CDs / CBB / Co composite photocatalyst based on ZIF-67 material and application of CDs / CBB / Co composite photocatalyst in CO2 reduction reaction

Abstract

The invention relates to the technical field of photocatalytic CO2 reduction reaction, in particular to a CDs / CBB / Co composite photocatalyst based on a ZIF-67 material and application of the CDs / CBB / Co composite photocatalyst in CO2 reduction reaction. A conventional photocatalyst is used as a catalyst for a CO2 reduction reaction, and the ethylene selectivity is generally lower than 35%. In order to solve the technical problems, the invention provides the CDs / CBB / Co composite photocatalyst based on the ZIF-67 material, a technology of'reduction calcination of ZIF-67 + electrostatic self-assembly of ternary heterojunction 'is provided, CDs quantum dots, CBB light quantum dots and a Co matrix are compounded, and efficient conversion from CO2 to C2H4 is realized through a multiple synergistic mechanism.

Description

Technical Field

[0001] The present invention relates to the technical field of alloy modifiers, and in particular to a CDs / CBB / Co composite photocatalyst based on ZIF-67 material and application thereof in a CO2 reduction reaction. Background Art

[0002] Photocatalytic CO2 reduction is a key technology for achieving the "dual carbon" strategy, but existing catalysts generally have three major bottlenecks: low product selectivity (ethylene selectivity <35%), high carrier recombination rate (>90%), and poor stability (cycle decay >40%). Specifically,

[0003] 1. Inherent defects of single-component catalysts

[0004] ZIF-67 derived Co3O4: High temperature calcination (>400℃) causes the pore structure to collapse, and the specific surface area decreases from 1400 to 200m 2 / g, CO2 adsorption capacity decreased by about 80%;

[0005] CBB photonic quantum dots: Quantum dot agglomeration leads to exciton self-trapping, and the photogenerated electron lifetime is less than 2ns;

[0006] Carbon dots (CDs): lack catalytic active sites and only act as electron mediators, with a CO2 activation energy barrier > 1.5 eV;

[0007] 2. Synergistic failure of binary composite materials

[0008] CBB / Co3O4: The deep valence band of Co3O4 (+2.3eV) and the shallow valence band of CBB (+1.6eV) have an energy mismatch, the hole transport barrier reaches 0.7eV, and the interface recombination is intensified;

[0009] CDs / CBB: The LUMO of CDs (-1.1 eV) is higher than the conduction band of CBB (-0.8 eV), electrons flow back in the opposite direction, and the reduction efficiency drops by 50%.

[0010] 3. Performance degradation caused by preparation process

[0011] High temperature pyrolysis destroys the structure: the traditional ZIF-67 calcination temperature is greater than 400℃, Co 2+ Oxidized to Co 3+ , loss of CO2 activation sites;

[0012] Quantum dot synthesis defects: The DMF solvothermal method for preparing CBB produces bromine vacancies (V_Br), which become carrier recombination centers (PL quenching rate > 85%). Summary of the Invention

[0013] A problem with the prior art is that conventional photocatalysts used as catalysts for CO2 reduction reactions typically have ethylene selectivity below 35%. To address this technical issue, the present invention provides a CDs / CBB / Co composite photocatalyst based on ZIF-67 material, the preparation of which comprises the following steps:

[0014] (1) ZIF-67 is subjected to high-temperature calcination under a reducing atmosphere after removing moisture to obtain a Co matrix;

[0015] (2) The Co matrix was evenly dispersed in isopropanol to obtain a Co suspension, and then CDs and CBB quantum dots were added to the Co suspension in sequence. After the electrostatic self-assembly was completed, the CDs / CBB / Co composite photocatalyst was obtained after centrifugation, water washing, and vacuum drying.

[0016] Preferably, the reducing atmosphere is a mixture of 95% by volume of Ar and 5% by volume of H2.

[0017] Preferably, the temperature of the high-temperature degassing is 120° C., the degassing time is 2 hours, the temperature of the high-temperature calcination is 550° C., and the high-temperature calcination time is 2 hours.

[0018] Preferably, the CDs are synthesized by an electrochemical method, which specifically comprises the following steps:

[0019] Two high-purity graphite rods are used as anode and cathode, respectively, and connected to the positive and negative poles of a DC power supply. Ultrapure water is used as the electrolyte. A constant voltage is applied between the electrodes for continuous electrolysis. During the entire electrolysis process, stirring is continued until the graphite rods are gradually corroded and the initially colorless electrolyte turns into a dark solution. The dark solution is subjected to high-speed centrifugation and dialyzation to remove the precipitated graphite oxide and large graphite particles to obtain an aqueous solution of carbon dots. Finally, the aqueous solution of carbon dots is freeze-dried to obtain powdered carbon dots, namely CDs.

[0020] Preferably, the voltage of electrolysis is 30V.

[0021] Preferably, the mass ratio of CDs to CBB and Co matrix in the Co suspension is 1:5:10, and the mass concentration of the Co suspension is 0.25 mg / mL.

[0022] Preferably, CBB is a quantum dot synthesized by a solvothermal method using CsBr and BiBr3 as reaction raw materials and dimethyl sulfoxide as reaction solvent.

[0023] Preferably, the preparation method of CBB comprises the following steps:

[0024] (1) 0.9 mmol CsBr and 0.6 mmol BiBr3 were added to a 100 mL round-bottom flask along with 20 mL dimethyl sulfoxide. The mixture was heated from room temperature to 60°C in an oil bath and kept at 60°C until the reaction materials were completely dissolved to obtain a clear precursor solution.

[0025] (2) The clarified precursor solution obtained in step (1) was then quickly added to 500 mL of isopropanol and stirred vigorously to obtain a turbid light yellow liquid. The light yellow liquid was then centrifuged at 4500 rpm to collect the supernatant. The supernatant was then centrifuged at 12000 rpm for at least 10 min, the supernatant was discarded, and the remaining precipitate was washed with isopropanol at least 3 times. Finally, a light yellow precipitate was collected and vacuum dried to obtain light yellow powdered CBB quantum dots.

[0026] Preferably, the preparation method of ZIF-67 comprises the following steps:

[0027] (1) 73.08 mmol of 2-methylimidazole and 10.31 mmol of Co(NO3)2·6H2O were dispersed in 40 mL of methanol and uniformly dispersed by ultrasonication to obtain a 2-methylimidazole solution and a Co(NO3)2·6H2O suspension, respectively;

[0028] (2) 2-Methylimidazole solution was added to the Co(NO3)2·6H2O suspension and stirred at room temperature for at least 24 h. The solid product was then collected by centrifugation. The solid product was then washed with methanol at least three times and dried to obtain ZIF-67.

[0029] The present invention has the following beneficial effects:

[0030] The present invention has developed a "reduction-calcined ZIF-67 + electrostatic self-assembled ternary heterojunction" technology, which achieves efficient conversion of CO2 to C2H4 through multiple synergistic mechanisms:

[0031] 1. Structural innovation of ZIF-67-derived Co matrix

[0032] Reduction calcination: Preserving the dodecahedral framework of ZIF-67 under Ar / H2 while partially reducing Co 2+ →Co 0 , forming a metallic Co core@CoO shell structure ( Figure 1 a) Both:

[0033] (1) High specific surface area promotes CO2 adsorption;

[0034] (2) Metal Co provides d-orbital electrons, lowering the COOH→CO energy barrier (from 1.28→0.75 eV).

[0035] 2. Quantum dot synthesis process optimization

[0036] Synthesis of Cs3Bi2Br9 (CBB) by non-coordinating solvent method: DMSO dissolution + isopropanol anti-solvent precipitation was used, and the bromine vacancy concentration was significantly reduced (compared to 8.7×10 17 cm -3 ), and the fluorescence quantum yield is significantly improved.

[0037] 3. Ternary heterojunction energy level engineering design

[0038] The stepped energy levels are arranged as follows:

[0039] CDs(LUMO:-1.1eV)→CBB(CB:-0.8eV)→Co(Fermi:-0.2eV)

[0040] The directional migration of photogenerated electrons is achieved, and the interface recombination rate is significantly reduced.

[0041] Electrostatic self-assembly process: Control the mass ratio of CDs:CBB:Co=1:5:10 to form a heterojunction in isopropyl alcohol, which significantly improves the hole isolation efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] Figure 1 1 is the XRD pattern of CDs / CBB / Co and CDs obtained in Example 1 of the present invention.

[0043] Figure 2 These are performance test graphs of the photocatalytic carbon dioxide reduction reaction of CDs / CBB / Co, Co matrix, and CBB obtained in Example 1 of the present invention, and CBB / Co, CDs / CBB, and CDs / Co obtained in Comparative Examples 1-3, respectively.

[0044] Figure 3 These are the SEM and TEM images of the Co matrix obtained in Example 1 of the present invention and the CBB / Co obtained in CDs / CBB / Co Comparative Example 1. Figure 3 a is the SEM image of Co matrix, Figure 3 b is the TEM image of Co matrix, Figure 3 c is the TEM image of CBB / Co, Figure 3 d is the TEM image of CDs / CBB / Co. DETAILED DESCRIPTION

[0045] The present invention will be described in detail below with reference to the following examples. However, it should be understood that the following examples are merely illustrative of the embodiments of the present invention and are not intended to limit the scope of the present invention.

[0046] Example 1

[0047] A CDs / CBB / Co composite photocatalyst based on ZIF-67 material, the preparation method is as follows:

[0048] (1) ZIF-67 is subjected to high-temperature degassing under a reducing atmosphere and then subjected to high-temperature calcination to obtain a Co matrix, wherein the reducing atmosphere is a mixture of Ar and H2, wherein the volume ratio of Ar to H2 in the mixture is 95:5, the high-temperature degassing temperature is 120°C, the degassing time is 2 hours, and the high-temperature calcination temperature is 550°C, and the high-temperature calcination time is 2 hours;

[0049] (2) 40 mg of Co matrix was evenly dispersed in 40 mL of isopropanol to obtain a Co suspension. CDs and CBB quantum dots were then added to the Co suspension in sequence. The mass ratio of CDs to CBB and Co matrix in the Co suspension was 1:5:10, and the mass concentration of the Co suspension was 0.25 mg / mL. After constant temperature stirring at 40°C for 12 hours, the reaction solution was centrifuged at 8000 rpm for 3 minutes, washed twice with ultrapure water, and vacuum dried at 60°C to obtain a CDs / CBB / Co composite photocatalyst.

[0050] The CDs are synthesized by electrochemical method, and the specific method is as follows:

[0051] High-purity graphite rods were ultrasonically cleaned in ultrapure water for 15 minutes to remove surface impurities. Two high-purity graphite rods (purity 99.995%) were used as anode and cathode, respectively, and connected to the positive and negative poles of a DC power supply, respectively. 1000 mL of ultrapure water was used as the electrolyte, and a constant voltage of 30 V was applied between the electrodes for continuous electrolysis. During the entire electrolysis process, stirring was continued until the graphite rods gradually corroded and the initially colorless electrolyte turned into a dark solution. The dark solution was subjected to high-speed (20,000 rpm) centrifugation and dialyzed (molecular weight cutoff 1000 Da) to remove precipitated graphite oxide and large graphite particles to obtain an aqueous solution of carbon dots. Finally, the aqueous solution of carbon dots was freeze-dried to obtain powdered carbon dots, i.e., CDs.

[0052] The preparation method of CBB is as follows:

[0053] (1) 0.9 mmol CsBr and 0.6 mmol BiBr3 were added to a 100 mL round-bottom flask along with 20 mL dimethyl sulfoxide. The mixture was heated from room temperature to 60°C in an oil bath and kept at 60°C until the reaction materials were completely dissolved to obtain a clear precursor solution.

[0054] (2) The clarified precursor solution obtained in step (1) was then quickly added to 500 mL of isopropanol and stirred vigorously to obtain a turbid light yellow liquid. The light yellow liquid was then centrifuged at 4500 rpm to collect the supernatant. The supernatant was then centrifuged at 12000 rpm for 10 min, the supernatant was discarded, and the remaining precipitate was washed three times with isopropanol. Finally, a light yellow precipitate was collected. The obtained light yellow precipitate was vacuum dried to obtain light yellow powdered CBB quantum dots (denoted as CBB).

[0055] The preparation method of ZIF-67 is as follows:

[0056] (1) 73.08 mmol of 2-methylimidazole and 10.31 mmol of Co(NO3)2·6H2O were dispersed in 40 mL of methanol and uniformly dispersed by ultrasonication to obtain a 2-methylimidazole solution and a Co(NO3)2·6H2O suspension, respectively;

[0057] (2) 2-Methylimidazole solution was added to the Co(NO3)2·6H2O suspension and stirred at room temperature for 24 h. The solid product was then collected by centrifugation. The solid product was then washed three times with methanol and dried to obtain ZIF-67.

[0058] Comparative Example 1 is a CBB / Co composite photocatalyst, and the preparation method is as follows:

[0059] 40 mg of Co matrix was uniformly dispersed in 40 mL of isopropanol under continuous stirring to obtain a Co suspension. Then, 8 mg of CBB quantum dots were added to the Co suspension and stirred at 40°C for 6 h. The reactant was centrifuged at 8000 rpm for 3 min, washed twice with ultrapure water, and vacuum dried at 60°C for 6 h to obtain CBB / Co.

[0060] The preparation method of the Co matrix is ​​the same as that of Example 1, and the preparation method of the CBB quantum dots is the same as that of Example 1.

[0061] Comparative Example 2 is a CDs / CBB composite photocatalyst, and the preparation method is as follows:

[0062] 4 mg of CDs and 8 mg of CBB were uniformly dispersed in 10 mL of isopropanol. Subsequently, the mixture was stirred at 40°C for 6 h, and the reaction product was centrifuged at 8000 rpm for 3 min. After that, the solid product was washed twice with ultrapure water and vacuum dried at 60°C for 6 h to obtain CDs / CBB.

[0063] Comparative Example 3 is a CDs / Co composite photocatalyst, and the preparation method is as follows:

[0064] 40 mg of Co matrix was uniformly dispersed in 40 mL of isopropanol under continuous stirring to obtain a Co suspension. Then, 4 mg of CDs was added to the Co suspension and stirred at 40°C for 6 h. The reactant was centrifuged at 8000 rpm for 3 min, washed twice with ultrapure water, and vacuum dried at 60°C for 6 h to obtain CDs / Co.

[0065] Comparative Example 4 is the same as Example 1, except that the mass ratio of CDs to CBB and Co matrix in the Co suspension in Comparative Example 4 is 1:1:1.

[0066] Comparative Example 5 is the same as Example 1, except that the mass ratio of CDs to CBB and Co matrix in the Co suspension in Comparative Example 5 is 5:1:10.

[0067] Comparative Example 6 is the same as Example 1, except that the temperature of high-temperature calcination of ZIF-67 in a reducing atmosphere in Comparative Example 6 is 200°C.

[0068] Comparative Example 7 is the same as Example 1, except that the Co matrix in Example 1 is replaced by the same amount of ZIF-67.

[0069] Comparative Example 8 is the same as Example 1, except that the Co matrix in Example 1 is replaced by other MOF materials in the same amount.

[0070] The XRD patterns of CDs and CDs / CBB / Co composite photocatalysts obtained in Example 1 of the present invention are shown in the attached specification. Figure 1 shown.

[0071] The CDs, CBB, Co matrix and CDs / CBB / Co composite photocatalyst obtained in Example 1 of the present invention and the CBB / Co composite photocatalyst, CDs / CBB composite photocatalyst and CDs / Co composite photocatalyst obtained in Comparative Examples 1-3 were used as photocatalysts for CO2 reduction reaction, and the catalytic activity and selectivity of each photocatalyst were evaluated under the same reaction conditions and dosage. The specific test results are shown in the attached specification. Figure 2 The specific test process is as follows:

[0072] Photocatalytic CO2 conversion was carried out in a 25 mL sealed quartz reactor equipped with a glass-enclosed gas circulation system. 10 mg of catalyst and 3 mL of deionized water were added back into the reactor, and the reaction temperature was maintained at 20°C by a circulating water layer. Before irradiation, the reactor was vacuumed for 30 minutes and high-purity CO2 (99.999%) was bubbled into the reactor. A 300 W xenon lamp equipped with an AM1.5G filter was irradiated from the top of the reactor. The gaseous products were analyzed using a GC-7860Plus gas chromatograph equipped with a TCD and FID detector.1 The liquid products (such as HCOOH and CH3OH) were detected by H NMR. The reusability of the catalyst in photocatalytic CO2 reduction was investigated by a 5 h cycle reaction. Gas chromatography-mass spectrometry (GC-MS, Agilent 8860) was used for 13 CO2 isotope testing.

[0073] The SEM and TEM images of the Co matrix obtained in Example 1 of the present invention are respectively Figure 3 middle Figure 3 a and Figure 3 b. TEM images of CBB / Co composite photocatalyst and CDs / CBB / Co composite photocatalyst are shown in the attached manual. Figure 3 in Figure 3 As shown in d.

[0074] The catalytic activity and selectivity of the CDs / CBB / Co composite photocatalyst obtained in Example 1 of the present invention and the photocatalysts obtained in Comparative Examples 1 to 8 were evaluated under the same reaction conditions and dosages. The specific test results are shown in Table 1.

[0075] Table 1

[0076]

[0077] The test results show that gaseous products including CO, CH4, C2H4 and C2H6 were detected in the best three-way catalyst (CDs / CBB / Co), and showed a linear growth trend under 5 hours of light (see the attached manual). Figure 2 a), no liquid product was detected in the photocatalytic system. Figure 2 As shown in b, Co and CDs / Co exhibit negligible activity, the conversion rate of CBB to C1 (CO and CH4) products is slow, and no C2 products are detected in CBB. Loading CBB onto Co nanocages can significantly increase the yield of C1 products, with the CO and CH4 generation rates of 10-CBB / Co being 87.90 and 43.14 μmol g, respectively. -1 h -1 , which are 18.16 and 80.36 times that of pure CBB respectively.

[0078] Interestingly, when CDs were combined with CBB, C2 products were rapidly generated, with the C2H4 gas production rate of CDs / CBB being 8.23 ​​μmol g -1 h -1 , the C2H6 gas production rate was 2.02 μmol g -1 h -1It is worth noting that when Co nanocages were introduced to form a ternary CDs / CBB / Co catalyst, the gas production rate and selectivity of C2 products were significantly improved. By adjusting the CDs loading on the CDs / CBB / Co catalyst, we found that the C2H4 gas production rate of the 5-CDs / CBB / Co catalyst could reach 57.60 μmol g -1 h -1 .

[0079] As the instruction manual Figure 1 As shown in Figure a, the diffraction peaks of the Co matrix appear at 44.2°, 51.4° and 75.8°, corresponding to the (111), (200) and (220) crystal planes, respectively (PDF#15-0806); the CBB exhibits characteristic diffraction peaks at 15.7°, 22.2°, 27.2°, 31.7°, 39.0° and 45.3°, corresponding to the (101), (102), (003), (202), (212) and (204) crystal planes, respectively (PDF#44-0714). In addition, two broad CD peaks appear at 23° and 42° (see the attached manual). Figure 1 b), corresponding to the (002) and (100) graphitic carbon planes, respectively. Distinct diffraction peaks of Co and CBB were observed in CBB / Co, indicating that CBB was successfully loaded into the Co nanocage. As expected, CDs / CBB / Co exhibited similar diffraction peaks to CBB / Co, but no characteristic peaks associated with CDs were observed, likely due to the low content, uniform distribution, and small particle size of CDs.

[0080] The morphology of the Co, CBB / Co, and CDs / CBB / Co photocatalysts obtained in Example 1 is shown in the attached manual. Figure 3 shown.

[0081] The morphologies of the Co matrix, CBB / Co, and CDs / CBB / Co obtained in Example 1 were observed using scanning electron microscopes and transmission electron microscopes. Figure 3 As shown, Figure 3 a is the SEM image of Co matrix, Figure 3 b is the TEM image of Co matrix, Figure 3 c is the TEM image of CBB / Co, Figure 3 d is the TEM image of CDs / CBB / Co. As can be seen from the figure, the morphology of the CDs / CBB / Co photocatalyst obtained in Example 1 is that the smaller CDs and CBB nanoparticles are tightly wrapped on the Co surface, and a small amount of larger nanoparticles are embedded in the Co and are relatively evenly distributed.

[0082] With the above-described preferred embodiments of the present invention as a guide, and with reference to the above description, relevant personnel are fully capable of making various changes and modifications without departing from the technical scope of this invention. The technical scope of this invention is not limited to the contents of the specification and must be determined according to the scope of the claims.

Claims

1. A CDs / CBB / Co composite photocatalyst based on ZIF-67 material, characterized in that: The preparation method comprises the following steps: (1) ZIF-67 is subjected to high-temperature calcination under a reducing atmosphere after removing moisture to obtain a Co matrix; (2) The Co matrix is ​​uniformly dispersed in a small molecule alcohol solvent to obtain a Co suspension. CDs and CBB quantum dots are then added to the Co suspension in sequence. After electrostatic self-assembly is completed, the CDs / CBB / Co composite photocatalyst is obtained after centrifugation, water washing, and drying.

2. The CDs / CBB / Co composite photocatalyst based on ZIF-67 material according to claim 1, characterized in that: The reducing atmosphere is a mixed gas formed by 95% by volume of Ar and 5% by volume of H2.

3. The CDs / CBB / Co composite photocatalyst based on ZIF-67 material according to claim 1, characterized in that: The high-temperature degassing temperature is 120° C., the degassing time is at least 2 hours, the high-temperature calcination temperature is 550° C., and the high-temperature calcination time is at least 2 hours.

4. The CDs / CBB / Co composite photocatalyst based on ZIF-67 material according to claim 1, characterized in that: The CDs are synthesized by an electrochemical method, which specifically comprises the following steps: Two high-purity graphite rods are used as the anode and cathode, respectively, and connected to the positive and negative poles of a DC power supply. Ultrapure water is used as the electrolyte. A constant voltage is applied between the electrodes for continuous electrolysis. During the entire electrolysis process, stirring is continued until the graphite rods are gradually corroded, and the initial colorless electrolyte turns into a dark solution. The dark solution is subjected to high-speed centrifugation and dialyzation to remove the precipitated graphite oxide and large graphite particles to obtain an aqueous solution of carbon dots. Finally, the aqueous solution of carbon dots is freeze-dried to obtain powdered carbon dots, namely CDs.

5. The CDs / CBB / Co composite photocatalyst based on ZIF-67 material according to claim 4, characterized in that: The voltage of electrolysis is at least 30V.

6. The CDs / CBB / Co composite photocatalyst based on ZIF-67 material according to claim 1, characterized in that: The mass ratio of CDs to CBB and Co matrix in the Co suspension is 1:5:10, and the mass concentration of the Co suspension is 0.25 mg / mL.

7. The CDs / CBB / Co composite photocatalyst based on ZIF-67 material according to claim 1, characterized in that: CBB is a quantum dot synthesized by solvothermal method using CsBr and BiBr3 as reaction raw materials and dimethyl sulfoxide as reaction solvent.

8. The CDs / CBB / Co composite photocatalyst based on ZIF-67 material according to claim 7, characterized in that: The preparation method of CBB comprises the following steps: (1) 0.9 mmol CsBr and 0.6 mmol BiBr3 were added to a 100 mL round-bottom flask along with 20 mL dimethyl sulfoxide. The mixture was heated from room temperature to 60°C in an oil bath and kept at 60°C until the reaction materials were completely dissolved to obtain a clear precursor solution. (2) The clarified precursor solution obtained in step (1) was then quickly added to 500 mL of isopropanol and stirred vigorously to obtain a turbid light yellow liquid. The light yellow liquid was then centrifuged at 4500 rpm to collect the supernatant. The supernatant was then centrifuged at 12000 rpm for at least 10 min, the supernatant was discarded, and the remaining precipitate was washed with isopropanol at least 3 times. Finally, a light yellow precipitate was collected and vacuum dried to obtain light yellow powdered CBB quantum dots.

9. The CDs / CBB / Co composite photocatalyst based on ZIF-67 material according to claim 1, characterized in that: The preparation method of ZIF-67 comprises the following steps: (1) 73.08 mmol of 2-methylimidazole and 10.31 mmol of Co(NO3)2·6H2O were dispersed in 40 mL of methanol and uniformly dispersed by ultrasonication to obtain a 2-methylimidazole solution and a Co(NO3)2·6H2O suspension, respectively; (2) 2-Methylimidazole solution was added to the Co(NO3)2·6H2O suspension and stirred at room temperature for at least 24 h. The solid product was then collected by centrifugation. The solid product was then washed with methanol at least three times and dried to obtain ZIF-67.

10. A CO2 reduction reaction, characterized in that The CDs / CBB / Co composite photocatalyst according to any one of claims 1 to 9 is used as a photocatalyst for CO2 reduction reaction.

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