-PdO / ZnO. 10 mg of the catalyst was ultrasonically dissolved in 30 mL of deionized water, filled with a methane-oxygen ratio of 29:1, a total pressure of 3 MPa, and a reaction temperature of 25 °C. The reaction was carried out under the irradiation of a xenon lamp with a wavelength range of 350 - 780 nm for 1 h, and the yield of C1 products was 43.62 μmol h -1 , and the selectivity was 82.17% (C1 product selectivity = (yield of HCHO + HCOOH) / (yield of HCHO + HCOOH + CH3CHO + CO2)). Example 2

[0020] First, 0.9 g of zinc acetate (Zn(AC)2) was dispersed in 50 - 200 mL of deionized water and stirred until all Zn(AC)2 was dissolved, denoted as solution A. 0.6 - 3.0 g of sodium hydroxide (NaOH) was weighed and dispersed in 10 - 50 mL of deionized water and stirred until all NaOH was dissolved, denoted as solution B. Solution B was added dropwise to solution A, and the mixture was stirred for 2 h for full reaction. After stirring, it was left standing for 1 h. It was washed with deionized water and ethanol respectively, then centrifuged, dried overnight at 60 °C in an oven, and then transferred to a muffle furnace. It was heated to 500 °C at a heating rate of 5 °C / min and held at a constant temperature for 4 h to obtain ZnO. 1 g of ZnO was dispersed in 50 - 200 mL of deionized water, denoted as solution C. 14.1 mL of PdCl2 solution was added to 50 - 200 mL of aqueous solution, and then 0.0025 - 0.01 g of polyvinylpyrrolidone (PVP) was added to the solution. It was stirred for 1 h until evenly dispersed, denoted as solution D. Solution D was added dropwise to solution C, and at the same time, an LED array light was applied. Using the ultraviolet light response characteristics of ZnO, electron-hole pairs were generated to drive the selective reduction deposition of Pd 2+ for 2 h for full reaction. After stirring, it was left standing for 1 h. It was washed with deionized water and ethanol respectively, then centrifuged, and dried by supercritical CO2. The sample was transferred to a muffle furnace and heated to 300 °C using a stepwise heating program and held at a constant temperature for 2 h to obtain 3% Pd-PdO / ZnO. 10 mg of the catalyst was ultrasonically dissolved in 30 mL of deionized water, filled with a methane-oxygen ratio of 29:1, a total pressure of 3 MPa, and a reaction temperature of 25 °C. It was reacted for 1 h under the irradiation of a xenon lamp with a light source in the wavelength range of 350 - 780 nm. The yield of C1 products was 53.36 μmol h -1 , and the selectivity was 81.62%. Example 3

[0021] First, 0.9 g of zinc acetate (Zn(AC)2) was dispersed in 50 - 200 mL of deionized water and stirred until all Zn(AC)2 was dissolved, denoted as solution A; 0.6 - 3.0 g of sodium hydroxide (NaOH) was weighed and dispersed in 10 - 50 mL of deionized water and stirred until all NaOH was dissolved, denoted as solution B; solution B was added dropwise to solution A, and after stirring for a full reaction for 2 h, it was left to stand for 1 h after the stirring ended. It was washed with deionized water and ethanol respectively, then centrifuged, dried overnight at 60 °C in an oven and then transferred to a muffle furnace, and heated to 500 °C at a heating rate of 5 °C / min and held at a constant temperature for 4 h to obtain ZnO. 1 g of ZnO was dispersed in 50 - 200 mL of deionized water, denoted as solution C; 23.5 mL of PdCl2 solution was added to 50 - 200 mL of aqueous solution, and then 0.0025 - 0.01 g of polyvinylpyrrolidone (PVP) was added to the solution and stirred for 1 h until uniformly dispersed, denoted as solution D; solution D was added dropwise to solution C, and at the same time, an LED array light was applied. Using the ultraviolet light response characteristics of ZnO, electron - hole pairs were generated to drive the 2+ selective reduction deposition of Pd. After stirring for a full reaction for 2 h, it was left to stand for 1 h after the stirring ended. It was washed with deionized water and ethanol respectively, then centrifuged, and dried using supercritical CO2. The sample was transferred to a muffle furnace and heated to 300 °C using a stepped heating program and held at a constant temperature for 2 h to obtain 5% Pd - PdO / ZnO. 10 mg of the catalyst was ultrasonically dissolved in 30 mL of deionized water, filled with a methane - oxygen ratio of 29:1, a total pressure of 3 MPa, and a reaction temperature of 25 °C, and reacted for 1 h under the irradiation of a xenon lamp with a light source in the wavelength range of 350 - 780 nm. The yield of C1 products was 45.65 μmol h -1 -1, and the selectivity was 64.55%. Example 4

[0022] Disperse 0.9 g of zinc acetate (Zn(AC)₂) in 50 - 200 mL of deionized water, and stir until all Zn(AC)₂ is dissolved, denoted as solution A; weigh 0.6 - 3.0 g of sodium hydroxide (NaOH) and disperse it in 10 - 50 mL of deionized water, stir until all NaOH is dissolved, denoted as solution B; add solution B dropwise to solution A, stir for 2 h for full reaction, after stirring, let it stand for 1 h, wash with deionized water and ethanol respectively, then centrifuge, dry overnight at 60 °C in an oven and transfer to a muffle furnace, heat it to 500 °C at a heating rate of 5 °C / min, and keep it at a constant temperature for 4 h to obtain ZnO. Disperse 1 g of ZnO in 50 - 200 mL of deionized water, denoted as solution C; take 32.9 mL of PdCl₂ solution and add it to 50 - 200 mL of aqueous solution, then add 0.0025 - 0.01 g of polyvinylpyrrolidone (PVP) to the solution, stir for 1 h until evenly dispersed, denoted as solution D; add solution D dropwise to solution C, simultaneously apply LED array illumination, utilize the ultraviolet light response characteristics of ZnO to generate electron-hole pairs, and drive the selective reduction deposition of Pd 2+ After stirring for 2 h for full reaction, after stirring, let it stand for 1 h. Wash with deionized water and ethanol respectively, then centrifuge, and use supercritical CO₂ drying. Transfer the sample to a muffle furnace, heat it to 300 °C using a stepped heating program, and keep it at a constant temperature for 2 h to obtain 7% Pd-PdO / ZnO. Take 10 mg of the catalyst and ultrasonically dissolve it in 30 mL of deionized water, charge a methane-oxygen ratio of 29:1, a total pressure of 3 MPa, a reaction temperature of 25 °C, and react for 1 h under the irradiation of a xenon lamp with a wavelength range of 350 - 780 nm. The yield of C1 products is 44.67 μmol h -1 ⁻¹, and the selectivity is 59.36%.

[0023] Comparative Example 1 First, disperse 0.9 g of Zn(AC)₂ in 50 mL of deionized water, stir until all Zn(AC)₂ is dissolved, denoted as solution A; weigh 0.6 g of NaOH and disperse it in 10 mL of deionized water, stir until all NaOH is dissolved, denoted as solution B; add solution B dropwise to solution A, stir for 2 h for full reaction, after stirring, let it stand for 1 h, wash with deionized water and ethanol respectively, then centrifuge, dry overnight at 60 °C in an oven and transfer to a muffle furnace, heat it to 500 °C at a heating rate of 5 °C / min, and keep it at a constant temperature for 4 h to obtain ZnO. Take 10 mg of the catalyst and ultrasonically dissolve it in 30 mL of deionized water, charge a methane-oxygen ratio of 29:1, a total pressure of 3 MPa, a reaction temperature of 25 °C, and react for 1 h under the irradiation of a xenon lamp with a wavelength range of 350 - 780 nm. The yield of C1 products is 7.11 μmol h -1, with a selectivity of 78.20%.

[0024] Figure 1 XRD patterns of the catalysts of Example 1, Example 2, Example 3, Example 4 and Comparative Example 1 are shown. All five catalysts show distinct diffraction peaks at 2θ = 33.82°, 34.46°, 36.32°, 47.65°, 56.56° and 62.93°, which are attributed to the (100), (002), (101), (102), (110) and (103) crystal planes of hexagonal ZnO. No characteristic peaks of Pd species are observed, probably because Pd species are highly dispersed or the content of Pd species is low.

[0025] Figure 2 SEM images of Comparative Example 1 and Example 2 are shown. Both samples exhibit an irregular rod-like structure, indicating that the addition of Pd species has no effect on the morphology of ZnO.

[0026] Figure 3 TEM images of Comparative Example 1 and Example 2 are shown. Lattice fringes with a lattice spacing of 0.248 nm attributed to the ZnO(101) crystal plane are observed in Comparative Example 1. In Example 2, lattice fringes of 0.225 nm attributed to Pd(111), lattice fringes of PdO(101) with d = 0.265 nm and three lattice fringes of ZnO(101) are simultaneously observed, and an obvious contact interface indicates the formation of a heterostructure.

[0027] Figure 4 Comparison diagrams of the yields and selectivities of photocatalytic oxidation of methane to C1 products by photocatalysts with different loading ratios of Example 1, Example 2, Example 3, Example 4 and Comparative Example 1 are shown. Among them, after one hour of light irradiation, the highest yield of C1 products of 3%Pd-PdO / ZnO is 53.36 μmol h -1 , with a selectivity of 81.62%. As the Pd content continues to increase, the yield of C1 products gradually decreases. Therefore, the optimal doping amount of Pd is 3 wt.%. Example 5

[0028] 10 mg of 3%Pd-PdO / ZnO is ultrasonically dissolved in 30 mL of deionized water, and methane and oxygen are charged at a ratio of 26:4, with a total pressure of 3 MPa and a reaction temperature of 25°C. The reaction is carried out for 1 h under irradiation with a xenon lamp with a wavelength range of 350 - 780 nm. The yield of C1 products is 64.38 μmol h -1 , with a selectivity of 82.88%. Example 6

[0029] 10 mg of 3% Pd-PdO / ZnO was ultrasonically dissolved in 30 mL of deionized water. Methane and oxygen were charged in a ratio of 24:6, the total pressure was 3 MPa, the reaction temperature was 25 °C, and the reaction was carried out for 1 h under irradiation of a xenon lamp with a wavelength range of 350 - 780 nm. The yield of C1 products was 62.03 μmol h -1 , and the selectivity was 87.39%. Example 7

[0030] 10 mg of 3% Pd-PdO / ZnO was ultrasonically dissolved in 30 mL of deionized water. Methane and oxygen were charged in a ratio of 21:9, the total pressure was 3 MPa, the reaction temperature was 25 °C, and the reaction was carried out for 1 h under irradiation of a xenon lamp with a wavelength range of 350 - 780 nm. The yield of C1 products was 90.07 μmol h -1 , and the selectivity was 87.44%. Example 8

[0031] 10 mg of 3% Pd-PdO / ZnO was ultrasonically dissolved in 30 mL of deionized water. Methane and oxygen were charged in a ratio of 19:11, the total pressure was 3 MPa, the reaction temperature was 25 °C, and the reaction was carried out for 1 h under irradiation of a xenon lamp with a wavelength range of 350 - 780 nm. The yield of C1 products was 69.24 μmol h -1 , and the selectivity was 87.10%.

[0032] Figure 5 It is a comparison chart of the yield and selectivity of photocatalytic methane oxidation to C1 products with different gas ratios in Example 5, Example 6, Example 7, and Example 8. As the O2 ratio increases, the selectivity of C1 products gradually increases, and the yield of C1 products also shows a "volcano-shaped" curve. When the CH4 / O2 ratio is 21 / 9, the highest yield of C1 products is 90.07 μmol·h -1 , and the selectivity is 87.44%. Example 9

[0033] 10 mg of 3% Pd-PdO / ZnO was ultrasonically dissolved in 5 mL of deionized water. Methane and oxygen were charged in a ratio of 21:9, the total pressure was 3 MPa, the reaction temperature was 25 °C, and the reaction was carried out for 1 h under irradiation of a xenon lamp with a wavelength range of 350 - 780 nm. The yield of C1 products was 34.74 μmol h -1 , and the selectivity was 90.70%. Example 10

[0034] 10 mg of 3% Pd-PdO / ZnO was ultrasonically dissolved in 10 mL of deionized water. Methane and oxygen were charged in a ratio of 21:9, the total pressure was 3 MPa, the reaction temperature was 25 °C, and the reaction was carried out for 1 h under irradiation of a xenon lamp with a wavelength range of 350 - 780 nm. The yield of C1 products was 50.20 μmol h -1 , and the selectivity was 90.38%. Example 11

[0035] 10 mg of 3% Pd-PdO / ZnO was ultrasonically dissolved in 20 mL of deionized water. Methane and oxygen were charged in a ratio of 21:9, the total pressure was 3 MPa, the reaction temperature was 25 °C, and the reaction was carried out for 1 h under irradiation of a xenon lamp with a wavelength range of 350 - 780 nm. The yield of C1 products was 74.74 μmol h -1 , and the selectivity was 88.08%. Example 12

[0036] 10 mg of 3% Pd-PdO / ZnO was ultrasonically dissolved in 30 mL of deionized water. Methane and oxygen were charged in a ratio of 21:9, the total pressure was 3 MPa, the reaction temperature was 25 °C, and the reaction was carried out for 1 h under irradiation of a xenon lamp with a wavelength range of 350 - 780 nm. The yield of C1 products was 90.07 μmol h -1 , and the selectivity was 87.44%.

[0037] Figure 6 It is a comparison chart of the yield and selectivity of photocatalytic oxidation of methane to C1 products with different water amounts in Example 9, Example 10, Example 11, and Example 12. When the water amount increased from 5 mL to 30 mL, the yield of C1 increased from 34.74 μmol·h -1 to 90.07 μmol·h -1 , mainly because the increase in water amount would promote the mass transfer of the reaction system, and the more water there was, the more it would promote the desorption of products. Example 13

[0038] 10 mg of 3% Pd-PdO / ZnO was ultrasonically dissolved in 30 mL of deionized water. Methane and oxygen were charged in a ratio of 21:9, the total pressure was 0.5 MPa, the reaction temperature was 25 °C, and the reaction was carried out for 1 h under irradiation of a xenon lamp with a wavelength range of 350 - 780 nm. The yield of C1 products was 62.42 μmol h -1 , and the selectivity was 87.47%. Example 14

[0039] Dissolve 10 mg of 3% Pd-PdO / ZnO ultrasonically in 30 mL of deionized water, charge a methane-oxygen ratio of 21:9, with a total pressure of 1 MPa, a reaction temperature of 25 °C, and react for 1 h under irradiation by a xenon lamp with a wavelength range of 350 - 780 nm. The yield of C1 products is 70.26 μmol h -1 , and the selectivity is 87.77%. Example 15

[0040] Dissolve 10 mg of 3% Pd-PdO / ZnO ultrasonically in 30 mL of deionized water, charge a methane-oxygen ratio of 21:9, with a total pressure of 2 MPa, a reaction temperature of 25 °C, and react for 1 h under irradiation by a xenon lamp with a wavelength range of 350 - 780 nm. The yield of C1 products is 78.67 μmol h -1 , and the selectivity is 87.45%. Example 16

[0041] Dissolve 10 mg of 3% Pd-PdO / ZnO ultrasonically in 30 mL of deionized water, charge a methane-oxygen ratio of 21:9, with a total pressure of 3 MPa, a reaction temperature of 25 °C, and react for 1 h under irradiation by a xenon lamp with a wavelength range of 350 - 780 nm. The yield of C1 products is 90.07 μmol h -1 , and the selectivity is 87.45%.

[0042] Figure 7 It is a comparison chart of the yields and selectivities of photocatalytic methane oxidation to C1 products at different total pressures in Example 13, Example 14, Example 15, and Example 16. When the reaction pressure increases, the solubilities of CH4 and O2 in water both increase. When the reaction pressure is 30 bar, the yield of C1 products is 90.07 μmol·h -1 , and the selectivity is 87.45%. Example 17

[0043] Dissolve 5 mg of 3% Pd-PdO / ZnO ultrasonically in 30 mL of deionized water, charge a methane-oxygen ratio of 21:9, with a total pressure of 3 MPa, a reaction temperature of 25 °C, and react for 1 h under irradiation by a xenon lamp with a wavelength range of 350 - 780 nm. The yield of C1 products is 69.30 μmol h -1 , and the selectivity is 88.08%. Example 18

[0044] 10 mg of 3% Pd-PdO / ZnO was ultrasonically dissolved in 30 mL of deionized water. Methane and oxygen were introduced at a ratio of 21:9, the total pressure was 3 MPa, the reaction temperature was 25 °C, and the reaction was carried out for 1 h under irradiation of a xenon lamp with a wavelength range of 350 - 780 nm. The yield of C1 products was 90.07 μmol h -1 , and the selectivity was 87.44%. Example 19

[0045] 20 mg of 3% Pd-PdO / ZnO was ultrasonically dissolved in 30 mL of deionized water. Methane and oxygen were introduced at a ratio of 21:9, the total pressure was 3 MPa, the reaction temperature was 25 °C, and the reaction was carried out for 1 h under irradiation of a xenon lamp with a wavelength range of 350 - 780 nm. The yield of C1 products was 105.58 μmol h -1 , and the selectivity was 89.52%. Example 20

[0046] 30 mg of 3% Pd-PdO / ZnO was ultrasonically dissolved in 30 mL of deionized water. Methane and oxygen were introduced at a ratio of 21:9, the total pressure was 3 MPa, the reaction temperature was 25 °C, and the reaction was carried out for 1 h under irradiation of a xenon lamp with a wavelength range of 350 - 780 nm. The yield of C1 products was 102.03 μmol h -1 , and the selectivity was 87.74%.

[0047] Figure 8 Figure for comparing the yields and selectivities of photocatalytic oxidation of methane to C1 products using different masses of catalysts in Example 17, Example 18, Example 19, and Example 20. When the mass of the catalyst was continuously increased, the yield of C1 products increased from 69.26 μmol·h -1 to 105.58 μmol·h -1 . When the mass of the catalyst was further increased to 30 mg, both the yield and selectivity of C1 products decreased. This is because when the amount of catalyst is small, the number of reactive oxygen species generated during the reaction is small, which is not conducive to the conversion of CH4 or •CH3, so the yield of HCHO is low; when the amount of catalyst is too large, the light transmittance of the system decreases, and the light absorption rate of the catalyst decreases, so the yield and selectivity of C1 will decrease. Example 21

[0048] 20 mg of 3% Pd-PdO / ZnO was ultrasonically dissolved in 30 mL of deionized water. Methane and oxygen were introduced at a ratio of 21:9, the total pressure was 3 MPa, the reaction temperature was 25 °C, and the reaction was carried out for 0.5 h under irradiation of a xenon lamp with a wavelength range of 350 - 780 nm. The yield of C1 products was 27.06 μmol, and the selectivity was 84.16%. Example 22

[0049] 20 mg of 3% Pd-PdO / ZnO was ultrasonically dissolved in 30 mL of deionized water. Methane and oxygen were charged at a ratio of 21:9, the total pressure was 3 MPa, the reaction temperature was 25 °C, and the reaction was carried out for 1 h under irradiation of a xenon lamp with a wavelength range of 350 - 780 nm. The yield of C1 products was 105.58 μmol h -1 , and the selectivity was 89.52%. Example 23

[0050] 20 mg of 3% Pd-PdO / ZnO was ultrasonically dissolved in 30 mL of deionized water. Methane and oxygen were charged at a ratio of 21:9, the total pressure was 3 MPa, the reaction temperature was 25 °C, and the reaction was carried out for 2 h under irradiation of a xenon lamp with a wavelength range of 350 - 780 nm. The yield of C1 products was 184.29 μmol, and the selectivity was 90.94%. Example 24

[0051] 20 mg of 3% Pd-PdO / ZnO was ultrasonically dissolved in 30 mL of deionized water. Methane and oxygen were charged at a ratio of 21:9, the total pressure was 3 MPa, the reaction temperature was 25 °C, and the reaction was carried out for 3 h under irradiation of a xenon lamp with a wavelength range of 350 - 780 nm. The yield of C1 products was 218.38 μmol, and the selectivity was 88.95%. Example 25

[0052] 20 mg of 3% Pd-PdO / ZnO was ultrasonically dissolved in 30 mL of deionized water. Methane and oxygen were charged at a ratio of 21:9, the total pressure was 3 MPa, the reaction temperature was 25 °C, and the reaction was carried out for 4 h under irradiation of a xenon lamp with a wavelength range of 350 - 780 nm. The yield of C1 products was 282.13 μmol, and the selectivity was 91.97%. Example 26

[0053] 20 mg of 3% Pd-PdO / ZnO was ultrasonically dissolved in 30 mL of deionized water. Methane and oxygen were charged at a ratio of 21:9, the total pressure was 3 MPa, the reaction temperature was 25 °C, and the reaction was carried out for 5 h under irradiation of a xenon lamp with a wavelength range of 350 - 780 nm. The yield of C1 products was 302.42 μmol, and the selectivity was 92.09%. Example 27

[0054] 20 mg of 3% Pd-PdO / ZnO was ultrasonically dissolved in 30 mL of deionized water. Methane and oxygen were charged at a ratio of 21:9, the total pressure was 3 MPa, the reaction temperature was 25 °C, and the reaction was carried out for 6 h under irradiation of a xenon lamp with a light source in the wavelength range of 350 - 780 nm. The yield of C1 products was 304.02 μmol and the selectivity was 91.59%.

[0055] Figure 9 It is a comparison chart of the yield and selectivity of photocatalytic oxidation of methane to C1 products in Examples 21, 22, 23, 24, 25, 26, and 27 with different reaction times. As the time increased from 0.5 h to 5 h, the yield of C1 products gradually increased. When the reaction time was 5 h, the yield of C1 products reached the highest at 302.42 μmol and the selectivity was 92.09%. Continuing to increase the reaction time, the selectivity of C1 products began to decline. With the increase of the reaction time, the C1 products in the system were over-oxidized, and the CO2 yield increased sharply at 6 h while the selectivity of C1 products decreased.

[0056] In this invention, a Pd-PdO / ZnO heterojunction photocatalyst was prepared by the co-precipitation method and the LED array photodeposition method. The interaction between Pd-PdO and the formation of the PdO / ZnO heterojunction accelerated the separation of photogenerated carriers. Under the optimal reaction conditions, the yield of C1 products was 105.58 μmol h -1 , and the selectivity was 89.52%.

[0057] The above are only the preferred embodiments of the present invention, and it is not limited to the present invention in any other form. Any modification or equivalent change made according to the technical essence of the present invention still belongs to the scope protected by the present invention.

Claims

1. A zinc oxide heterojunction photocatalyst, characterized in that, The structural formula is Pd-PdO / ZnO, where the loading amount of the noble metal is 1.0-7.0 wt%, and ZnO has an irregular rod-like structure.

2. The preparation method of a zinc oxide heterojunction photocatalyst according to claim 1, characterized in that, The specific steps are as follows: S1: First, disperse 0.9-4.5 g of zinc acetate (Zn(AC)2) in 50-200 mL of deionized water, stir until Zn(AC)2 is completely dissolved, and denote it as solution A; weigh 0.6-3.0 g of sodium hydroxide (NaOH) and disperse it in 10-50 mL of deionized water, stir until NaOH is completely dissolved, and denote it as solution B; slowly add solution B dropwise to solution A, stir and react fully for 2 h, after stirring, let it stand for 1 h, wash it with deionized water and ethanol respectively, then centrifuge, dry it overnight at 60 °C in an oven, and then transfer it to a muffle furnace, heat it up to 500 °C at a heating rate of 5 °C / min, and keep it at a constant temperature for 4 h to obtain ZnO; S2: Disperse 1 g of ZnO in 50 - 200 mL of deionized water, denoted as solution C; Take 4.7 - 32.9 mL of Pd precursor solution and add it to 50 - 200 mL of aqueous solution, then add 0.0025 - 0.01 g of polyvinylpyrrolidone (PVP) to the solution, stir for 1 h until uniformly dispersed, denoted as solution D; Drop solution D into solution C, simultaneously apply LED array illumination, utilize the ultraviolet light response characteristics of ZnO to generate electron - hole pairs, drive the selective reduction deposition of Pd 2+ , stir for full reaction for 2 h, and let it stand for 1 h after stirring ends; S3: Wash it with deionized water and ethanol respectively, then centrifuge, dry it by supercritical CO2 drying, transfer the sample to a muffle furnace, and heat it up by a stepwise heating program to calcine to obtain Pd-PdO / ZnO.

3. The preparation method of a zinc oxide heterojunction photocatalyst according to claim 2, characterized in that, In the step S1, the molar ratio of zinc acetate to sodium hydroxide is (1-1.5):

3.

4. The preparation method of a zinc oxide heterojunction photocatalyst according to claim 2, wherein The Pd precursor in the step S2 is any one of Pd chlorides, nitrates, and sulfates.

5. The preparation method of a zinc oxide heterojunction photocatalyst according to any one of claims 2 or 4, characterized in that, The concentration of the Pd precursor solution in the step S2 is 0.005-0.01 mg / mL, and the wavelength of the LED light is 350-480 nm.

6. The preparation method of a zinc oxide heterojunction photocatalyst according to claim 2, characterized in that, In the step S3, the drying temperature of supercritical CO2 is 40-60 °C, and the stepwise heating program in the muffle furnace is to heat up to 150 °C at 1 °C / min, and then heat up to 300-500 °C at 5 °C / min, and the continuous heating time is 2-5 h.

7. Use of the zinc oxide heterojunction photocatalyst according to claim 1, characterized in that, It is used for the direct photocatalytic oxidation of methane to C1 compounds. The specific steps are as follows: Take 5-30 mg of the catalyst, dissolve it in 10-30 mL of deionized water by ultrasonic method, the methane pressure is 0.1-3.0 MPa, use oxygen as the oxidant, the oxygen pressure is 0-1.0 MPa, the total pressure is 0.1-3.0 MPa, the light source is a xenon lamp with a wavelength range of 350-780 nm, the reaction is carried out at room temperature, and the reaction is carried out for 1-6 h under the irradiation of the xenon lamp, and the contents of the gas phase and liquid phase products are detected.

8. Use of the zinc oxide heterojunction photocatalyst according to claim 7, characterized in that, The volume ratio of methane to oxygen is 21:9.

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