2,2'-dipyridyl modified single-site rh / carbon nitride photocatalyst and method for catalytic reduction of co2

Abstract

The application discloses a 2,2'-bipyridine modified single-point Rh / carbon nitride photocatalyst and a method for catalytic reduction of CO2, and the photocatalyst takes carbon nitride as a carrier, and a single-point Rh loaded carbon nitride (Rh1 / CN) is obtained through a photodeposition method; the Rh1 / CN is used as a pre-catalyst, and preparation of a single-point Rh and bipyridine grafted carbon nitride photocatalyst (Rh1 / CN-bpy) is realized in a CO2 photocatalytic reduction reaction. In the catalytic process, 2,2'-bipyridine is used as an electron acceptor and an electron reservoir to change the electronic structure of Rh, so that the reaction path is changed, the electron transfer from CN to Rh-bpy is facilitated, and the interaction between Rh-bpy and CO2 is superior to the interaction with a proton, so that higher CO selectivity is obtained. In the application, N on the surface of carbon nitride is used as an anchoring point, and single-point Rh and bipyridine are grafted, the method is simple to operate, and a new idea for improving the product selectivity of photocatalytic reduction of CO2 is provided.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of preparation of photocatalysts for photocatalytic reduction of CO2 by solar energy, and particularly relates to a 2,2'-bipyridine modified single-point Rh / carbon nitride photocatalyst and a method for photocatalytic reduction of CO2. BACKGROUND

[0002] The combustion of fossil fuels leads to a sharp increase in the content of CO2 in the air, and the problem of greenhouse effect is serious. Photocatalytic reduction of CO2 can convert it into valuable chemical fuels, thereby reducing the content of CO2. At present, the two major problems in this field are poor selectivity and low efficiency, so it is particularly important to select an efficient catalyst. Among the many semiconductor catalyst materials, g-C3N4 has attracted widespread attention due to its low cost, visible light absorption, simple preparation process, stable structure and easy modification.

[0003] Single g-C3N4 has been widely studied in this field, but its photocatalytic efficiency is limited due to its small specific surface area, easy recombination of photo-generated electrons and holes, and insufficient catalytic active sites. Therefore, g-C3N4 needs to be modified. Current modification methods mainly include morphology control, element doping, loading of cocatalysts, and construction of heterojunctions. Among them, loading of cocatalysts is an effective method, especially single-atom / single-point metal cocatalysts, which have emerged in recent years due to their high atom utilization rate, high catalytic activity, and clear structure. In addition, g-C3N4 has a high nitrogen content, which can form a coordination bond (M-N x ) (APL Materials, 2020, 8(12): 120703) with metal atoms and anchor them on the surface of g-C3N4, making them more stable. Single-atom / single-point can adjust the energy band structure of g-C3N4, enhance the light absorption capacity of g-C3N4, and endow it with unique high-activity catalytic sites. On the other hand, it can also improve the separation and transfer of photo-generated electron-hole pairs, thereby meeting the demand for conversion of solar energy to chemical energy. However, due to the nature of metal cocatalysts, they tend to promote the reduction of protons to H2 in the competition process of proton reduction and CO2 reduction, resulting in low efficiency of photocatalytic reduction of CO2 and low CO / H2 selectivity. For example, Wang et al. (Journal of Alloys and Compounds, 2019, 786:149-154) used a photodeposition method to load cocatalyst Ag on the surface of H2SrTa2O7 (HST), and the maximum selectivity of the catalyst Ag / HST for CO could only reach 60.9%. et al. (Applied Surface Science, 2020, 503: 144426) loaded Pt on g-C3N4 by photodeposition, and the rate of 8 h photocatalytic CO2 reduction to CO and CH4 was 32 μmol·g -1 and 28 μmol·g -1 , but the yield of H2 was as high as 275 μmol·g -1 Therefore, it is particularly important to control the proton reduction process, regulate the reaction pathway, inhibit the production of H2, and promote the generation of CO in the presence of CO2 and H2O.

[0004] Based on the above selectivity problems, a large number of studies have shown that the modification of semiconductors with ligands or metals of complexes can change the active sites and pathways of the reaction, which may solve the above problems. There are currently three main modification methods: 1) combining metal molecular complexes and semiconductors through electrostatic interaction (Journal of the American Chemical Society, 2020, 142(45): 19249-19258); 2) combining molecular complexes and semiconductors through covalent bonds (Journal of the American Chemical Society, 2020, 142(13): 6188-6195); 3) using ligands to directly modify the metal cocatalyst on the surface of the carrier (Biomaterials, 2021, 276: 121064). Compared with the first two combination methods, the stability of method 3 is higher, and the transmission of photoexcited electrons is easier. As an electron acceptor or electron reservoir, the ligand can change the electronic structure of the metal, which can promote the transfer of electrons from the semiconductor material to the metal center, thereby improving the separation efficiency of photoinduced carriers and effectively inhibiting the recombination of charges. For example, Wei et al. (Biomaterials, 2021, 276: 121064) constructed [Ru(bpy)2] 2+A new multifunctional material coordinated with g-C3N4 nanosheets. Compared with the material assembled by direct π-π stacking of the two, Ru-g-C3N4 has a longer luminescence lifetime, which helps to separate the photo-induced electron and hole pairs, greatly improving the photocatalytic efficiency. The research on pyridine ligands is the most extensive at present. It acts as an electron acceptor or electron reservoir in the catalytic process, which can promote the transfer of semiconductor conduction band electrons to the metal center, improve the separation efficiency of electrons and holes, and also adjust the electronic or spatial properties of the metal center to regulate the surface active site, thereby changing the reaction path and controlling the selectivity of the product, achieving efficient photocatalytic CO2 reduction. Wei et al. (Nanoscale, 2023, 15(10):5036-5043) integrated 2,2'-bipyridine into the g-CN matrix through an imine bond, and the generated g-CN was coordinated with the abundant transition metal cobalt on earth to generate g-CN-bpy-Co. Experiments showed that the catalyst could effectively reduce CO2 to CO, and had high activity (106.3 μmol g -1 h -1 ) and durability under visible light irradiation. However, the photocatalyst prepared by this method has a long photoexcited electron transport path, and it is relatively difficult for photoexcited electrons to transport, and bpy and CN are combined in the form of a covalent bond, so the catalyst is relatively unstable. SUMMARY

[0005] The purpose of the present application is to provide a 2,2'-bipyridine modified single-point Rh / nitrogen carbon photocatalyst by coordinating 2,2'-bipyridine and metal-loaded nitrogen carbon, so as to obtain a 2,2'-bipyridine modified single-point Rh / nitrogen carbon photocatalyst, and at the same time, the preparation of the catalyst can realize photocatalytic reduction of CO2, promote the generation of CO, inhibit the generation of H2, and thus improve the selectivity of the CO product.

[0006] In order to achieve the above purpose, the method for preparing a 2,2'-bipyridine modified single-point Rh / nitrogen carbon photocatalyst while reducing CO2 comprises the following steps:

[0007] Step 1: trimerize melamine under air atmosphere at 300-500°C for 2-4 hours, grind the powder product and mix with potassium chloride and lithium chloride in anhydrous and oxygen-free environment, grind uniformly, and calcine at 450-550°C under inert atmosphere for 3-4 hours. The obtained product is washed with water and dried to obtain nitrogen carbon;

[0008] Step 2: carbon nitride is uniformly dispersed into deionized water to obtain a carbon nitride dispersion; sodium chloride solid is dissolved in deionized water, and an aqueous solution of RhCl3 is added to form a uniform solution; the obtained uniform solution is added to the carbon nitride dispersion, stirring is carried out at 70-90℃ for 6-10 hours, the obtained product is washed with water, dried, and then dispersed into deionized water, and irradiated under visible light for 40-80 minutes, and then washed with water and dried to obtain single-point Rh / carbon nitride; the mass fraction of Rh in the single-point Rh / carbon nitride is 0.25%-2%;

[0009] Step 3: the single-point Rh / carbon nitride is added into a reaction tube, 2,2'-bipyridine, acetonitrile, triethanolamine, and deionized water are added, and irradiation is carried out under CO2 atmosphere by using ultraviolet-visible light for 10-12 hours to obtain a 2,2'-bipyridine-modified Rh1 / CN photocatalyst and simultaneously to make CO2 photocatalytically reduced to generate CO. The obtained catalyst is directly used for photocatalytic reduction of CO2 to generate CO under irradiation of ultraviolet-visible light, that is, the catalyst is added into a reaction tube, irradiation is carried out under CO2 atmosphere by using ultraviolet-visible light for 10-12 hours, and CO2 is reduced to generate CO.

[0010] In the above step 1, preferably, melamine is heated to 500℃ at a temperature increasing rate of 8-12℃ / min under air atmosphere, the obtained powder product after grinding is mixed with potassium chloride and lithium chloride in anhydrous and oxygen-free environment, and grinding is uniformly carried out, and heating is carried out to 550℃ at a temperature increasing rate of 4-6℃ / min under inert atmosphere, and constant temperature calcination is carried out for 4 hours.

[0011] In the above step 1, further preferably, the mass ratio of the carbon nitride to potassium chloride and lithium chloride is 1:5-6:4-5.

[0012] In the above step 2, preferably, the obtained uniform solution is added to the carbon nitride dispersion, stirring is carried out at 80℃ for 8 hours, the obtained product is washed with water, dried, and then dispersed into deionized water, and irradiation is carried out under visible light with λ>420nm for 60 minutes.

[0013] In the above step 2, further preferably, the mass ratio of the carbon nitride to RhCl3 and sodium chloride is 100:0.25-2:15-30.

[0014] In the above step 3, preferably, the mass ratio of the 2,2'-bipyridine to the single-point Rh / carbon nitride is 1:1-6, and preferably, the volume ratio of the acetonitrile, triethanolamine, and deionized water is 3:1:1.

[0015] The beneficial effects of the present application are as follows:

[0016] This invention uses carbon nitride as a support. First, single-site Rh-supported carbon nitride (Rh1 / CN) is obtained via photodeposition. Then, using Rh1 / CN as a precatalyst, a single-site Rh and 2,2′-bipyridine-grafted carbon nitride photocatalyst (Rh1 / CN-bpy) is prepared in the CO2 photocatalytic reduction reaction. During the catalytic process, 2,2′-bipyridine acts as an electron acceptor and electron reservoir, altering the electronic structure of Rh and thus changing the reaction pathway. This facilitates the transfer of electrons from CN to Rh-bpy, and the interaction between Rh-bpy and CO2 is superior to its interaction with protons. Therefore, compared to Rh1 / CN, Rh1 / CN-bpy effectively suppresses hydrogen production and promotes CO formation, resulting in higher CO selectivity. The photoreduction activity and selectivity of CO2 to CO are increased by 93 and 98 times, respectively. This invention uses the N-site on the carbon nitride surface as an anchoring point to graft single-site Rh and 2,2′-bipyridine to obtain the Rh1 / CN-bpy catalyst. On the one hand, this method is simple to operate and produces a more stable catalyst; on the other hand, the photoexcited electron transport path of the catalyst is short, and 2,2′-bipyridine, acting as an electron acceptor or electron reservoir, can alter the electronic structure of metallic Rh, promoting the transfer of electrons from CN to the Rh-bpy center, thereby improving the separation efficiency of photogenerated carriers and effectively suppressing charge recombination. Furthermore, the interaction between Rh-bpy and CO2 is superior to its interaction with protons; therefore, compared to Rh1 / CN, Rh1 / CN-bpy effectively suppresses hydrogen production and promotes CO generation, thus achieving higher CO selectivity. In addition, this method can also provide a new approach to improving the selectivity of photocatalytic CO2 reduction products. Attached Figure Description

[0017] Figure 1 These are the XRD patterns of CN, Rh1 / CN, Rh1 / CN-bpy in Example 1, CN / RhCl3-bpy in Comparative Example 2, and CN-bpy in Comparative Example 3.

[0018] Figure 2 This is a SEM image of Rh1 / CN-bpy in Example 1.

[0019] Figure 3 This is a TEM image of Rh1 / CN-bpy in Example 1.

[0020] Figure 4 This is the HAADF and elemental mapping diagram of Rh1 / CN-bpy in Example 1, representing the elements Rh, C, O, N, and Cl.

[0021] Figure 5 These are the FTIR spectra of CN, Rh1 / CN, Rh1 / CN-bpy in Example 1, CN / RhCl3-bpy in Comparative Example 2, and CN-bpy and bpy in Comparative Example 3.

[0022] Figure 6 This is a comparison chart of the photocatalytic hydrogen production activity and photocatalytic CO2 reduction activity of CN, Rh1 / CN, Rh1 / CN-bpy in Example 1, CN / RhCl3 in Comparative Example 1, CN / RhCl3-bpy in Comparative Example 2, and CN-bpy in Comparative Example 3 under ultraviolet-visible light.

[0023] Figure 7 This is a comparison chart of the photocatalytic hydrogen production activity and photocatalytic CO2 reduction activity of Rh1 / CN-bpy under ultraviolet-visible light in Example 1.

[0024] Figure 8 This is a comparison of the photocatalytic hydrogen production activity and photocatalytic CO2 reduction activity of different bipyridine-modified Rh1 / CN photocatalysts under UV-Vis light (1: 2,2′-bipyridine, 2: 4,4′-di-tert-butyl-2,2′-bipyridine, 3: 4,4′-dibromo-2,2′-bipyridine, 4: 2,2′:6′2”-terpyridine, 5: 4,4′-dimethyl-2,2′-bipyridine, 6: 4′-chloro-2,2′:6′2”-terpyridine, 7: 2,2′-bipyridine-4,4′-dicarboxylic acid, 8: 5,5′-dimethyl-2,2′-bipyridine).

[0025] Figure 9 This is a cycle diagram of the photocatalytic hydrogen production activity and photocatalytic CO2 reduction activity of Rh1 / CN-bpy under ultraviolet-visible light in Example 1. Detailed Implementation

[0026] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments, but the scope of protection of the present invention is not limited to these embodiments.

[0027] Example 1

[0028] Step 1: Place 10g of melamine in a covered crucible and place it in a muffle furnace. Under an air atmosphere, heat the mixture to 500℃ at a heating rate of 10℃ / min and calcine at this temperature for 4 hours. After grinding, obtain a powder product. Take 600mg of the obtained powder product, 3.3g of KCl, and 2.7g of LiCl, mix and grind them in a glove box for 15 minutes, and heat the mixture to 550℃ under an argon atmosphere in a tube furnace at a heating rate of 5℃ / min. Calcine at this temperature for 4 hours. Wash the obtained product several times with boiling water and then dry it under vacuum at 60℃ to obtain carbon nitride (CN).

[0029] Step 2: In a round-bottom flask, disperse 100 mg CN in 30 mL of deionized water and stir vigorously until homogeneous to obtain a CN dispersion. Separately, dissolve 20 mg NaCl solid in 4 mL of deionized water and add 300 μL of 2.5 mg / mL RhCl3 aqueous solution. Sonicate for 5 min to form a homogeneous solution. Then add the obtained homogeneous solution to the CN dispersion in the round-bottom flask and stir at 80 °C for 8 h. Wash the resulting product three times with deionized water, dry under vacuum at 60 °C, grind into powder, disperse the powder in a beaker containing 15 mL of deionized water, irradiate under visible light (λ>420 nm) for 1 h, wash three times with deionized water, and dry under vacuum at 60 °C to obtain a single-point Rh / carbon nitride. The mass fraction of Rh in the single-point Rh / carbon nitride is 0.75%, denoted as Rh1 / CN.

[0030] Step 3: Add 20 mg of Rh1 / CN to the reaction tube, along with 15 mg of 2,2′-bipyridine (bpy), 3 mL of acetonitrile, 1 mL of LTEOA, and 1 mL of H2O. Seal the reaction tube with a rubber stopper. While vigorously stirring, purge the tube with CO2 gas to expel air. After sealing, irradiate the tube for 10 hours under a 300W xenon lamp (PLS-SXE 300UV, Perfectlight, full-band) to perform the photocatalytic CO2 reduction reaction. After the reaction, extract 400 μL of the generated gas using a syringe and test the gas yield using a gas chromatograph (GC-700). The resulting solid product is washed twice with water, vacuum dried at 60°C, and then ground to obtain the pyridine-ligand-modified single-site Rh / carbon nitride photocatalyst, denoted as Rh1 / CN-bpy. Rh1 / CN-bpy can be further used directly for photocatalytic CO2 reduction to CO under UV-Vis irradiation.

[0031] Comparative Example 1

[0032] 20 mg CN was added to a reaction tube, along with 3 mL acetonitrile, 1 mL H₂O, 1 mL TEOA, and 300 μL of a 2.5 mg / mL RhCl₃ aqueous solution. The reaction tube was sealed with a rubber stopper. CO₂ gas was introduced into the tube during vigorous stirring to purge air. After sealing, the tube was irradiated for 10 h under a 300 W xenon lamp (PLS-SXE 300UV, Perfectlight, full-band) to perform a photocatalytic CO₂ reduction reaction. 400 μL of the gas produced after the reaction was extracted using a syringe and the gas yield was measured using a gas chromatograph (GC-700). The resulting solid product was washed twice with water, vacuum dried at 60 °C, and then ground to obtain CN / RhCl₃.

[0033] Comparative Example 2

[0034] 20 mg of CN was added to a reaction tube, along with 15 mg of 2,2′-bipyridine, 3 mL of acetonitrile, 1 mL of H₂O, 1 mL of TEOA, and 300 μL of a 2.5 mg / mL RhCl₃ aqueous solution. The reaction tube was sealed with a rubber stopper. CO₂ gas was introduced into the tube during vigorous stirring to purge air. After sealing, the tube was irradiated for 10 h under a 300 W xenon lamp (PLS-SXE 300UV, Perfectlight, full-band) to perform a photocatalytic CO₂ reduction reaction. 400 μL of the gas produced after the reaction was extracted using a syringe and the gas yield was measured using a gas chromatograph (GC-700). The resulting solid product was washed twice with water, vacuum dried at 60 °C, and then ground to obtain CN / RhCl₃-bpy.

[0035] Comparative Example 3

[0036] 20 mg of CN was added to a reaction tube, along with 15 mg of 2,2′-bipyridine, 3 mL of acetonitrile, 1 mL of TEOA, and 1 mL of H2O. The reaction tube was sealed with a rubber stopper. During vigorous stirring, CO2 gas was introduced into the reaction tube to purge air. After sealing, the tube was irradiated for 10 hours under a 300 W xenon lamp (PLS-SXE 300UV, Perfectlight, full-band) to perform a photocatalytic CO2 reduction reaction. 400 μL of the gas produced after the reaction was extracted using a syringe and the gas yield was measured using a gas chromatograph (GC-700). The resulting solid product was washed twice with water, vacuum dried at 60 °C, and then ground to obtain CN-bpy.

[0037] Comparative Example 4

[0038] 20 mg Rh1 / CN was added to the reaction tube, along with 15 mg of bipyridine (selected from 4,4′-di-tert-butyl-2,2′-bipyridine, 4,4′-dibromo-2,2′-bipyridine, 2,2′:6′2”-terpyridine, 4,4′-dimethyl-2,2′-bipyridine, 4′-chloro-2,2′:6′2”-terpyridine, 2,2′-bipyridine-4,4′-dicarboxylic acid, and 5,5′-dimethyl-2,2′-bipyridine), 3 mL acetonitrile, 1 mL TEOA, and 1 mL H2O. The reaction tube was sealed with a rubber stopper. CO2 gas was introduced into the reaction tube during vigorous stirring to purge the air inside. After sealing, the tube was irradiated for 10 h under a 300W xenon lamp (PLS-SXE 300UV, Perfectlight, full-band) to carry out the photocatalytic CO2 reduction reaction. After the reaction, 400 μL of the generated gas was extracted using a syringe and the gas yield was measured using a gas chromatograph (GC-700). The resulting solid product was washed twice with water, dried under vacuum at 60 °C, and then ground to obtain Rh1 / CN modified with different bipyridines.

[0039] The structures of the samples prepared in Example 1 and Comparative Examples 1-4 were characterized, and the results are shown in the figure. Figures 1-9 .like Figure 1 As shown, the diffraction peaks of pure CN and all composite materials have only two characteristic peaks at 2θ = 8.02° and 28.2°, which correspond to the (100) and (002) crystal planes of CN, respectively. It can also be seen from the figure that no diffraction peaks related to Rh appeared in all samples containing Rh. Therefore, it is preliminarily believed that Rh may be deposited on CN in a single-point manner.

[0040] Depend on Figure 2 It is known that Rh1 / CN has a nanorod-like structure, but SEM testing alone cannot determine the Rh loading. Therefore, further transmission electron microscopy (TEM) testing is required, such as... Figure 3 As shown, observation reveals that no Rh nanoparticles or clusters exist in the Rh1 / CN-bpy sample, indicating that Rh is highly dispersed on the CN support. This is consistent with the results obtained from XRD, proving that the Rh prepared by photodeposition is loaded onto the CN in a single-point manner.

[0041] Figure 4 The aberration-corrected electron microscopy (HAAD-STEM) results further confirm that Rh in the Rh1 / CN-bpy catalyst exists as single-point sites. The circles in the figure mark the locations of the corresponding single-point Rh spots, all of which are atomic-sized bright spots. It can be clearly seen that the single-point Rh is uniformly dispersed on the CN support in the Rh1 / CN-bpy sample. The above characterization results indicate that Rh does not exhibit obvious clusters and is mainly loaded onto CN in a single-point form, demonstrating the successful preparation of CN modified with single-point Rh.

[0042] Further Fourier transform infrared (FTIR) measurements were used to determine the modification of Rh1 / CN by 2,2′-bipyridine. Several samples, including CN, Rh1 / CN, Rh1 / CN-bpy, CN-bpy, CN / RhCl3-bpy, and bpy, were characterized. Figure 5 As shown. In the FTIR spectrum, pure CN appears at 809 cm⁻¹. -1 892cm -1 and 1000-1700cm -1 The peaks at these locations are attributed to the breathing vibrations of the s-triazine ring, the deformation vibrations of NH, and the stretching vibrations of CN and C=N in the CN heterocycle, respectively. The peaks at 3000-3500 cm⁻¹ are also relevant. -1The broad peak at 1415 cm⁻¹ is attributed to the NH₂ group at the CN terminus. The typical tensile vibration peak of CN in the sample Rh1 / CN after loading with single-point Rh showed no significant change, indicating that Rh loading did not affect the major structural unit of CN. The peaks at 1415 cm⁻¹ in the three samples Rh1 / CN-bpy, CN-bpy, and CN / RhCl₃-bpy after adding 2,2′-bipyridine were also observed. -1 1458cm -1 and 1552cm -1 Three new peaks appeared at 1415cm. -1 and 1458cm -1 The two peaks are attributed to the CH deformation vibration of 2,2′-bipyridine, 1552 cm⁻¹ -1 The peaks are attributed to the deformation vibrations of the C=N and C=C rings of the 2,2′-bipyridine ring, and the peaks in the three samples after the addition of 2,2′-bipyridine are at 1239 cm⁻¹. -1 The tensile vibrations of CN and C=N in the CN heterocycle become more pronounced, due to the in-plane deformation vibrations of the 2,2′-bipyridine ring (1248 cm⁻¹). -1 This is caused by superposition. The above demonstrates the presence of 2,2′-bipyridine in the three samples: Rh1 / CN-bpy, CN-bpy, and CN / RhCl3-bpy. Furthermore, it was found that the addition of 2,2′-bipyridine resulted in a 2179 cm⁻¹... -1 The peak value weakened, which may be due to the relatively lower CN content after the addition of 2,2′-bipyridine.

[0043] Depend on Figure 6As can be seen, with triethanolamine as the hole sacrificial agent, pure CN showed no activity in the system after 10 hours of UV-Vis irradiation. In Comparative Example 3, even with the addition of 2,2′-bipyridine, the CO yield was only 6 μmol / g. In Comparative Example 1, after loading single-site Rh onto CN via photodeposition, although H2 and CO were produced simultaneously, the main product was H2 (yield of 13647.5 μmol / g after 10 hours), while CO was only produced at 1.4 μmol / g. When 2,2′-bipyridine was added to the system in Example 1 to coordinate with a single-site Rh, forming a co-catalyst with Rh-bpy as the single-site molecule, the CO generation rate increased significantly (131.5 μmol / g yield in 10 h, 93 times and 98 times higher in activity and selectivity than Rh1 / CN, respectively), while H2 generation was also significantly inhibited (4814.5 μmol / g), indicating that coordination between 2,2′-bipyridine and the single-site Rh is necessary. Further photocatalytic CO2+H2O reduction activity tests were conducted on the one-pot prepared catalysts CN / RhCl3 (Comparative Example 1) and CN / RhCl3-bpy (Comparative Example 2). The results showed that the catalysts prepared in this way also exhibited similar characteristics, although their performance was lower compared to Rh1 / CN and Rh1 / CN-bpy prepared by photodeposition.

[0044] We also investigated the effect of different Rh contents on the reactivity, from Figure 7 It can be seen that under UV-Vis irradiation, the amount of CO generated by Rh1 / CN-bpy-x gradually increases with the increase of Rh content. When the deposition amount of Rh at a single point increases to 0.75% (mass fraction), the CO yield reaches 131.5 μmol / g after 10 h. As the Rh content further increases, the CO yield decreases, indicating that excessive Rh deposition may become a recombination center for photogenerated carriers, hindering the light absorption of CN.

[0045] In addition, we conducted a series of control experiments. First, under ultraviolet-visible light irradiation, we studied the effects of different types of bipyridine prepared in Comparative Example 4 on Rh. 1 / The effect of CN on the photocatalytic activity and selectivity of CO2+H2O reduction, from Figure 8 As shown in (a) and (b), all seven added bipyridines promoted the reduction of CO2 to CO while significantly inhibiting H2 production. Even the relatively less effective 2,2′-bipyridine-4,4′-dicarboxylic acid still greatly promoted CO generation, with activity (CO yield of 1.4 μmol / g within 10 h) and selectivity increased by 45 times and 156 times, respectively, compared to Rh1 / CN. These comparative experiments demonstrate the versatility of 2,2′-bipyridine-modified carbon nitride surface single-site Rh catalysts in selectively controlling the photocatalytic CO2 + H2O reduction products.

[0046] Finally, we also tested the stability of the catalyst Rh1 / CN-bpy prepared in Example 1 under UV-Vis irradiation. Each cycle lasted 10 hours. Figure 9 As shown, after 5 cycles, the photocatalytic activity for reducing CO2 to CO did not change significantly. Figure 9 (b)), but after two cycles, the yield of the competing reaction H2 decreased significantly. Figure 9 (a) indicates that Rh1 / CN-bpy has good stability for photocatalytic CO2 reduction.

Claims

1. A 2,2'-bipyridine-modified single-site Rh / carbon nitride photocatalyst, characterized in that... The photocatalyst was prepared by the following method: Step 1: Calcining melamine at 300-500℃ for 2-4 hours in air atmosphere, grinding the powdered product with potassium chloride and lithium chloride in an anhydrous and oxygen-free environment until uniform, calcining at 450-550℃ for 3-4 hours in an inert atmosphere, and washing and drying the obtained product to obtain carbon nitride. Step 2: Disperse carbon nitride uniformly in deionized water to obtain a carbon nitride dispersion; dissolve sodium chloride solid in deionized water and add RhCl3 aqueous solution to form a homogeneous solution; add the obtained homogeneous solution to the carbon nitride dispersion and stir at 70-90℃ for 6-10 hours; wash and dry the obtained product with water, then disperse it in deionized water and irradiate it under visible light for 40-80 minutes; wash and dry it to obtain a single-point Rh / carbon nitride; the mass fraction of Rh in the single-point Rh / carbon nitride is 0.25%-2%; Step 3: Add single-site Rh / carbon nitride to the reaction tube, along with 2,2'-bipyridine, acetonitrile, triethanolamine, and deionized water. Irradiate with UV-Vis light for 10–12 hours under a CO2 atmosphere. After washing, drying, and grinding the obtained product, obtain the 2,2'-bipyridine-modified single-site Rh / carbon nitride photocatalyst.

2. The 2,2'-bipyridine-modified single-site Rh / carbon nitride photocatalyst according to claim 1, characterized in that, In step 1, melamine is heated to 500°C in air at a heating rate of 8–12°C / min and calcined at a constant temperature for 4 hours. The ground powder product is mixed and ground evenly with potassium chloride and lithium chloride in an anhydrous and oxygen-free environment, and then heated to 550°C in an inert atmosphere at a heating rate of 4–6°C / min and calcined at a constant temperature for 4 hours.

3. The 2,2'-bipyridine-modified single-site Rh / carbon nitride photocatalyst according to claim 1 or 2, characterized in that, In step 1, the mass ratio of carbon nitride to potassium chloride and lithium chloride is 1:5-6:4-5.

4. The 2,2'-bipyridine-modified single-site Rh / carbon nitride photocatalyst according to claim 1, characterized in that, In step 2, the obtained homogeneous solution is added to the carbon nitride dispersion and stirred at 80°C for 8 hours. The resulting product is washed with water, dried, and then dispersed in deionized water and irradiated under visible light with λ>420nm for 60 minutes.

5. The 2,2'-bipyridine-modified single-site Rh / carbon nitride photocatalyst according to claim 1 or 4, characterized in that, In step 2, the mass ratio of carbon nitride to RhCl3 and sodium chloride is 100:0.25-2:15-30.

6. The 2,2'-bipyridine-modified single-site Rh / carbon nitride photocatalyst according to claim 1, characterized in that, In step 3, the mass ratio of 2,2'-bipyridine to single-point Rh / carbon nitride is 1:1 to 6.

7. The 2,2'-bipyridine-modified single-site Rh / carbon nitride photocatalyst according to claim 1, characterized in that, In step 3, the volume ratio of acetonitrile, triethanolamine and deionized water is 3:1:

1.

8. A method for catalytic reduction of CO2, characterized in that... Includes the following steps: Step 1: Calcining melamine at 300-500℃ for 2-4 hours in air atmosphere, grinding the powdered product with potassium chloride and lithium chloride in an anhydrous and oxygen-free environment until uniform, calcining at 450-550℃ for 3-4 hours in an inert atmosphere, and washing and drying the obtained product to obtain carbon nitride. Step 2: Disperse carbon nitride uniformly in deionized water to obtain a carbon nitride dispersion; dissolve sodium chloride solid in deionized water and add RhCl3 aqueous solution to form a homogeneous solution; add the obtained homogeneous solution to the carbon nitride dispersion and stir at 70-90℃ for 6-10 hours; wash and dry the obtained product with water, then disperse it in deionized water and irradiate it under visible light for 40-80 minutes; wash and dry it to obtain a single-point Rh / carbon nitride; the mass fraction of Rh in the single-point Rh / carbon nitride is 0.25%-2%; Step 3: Add single-point Rh / carbon nitride to the reaction tube, along with 2,2'-bipyridine, acetonitrile, triethanolamine, and deionized water. Introduce CO2 gas and irradiate under UV-Vis light for 10–12 hours. The photocatalytic reduction of CO2 to CO yields a 2,2'-bipyridine-modified Rh1 / CN photocatalyst, which can be directly used for photocatalytic reduction of CO2 to CO under UV-Vis light irradiation.

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