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CN117299214B - CQD/Ni-MOFVCatalyst, preparation method and application thereof - Google Patents

CQD/Ni-MOFVCatalyst, preparation method and application thereof Download PDF

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CN117299214B
CN117299214B CN202311247097.1A CN202311247097A CN117299214B CN 117299214 B CN117299214 B CN 117299214B CN 202311247097 A CN202311247097 A CN 202311247097A CN 117299214 B CN117299214 B CN 117299214B
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mof
catalyst
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CN117299214A (en
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冯彩霞
王紫琼
王岩
蔡晓强
段海静
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Henan University
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    • B01J31/00—Catalysts comprising hydrides, coordination complexes or organic compounds
    • B01J31/16—Catalysts comprising hydrides, coordination complexes or organic compounds containing coordination complexes
    • B01J31/1691—Coordination polymers, e.g. metal-organic frameworks [MOF]
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    • B01J31/00—Catalysts comprising hydrides, coordination complexes or organic compounds
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    • B01J31/2204—Organic complexes the ligands containing oxygen or sulfur as complexing atoms
    • B01J31/2208—Oxygen, e.g. acetylacetonates
    • B01J31/2226—Anionic ligands, i.e. the overall ligand carries at least one formal negative charge
    • B01J31/223—At least two oxygen atoms present in one at least bidentate or bridging ligand
    • B01J31/2239—Bridging ligands, e.g. OAc in Cr2(OAc)4, Pt4(OAc)8 or dicarboxylate ligands
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Abstract

The invention belongs to the technical field of nano material photocatalysis, relates to a CQD/Ni-MOF V catalyst, and a preparation method and application thereof, and aims to solve the problem that a CO 2 product is difficult to convert into CH 4 by solar photocatalytic reduction. The method comprises the following steps: adding carbon dot solution into precursor of synthesizing Ni-MOF V by nickel chloride hexahydrate, terephthalic acid, ammonium chloride and the like, and preparing CQD/Ni-MOF V series catalyst by hydrothermal reaction. The method is simple and easy to operate; and the activity of the composite catalyst on CO 2 photocatalytic reduction can be obviously improved by simply controlling the content of the carbon dot solution, so that the selectivity of a product CH 4 is improved. The invention provides a new thought for preparing a catalyst for synthesizing a desired product by photocatalytic reduction of CO 2, and provides a preparation method of a photocatalyst with controllable product, which has very good prospect in the field of artificial photosynthesis.

Description

CQD/Ni-MOF V catalyst and preparation method and application thereof
Technical Field
The invention belongs to the technical field of nano material photocatalysis, and relates to a CQD/Ni-MOF V catalyst, a preparation method and application thereof.
Background
The combustion of fossil fuels results in excessive emissions of CO 2, and excessive emissions of CO 2 result in environmental problems such as global warming, destruction of ecosystems, elevation of sea level, and the like. In order to reduce the atmospheric concentration of CO 2, alleviate serious environmental problems, increase energy supply, search for sustainable and energy-saving methods to directly convert carbon dioxide into valuable chemicals has become the primary topic of environmental improvement. However, due to the thermodynamic stability and kinetic inertness of the CO 2 molecules and the complex multi-proton coupled electron transfer during the reaction, the design of a high performance catalyst capable of capturing and activating CO 2 molecules simultaneously remains a great challenge.
The CO 2 reduction process is a multiprotocol coupled electron transfer process, and the final product formation is generally determined by the kinetic and thermodynamic parameters of the CO 2 reduction pathway. Taking CH 4 and CO as examples, CH 4 formation is thermodynamically favored over CO formation because the reaction of the former occurs at a lower potential. However, from a kinetic point of view, the formation of CH 4, which requires eight electrons, is more difficult than the two electron reduction of CO 2 to CO. Equations (1) and (2) for the reaction to generate CH 4 and CO are as follows (at ph=7):
In the chemical industry, CO is an important raw material for synthesizing a series of basic organic chemical products and intermediates as a main component of synthesis gas and various types of gas. Starting from CO, almost all of the basic chemicals present, such as ammonia, phosgene, and alcohols, acids, anhydrides, esters, ethers, amines, alkanes, alkenes, etc., can be produced. Methane is a high quality gaseous fuel with relatively high combustion values and is also an important feedstock for the production of synthesis gas and many chemical products. Therefore, the catalyst is designed to regulate and control the proportion of the CO and CH 4 products, so that the catalyst has very important practical application value.
Metal Organic Frameworks (MOFs) consist of porous three-dimensional crystalline structures between metal-containing nodes and organic bridging linkers, which have led to rapid developments in a number of fields such as selective adsorption and separation, light trapping, sensors and photocatalysis due to their adjustable porosity, high surface area, structural uniformity and well-defined porous structures. In order to improve the photocatalytic activity of the metal organic framework material, patent CN114832830A discloses an oxide heterojunction with a B/A/B structure derived from MOF, and a preparation method and application thereof, wherein the preparation method of the oxide heterojunction is complex, and a high-temperature calcination process is required, so that great resource waste is caused; and the photocatalytic reduction carbon dioxide product is not easy to regulate and control, and the selectivity is low.
The Carbon Quantum Dots (CQD) are novel carbon nano materials and have the advantages of low cost, low toxicity, easiness in functionalization, good electron conductivity and the like. According to related reports, the introduction of CQD on a photocatalyst can increase the charge transfer rate, reduce charge recombination, and effectively improve the light absorption efficiency. The study demonstrates that loading CQD on MOF can greatly improve photocatalytic efficiency. Patent CN112808313A discloses a nitrogen-doped carbon quantum dot/metal organic framework material MOF-5 photocatalyst, and a preparation method and application thereof, and realizes the application of the photocatalyst in photocatalytic degradation of chromium-containing wastewater; patent CN114042476A discloses a preparation method of a MOF-TiO 2/graphene quantum dot nano composite photocatalyst, which is prepared by filling TiO 2 in a MOF under the condition of multiple deposition cycles by using an ALD atomic layer deposition system and then mixing with a graphene quantum dot solution, and has a complex preparation process. Based on the background, the photocatalyst which is simple in preparation process and can be regulated and controlled with high selectivity of a photocatalytic product is provided with great significance.
Disclosure of Invention
Aiming at the technical problems of uncontrollable and low selectivity of a photocatalytic reduction carbon dioxide product in the field of artificial photosynthesis, the invention provides a CQD/Ni-MOF V catalyst, a preparation method and application thereof, and CQD/Ni-MOF V series catalysts are prepared by constructing carbon points with different contents on the surface of the Ni-MOF rich in oxygen vacancies, thereby realizing the regulation and control of the selectivity of the photocatalytic reduction CO 2 product. The catalyst provided by the invention has the advantages of simple preparation mode and low cost.
In order to achieve the above object, the technical scheme of the present invention is as follows:
A method for preparing a CQD/Ni-MOF V catalyst, which comprises the following steps:
(1) Placing N, N-Dimethylformamide (DMF), ethanol (CH 3CH2 OH) and water into a container, stirring to uniformly mix the materials, and preparing a mixed solvent;
(2) Adding terephthalic acid into the mixed solvent in the step (1), and performing ultrasonic dissolution to obtain a colorless solution I;
(3) Adding nickel chloride hexahydrate (NiCl 2·6H2 O) into the colorless solution I obtained in the step (2), and performing ultrasonic dissolution to obtain a pale green solution I;
(4) Adding ammonium chloride (NH 4 Cl) into the light green solution in the step (3), carrying out ultrasonic treatment, stirring and dissolving to obtain a light green solution II;
(5) Adding the CQD solution into the light green solution II in the step (4), stirring for 15-20min, then placing the solution into a reaction kettle, performing hydrothermal reaction at 140-180 ℃ for 20-24h, centrifuging after the reaction is finished, and drying to obtain the CQD/Ni-MOF V catalyst.
Further, in the step (1), the volume ratio of DMF, CH 3CH2 OH and water is (14-16) to 1:1, and the stirring time is 5-10min.
Further, the molar ratio of terephthalic acid to nickel chloride hexahydrate in the steps (2) and (3) is 1:1, and the solution is dissolved by ultrasonic treatment for 10-15 min.
Further, the molar concentration of the nickel chloride hexahydrate in the step (3) is 0.021mol/L.
Further, in the step (4), the molar concentration of NH 4 Cl is 0.093mol/L, and the ultrasonic treatment is carried out for 15-20min, and the stirring is carried out for 20min.
Further, the volume ratio of the CQD solution to the light green solution II in the step (5) is (2-15): (6400-7200).
The preparation method of the CQD solution comprises the following steps:
(1) Placing Citric Acid (CA), thiourea (CH 4N2 S) and water in a container, dissolving by ultrasonic for 10-15min, stirring for 15-20min to uniformly mix them, and obtaining colorless solution; the molar ratio of the CA to the CH 4N2 S is 1:3, and the molar concentration of the CA is 0.1mol/L.
(2) And (3) putting the colorless solution in the step (1) into a reaction kettle, performing hydrothermal reaction at 180 ℃ for 6 hours, centrifuging after the reaction is finished, and collecting a supernatant to obtain a CQD solution.
The CQD/Ni-MOF V catalyst is prepared by the preparation method.
The application of the CQD/Ni-MOF V catalyst in the photocatalytic reduction of CO 2.
The invention has the beneficial effects that:
(1) The CQD/Ni-MOF V catalyst containing different carbon quantum dot contents is prepared by a hydrothermal method through one-step reaction, and the preparation process is simple.
(2) The CQD/Ni-MOF V catalyst obtained by the preparation method provided by the invention has the advantages that the selectivity of CH 4 generated by the CQD/Ni-MOF V catalyst is increased and then reduced along with the introduction of the CQD. When 25 μl of CQD solution was added, the selectivity of the catalyst for reduction of CO 2 to CH 4 was maximized, reaching 98.92%.
(3) The CQD/Ni-MOF V catalyst prepared by the preparation method can greatly improve the migration rate of photo-generated electrons, is favorable for the reaction of generating methane by multiple electrons, and can effectively improve the selectivity of generating CH 4 by photo-catalytic reduction of CO 2.
Drawings
In order to more clearly illustrate the embodiments of the invention or the technical solutions in the prior art, the drawings that are required in the embodiments or the description of the prior art will be briefly described, it being obvious that the drawings in the following description are only some embodiments of the invention, and that other drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
FIG. 1 is an X-ray diffraction (XRD) pattern of Ni-MOF V prepared in comparative example 1 and of CQD/Ni-MOF V catalysts prepared in examples 1-4.
FIG. 2 is an SEM image of a Ni-MOF V prepared according to comparative example 1 and a CQD/Ni-MOF V catalyst prepared according to examples 1-4; wherein (a) Ni-MOF V catalyst, (b) CQD-10/Ni-MOF V catalyst, (c) CQD-25/Ni-MOF V catalyst, (d) CQD-50/Ni-MOF V catalyst, (e) CQD-75/Ni-MOF V catalyst.
FIG. 3 is a TEM image of a CQD of individual carbon dots and a CQD/Ni-MOF V catalyst prepared in examples 1-4; wherein (a) a single carbon point CQD, (b) a CQD-10/Ni-MOF V catalyst, (c) a CQD-25/Ni-MOF V catalyst, (d) a CQD-50/Ni-MOF V catalyst, and (e) a CQD-75/Ni-MOF V catalyst.
FIG. 4 is an ESR plot of the Ni-MOF V prepared in comparative example 1 and the CQD/Ni-MOF V catalysts prepared in examples 1-4.
FIG. 5 is a graph of the photocatalytic reduction CO 2 activity of the Ni-MOF V prepared in comparative example 1 and the CQD/Ni-MOF V catalysts prepared in examples 1-4; wherein (A) the activity diagram of CO 2 is generated by photocatalytic reduction of CO, and (B) the activity diagram of CH 4 is generated by photocatalytic reduction of CO 2.
FIG. 6 is a graph (a) of the yield of CH 4 produced by reduction of CO 2 after 6 hours of photocatalytic reaction versus the volume of carbon dot solution added during synthesis for the strong ESR peaks of the Ni-MOF V prepared in comparative example 1 and the CQD/Ni-MOF V catalysts prepared in examples 1-4; and (b) a graph of the yield of CO produced by reduction of CO 2 after 6h of reaction, which is strong in ESR peak of the Ni-MOF V and CQD/Ni-MOF V catalysts, versus the volume of the carbon quantum dot solution added in the synthesis process.
FIG. 7 is an EIS diagram of the Ni-MOF V prepared in comparative example 1 and the CQD/Ni-MOF V catalysts prepared in examples 1-4.
Detailed Description
The technical solutions of the present invention will be clearly and completely described in conjunction with the embodiments of the present invention, and it is apparent that the described embodiments are only some embodiments of the present invention, but not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without any inventive effort, are intended to be within the scope of the invention.
Example 1
The preparation method of the CQD/Ni-MOF V catalyst comprises the following steps:
(1) Preparation of CQD solution: dissolving 4mmol (0.7685 g) of CA and 12mmol (0.9134 g) of CH 4N2 S in 40mL of distilled water, dissolving by ultrasonic for 10min, stirring for 20min to uniformly mix to form a colorless solution, placing the colorless solution into a reaction kettle for hydrothermal reaction, placing the reaction kettle into an oven for reaction for 6h at the temperature of 180 ℃, centrifuging after the reaction is finished, and taking the supernatant to obtain the CQD solution.
(2) Preparation of CQD/Ni-MOF V catalyst: firstly, placing 32mL of DMF, 2mL of ethanol and 2mL of water into a 50mL small beaker for mixing, and uniformly mixing to form a colorless solution I after ultrasonic treatment for 5 min; secondly, dissolving 0.75mmol of terephthalic acid in the colorless solution I, and performing ultrasonic treatment for 10min to obtain a colorless solution II; then, adding 0.75mmol of nickel chloride hexahydrate into the colorless solution II, and performing ultrasonic treatment for 10min to obtain a light green solution I; then adding 0.18g of NH 4 Cl into the pale green solution I, carrying out ultrasonic treatment for 15min, stirring for 20min, and obtaining a pale green solution II after the solid is completely dissolved; finally, 10 mu L of the CQD solution prepared in the step (1) is added into the light green solution II, stirred for 20min, then transferred into a reaction kettle, placed into an oven to react for 24h at the temperature of 140 ℃, centrifuged and dried to obtain the composite material, and named CQD-10/Ni-MOF V.
Example 2
The preparation method of the CQD/Ni-MOF V catalyst comprises the following steps:
(1) Preparation of CQD solution: dissolving 4mmol (0.7685 g) of CA and 12mmol (0.9134 g) of CH 4N2 S in 40mL of distilled water, dissolving by ultrasonic for 10min, stirring for 20min to uniformly mix to form a colorless solution, placing the colorless solution into a reaction kettle for hydrothermal reaction, placing the reaction kettle into an oven for reaction for 6h at the temperature of 180 ℃, centrifuging after the reaction is finished, and taking the supernatant to obtain the CQD solution.
(2) Preparation of CQD/Ni-MOF V catalyst: firstly, placing 32mL of DMF, 2mL of ethanol and 2mL of water into a 50mL small beaker for mixing, and uniformly mixing to form a colorless solution I after ultrasonic treatment for 5 min; secondly, dissolving 0.75mmol of terephthalic acid in the colorless solution I, and performing ultrasonic treatment for 10min to obtain a colorless solution II; then, adding 0.75mmol of nickel chloride hexahydrate into the colorless solution II, and performing ultrasonic treatment for 10min to obtain a light green solution I; then adding 0.18g of NH 4 Cl into the pale green solution I, carrying out ultrasonic treatment for 15min, stirring for 20min, and obtaining a pale green solution II after the solid is completely dissolved; finally, 25 mu L of the CQD solution prepared in the step (1) is added into the light green solution II, stirred for 20min, then transferred into a reaction kettle, placed into an oven to react for 24h at the temperature of 140 ℃, centrifuged and dried to obtain the composite material, and named CQD-25/Ni-MOF V.
Example 3
The preparation method of the CQD/Ni-MOF V catalyst comprises the following steps:
(1) Preparation of CQD solution: dissolving 4mmol (0.7685 g) of CA and 12mmol (0.9134 g) of CH 4N2 S in 40mL of distilled water, dissolving by ultrasonic for 10min, stirring for 20min to uniformly mix to form a colorless solution, placing the colorless solution into a reaction kettle for hydrothermal reaction, placing the reaction kettle into an oven for reaction for 6h at the temperature of 180 ℃, centrifuging after the reaction is finished, and taking the supernatant to obtain the CQD solution.
(2) Preparation of CQD/Ni-MOF V catalyst: firstly, placing 32mL of DMF, 2mL of ethanol and 2mL of water into a 50mL small beaker for mixing, and uniformly mixing to form a colorless solution I after ultrasonic treatment for 5 min; secondly, dissolving 0.75mmol of terephthalic acid in the colorless solution I, and performing ultrasonic treatment for 10min to obtain a colorless solution II; then, adding 0.75mmol of nickel chloride hexahydrate into the colorless solution II, and performing ultrasonic treatment for 10min to obtain a light green solution I; then adding 0.18g of NH 4 Cl into the pale green solution I, carrying out ultrasonic treatment for 15min, stirring for 20min, and obtaining a pale green solution II after the solid is completely dissolved; finally, 50 mu L of the CQD solution prepared in the step (1) is added into the light green solution II, stirred for 20min, then transferred into a reaction kettle, placed into an oven to react for 24h at the temperature of 140 ℃, centrifuged and dried to obtain the composite material, and named CQD-50/Ni-MOF V.
Example 4
The preparation method of the CQD/Ni-MOF V catalyst comprises the following steps:
(1) Preparation of CQD solution: dissolving 4mmol (0.7685 g) of CA and 12mmol (0.9134 g) of CH 4N2 S in 40mL of distilled water, dissolving by ultrasonic for 10min, stirring for 20min to uniformly mix to form a colorless solution, placing the colorless solution into a reaction kettle for hydrothermal reaction, placing the reaction kettle into an oven for reaction for 6h at the temperature of 180 ℃, centrifuging after the reaction is finished, and taking the supernatant to obtain the CQD solution.
(2) Preparation of CQD/Ni-MOF V catalyst: firstly, placing 32mL of DMF, 2mL of ethanol and 2mL of water into a 50mL small beaker for mixing, and uniformly mixing to form a colorless solution I after ultrasonic treatment for 5 min; secondly, dissolving 0.75mmol of terephthalic acid in the colorless solution I, and performing ultrasonic treatment for 10min to obtain a colorless solution II; then, adding 0.75mmol of nickel chloride hexahydrate into the colorless solution II, and performing ultrasonic treatment for 10min to obtain a light green solution I; then adding 0.18g of NH 4 Cl into the pale green solution I, carrying out ultrasonic treatment for 15min, stirring for 20min, and obtaining a pale green solution II after the solid is completely dissolved; finally, 75 mu L of the CQD solution prepared in the step (1) is added into the light green solution II, stirred for 20min, then transferred into a reaction kettle, placed into an oven to react for 24h at the temperature of 140 ℃, centrifuged and dried to obtain the composite material, and named CQD-75/Ni-MOF V.
Example 5
The preparation method of the CQD/Ni-MOF V catalyst comprises the following steps:
(1) Preparation of CQD solution: dissolving 4mmol (0.7685 g) of CA and 12mmol (0.9134 g) of CH 4N2 S in 40mL of distilled water, dissolving by ultrasonic for 15min, stirring for 15min to uniformly mix to form a colorless solution, placing the colorless solution into a reaction kettle for hydrothermal reaction, placing the reaction kettle into an oven for reaction for 6h at the temperature of 180 ℃, centrifuging after the reaction is finished, and taking the supernatant to obtain the CQD solution.
(2) Preparation of CQD/Ni-MOF V catalyst: firstly, placing 28mL of DMF, 2mL of ethanol and 2mL of water into a 50mL small beaker for mixing, and uniformly mixing to form a colorless solution I after ultrasonic treatment for 10 min; secondly, dissolving 0.67mmol of terephthalic acid in the colorless solution I, and performing ultrasonic treatment for 15min to obtain a colorless solution II; then, adding 0.67mmol of nickel chloride hexahydrate into the colorless solution II, and performing ultrasonic treatment for 15min to obtain a light green solution I; then adding 0.16g of NH 4 Cl into the pale green solution I, carrying out ultrasonic treatment for 20min, stirring for 20min, and obtaining a pale green solution II after the solid is completely dissolved; finally, 10 mu L of the CQD solution prepared in the step (1) is added into the light green solution II, stirred for 15min, then transferred into a reaction kettle, placed into an oven to react for 20h at the temperature of 180 ℃, centrifuged and dried to obtain the composite material, and named CQD-10/Ni-MOF V.
Example 6
The preparation method of the CQD/Ni-MOF V catalyst comprises the following steps:
(1) Preparation of CQD solution: dissolving 4mmol (0.7685 g) of CA and 12mmol (0.9134 g) of CH 4N2 S in 40mL of distilled water, dissolving by ultrasonic for 15min, stirring for 20min to uniformly mix to form a colorless solution, placing the colorless solution into a reaction kettle for hydrothermal reaction, placing the reaction kettle into an oven for reaction for 6h at the temperature of 180 ℃, centrifuging after the reaction is finished, and taking the supernatant to obtain the CQD solution.
(2) Preparation of CQD/Ni-MOF V catalyst: firstly, placing 28mL of DMF, 2mL of ethanol and 2mL of water into a 50mL small beaker for mixing, and uniformly mixing to form a colorless solution I after ultrasonic treatment for 10 min; secondly, dissolving 0.67mmol of terephthalic acid in the colorless solution I, and performing ultrasonic treatment for 15min to obtain a colorless solution II; then, adding 0.67mmol of nickel chloride hexahydrate into the colorless solution II, and performing ultrasonic treatment for 15min to obtain a light green solution I; then adding 0.16g of NH 4 Cl into the pale green solution I, carrying out ultrasonic treatment for 20min, stirring for 20min, and obtaining a pale green solution II after the solid is completely dissolved; finally, 75 mu L of the CQD solution prepared in the step (1) is added into the light green solution II, stirred for 15min, then transferred into a reaction kettle, placed into an oven to react for 20h at the temperature of 180 ℃, centrifuged and dried to obtain the composite material, and named CQD-75/Ni-MOF V.
Example 7
The preparation method of the CQD/Ni-MOF V catalyst comprises the following steps:
(1) Preparation of CQD solution: dissolving 4mmol (0.7685 g) of CA and 12mmol (0.9134 g) of CH 4N2 S in 40mL of distilled water, dissolving by ultrasonic for 10min, stirring for 20min to uniformly mix to form a colorless solution, placing the colorless solution into a reaction kettle for hydrothermal reaction, placing the reaction kettle into an oven for reaction for 6h at the temperature of 180 ℃, centrifuging after the reaction is finished, and taking the supernatant to obtain the CQD solution.
(2) Preparation of CQD/Ni-MOF V catalyst: firstly, placing 30mL of DMF, 2mL of ethanol and 2mL of water into a 50mL small beaker for mixing, and uniformly mixing to form a colorless solution I after ultrasonic treatment for 5 min; secondly, dissolving 0.71mmol of terephthalic acid in the colorless solution I, and performing ultrasonic treatment for 13min to obtain a colorless solution II; then, adding 0.71mmol of nickel chloride hexahydrate into the colorless solution II, and performing ultrasonic treatment for 10min to obtain a light green solution I; then adding 0.17g of NH 4 Cl into the pale green solution I, carrying out ultrasonic treatment for 10min, stirring for 15min, and obtaining a pale green solution II after the solid is completely dissolved; finally, 50 mu L of the CQD solution prepared in the step (1) is added into the light green solution II, stirred for 20min, then transferred into a reaction kettle, placed into an oven to react for 22h at 160 ℃, centrifuged and dried to obtain the composite material, and named CQD-50/Ni-MOF V.
Comparative example 1
The preparation method of the Ni-MOF V catalyst of the comparative example comprises the following steps:
Firstly, placing 32mL of DMF, 2mL of ethanol and 2mL of water into a 50mL small beaker for mixing, and uniformly mixing to form a colorless solution I after ultrasonic treatment for 5 min; secondly, dissolving 0.75mmol of terephthalic acid in the colorless solution I, and performing ultrasonic treatment for 10min to obtain a colorless solution II; then, adding 0.75mmol of nickel chloride hexahydrate into the colorless solution II, and performing ultrasonic treatment for 10min to obtain a light green solution I; finally, adding 0.18g of NH 4 Cl into the light green solution I, carrying out ultrasonic treatment for 15min, stirring for 20min, transferring into a reaction kettle after the solid is completely dissolved, placing the reaction kettle into an oven to react for 24h at the temperature of 140 ℃, centrifuging, and drying to obtain the Ni-MOF V catalyst.
Implementation effect analysis
(1) Characterization of the catalyst Crystal Structure
FIG. 1 is an XRD pattern for Ni-MOF V and CQD/Ni-MOF V composites of varying loadings. The diffraction peak of Ni-MOF V is consistent with literature reports, indicating that Ni-MOF materials were successfully prepared. The characteristic diffraction peaks of all the samples (Ni-MOF V and CQD/Ni-MOF V) in FIG. 1 were evident as absorption peaks at 2 theta values of 8.3, 15.0, 15.9, 17.0, 25.8, 30, which are consistent with the Ni-MOF (CCDC No. 985792) standard card, without any impurity peaks. The characteristic diffraction peaks for CQDs were not found in the XRD pattern of the CQD/Ni-MOF V composite, probably due to the low crystallinity and content of CQDs in the CQD/Ni-MOF V composite.
(2) Analysis of catalyst surface topography
FIG. 2 is an SEM image of Ni-MOF V and CQD/Ni-MOF V composites of varying content. In FIG. 2 (a), the Ni-MOF V is shown as elliptical particles. In FIGS. 2 (b) - (e) are scanning electron microscope images of CQD-10/Ni-MOF V,CQD-25/Ni-MOFV,CQD-50/Ni-MOFV and CQD-75/Ni-MOF V, respectively, it can be seen that the addition of CQD does not change its morphology. However, no CQD was found in the SEM image due to magnification limitations, and the presence of CQD on Ni-MOF V was further confirmed by high-resolution TEM.
FIG. 3 is a TEM image of a CQD/Ni-MOF V composite with individual carbon dots and varying loadings, where (a), (b), (c), (d), and (e) are TEM images of the individual carbon dots CQD, CQD-10/Ni-MOF V,CQD-25/Ni-MOFV,CQD-50/Ni-MOFV,CQD-75/Ni-MOFV, respectively. From the graph, the carbon dots are nano-microspheres with the particle size of about 5nm, and the more the microspheres on the surface of the CQD/Ni-MOF V composite material are along with the increase of the volume addition amount of the carbon dot solution in the synthesis process, the successful loading of the CQD on the Ni-MOF V series material is shown.
(3) ESR analysis of catalysts
Electron paramagnetic resonance (ESR) is the detection of the presence of paramagnetic species in the catalyst by a Bruker ESR a300-10/12 spectrometer in the dark state at room temperature (298K). The signal peak of g=2.003 corresponds to the presence of oxygen vacancies with single electron tethering.
FIG. 4 is an ESR plot of Ni-MOF V and CQD/Ni-MOF V series catalysts. From the figure, it can be seen that at g=2.003, all samples have a significant signal peak of oxygen vacancies, which proves that the prepared catalyst contains oxygen vacancies. And it can also be seen from the graph that the signal peak intensity of ESR, I CQD-25/Ni-MOFV>ICQD-10/Ni-MOFV>INi-MOFV>ICQD-75/Ni-MOFV≈ ICQD-50/Ni-MOFV. This is probably because the carbon dot solution contains unreacted thiourea, and part of the thiourea can be decomposed at high temperature in the reaction kettle to generate reductive ammonia gas, which is helpful for generating oxygen vacancies. However, too many carbon quantum dots may fill oxygen vacancies such that the concentration of oxygen vacancies is reduced and the ESR peak is reduced.
(4) Activity test and analysis of catalyst photocatalytic reduction CO 2 product
The photocatalytic reduction experiments of CO 2 were carried out in a quartz glass reactor (100 mL), the light source of visible light being a 300W xenon lamp (PLS-SXE 300) fitted with a 420nm cut-off filter. Prior to the reaction, 5mg of the catalyst and 15mg of [ Ru (bpy) 3]Cl2·6H2 O were dispersed in 6mL of a mixed solution of acetonitrile (CH 3CN;3mL) ,H2 O (2 mL) and triethanolamine (TEOA; 1 mL). The TEOA was used as a sacrificial agent and [ Ru (bpy) 3]2+ was used as an electron mediator. Then, the reactor was evacuated and a CO 2 gas of 99.9% purity was introduced and the reaction was illuminated for 6h. The gas product was detected by gas chromatography equipped with TCD and FID (GC 7900).
In addition, the selectivity of CO and CH 4 can be calculated according to equation (1) and equation (2).
Wherein, the selectivity (%) of S CO to CO, the selectivity (%) of S CH4 to CH 4, and the reaction rate of n (CO) to CO (mmol g -1h-1）,n（CH4) to CH 4 (mmol g -1h-1) are all shown in the specification.
FIG. 5 is a graph showing the activities of the composite materials of Ni-MOF V and CQD/Ni-MOF V in photocatalytic reduction of carbon dioxide to different products. FIG. 5A is a graph comparing the activities of the products of Ni-MOF V and CQD/Ni-MOF V in photocatalytic reduction of CO 2 to CO, and shows that the pure Ni-MOF V catalyst without CQD has the highest CO production by reduction of CO 2, and can reach 1.99 mmol.g -1 after 6 hours of reaction. As can be seen from fig. 5A and 5B, when the CQD loading is small, the yield of CO produced by catalytic reduction of CO 2 by the novel Ni-MOF catalyst gradually decreases (η CO,Ni-MOFV>ηCO,CQD-10/Ni-MOFV>ηCO,CQD-25/Ni-MOFV), the yield of CH 4 produced by catalytic reduction of CO 2 gradually increases (η CH4,Ni-MOFV<ηCH4,CQD-10/Ni-MOFV<ηCH4,CQD-25/Ni-MOFV), the yield of CO produced by catalytic reduction of CO 2 by the Ni-MOF catalyst gradually increases as the carbon point loading continues to increase, the yield of CH 4 produced by catalytic reduction of CO 2 gradually decreases, this law of variation substantially coincides with the law of variation of the concentration of oxygen vacancies, the greater the concentration of oxygen vacancies, the higher the yield of reduction of CO 2 to CH 4 and the lower the yield of CO. The yield of CO 2 produced by catalytic reduction of CO 2 by Ni-MOF V alone is the greatest, the selectivity of CO product reaches 21.19% after reaction for 6h, namely 1.99 mmol.g -1; the yield of the catalytic reduction CO 2 to CH 4 is minimum, the reaction time reaches 1.88 mmol.g -1 after 6 hours, and the selectivity of the product CH 4 is about 78.81 percent. The minimum amount of CO generated after the CQD-25/Ni-MOF V with the maximum concentration of oxygen vacancies reacts for 6 hours is 0.26 mmol.g -1, and the selectivity of CO of the product is 1.08%; the maximum amount of CH 4 is 6.02 mmol g -1, and the selectivity of the product methane can reach 98.92%. This suggests that the loading of carbon sites affects the concentration of oxygen vacancies, which in turn has a direct effect on the selectivity of the catalyst to reduce CO 2 to methane and CO (fig. 6 a and 6 b), with greater concentrations of oxygen vacancies having a higher selectivity to methane. The selectivity of the catalytic product is regulated and controlled by loading different amounts of carbon points on the surface of the Ni-MOF.
(5) EIS analysis of catalysts
The electrochemical impedance spectrum of the catalyst was measured using an electrochemical workstation (CHI 630E, china) equipped with a standard three-electrode photoelectrochemical cell. The working electrode was prepared as follows: 20mg of the catalyst to be tested was dispersed in 0.5mL of ultrapure water, followed by addition of 50mL Nafion solution (5 wt%) to form a uniform slurry. The obtained slurry was uniformly coated on indium tin oxide (2.0 cm. Times.1.5 cm) glass to obtain a working electrode. An Ag/AgCl electrode and Pt filament were used as reference and counter electrodes, respectively. Electrochemical Impedance Spectroscopy (EIS) was measured using Zennium pro electrochemical workstation (ZAHNER, germany) under visible light irradiation in the frequency range of 100 kHz-0.1Hz, using a solution of 5mM [ Fe (CN) 6]3-/[Fe（CN）6]4- and 0.5M KCl as electrode solution.
FIG. 7 is an EIS spectrum of the Ni-MOF V and CQD/Ni-MOF V catalysts prepared. Under visible light irradiation, the arc radius of the electrochemical impedance plot of CQD/Ni-MOF V is smaller than that of Ni-MOF V, and the arc radius of CQD-25/Ni-MOF V is smallest, which indicates that the resistance to photo-generated charge migration on the CQD-25/Ni-MOF V catalyst is smallest, the number of photo-generated electrons migrating to the catalyst surface is greatest, and the reaction of multi-electron reaction to methane is most likely to occur.
The foregoing description of the preferred embodiments of the invention is not intended to be limiting, but rather is intended to cover all modifications, equivalents, alternatives, and improvements that fall within the spirit and scope of the invention.

Claims (8)

1. A method for preparing a CQD/Ni-MOF V catalyst for photocatalytic reduction of CO 2, comprising the steps of:
(1) Adding terephthalic acid into a mixed solvent, ultrasonically dissolving, then adding nickel chloride hexahydrate, and ultrasonically dissolving to obtain a light green solution I;
(2) Adding ammonium chloride into the light green solution I in the step (1), and stirring and dissolving by ultrasonic to obtain a light green solution II;
(3) Adding the CQD solution into the light green solution II in the step (2), performing hydrothermal reaction for 20-24h at 140-180 ℃, centrifuging and drying to obtain the CQD/Ni-MOF V catalyst.
2. The method for preparing a CQD/Ni-MOF V catalyst for photocatalytic reduction of CO 2 according to claim 1, wherein the mixed solvent in step (1) is a mixed solvent of N, N-dimethylformamide, ethanol and water, and the volume ratio of the three is (14-16): 1:1.
3. The method for preparing a CQD/Ni-MOF V catalyst for photocatalytic reduction of CO 2 according to claim 2, characterized in that in step (1), the molar ratio of nickel chloride hexahydrate to terephthalic acid is 1:1, the molar concentration of nickel chloride hexahydrate is 0.021mol/L; the molar concentration of ammonium chloride in the step (2) is 0.093mol/L.
4. The method for preparing a CQD/Ni-MOF V catalyst for photocatalytic reduction of CO 2 according to claim 3, wherein the method for preparing a CQD solution in step (3) is: dissolving citric acid and thiourea in water, performing hydrothermal reaction at 180 ℃ for 6 hours, centrifuging after the reaction is finished, and taking supernatant to obtain CQD solution.
5. The method for preparing a CQD/Ni-MOF V catalyst for photocatalytic reduction of CO 2 according to claim 4, characterized in that the molar ratio of citric acid to thiourea is 1:3, the molar concentration of citric acid is 0.1mol/L.
6. The method for preparing a CQD/Ni-MOF V catalyst for photocatalytic reduction of CO 2 according to claim 1, wherein the volume ratio of CQD solution to pale green solution ii in step (3) is (2-15): (6400-7200).
7. A CQD/Ni-MOF V catalyst prepared by the method of any one of claims 1-6.
8. Use of the CQD/Ni-MOF V catalyst of claim 7 for photocatalytic reduction of CO 2 to CH 4.
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