Multilayer Co (OH) 2 / CeO2-g-C3N4 ternary synergistic heterostructure photocatalyst, preparation method and application

Abstract

The invention relates to a multi-layer Co (OH) 2 / CeO2-g-C3N4 ternary synergistic heterostructure photocatalyst, a preparation method and application, the multi-layer Co (OH) 2 / CeO2-g-C3N4 ternary synergistic heterostructure photocatalyst is characterized in that preparation raw materials of the catalyst mainly comprise g-C3N4 (CN), Co salt and Ce salt, the obtained catalyst is marked as xCoCe-CN, x is the sum of the mass percent of Co salt and the mass percent of Ce salt, x is the sum of the mass percent of Co salt and the mass percent of Ce salt, and x is the sum of the mass percent of Ce salt and the mass percent of g-C3N4. The specific mass percent numerical range of x is 10-70%; according to the scheme, g-C3N4 is combined with CeO2 and Co (OH) 2, so that the photocatalytic performance of g-C3N4 is effectively improved, and the purpose of constructing p-n type ternary heterogeneity is achieved.

Description

Technical Field

[0001] This application belongs to the technical field of photocatalytic materials, and specifically relates to a multi-layer Co(OH) 2 / CeO 2 -g-C 3 N 4 ternary synergistic heterostructure photocatalyst, preparation method and application. Background Art

[0002] In recent years, the problem of indoor air pollution has attracted much attention, and nitrogen oxides (NOx) are one of the main harmful gases. Although the indoor NO concentration is usually low (in the ppb range), long-term exposure can cause health problems such as respiratory diseases and cancer, and has an important impact on global climate change. At present, traditional physical adsorption methods and chemical washing methods are difficult to meet the increasingly strict environmental protection requirements due to problems such as low treatment efficiency, high energy consumption and secondary pollution. Therefore, the development of efficient and green photocatalytic materials for the removal of low-concentration NO has become a research hotspot.

[0003] Photocatalytic technology has been widely used in the field of air pollution control due to its advantages such as using sunlight as a driving force, mild operating conditions and environmental friendliness. Among them, graphitic carbon nitride (g-C 3 N 4 ) has become a research hotspot in recent years due to its moderate band gap, excellent chemical stability and low cost. However, the photocatalytic performance of g-C 3 N 4 is limited by its inherent defects, such as rapid recombination of electron-hole pairs, low specific surface area and limited light absorption range. These problems seriously restrict its application in the field of low-concentration NO oxidation.

[0004] In order to improve the performance of g-C 3 N 4 , researchers have tried various modification strategies, including surface chemical modification, element doping, and compounding with other semiconductors. For example, g-C 3 N 4 / SnO 2 achieved more efficient separation of photo-generated carriers by constructing a Z-scheme heterojunction, but its photocatalytic efficiency is still only about 35%. Although such modification methods have achieved certain results, how to further improve the performance of g-C 3 N 4 remains a technical problem to be solved urgently.

[0005] Rare earth oxides (such as CeO 2 ) have become one of the photocatalytic materials that have attracted much attention due to their excellent oxygen storage capacity and Ce 3+ / Ce 4+ redox characteristics. However, CeO2 The wide band gap limits its absorption capacity in the visible light band, and the recombination rate of photogenerated electrons and holes is high.

[0006] Therefore, researchers proposed to improve CeO by constructing heterostructures. 2 For example, Bi 0 / CeO 2 The -delta photocatalyst has achieved an efficiency of 43% in the NO oxidation reaction, but it is still far from the requirements for industrial application.

[0007] In addition, cobalt-based materials such as Co(OH) 2 ) has gradually become a potential photocatalytic co-catalyst due to its low cost and easy availability. Co(OH) 2 As a p-type semiconductor, it can effectively form a pn heterojunction when combined with an n-type material, improving the separation efficiency of photogenerated carriers and inhibiting their recombination. For example, in the photocatalytic water splitting reaction, Co(OH) 2 It shows strong electron transport ability and carrier adsorption ability, but its application in NO oxidation reaction has not been deeply studied.

[0008] Compared with single or binary catalysts, ternary heterostructures can further improve the light absorption range, carrier separation efficiency, and catalytic reaction efficiency. 3 N 4 @Ag / BiVO 4 Ternary heterostructures show excellent photocatalytic performance under visible light conditions, but their industrial application is limited due to the high cost of Ag. This shows that building more efficient ternary heterostructures by introducing low-cost metal or non-metal elements is an effective strategy to improve catalyst performance.

[0009] In summary, how to improve gC by rationally designing catalyst structure and optimizing preparation process? 3 N 4 Improving the performance of photocatalysts and achieving low-cost and efficient NO photocatalytic oxidation are issues that urgently need to be addressed in the current technical field. Summary of the invention

[0010] In view of the above-mentioned deficiencies in the prior art, the present application provides a method of 3 N 4 With CeO 2 and Co(OH) 2 Combined to effectively improve gC 3 N 4 The photocatalytic performance of pn-type ternary heterojunction multilayer Co(OH) 2 / CeO 2 -gC3 N 4 Triple-component synergistic heterostructure photocatalyst.

[0011] To solve the above technical problems, the technical solution adopted in this application is: A multi-layer Co(OH) 2 / CeO 2 -g-C 3 N 4 triple-component synergistic heterostructure photocatalyst, and the main raw materials for preparing the catalyst include: g-C 3 N 4 (CN), Co salt and Ce salt, and the obtained catalyst is labeled as xCoCe-CN, where x is the sum of the mass percentages of the Co salt and the Ce salt, and the specific mass percentage value range of x is 10-70%.

[0012] Further, the x is 10%, 30%, 50% or 70%.

[0013] Further, the mass percentages of the Co salt and the Ce salt are equal.

[0014] Furthermore, the mass percentages of the Co salt and the Ce salt are 5%, 15%, 25% and 35% respectively.

[0015] Further, the Co salt is Co(NO 3 ) 2 ·6H 2 O, and the Ce salt is Ce(NO 3 ) 3 ·6H 2 O.

[0016] This application also provides a preparation method for a multi-layer Co(OH) 2 / CeO 2 -g-C 3 N 4 triple-component synergistic heterostructure photocatalyst, specifically including:

[0017] (1) g-C 3 N 4 (CN): Put melamine into an aluminum oxide crucible, heat it to 500-600 °C and keep it for 3-5 hours;

[0018] (2) Co(OH) 2 / CeO 2 / g-C 3 N 4 (CoCe-CN): Mix g-C 3 N 4 with deionized water and ethanol; then add the calculated Co(NO 3) 2 ·6H 2 O and Ce(NO 3 ) 3 ·6H 2 O is added to the above-mentioned g-C 3 N 4 mixture, ultrasonicated for 5 - 20 minutes for the first time and stirred for 20 - 40 minutes; then NaOH solution is added, ultrasonicated for 2 - 10 minutes and stirred for 10 - 20 minutes for the second time; finally, the solution is placed in an autoclave, heated to 90 - 110 °C and maintained for 5 - 7 hours to obtain a suspension; then the suspension is centrifuged and washed alternately with deionized water and ethanol until the pH value of the mixture is close to 7; finally, the mixture is dried in vacuo to obtain the target catalyst.

[0019] Further, in step (1), it is heated to 540 - 560 °C and maintained for 3 - 4 hours.

[0020] Further, the g-C 3 N 4 in step (2) has a dosage ratio with deionized water and ethanol of: 1 g : 400 - 500 ml : 40 - 60 ml.

[0021] Further, in step (2), the first sonication is for 12 - 15 min and the stirring is for 25 - 35 min.

[0022] Further, in step (2), the second sonication is for 4 - 6 min and the stirring is for 12 - 18 min.

[0023] Further, in step (2), it is heated to 95 - 105 °C and maintained for 5 - 7 hours.

[0024] Further, the vacuum drying in step (2) is drying at 55 - 65 °C for 10 - 15 hours.

[0025] This application also provides an application of the above multi-layer Co(OH) 2 / CeO 2 -g-C 3 N 4 ternary synergistic heterostructure photocatalyst in the photocatalytic oxidation of NO.

[0026] Advantages and beneficial effects of this application:

[0027] 1. Aiming at the problem of rapid recombination of electron / hole pairs under visible light, this application promotes NO oxidation by constructing a ternary heterojunction photocatalyst; this research combines g-C 3 N 4 with CeO 2 and Co(OH) 2Combination improves the photocatalytic performance of g-C 3 N 4 , thus constructing a p-n type ternary heterojunction; among them, CeO 2 has good redox performance and UV light absorption ability, and at the same time can enhance the oxidation ability of the photocatalyst to NO; g-C 3 N 4 is a visible light-responsive material that can utilize most of the energy in sunlight. Its combination with CeO 2 and Co(OH) 2 realizes the synergistic utilization of ultraviolet light and visible light, broadening the light absorption range; Co(OH) 2 promotes the separation and transfer of photo-generated charges, reduces the recombination of photo-generated electrons and holes, and enables more active species to participate in the reaction; in the CeO 2 / Co(OH) 2 / g-C 3 N 4 system, heterojunctions are formed between the components, improving the separation efficiency of photo-generated electrons and holes; CeO 2 has a relatively low conduction band (CB) position, which is conducive to the transfer of electrons to g-C 3 N 4 , while the holes in g-C 3 N 4 can be transferred to CeO 2 . This charge migration path reduces the recombination probability, thereby improving the catalytic performance; among them, Co(OH) 2 , as a p-type semiconductor, exists between the surfaces of CeO 2 and g-C 3 N 4 . It not only provides a fast electron transfer path for CeO 2 and g-C 3 N 4 , but also further reduces the recombination of electron / hole pairs by forming a heterostructure; in addition, this system simultaneously provides an additional OH - reaction pathway for photocatalytic oxidation of NO, accelerating the photocatalytic oxidation of NO; this scheme not only optimizes the property of poor visible light response ability of CeO 2 materials, but also replaces noble metal materials, reducing costs while further enhancing the photocatalytic efficiency.

[0028] 2. This application constructs a ternary heterostructure by simultaneously loading CeO 3 N 4 and Co(OH) 2 on the surface of g-C 2 , achieving the following technical effects: 2.1. Z-type heterojunction design: Co(OH)2 As an electron donor-acceptor center, it effectively improves the separation efficiency of photo-generated carriers and reduces the recombination rate of electron-hole pairs; 2.2. Multilayer structure: CeO 2 and Co(OH) 2 are uniformly distributed on the surface of g-C 3 N 4 surface, providing more active sites and further enhancing the catalytic performance through mutual synergy; 2.3. Formulation optimization: By regulating the mass ratio of Ce and Co to 25% (50CoCe-CN), the performance of the catalyst reaches the optimum, avoiding the problem of blocked active sites caused by excessive loading; Use of transition metals: Using cobalt and cerium as the main active components reduces the cost of the catalyst.

[0029] 3. The catalyst with this specific structure in this application has a simple and low-cost specific preparation process. It uses a one-step hydrothermal method without complex multi-step reactions or expensive equipment, and has high process reproducibility; Mild conditions: The reaction temperature only needs to be 100–120 °C, avoiding the destruction of the catalyst structure under high-temperature conditions; Green and environmentally friendly: There is no need to use toxic solvents or dangerous reagents in the reaction system, which conforms to the principles of green chemistry.

[0030] 4. The catalyst with this specific structure in this application, through photocatalytic performance testing and characterization, shows that the catalyst in this application has significant advantages in the photocatalytic oxidation of low-concentration NO: 4.1. Improved photocatalytic efficiency: Under the condition of 600 ppb NO concentration, the photocatalytic oxidation efficiency of 50CoCe-CN reaches 53.5%, which is 1.82 times that of pure g-C 3 N 4 and much higher than that of currently reported g-C 3 N 4 / SnO 2 and other binary photocatalysts (about 35%); 4.2. High selectivity: Experimental results show that the selectivity of this catalyst for NO 3 - is much higher than that of NO 2 , avoiding the generation of secondary pollutants; 4.3. Strong stability: After 5 cycles of use, the performance retention rate of the catalyst exceeds 95%, and its crystal structure and morphology are verified by XRD characterization to have no obvious changes, showing excellent long-term stability. Description of the Drawings

[0031] Figure 1 (a) Schematic diagram of the synthesis of CN and CoCe-CN, (b) XRD patterns of CN, Co-CN, Ce-CN and CoCe-CN photocatalysts, and (c) FT-IR spectra.

[0032] Figure 2(a, b) High-resolution transmission electron microscopy (HRTEM) images of CoCe-CN, (c-g) energy-dispersive X-ray spectroscopy (EDS) mapping of CoCe-CN.

[0033] Figure 3 (a) NO conversion rates of 10 / 30 / 50 / 70CoCe-CN and CN, (b) NO 2 concentration and selectivity of CN and 50CoCe-CN, (c) NO conversion rates of 50Co-CN, 50Ce-CN and 50CoCe-CN, (d) Trapping experiments on the photocatalytic removal of NO by CoCe-CN.

[0034] Figure 4 (a) Ultraviolet-visible spectra (UV-vis spectra) of the photocatalyst, (b) Photoluminescence spectra (PL spectra) of the photocatalyst, (c) Electrochemical impedance spectra (EIS plots) of the photocatalyst, (d) Transient photocurrent responses of the photocatalyst, (e) Tauc plots of the N photocatalyst, (f) Mott-Schottky plots (M-S plots) of the photocatalyst. Detailed implementation manners

[0035] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the embodiments and the accompanying drawings. Obviously, the described embodiments are only preferred embodiments, rather than all embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments in the present application without creative efforts shall fall within the protection scope of the present application.

[0036] The g-C 3 N 4 used in the following examples was obtained by the following method: 4 grams of melamine was placed in a 30 ml aluminum oxide crucible (alumina crucible) with a lid, and then heated to 550 °C and maintained for 4 hours to obtain the final g-C 3 N 4 ; the obtained powder was ground for standby.

[0037] Example 1:

[0038] 0.4 grams of g-C 3 N 4 was mixed with 180 ml of deionized water and 20 ml of ethanol. Then 0.987 g and 0.748 g of Co(NO 3 ) 2 ·6H 2 O and Ce(NO 3 )3 ·6H 2 O is added to the above g-C 3 N 4 mixture, and it is ultrasonicated for 10 minutes and stirred for 30 minutes. Subsequently, 4 mL of 3 mol / L NaOH solution is added, and it is ultrasonicated again for 5 minutes and stirred for 15 minutes. The final solution is placed in a 500 mL autoclave, heated to 100 °C and maintained for 6 hours. Then the suspension is centrifuged and washed alternately with deionized water and ethanol until the pH value of the mixture is close to 7. Finally, the mixture is dried in vacuo at 60 °C for 12 hours; a catalyst with a mass percentage of 25% Co / 25% Ce / g-C 3 N 4 is obtained.

[0039] 0.2 g of the prepared catalyst is mixed with 15 mL of deionized water and ultrasonicated for 15 minutes. Then, the mixture is placed in a glass dish with a diameter of 12 cm and dried in vacuo at 60 °C for 6 hours. After that, the glass dish is placed at the center of the photocatalytic reactor. The light source is a 300 W xenon lamp equipped with a 420 nm cut-off filter, which is vertically placed 20 cm above the surface of the catalyst.

[0040] During the photocatalytic reaction, 50 ppm NO provided from a compressed gas cylinder (N 2 balance) is diluted to 600 ppb by a pure air stream and flows through the surface of the sample, and the total flow rate in the reactor is 2 L / min. When the adsorption-desorption equilibrium between NO and the catalyst is reached, the xenon lamp is turned on to start the photocatalytic NO oxidation reaction. The light intensity measured by a photometer vertically placed 20 cm below the lamp is 1.516 W.

[0041] During the whole reaction process, the concentrations of NO and NO 2 are measured and recorded by a NOx analyzer, and the photocatalytic efficiency of NO is calculated according to the following formula: where C 0 (ppb) is the concentration of NO at the start of the reaction, and C (ppb) is the concentration of NO at a given time.

[0042] Example 2:

[0043] 0.4 g of g-C 3 N 4 is mixed with 180 mL of deionized water and 20 mL of ethanol. Then 0.197 g and 0.149 g of Co(NO 3 ) 2 ·6H 2 O and Ce(NO 3 ) 3 ·6H 2 O are added to the above g-C 3 N4 In the mixture, ultrasonic treatment was carried out for 10 minutes and stirring for 30 minutes. Subsequently, 4 mL of 3 mol / L NaOH solution was added, and ultrasonic treatment was carried out again for 5 minutes and stirring for 15 minutes. The final solution was placed in a 500 mL autoclave, heated to 100 °C and maintained for 6 hours. Then the suspension was centrifuged and washed alternately with deionized water and ethanol until the pH value of the mixture was close to 7. Finally, the mixture was dried in vacuo at 60 °C for 12 hours. A catalyst with a mass percentage of 5% Co / 5% Ce / g-C 3 N 4 was obtained.

[0044] Subsequently, the photocatalytic oxidation activity test of NO was carried out according to the procedure of Example 1.

[0045] Example 3:

[0046] 0.4 g of g-C 3 N 4 was mixed with 180 mL of deionized water and 20 mL of ethanol. Then 0.592 g and 0.447 g of Co(NO 3 ) 2 ·6H 2 O and Ce(NO 3 ) 3 ·6H 2 O were added to the above g-C 3 N 4 mixture. Ultrasonic treatment was carried out for 10 minutes and stirring for 30 minutes. Subsequently, 4 mL of 3 mol / L NaOH solution was added, and ultrasonic treatment was carried out again for 5 minutes and stirring for 15 minutes. The final solution was placed in a 500 mL autoclave, heated to 100 °C and maintained for 6 hours. Then the suspension was centrifuged and washed alternately with deionized water and ethanol until the pH value of the mixture was close to 7. Finally, the mixture was dried in vacuo at 60 °C for 12 hours. A catalyst with a mass percentage of 15% Co / 15% Ce / g-C 3 N 4 was obtained.

[0047] Subsequently, the photocatalytic oxidation activity test of NO was carried out according to the procedure of Example 1.

[0048] Example 4:

[0049] 0.4 g of g-C 3 N 4 was mixed with 180 mL of deionized water and 20 mL of ethanol. Then 1.379 g and 1.04 g of Co(NO 3 ) 2 ·6H 2 O and Ce(NO 3 ) 3 ·6H 2 O were added to the above g-C3 N 4 In the mixture, ultrasonic treatment was carried out for 10 minutes and stirring for 30 minutes. Subsequently, 4 mL of 3 mol / L NaOH solution was added, and ultrasonic treatment was carried out again for 5 minutes and stirring for 15 minutes. The final solution was placed in a 500 mL autoclave, heated to 100 °C and maintained for 6 hours. Then the suspension was centrifuged and washed alternately with deionized water and ethanol until the pH value of the mixture was close to 7. Finally, the mixture was dried in vacuo at 60 °C for 12 hours. 35% Co / 35% Ce / g-C 3 N 4 catalyst with mass percentage.

[0050] Subsequently, the photocatalytic oxidation activity test of NO was carried out according to the procedure of Example 1.

[0051] Comparative Example 1

[0052] Prepare Co(OH) 2 / g-C 3 N 4 , the synthesis method was the same as that of CoCe-CN in Example 1 above, but Ce(NO 3 ) 3 ·6H 2 O was not used. In order to analyze whether there is a synergistic effect between cobalt and cerium, the mass ratio of cobalt was controlled at 10% Co, 30% Co, 50% Co and 70% Co, and labeled as yCo-CN, where y represents the mass ratio %.

[0053] Comparative Example 2

[0054] Prepare CeO 2 / g-C 3 N 4 , the synthesis method was the same as that of CoCe-CN in Example 1 above, but Co(NO 3 ) 3 ·7H 2 O was not added. The mass ratio of cerium was controlled at 10% Ce, 30% Ce, 50% Ce and 70% Ce, and labeled as zCe-CN, where z represents the mass ratio %.

[0055] Comparative Example 3

[0056] Prepare Co(OH) 2 / CeO 2 , the synthesis method was the same as that of CoCe-CN in Example 1 above, but g-C 3 N 4 was not added, and the sample was labeled as CoCe.

[0057] The products obtained in the examples and comparative examples were subjected to activity detection, and the method was as follows:

[0058] The activity of the catalyst for photocatalytic oxidation of NO was evaluated in a continuous-flow cubic reactor. The reactor was made of quartz (20×15×10 cm) and covered with aluminum foil on all sides except the top. Before each experiment, 0.2 g of the prepared catalyst was mixed with 15 mL of deionized water and sonicated for 15 minutes.

[0059] Then, the mixture was placed in a glass dish with a diameter of 12 cm and vacuum-dried at 60 °C for 6 hours. After that, the glass dish was placed at the center of the cubic reactor.

[0060] The light source was a 300 W xenon lamp equipped with a 420 nm cut-off filter, which was vertically placed 20 cm above the catalyst surface. During the photocatalytic reaction, 50 ppm NO provided from a compressed gas cylinder (N 2 equilibrium) was diluted to 600 ppb by a pure air stream and flowed through the sample surface, and the total flow rate in the reactor was 2 L / min. When the adsorption-desorption equilibrium between NO and the catalyst was reached, the xenon lamp was turned on to start the photocatalytic NO oxidation reaction. The light intensity measured by a photometer vertically placed 20 cm below the lamp was 1.516 W. During the whole reaction process, the concentrations of NO and NO 2 were measured and recorded by a NOx analyzer, and the photocatalytic efficiency of NO was calculated according to the following formula: where, C 0 (ppb) is the NO concentration at the start of the reaction, and C (ppb) is the NO concentration at a given time.

[0061] The detection results are shown in the appendix Figures 1-4 as follows:

[0062] Figure 1 (a) Schematic diagram of the synthesis of CN and CoCe-CN, (b) XRD patterns and (c) FT-IR spectra of CN, Co-CN, Ce-CN and CoCe-CN photocatalysts; the crystal structures of CN, Co-CN, Ce-CN and CoCe-CN catalysts were identified by the XRD patterns, as Figure 1 shown in (b). The XRD pattern of CN indicates the successful synthesis of g-C 3 N 4 , with two obvious diffraction peaks at 27.4° (002) and 13.1° (100) respectively. The XRD patterns of Ce-CN and Co-CN show obvious peaks attributed to the (001), (100), (101) planes of Co(OH) 2 and the (200), (220), (311) planes of CeO 2 , all of which confirm that cobalt and cerium exist as Co(OH) 2 and CeO 2The formal load is on g-C 3 N 4 surface. As for CoCe-CN, only g-C 3 N 4 , Co(OH) 2 and CeO 2 characteristic diffraction patterns were observed, indicating the coexistence of cobalt and cerium in g-C 3 N 4 . All these results consistently show that Co(OH) 2 and CeO 2 were successfully loaded on the surface of g-C 3 N 4 without destroying its crystal structure. The structure and chemical functional groups were studied by FT-IR spectroscopy ( Figure 1 (c)), which was consistent with the XRD results and proved the synthesis of CoCe-CN.

[0063] Figure 2 (a,b) High-resolution transmission electron microscopy (HRTEM) images of CoCe-CN, (c-g) energy-dispersive X-ray spectroscopy (EDS) mappings of CoCe-CN; The HRTEM images ( Figure 2 (a,b)) show that the lattice spacings of the (101) plane of Co(OH) 2 and the (111) plane of CeO 2 are 0.237 nm and 0.332 nm respectively, further confirming the presence of Co(OH) 2 and CeO 2 on the surface of g-C 3 N 4 . The EDS element mapping diagrams ( Figure 2 (c-g)) show that C and N are uniformly distributed on the surface of the sample, indicating that g-C 3 N 4 is the main component of the catalyst. In addition, it is further confirmed that Co and Ce exist in the form of Co(OH) 2 and CeO 2 respectively on the surface of g-C 3 N 4 , as shown by the similar distributions of O, Co and Ce elements in Figure 2 (c,f,g). In addition, their uniform element distribution also indicates an interaction between Co(OH) 2 and CeO 2 .

[0064] Figure 3 (a) NO conversion rates of 10 / 30 / 50 / 70CoCe-CN and CN (b) NO of CN and 50CoCe-CN 2The NO conversion rates of 50Co-CN, 50Ce-CN, and 50CoCe-CN in terms of concentration and selectivity (c); (d) Trapping experiments for the photocatalytic removal of NO by CoCe-CN; To evaluate the effects of Co(OH) 2 and CeO 2 and their mass fractions on the photocatalytic activity, we evaluated the NO photocatalytic oxidation efficiency of the photocatalysts; The results are as Figure 3 (a) shown. CN exhibited low photocatalytic activity, with a NO oxidation efficiency of approximately 29.4%. However, the introduction of cobalt and cerium significantly improved the NO oxidation efficiency; furthermore, the NO oxidation efficiency increased with the increase in the mass fractions of cobalt and cerium, and the performance was optimal when the mass fraction reached 25% (50CoCe-CN); in addition, compared with g-C 3 N 4 , 50CoCe-CN not only achieved the highest NO oxidation efficiency of 53.5% but also showed the highest NO 3 - selectivity, as Figure 3 (b) shown; We further increased the loading ratios of Co(OH) 2 and CeO 2 , but due to the aggregation of excessive Co(OH) 2 and CeO 2 particles and the blockage of active sites, the NO oxidation efficiency decreased. This indicates that the mass fraction of the loaded material is an important factor affecting the photocatalytic oxidation of NO. To confirm the synergistic effect of cobalt and cerium on g-C 3 N 4 , we also investigated the photocatalytic activities of Co-CN and Ce-CN, as Figure 3 (c) shown. It can be seen that the NO oxidation efficiencies of 50Co-CN and 50Ce-CN were 39.5% and 43.9% respectively, lower than 53.5% of 50CoCe-CN. This indicates that the interaction between cobalt and cerium further improved the NO oxidation efficiency.

[0065] Figure 4 (a) UV-vis spectra of the photocatalysts, (b) PL spectra of the photocatalysts, (c) EIS plots of the photocatalysts, (d) transient photocurrent responses of the photocatalysts, (e) Tauc plots of the N photocatalysts, (f) M-S plots of the photocatalysts; To evaluate the prepared CN, Co(OH) 2 , CeO 2, the optical absorption properties and band gaps of the 50Co-CN, 50Ce-CN, and 50CoCe-CN samples were analyzed by examining their UV-visible spectra; as can be seen from Figure 4 (a), all samples exhibited strong optical absorption intensity in the visible light range. After loading cobalt, the absorption intensity of the samples in the 200 - 800 nm range increased significantly, which may be due to the color change caused by the presence of cobalt; thus, it can be concluded that the loading of Co(OH) 2 can significantly enhance the light absorption of the photocatalyst, and Co(OH) 2 contributes to the observed increase in visible light absorption of 50CoCe-CN, rather than CeO 2 . Fluorescence analysis further evaluated the separation and recombination behavior of electron-hole carriers in the photocatalyst. Figure 4 (b) shows the fluorescence spectra of the photocatalysts prepared with an excitation wavelength of 419 nm. We found that the CN sample had the strongest fluorescence intensity; however, when CeO 2 or Co(OH) 2 was loaded, the intensity decreased, indicating that both components effectively inhibited the recombination of electron-hole pairs. In addition, the ternary heterostructure formed by the simultaneous loading of CeO 2 and Co(OH) 2 further improved the carrier transport and separation performance of the photocatalyst. Moreover, we further investigated the charge separation and recombination of e - / h + pairs at the photocatalyst interface through photocurrent response and EIS, as shown in Figure 4 (c,d). Our results showed that 50CoCe-CN had the minimum impedance and more free electrons compared to 50Co-CN and 50Ce-CN, which can be seen from the smallest semicircle diameter of the 50CoCe-CN catalyst in Figure 4 (c). Figure 4 (d) also shows the photocurrent density of each sample under visible light irradiation cycles, indicating that 50CoCe-CN had the strongest photocurrent density, which was about 3 times and 4 times that of 50Co-CN and 50Ce-CN, respectively.

[0066] Therefore, the catalysts obtained through the above examples and comparative examples were subjected to performance testing. The results showed that Co(OH) 2 and CeO 2 inhibited the electron-hole recombination of g-C 3 N 4 , promoted the separation of e - / h + pairs, and the co-loading of Co(OH) 2 and CeO 2 on g-C 3 N4 These performances can be further improved. This catalytic system also provides additional OH for photocatalytic oxidation of NO - reaction pathway, accelerating the photocatalytic oxidation of NO; This scheme not only optimizes the property of poor visible light response ability of CeO 2 material, but also replaces noble metal materials, reducing the cost while further enhancing the photocatalytic efficiency.

Claims

1. A multilayer Co(OH)2 / CeO2-g-C3N4 ternary synergistic heterostructure photocatalyst, characterized by: The raw materials for preparing the catalyst mainly include: g-C3N4(CN), Co salt and Ce salt. The obtained catalyst is marked as xCoCe-CN, wherein x is the sum of the mass percentages of the Co salt and the Ce salt, and the specific mass percentage value range of x is 10-70%.

2. The multilayer Co(OH)2 / CeO2-g-C3N4 ternary synergistic heterostructure photocatalyst according to claim 1, characterized in that: The x is 10%, 30%, 50% or 70%.

3. The multilayer Co(OH)2 / CeO2-g-C3N4 ternary synergistic heterostructure photocatalyst according to claim 1, characterized in that: The mass percentage contents of the Co salt and the Ce salt are equal; the Co salt is Co(NO3)2·6H2O, and the Ce salt is Ce(NO3)3·6H2O.

4. The multilayer Co(OH)2 / CeO2-g-C3N4 ternary synergistic heterostructure photocatalyst according to claim 3, characterized in that: The mass percentage contents of the Co salt and the Ce salt are 5%, 15%, 25% and 35% respectively.

5. A method for preparing the multilayer Co(OH)2 / CeO2-g-C3N4 ternary synergistic heterostructure photocatalyst according to any one of claims 1 to 4, characterized in that: Specific examples include: (1) g-C3N4(CN): Melamine was placed in an aluminum oxide crucible, heated to 500-600°C and maintained for 3-5 hours; (2) Co(OH)2 / CeO2 / g-C3N4 (CoCe-CN): g-C3N4 was mixed with deionized water and ethanol; then the calculated Co(NO3)2·6H2O and Ce(NO3)3·6H2O were added to the above g-C3N4 mixture, the first ultrasonic treatment was performed for 5-20 minutes, and stirring was performed for 20-40 minutes; then NaOH solution was added, the second ultrasonic treatment was performed for 2-10 minutes and stirring was performed for 10-20 minutes; the final solution was placed in an autoclave, heated to 90-110°C and maintained for 5-7 hours to obtain a suspension; then the suspension was centrifuged and washed alternately with deionized water and ethanol until the pH value of the mixture was close to 7; finally, the mixture was vacuum dried to obtain the target catalyst.

6. The method for preparing the multilayer Co(OH)2 / CeO2-g-C3N4 ternary synergistic heterostructure photocatalyst according to claim 5, characterized in that: The heating in step (1) is to 540-560° C. and maintained for 3-4 hours.

7. The method for preparing the multilayer Co(OH)2 / CeO2-g-C3N4 ternary synergistic heterostructure photocatalyst according to claim 5, characterized in that: The usage ratio of g-C3N4 to deionized water and ethanol in step (2) is: 1g:400-500ml:40-60ml.

8. The method for preparing the multilayer Co(OH)2 / CeO2-g-C3N4 ternary synergistic heterostructure photocatalyst according to claim 5, characterized in that: The first ultrasonic treatment in step (2) is performed for 12-15 minutes, followed by stirring for 25-35 minutes; the second ultrasonic treatment in step (2) is performed for 4-6 minutes, followed by stirring for 12-18 minutes.

9. The method for preparing the multilayer Co(OH)2 / CeO2-g-C3N4 ternary synergistic heterostructure photocatalyst according to claim 5, characterized in that: The heating to 95-105°C and maintaining for 5-7 hours in step (2) is as follows; the vacuum drying in step (2) is performed at 55-65°C for 10-15 hours.

10. Use of the multilayer Co(OH)2 / CeO2-g-C3N4 ternary synergistic heterostructure photocatalyst according to any one of claims 1 to 4 in the photocatalytic oxidation of NO.

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