Polyionic liquid modified silver nanoparticles as well as preparation method and application thereof

Abstract

The invention discloses polyionic liquid modified silver nanoparticles and a preparation method and application thereof. The preparation method comprises the following steps: respectively dispersing silver nanoparticles, vinyl-substituted ionic liquid monomers, a cross-linking agent and a free radical initiator in an organic solvent; carrying out in-situ free radical polymerization at high temperature under a deoxidizing condition, so that an ionic liquid monomer and the cross-linking agent are polymerized on the surfaces of the silver nanoparticles to form a covalent bond or coordinate bond combined cross-linked polyionic liquid shell layer; and carrying out centrifugal separation, solvent washing and vacuum drying on the product to obtain the polyion liquid modified silver nanoparticles. In the reaction for preparing CO by electrocatalytic CO2 reduction at normal temperature and normal pressure, the Faraday efficiency can reach 70% or above. According to the method, by constructing a chemical bond combined polyionic liquid shell layer, the product selectivity and the hydrogen evolution resistance of the silver nanoparticles are effectively improved, the problem that a physical adsorption type ionic liquid modification layer is prone to falling off is solved, and remarkable innovativeness and application value are achieved.

Description

Technical Field

[0001] This invention belongs to the field of electrocatalysis technology, specifically relating to a method for preparing and applying a highly stable polyionic liquid-modified silver nanoparticle catalyst with a covalently cross-linked core-shell structure that efficiently and selectively converts CO2 into CO. Background Technology

[0003] Currently, carbon dioxide conversion technologies mainly include three types: photocatalysis, thermocatalysis, and electrocatalysis. Among them, electrocatalysis has attracted much attention due to its mild reaction conditions and the ability to directly utilize renewable energy. The reduction products of CO2 via electrocatalysis include not only CO, but also methane, methanol, formic acid, and ethanol. CO is a core component of syngas (H2 / CO) and a key intermediate in the production of methanol and various fine chemicals, resulting in strong market demand. However, how to efficiently and selectively electrocatalytically reduce CO2 to CO remains one of the key challenges currently facing the industry.

[0004] Studies have found that noble metal-based catalysts such as gold and silver have suitable adsorption strength for CO intermediates generated from CO2 reduction, and are considered preferred catalysts for the electrocatalytic reduction of CO2 to CO. However, in aqueous electrolytes, the competitive hydrogen evolution reaction (HER) typically significantly reduces the Faraday efficiency and selectivity of CO generation.

[0005] To address this issue, researchers have attempted to modulate the surface microenvironment of catalysts through surface modification strategies to improve their electrocatalytic performance in reducing CO2 to CO. Ionic liquids (ILs) and their polymers (PILs) are increasingly being applied to the field of electrocatalysis due to their unique physicochemical properties (such as high CO2 solubility and wide electrochemical window). Existing ionic liquid modification methods mainly employ physical adsorption, dip coating, solution mixing, and other methods to apply ionic liquids or pre-synthesized polymers to the catalyst surface or to directly apply ionic liquids to the electrolyte.

[0006] The patent, CN116145155A, describes a method for efficiently electrocatalytically reducing CO2 to CO by using a cadmium-based catalyst, which is in situ electrodeposited on carbon paper, as the cathode material, in a mixed electrolyte composed of ionic liquid and acetonitrile.

[0007] The announcement number is CN114032581B. By electrochemically reconstructing the metal surface in an ionic liquid medium, the electrochemically active surface area of ​​the reconstructed metal is increased, thereby enabling electrocatalysis with high selectivity for formic acid / carbon monoxide and high current density.

[0008] However, the coatings formed by these methods have weak bonding forces with the catalyst core, which are usually non-covalent physical interactions. This makes the coatings prone to swelling, detachment, or reconstruction under harsh electrocatalytic reaction conditions (such as gas escape and ion migration), resulting in uneven exposure of catalyst active sites, rapid catalyst performance degradation, and short catalyst lifetime.

[0009] Therefore, developing a new method to construct a stable, uniform, and functionally tunable catalyst / polymer interface to address the fundamental shortcomings of traditional modification strategies is crucial for achieving efficient and long-lived CO2 electroreduction catalysts. Summary of the Invention

[0010] The purpose of this invention is to overcome the shortcomings of the prior art and provide a polyionic liquid-modified silver nanoparticle catalyst with a covalently cross-linked core-shell structure prepared by in-situ free radical polymerization, so that the ionic liquid and silver nanoparticles form chemical bonds. This method aims to solve the problems of coating instability and easy peeling caused by traditional physical modification methods.

[0011] Another object of the present invention is to provide the application of the catalyst in the electrocatalytic reduction of CO2 to CO.

[0012] To achieve the above objectives, the present invention provides a method for preparing a polyionic liquid-modified silver nanoparticle catalyst, comprising the following steps: Step S1: Disperse silver nanoparticles with an average particle size of 20-200 nm, vinyl-substituted ionic liquid monomers, divinylbenzene crosslinking agents, and azobisisobutyronitrile free radical initiators in DMF, respectively. The general structural formula of the ionic liquid monomer is shown below:

[0013] In the above structural formulas, A is an imidazole ionic liquid monomer, where R is independently selected from any one of C1-C16 alkyl, phenyl, or cycloalkyl groups; B is a thiazole ionic liquid monomer; and C is a quaternary ammonium ionic liquid monomer.

[0014] Step S2: Under deoxygenated conditions, an in-situ free radical polymerization reaction is carried out at 60-80°C, causing the ionic liquid monomer and the crosslinking agent to polymerize on the surface of the silver nanoparticles, forming a crosslinked polyionic liquid shell layer bonded by covalent or coordinate bonds. This "in-situ" polymerization is the core of this invention, ensuring that the polymer layer grows directly on the surface of the silver nanoparticles, forming a robust core-shell structure, fundamentally avoiding the problem of detachment associated with physical modification methods. S3. After centrifugation, washing with DMF and diethyl ether and vacuum drying, the product of step S2 is obtained as highly stable polyionic liquid modified silver nanoparticles with a covalently cross-linked core-shell structure.

[0015] The highly stable polyionic liquid-modified silver nanoparticle catalyst with a covalently cross-linked core-shell structure provided by this invention can electrocatalyze the reduction of CO2 to CO under mild conditions. Its beneficial effects are as follows: I. High structural stability: Through in-situ polymerization and cross-linking, a strong chemical bond is formed between the polyionic liquid coating layer and the silver nanoparticle core, which significantly improves the structural stability and durability of the catalyst in the electrolyte and overcomes the defect of easy coating peeling in the prior art.

[0016] II. Significantly Enhanced Catalytic Selectivity: The polymer coating layer, especially the polymer layer with long alkyl chains (such as C8-C16), constructs a hydrophobic confinement layer structure on the surface of silver nanoparticles. This microenvironment effectively repels water molecules and protons from approaching the catalytic active site, thereby greatly suppressing the competitive hydrogen evolution reaction and allowing more electrons to be used for CO2 reduction. Consequently, the Faradaic efficiency and selectivity of CO are significantly improved. Notably, with increasing alkyl chain length (C2 to C16), the CO Faradaic efficiency shows a near-linear increase, while the hydrogen evolution reaction rate decreases significantly, indicating that the long-chain polyionic liquid layer constructs a unique hydrophobic confinement layer structure.

[0017] III. Excellent catalytic activity: The catalyst prepared by this invention exhibits excellent catalytic activity under mild conditions (room temperature and pressure). Attached Figure Description

[0018] Figure 1 Characterization diagrams of the catalyst prepared for Example 1 (C16 alkyl chain) of the present invention. (a) Scanning electron microscope (SEM) image of silver nanoparticles before polymerization; (b) SEM image of the core-shell structure catalyst formed after polymerization; (c) Transmission electron microscope (TEM) image of the catalyst; (d) X-ray diffraction (XRD) pattern of the catalyst; (e) Thermogravimetric analysis (TGA) curve of the catalyst; (f) CO Faradaic efficiency diagram of the catalyst at different voltages.

[0019] Figure 2 Characterization images of the unmodified silver nanoparticles in Comparative Example 1. (a) Scanning electron microscopy (SEM) image; (b) CO Faraday efficiency plots at different voltages.

[0020] Figure 3 CO Faradaic efficiency plot of the catalyst prepared by physical mixing of Comparative Example 2 (C16 alkyl chain).

[0021] Figure 4 The CO Faradaic efficiency diagram of the catalyst prepared in Example 2 of this invention (C12 alkyl chain).

[0022] Figure 5The CO Faradaic efficiency diagram of the catalyst prepared in Example 3 (C8 alkyl chain) of the present invention.

[0023] Figure 6 The CO Faradaic efficiency diagram of the catalyst prepared in Example 4 (C6 alkyl chain) of the present invention.

[0024] Figure 7 The CO Faradaic efficiency diagram of the catalyst prepared in Example 5 (C4 alkyl chain) of the present invention.

[0025] Figure 8 The CO Faradaic efficiency diagram of the catalyst prepared in Example 6 (C2 alkyl chain) of the present invention.

[0026] Figure 9 The CO Faradaic efficiency diagram is shown for the catalyst prepared in Example 7 of this invention (different molar ratios).

[0027] Figure 10 The CO Faradaic efficiency diagram is shown for the catalyst prepared in Example 8 of this invention (different molar ratios).

[0028] Figure 11 The CO Faradaic efficiency diagram of the catalyst prepared for Example 9 (thiazole ionic liquid) of the present invention.

[0029] Figure 12 The CO Faradaic efficiency diagram is shown for the catalyst prepared in Example 10 (quaternary ammonia ionic liquid) of this invention. Detailed Implementation

[0030] The present invention will be described in detail below through specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0031] A method for preparing a polyionic liquid-modified silver nanoparticle catalyst includes the following steps: Step S1: Disperse silver nanoparticles with an average particle size of 20-200 nm, vinyl-substituted ionic liquid monomers, divinylbenzene crosslinking agents, and azobisisobutyronitrile free radical initiators in DMF, respectively. The general structural formula of the ionic liquid monomer is shown below:

[0032] In the above structural formulas, A is an imidazole ionic liquid monomer, where R is independently selected from any one of C1-C16 alkyl, phenyl, or cycloalkyl groups; B is a thiazole ionic liquid monomer; and C is a quaternary ammonium ionic liquid monomer.

[0033] Step S2: Under deoxygenated conditions, an in-situ free radical polymerization reaction is carried out at 60-80°C, causing the ionic liquid monomer and the crosslinking agent to polymerize on the surface of the silver nanoparticles, forming a crosslinked polyionic liquid shell layer bonded by covalent or coordinate bonds. This "in-situ" polymerization is the core of this invention, ensuring that the polymer layer grows directly on the surface of the silver nanoparticles, forming a robust core-shell structure, fundamentally avoiding the problem of detachment associated with physical modification methods. S3. After centrifugation, washing with DMF and diethyl ether and vacuum drying, the product of step S2 is obtained as highly stable polyionic liquid modified silver nanoparticles with a covalently cross-linked core-shell structure.

[0034] Preferably, the ionic liquid monomer of the present invention is a vinyl-containing imidazole, thiazole, or quaternary ammonium cationic salt. Further, the ionic liquid monomer of the present invention is 1-vinyl-3-R imidazole bromide, wherein R is a C1-C16 straight-chain alkyl group. Even further, R is a C8-C16 straight-chain alkane.

[0035] Preferably, the crosslinking agent of the present invention is divinylbenzene, which functions to form a crosslinked network structure during polymerization, thereby enhancing the mechanical strength and stability of the coating layer.

[0036] Preferably, the initiator of the present invention is azobisisobutyronitrile.

[0037] In the above preparation method, in step S1, the molar ratio of silver nanoparticles to ionic liquid monomer and crosslinking agent is 1.67:(0.2-2):(1-3). Further, the molar ratio of silver nanoparticles to ionic liquid monomer and crosslinking agent is 1.67:1:(1-2). Even further, the molar ratio of silver nanoparticles to ionic liquid monomer and crosslinking agent is 1.67:1:2.

[0038] In the above preparation method, in step S1, the content of the initiator accounts for 5% of the total mass of the ionic liquid monomer and the crosslinking agent.

[0039] This invention also protects the polyionic liquid-modified silver nanoparticle catalyst prepared by the above method, and its application as an electrocatalyst in the electrocatalytic reduction of CO2 to CO.

[0040] Example 1: The polyionic liquid-modified silver nanoparticles of this example were dispersed in 13 ml of DMF along with 0.2216 g of 1-vinyl-3-hexadecylimidazolium bromide (C16), 0.1445 g of divinylbenzene (crosslinking agent), 0.0183 g of azobisisobutyronitrile (initiator), and 0.1 g of silver nanoparticles (average particle size 20-200 nm). The mixed solution was frozen in liquid nitrogen and then subjected to three cycles of vacuuming and nitrogen purging to thoroughly remove oxygen. Subsequently, the mixture was magnetically stirred at 80 °C for 24 hours. After the reaction was complete, the product was centrifuged, washed alternately with DMF and diethyl ether, and finally vacuum dried at 60 °C to obtain a solid powdered catalyst.

[0041] Comparative Example 1: 0.1 g of silver nanoparticles (average particle size of 20-200 nm) were dispersed in 13 ml of DMF and treated under the same deoxygenation, heating and post-treatment conditions as in Example 1 for comparison.

[0042] Comparative Example 2: 0.2216 g of 1-vinyl-3-hexadecylimidazolium bromide (C16), 0.1445 g of divinylbenzene (crosslinking agent), and 0.0183 g of azobisisobutyronitrile (initiator) were dispersed in 13 ml of DMF. The mixture was frozen in liquid nitrogen and then subjected to three cycles of vacuum-nitrogen purging to completely remove oxygen. Subsequently, it was magnetically stirred at 80 °C for 24 hours. After the reaction was complete, the product was centrifuged, washed alternately with DMF and diethyl ether, and finally vacuum-dried at 60 °C to obtain a pure polymer powder. This powder was then thoroughly mixed with 0.1 g of silver nanoparticles (average particle size 20-200 nm) to obtain a physically mixed solid powder catalyst for comparison.

[0043] Examples 2-6: Effect of different alkyl chain lengths: Repeat the steps of Example 1, but use equimolar amounts of 1-vinyl-3-dodecylimidazolium bromide (C12, Example 2), 1-vinyl-3-octylimidazolium bromide (C8, Example 3), 1-vinyl-3-hexylimidazolium bromide (C6, Example 4), 1-vinyl-3-butylimidazolium bromide (C4, Example 5) and 1-vinyl-2-ethylimidazolium bromide (C2, Example 6) instead of 1-vinyl-3-hexadecylimidazolium bromide.

[0044] Examples 7-8: Effect of different molar ratios: Repeat the steps of Example 1, but adjust the molar ratio of the reactants. In Example 7, the molar ratio of silver nanoparticles: ionic liquid monomer (C16): crosslinking agent was 1.67:2:2. In Example 8, this molar ratio was 1.67:2:1.

[0045] Examples 9-10: The effects of different types of ionic liquids: Repeat the steps of Example 1, but use equimolar amounts of 3-vinyl-2,3-dimethylthiazolium iodide (thiazolium ionic liquid, Example 9) and 4-vinylbenzyltrimethylammonium chloride (quaternary ammonium ionic liquid, Example 10) instead of 1-vinyl-3-hexadecylimidazolium bromide.

[0046] Performance testing and results analysis: The catalysts obtained from the above examples and comparative examples were used to make working electrodes, and their electrocatalytic performance was tested in a CO2-saturated 0.5 mol / L potassium bicarbonate aqueous solution.

[0047] Structural and performance analysis: Figure 1 The SEM image in (b) clearly shows that, through the method of the present invention, silver nanoparticles ( Figure 1 (a) Figure 1 The surface of (c) is uniformly coated with a polymer, forming a core-shell structure. Figure 1 The XRD pattern in (d) confirms that the polymerization process did not change the crystal structure of silver. Figure 1 Thermogravimetric analysis of (e) indicates successful polymer grafting.

[0048] Comparison of core effects: Figure 1 (e) and Figure 2 (b) Figure 3 The comparison shows that the polyionic liquid-modified catalyst prepared in Example 1 achieved a maximum Faradaic efficiency of 76% for CO at -0.9 to -1.2 V (vs. RHE). In contrast, the unmodified silver nanoparticles in Comparative Example 1 and the physically mixed catalyst in Comparative Example 2 exhibited very low Faradaic efficiencies for CO under the same conditions, primarily undergoing hydrogen evolution reaction. This difference demonstrates that the highly stable polyionic liquid-modified silver nanoparticle catalyst with a covalently cross-linked core-shell structure formed by the in-situ polymerization modification of this invention represents a fundamental performance improvement.

[0049] Results of comparison of Examples 1-6 ( Figure 1 (f) and Figure 4-8A clear and significant trend can be observed: as the length of the alkyl chain on the imidazole ring increases (C2 to C16), the efficiency of the electrocatalytic reduction of CO2 to CO by the catalyst gradually increases. The efficiency of the C2 chain (Example 6) is relatively low, while the efficiency of the C16 chain (Example 1) is the highest. This result reveals an unexpected technical effect of the present invention: the strongly hydrophobic confinement layer structure formed by long alkyl chains (especially C8 to C16) in the polymer shell plays a decisive role in suppressing the competitive hydrogen evolution reaction. This hydrophobic layer acts as a "molecular barrier," hindering the transport of protons from the aqueous electrolyte to the silver catalyst surface, thereby creating more favorable conditions for the adsorption and reduction of CO2 molecules. This strong structure-performance correlation is difficult to achieve with simple physical mixing methods, and also demonstrates the non-obviousness of the specific ionic liquid monomer structure selection in the present invention.

[0050] Examples 7 and 8 ( Figure 9 and 10 The results show that adjusting the molar ratio of reactants also affects catalytic performance, proving that the molar ratio range required by this invention is an optimized effective range.

[0051] Examples 9 and 10 ( Figure 11 and 12 The results show that different types of ionic liquid monomers also affect catalytic performance, proving that the ionic liquid required by this invention is an optimized and effective type.

[0052] In summary, this invention successfully constructed an Ag@PILs catalyst with a stable core-shell structure through a novel in-situ polymerization method, and achieved high selectivity and high activity for electrocatalytic CO2 reduction to CO by controlling the hydrophobicity of the polymer shell, thus solving the key pain points of the prior art.

[0053] A highly stable polyionic liquid-modified silver nanoparticle with a covalently cross-linked core-shell structure is disclosed, characterized by the use of imidazole, thiazole, or quaternary ammonium polyionic liquids. First, silver nanoparticles with an average particle size of 20-200 nm are dispersed in an organic solvent along with vinyl-substituted ionic liquid monomers, a cross-linking agent, and a free radical initiator. Second, under deoxygenated conditions, in-situ free radical polymerization is carried out at high temperature, causing the ionic liquid monomers and the cross-linking agent to polymerize on the surface of the silver nanoparticles, forming a covalently or coordinately bonded cross-linked polyionic liquid shell. Finally, the product is centrifuged, washed with solvent, and vacuum dried to obtain the polyionic liquid-modified silver nanoparticles. The silver nanoparticles of this invention, when used as an electrocatalyst in the electrocatalytic reduction of CO2 to CO at room temperature and pressure, achieve a Faradaic efficiency of over 70%. This method, by constructing a chemically bonded polyionic liquid shell, effectively improves the product selectivity and hydrogen evolution resistance of the silver nanoparticles, overcoming the problem of easy detachment of physically adsorbed ionic liquid modification layers, and possesses significant innovation and application value.

Claims

1. A method for preparing polyionic liquid-modified silver nanoparticles, characterized in that, Includes the following steps: Step S1: Silver nanoparticles with an average particle size of 20-200 nm, vinyl-substituted ionic liquid monomers, divinylbenzene crosslinking agents, and azobisisobutyronitrile free radical initiators are dispersed in N,N-dimethylformamide, respectively. The general structural formula of the ionic liquid monomer is: In the above structural formulas, A is an imidazole ionic liquid monomer, where R is independently selected from any one of C1-C16 alkyl, phenyl, or cycloalkyl groups; B is a thiazole ionic liquid monomer; and C is a quaternary ammonium ionic liquid monomer. Step S2: Under deoxygenated conditions, an in-situ free radical polymerization reaction is carried out at 60-80℃ for 1-24 hours to polymerize the ionic liquid monomer and the crosslinking agent on the surface of the silver nanoparticles, forming a crosslinked polyionic liquid shell bonded by covalent or coordinate bonds. Step S3: After centrifugation, washing with N,N-dimethylformamide and diethyl ether, the product from step S2 is dried under vacuum at 50-60℃ to obtain polyionic liquid modified silver nanoparticles with a covalently cross-linked core-shell structure.

2. The preparation method according to claim 1, characterized in that: The ionic liquid monomer is a vinyl-containing imidazole, thiazole, or quaternary ammonium cationic salt.

3. The preparation method according to claim 2, characterized in that: The ionic liquid monomer is 1-vinyl-3-R imidazolium bromide, wherein R is a C1-C16 straight-chain alkyl group.

4. The preparation method according to claim 1, characterized in that: In step S1, the molar ratio of the silver nanoparticles to the ionic liquid monomer and crosslinking agent is 1.67:(0.2-2):(1-3).

5. The preparation method according to claim 4, characterized in that: The crosslinking agent is divinylbenzene.

6. The preparation method according to claim 1, characterized in that: In step S1, the amount of free radical initiator is 5% of the total mass of the ionic liquid monomer and the crosslinking agent.

7. The preparation method according to claim 6, characterized in that: The initiator is azobisisobutyronitrile.

8. The preparation method according to claim 1, characterized in that: The deoxygenation reaction in step S2 involves deoxygenating the reaction system under nitrogen protection, which means performing at least three vacuum-nitrogen-purging cycles on the reaction system.

9. The polyionic liquid modified silver nanoparticles prepared by the method according to any one of claims 1-8, wherein the thickness of the resulting polyionic liquid shell is 5-50 nm.

10. An application of the polyionic liquid-modified silver nanoparticles according to claim 9, characterized in that, The polyionic liquid modified silver nanoparticles are used as an electrocatalyst for the electrocatalytic reduction of CO2 to produce CO. Under normal temperature and pressure conditions, CO is generated by electrolysis in a CO2-saturated 0.5 mol / L potassium bicarbonate solution at -0.9 to -1.2 V (vs. RHE).

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