Preparation method and application of a bifunctional catalyst for direct carbon dioxide conversion

Abstract

The present invention relates to the technical field of catalysts, and discloses a preparation method and application of a bifunctional catalyst for direct conversion of carbon dioxide, comprising the following steps: (1) dispersing carbon nanotubes and amino small molecules in an organic solvent, wherein the mixed mass ratio of SWCNTs and MWCNTs in the carbon nanotubes is 1:3 to 3:1, and the amino small molecules are one or more of glutathione, cysteine and lysine; (2) then adding a copper salt, and stirring at -5 to 5 °C for 24 h or more; subsequently adding an aqueous solution of sodium borohydride at -5 to 5 °C, standing and aging for 2 to 4 h, and finally performing post-treatment to obtain the bifunctional catalyst. By using carbon nanotubes as a carrier and loading Cu nanoclusters thereon, the present invention obtains a Cu nanocluster bifunctional catalyst with high dispersion and high conductivity, which can adsorb and capture low-concentration CO2 and directly catalytically convert it into methanol, and at the same time has high methanol selectivity.

Description

Technical Field

[0001] The present invention relates to the technical field of catalysts, and in particular to a preparation method and application of a bifunctional catalyst for direct conversion of carbon dioxide. Background Art

[0002] A large amount of CO2 is emitted during the chemical production process, causing a series of ecological and environmental problems. It is worth noting that the concentration of CO2 in the industrial waste gas emitted is generally low (5-30%), and it is urgent to develop advanced and efficient CCUS technologies (CO2 capture, utilization and storage) to achieve carbon emission reduction in the chemical process. However, there are still many challenges in this technology. In terms of CO2 capture, absorption and adsorption methods are currently the most potential technical routes for large-scale capture of low-concentration CO2. However, the regeneration energy consumption of current CO2 absorbents / adsorption materials is relatively high, and the system operation energy consumption is as high as 4.0-6.0 MJ / kg CO2, which seriously hinders the large-scale application of CO2 capture and utilization technologies. In terms of catalytic conversion of CO2, the current main technologies are the production of products such as methanol and olefins by catalytic hydrogenation of CO2, and the dry reforming of CO2 and methane to produce syngas, etc. For example, the patent with the publication number CN106622252A discloses a catalyst for hydrogenation of CO2 to methanol, aiming to overcome the problems of low CO2 conversion rate and low methanol selectivity in the process of hydrogenation of CO2 to methanol. A catalyst mainly composed of Cu, ZnO, ZrO2 and graphene oxide (GO) was studied and prepared by co-current coprecipitation method. However, this catalyst is used for thermal catalytic conversion to methanol, and the reaction temperature is 200-300°C and the pressure is 4.0-12.0 MPa. Therefore, the reaction conditions are harsh (requiring high temperature and high pressure conditions), resulting in high energy consumption and cost.

[0003] Moreover, most of the existing CCUS technologies separately develop CO2 capture agents and CO2 catalysts. By developing capture agents with higher CO2 adsorption capacity and lower CO2 regeneration energy consumption, such as using amine solution absorbents to replace alkali solution absorbents to reduce CO2 regeneration energy consumption. At the same time, strategies such as the morphology effect, crystal plane effect, valence state effect and defect effect of the catalyst are used to design and prepare thermal / electro catalysts with higher selectivity and efficiency. Such a two-step process faces problems such as high CO2 regeneration energy consumption, complex process flow, high equipment investment and maintenance costs, resulting in the inability to widely promote the existing CO2 capture and conversion technologies. Summary of the Invention

[0004] In order to solve the problems existing in the two-step process, such as complex process flow, more energy consumption for CO2 regeneration and compression, long time cycle and low efficiency, the present invention provides a preparation method and application of a bifunctional catalyst for direct conversion of carbon dioxide. By using carbon nanotubes as a carrier and loading Cu nanoclusters thereon, a highly dispersed and highly conductive Cu nanocluster catalyst is obtained. This catalyst can directly catalytically convert low-concentration CO2 after adsorption and capture to obtain methanol, thus realizing the dual functions of a CO2 capture agent and a CO2 catalyst.

[0005] The object of the present invention is achieved by the following technical solutions:

[0006] In the first aspect, the present invention provides a preparation method of a bifunctional catalyst for direct conversion of carbon dioxide, comprising the following steps:

[0007] (1) Disperse carbon nanotubes and amino small molecules in an organic solvent. The mixing mass ratio of SWCNTs and MWCNTs in the carbon nanotubes is 1:3 to 3:1, and the amino small molecules are one or more of glutathione, cysteine, and lysine;

[0008] (2) Then add a copper salt and stir at -5 to 5 °C for 24 h or more; subsequently add an aqueous solution of sodium borohydride at -5 to 5 °C, stand for aging for 2 to 4 h, and finally obtain a bifunctional catalyst through post-treatment.

[0009] In the existing CCUS technology, the capture and conversion of CO2 are carried out independently in two steps. Low-concentration CO2 is first absorbed by an alkaline solution / amine solution, then heated for CO2 regeneration, and finally compressed and electro / thermally catalytically converted into high-value chemicals and fuels. This two-step process has problems such as complex process flow, more energy consumption for CO2 regeneration and compression, long time cycle and low efficiency. Therefore, in response to this problem, the bifunctional catalyst prepared in the present invention can directly catalytically convert low-concentration CO2 after adsorption and capture to obtain methanol, thus realizing the high efficiency of energy utilization and the intensiveness of the process flow.

[0010] The bifunctional catalyst is a Cu nanocluster catalyst with high dispersion and high conductivity obtained by using carbon nanotubes as a carrier and loading Cu nanoclusters thereon. Among them, the carbon nanotubes have a hollow pipe structure, providing a channel for mass transfer. There are a large number of atoms on its surface, providing rich and uniform adsorption sites and having a high specific surface area, which is beneficial to the adsorption of Cu nanoclusters, enhancing the stability and uniformity of the loading. Moreover, the carbon nanotubes have a large aspect ratio, and when loaded on the surface of carbon paper, a 3D intertwined network structure can be formed, enhancing the binding force and supportability, and the 3D intertwining can also form a pore structure, thereby realizing the efficient adsorption and enrichment of low-concentration CO2.

[0011] Meanwhile, the carbon nanotubes include SWCNTs and MWCNTs. SWCNTs have a large specific surface area, which can provide more adsorption sites for Cu nanoclusters. Due to their smaller tube diameters, the Cu nanoclusters are relatively evenly dispersed on the surface of single-walled carbon nanotubes. When SWCNTs and MWCNTs are mixed, the presence of MWCNTs can play a role in spacing and supporting. The multi-layer structure of MWCNTs can prevent the aggregation of Cu nanoclusters because the clusters can be better dispersed between the multi-layers or in the voids between MWCNTs and SWCNTs. From the perspective of surface chemistry, there are functional groups (such as carboxyl groups, hydroxyl groups, etc.) on the CNTs surface that can interact with Cu nanoclusters through chemical bonding (such as coordination bonds). The distribution and properties of the surface functional groups of SWCNTs and MWCNTs are slightly different. When the mixing ratio is appropriate, this difference can be used to enhance the adsorption and dispersion of Cu nanoclusters. The better dispersion of Cu nanoclusters on the carbon nanotube surface is more conducive to acting as "nodes" for electron transport, further enhancing the conductivity of the entire system.

[0012] Amino small molecules can coordinate with Cu(II) to form chelates and act as capping agents for Cu(II) to form Cu nanoclusters of different sizes. However, the amino molecular chain cannot be too large, otherwise a large steric hindrance effect will be generated, resulting in the formation of very large Cu clusters or even the inability to form clusters. In addition, coordinating large amino molecules around Cu(II) will hinder the contact and participation of CO2 molecules with the Cu cluster catalyst in the reaction. The mechanism of CO2 electroreduction is that CO2 is first activated and hydrogenated to intermediate, and further hydrogenated to , and if hydrogenation continues, it will reach methanol / methane. But if dimerizes, multi-carbon products such as ethylene / ethanol, etc. will be produced. By controlling the amounts of amino small molecules and Cu salts, Cu nanocluster catalysts with appropriate sizes can be prepared, thereby controlling the appropriate number and distance of Cu active sites. At the same time, by introducing amino groups, the adsorption behavior of CO2 electroreduction intermediates can be synergistically regulated, thereby regulating the selectivity of Cu nanoclusters to highly selectively produce methanol products.

[0013] Sodium borohydride is used to reduce Cu(II). At the same time, the aging process can promote full reaction, optimize the composition and structure of the reaction products, and can also promote the crystal growth and particle maturation of the generated metal nanoparticles, and there is enough time for lattice rearrangement and perfection, improving the crystallinity of the products. When highly crystalline metal nanocrystals are used as catalysts, the activity and stability of their surface atoms will be better. A good crystalline structure can provide a more suitable atomic arrangement and electronic structure, thereby improving the catalytic efficiency.

[0014] Preferably, the carbon nanotubes are pretreated with an acid solution; the acid is one of hydrochloric acid solution, sulfuric acid solution and nitric acid solution with a concentration of 0.6 mol / L; the pretreatment time is 24 to 48 h.

[0015] The impurities are removed by pretreatment with an acid solution.

[0016] Preferably, the mixing mass ratio of SWCNTs and MWCNTs in the carbon nanotubes is 1:3, 1:1 or 3:1.

[0017] SWCNTs itself has excellent electrical properties and fast electron transport. When SWCNTs are mixed with MWCNTs, a good conductive channel can be formed at the interface where the two contact. Electrons can be conducted from the Cu nanoclusters to the CNTs surface through chemical bonds (such as Cu - C bonds), and then quickly transported in the CNTs network. The multi-layer structure of MWCNTs also helps to increase the diversity of electron transport paths.

[0018] When the mixing mass ratio of SWCNTs and MWCNTs is 1:3, the relative content of MWCNTs is higher, and its internal multi-layer structure can provide more electron transport channels, and the synergistic effect between SWCNTs and MWCNTs can be utilized to better improve the overall conductivity.

[0019] Preferably, the addition ratio of the carbon nanotubes, amino small molecules and copper salt is 20 mg: 0.0625 to 1.0 mmol: 0.1 to 0.3 mmol; more preferably, the addition ratio of the carbon nanotubes, amino small molecules and copper salt is 20 mg: 0.2 to 1.0 mmol: 0.1 to 0.3 mmol.

[0020] Preferably, the SWCNTs are single-walled carbon nanotubes with a diameter of 0.4 to 2 nm; the MWCNTs are multi-walled carbon nanotubes with a diameter of 2 to 50 nm.

[0021] Preferably, the copper salt contains Cu 2+ ions of nitrate, chloride, carbonate, acetylacetonate or acetate.

[0022] Preferably, the addition ratio of the carbon nanotubes and the organic solvent is 20 mg: 30 to 50 mL.

[0023] Preferably, the volume ratio of the sodium borohydride aqueous solution and the organic solvent is 1:4 to 8.

[0024] Preferably, the concentration of the sodium borohydride aqueous solution is 0.1 to 1 mol / L.

[0025] Preferably, the organic solvent includes one or more of methanol, ethanol, ethylene glycol, acetone, and N,N-dimethylformamide.

[0026] Preferably, the post-treatment includes centrifugation, washing, and freeze-drying in sequence.

[0027] Preferably, the centrifugation speed is 8000 - 12000 rpm, and the centrifugation time is 5 - 15 min.

[0028] In a second aspect, the present invention also provides an application of the bifunctional catalyst prepared by the above preparation method in the catalytic conversion of carbon dioxide to methanol.

[0029] Preferably, the bifunctional catalyst can directly catalyze the conversion to methanol in industrial waste gas with a volume content of CO2 of 5 - 30%.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] (1) The bifunctional catalyst uses carbon nanotubes as a carrier and loads Cu nanoclusters on it to obtain a highly dispersed and highly conductive Cu nanocluster catalyst, which can adsorb and capture low-concentration CO2 and directly catalyze the conversion to methanol, thereby achieving the high efficiency of energy utilization and the intensiveness of the process flow;

[0032] (2) The carbon nanotubes include SWCNTs and MWCNTs. The distribution and properties of surface functional groups on SWCNTs and MWCNTs are slightly different, and this difference can be used to enhance the adsorption and dispersion of Cu nanoclusters. The dispersion of Cu nanoclusters on the surface of carbon nanotubes is better, which is more conducive to acting as a "node" for electron transfer and further enhancing the conductivity of the entire system;

[0033] (3) Amino small molecules can coordinate with Cu(II) to form chelates and act as capping agents for Cu(II) to form Cu nanoclusters of different sizes. By using the size effect of the nanocatalyst and the introduction of amino groups, the adsorption behavior of CO2 electroreduction intermediates is synergistically regulated, thereby regulating the selectivity of Cu nanoclusters and highly selectively generating methanol products. Description of the Drawings

[0034] Figure 1 SEM image of the catalyst without loaded Cu nanoclusters prepared in Example 1.

[0035] Figure 2 SEM image of the catalyst loaded with Cu nanoclusters prepared in Example 1. Detailed Embodiments

[0036] The following specific embodiments are used to illustrate the technical solutions of the present invention, but the protection scope of the present invention is not limited thereto.

[0037] The preparation of the bifunctional catalyst for direct conversion of carbon dioxide in the present invention comprises the following steps:

[0038] (1) Immerse carbon nanotubes in an acid solution for pretreatment for 24 - 48 h to remove impurities, and then wash with deionized water and freeze-dry for standby;

[0039] (2) Disperse the pretreated carbon nanotubes and amino small molecules in an organic solvent. The addition ratio of carbon nanotubes to the organic solvent is 20 mg : 30 - 50 mL. The mixing mass ratio of single-walled carbon nanotubes (SWCNTs, with a diameter of 0.4 - 2 nm) and multi-walled carbon nanotubes (MWCNTs, with a diameter of 2 - 50 nm) in the carbon nanotubes is 1:3 - 3:1. The amino small molecules are one or more of glutathione, cysteine, lysine, and proline;

[0040] (3) Then add a copper salt. The addition ratio of carbon nanotubes, amino small molecules, and the copper salt is 20 mg : 0.0625 - 1.0 mmol : 0.1 - 0.3 mmol. Stir at -5 - 5 °C for 24 h or more; subsequently, add an aqueous solution of sodium borohydride with a concentration of 0.1 - 1 mol / L at -5 - 5 °C. The volume ratio of the sodium borohydride aqueous solution to the organic solvent is 5 : 20 - 40, and let it stand for aging for 2 - 4 h;

[0041] (4) Finally, after post-treatment, centrifuge at a rotation speed of 8000 - 12000 rpm for 5 - 15 min, and then wash and freeze-dry in sequence to obtain the bifunctional catalyst.

[0042] In the specific embodiment of the present invention, the diameter of the single-walled carbon nanotubes (SWCNTs) used is 1 - 2 nm, and the diameter of the multi-walled carbon nanotubes (MWCNTs) used is 30 - 50 nm.

[0043] In the specific embodiment of the present invention, the acid solution used is one of hydrochloric acid solution, sulfuric acid solution, and nitric acid solution with a concentration of 0.6 mol / L.

[0044] In the specific embodiment of the present invention, the organic solvent includes one or more of methanol, ethanol, ethylene glycol, acetone, and N,N-dimethylformamide.

[0045] In the specific embodiment of the present invention, the copper salt is a nitrate, chloride, carbonate, acetylacetonate, or acetate containing Cu 2+ ions.

[0046] Example 1

[0047] (1) The carbon nanotubes were pretreated with a hydrochloric acid solution at a concentration of 0.6 mol / L for 24 h;

[0048] (2) 20 mg of the pretreated carbon nanotubes (a mixture of 5 mg of single-walled carbon nanotubes (SWCNTs) and 15 mg of multi-walled carbon nanotubes (MWCNTs), SWCNTs:MWCNTs = 1:3) and 0.5 mmol of an amino small molecule (glutathione) were added to 40 mL of a methanol solution and ultrasonically dispersed;

[0049] (3) Subsequently, 0.2 mmol of copper acetate was added, and the uniformly mixed solution was placed at 0 °C and vigorously stirred for 24 h. Then, 8 mL of a sodium borohydride solution with a concentration of 0.2 mol / L (at 0 °C) was slowly added dropwise, and it was left to age for 2 h;

[0050] (4) Finally, after multiple centrifugation, washing, and freeze-drying treatments, a bifunctional catalyst was obtained.

[0051] The prepared bifunctional catalyst was hot-pressed and loaded on a proton membrane as the working cathode, a standard solid-state Ag / AgCl electrode was used as the reference electrode, and a commercial iridium-coated titanium mesh was used as the counter electrode; 1 M sulfuric acid solution was used as the electrolyte for the anode, and 1 M KHCO3 solution was used as the electrolyte for the cathode; a simulated natural gas with a CO2 volume content of 15% was used as the raw material gas, and the electroreduction of CO2 was tested at a constant potential. Gas chromatography was used to detect the gas-phase products, and nuclear magnetic resonance hydrogen spectroscopy was used to detect the liquid-phase products.

[0052] Example 2

[0053] (1) The carbon nanotubes were pretreated with a nitric acid solution at a concentration of 0.6 mol / L for 24 h;

[0054] (2) 20 mg of the pretreated carbon nanotubes (a mixture of 10 mg of single-walled carbon nanotubes (SWCNTs) and 10 mg of multi-walled carbon nanotubes (MWCNTs), SWCNTs:MWCNTs = 1:1) and 0.3 mmol of an amino small molecule (cysteine) were added to 30 mL of an N,N-dimethylformamide (DMF) solution and ultrasonically dispersed;

[0055] (3) Subsequently, 0.2 mmol of copper acetylacetonate was added, and the uniformly mixed solution was placed at 0 °C and vigorously stirred for 24 h. Then, 6 mL of a sodium borohydride solution with a concentration of 0.5 mol / L (at 0 °C) was slowly added dropwise, and it was left to age for 2 h;

[0056] (4) Finally, after multiple centrifugation, washing, and freeze-drying treatments, a bifunctional catalyst was obtained.

[0057] The prepared bifunctional catalyst was hot-pressed and loaded on the proton membrane as the working cathode, a standard solid-state Ag / AgCl electrode was used as the reference electrode, and a commercial iridium-coated titanium mesh was used as the counter electrode; 1 M sulfuric acid solution was used as the electrolyte for the anode, and 1 M KHCO3 solution was used as the electrolyte for the cathode; simulated natural gas with a CO2 volume content of 10% was used as the raw material gas, and the electroreduction of CO2 was tested under constant potential. Gas chromatography was used to detect the gas-phase products, and nuclear magnetic resonance hydrogen spectroscopy was used to detect the liquid-phase products.

[0058] Example 3

[0059] (1) The carbon nanotubes were pretreated with a hydrochloric acid solution with a concentration of 0.6 mol / L for 24 h;

[0060] (2) 20 mg of the pretreated carbon nanotubes (a mixture of 5 mg of single-walled carbon nanotubes (SWCNTs) and 15 mg of multi-walled carbon nanotubes (MWCNTs), SWCNTs:MWCNTs = 1:3) and 0.8 mmol of amino small molecule (glutathione) were added to 40 mL of ethylene glycol solution and ultrasonically dispersed;

[0061] (3) Subsequently, 0.3 mmol of copper acetylacetonate was added, and the uniformly mixed solution was placed under vigorous stirring at 0 °C for 24 h, then 10 mL of a sodium borohydride solution with a concentration of 0.4 mol / L (at 0 °C) was slowly added dropwise, and it was left to age for 2 h;

[0062] (4) Finally, after multiple centrifugation, washing, and freeze-drying treatments, a bifunctional catalyst was obtained.

[0063] The prepared bifunctional catalyst was hot-pressed and loaded on the proton membrane as the working cathode, a standard solid-state Ag / AgCl electrode was used as the reference electrode, and a commercial iridium-coated titanium mesh was used as the counter electrode; 1 M sulfuric acid solution was used as the electrolyte for the anode, and 1 M KHCO3 solution was used as the electrolyte for the cathode; simulated natural gas with a CO2 volume content of 15% was used as the raw material gas, and the electroreduction of CO2 was tested under constant potential. Gas chromatography was used to detect the gas-phase products, and nuclear magnetic resonance hydrogen spectroscopy was used to detect the liquid-phase products.

[0064] Example 4

[0065] The difference from Example 1 is that: SWCNTs:MWCNTs = 1:1.

[0066] (1) The carbon nanotubes were pretreated with a hydrochloric acid solution with a concentration of 0.6 mol / L for 24 h;

[0067] (2) Add 20 mg of pretreated carbon nanotubes (a mixture of 10 mg of single-walled carbon nanotubes (SWCNTs) and 10 mg of multi-walled carbon nanotubes (MWCNTs), SWCNTs:MWCNTs = 1:1) and 0.5 mmol of amino small molecule (glutathione) into 40 mL of methanol solution and disperse them by ultrasonic treatment;

[0068] (3) Subsequently, add 0.2 mmol of copper acetate, place the well-mixed solution at 0 °C and stir vigorously for 24 h, then slowly dropwise add 8 mL of sodium borohydride solution with a concentration of 0.2 mol / L (at 0 °C), and let it age for 2 h;

[0069] (4) Finally, after centrifugation, washing, and freeze-drying for multiple times, a bifunctional catalyst is obtained.

[0070] The prepared bifunctional catalyst is hot-pressed and loaded on a proton membrane as the working cathode, a standard solid-state Ag / AgCl electrode is used as the reference electrode, and a commercial iridium-coated titanium mesh is used as the counter electrode; 1 M sulfuric acid solution is used as the electrolyte for the anode, and 1 M KHCO3 solution is used as the electrolyte for the cathode; a simulated natural gas with a CO2 volume content of 15% is used as the raw material gas, and the electroreduction of CO2 is tested under a constant potential. Gas chromatography is used to detect the gas-phase products, and nuclear magnetic resonance hydrogen spectroscopy is used to detect the liquid-phase products.

[0071] Example 5

[0072] The difference from Example 1 is that: SWCNTs:MWCNTs = 3:1.

[0073] (1) Pretreat the carbon nanotubes with a hydrochloric acid solution with a concentration of 0.6 mol / L for 24 h;

[0074] (2) Add 20 mg of pretreated carbon nanotubes (a mixture of 15 mg of single-walled carbon nanotubes (SWCNTs) and 5 mg of multi-walled carbon nanotubes (MWCNTs), SWCNTs:MWCNTs = 3:1) and 0.5 mmol of amino small molecule (glutathione) into 40 mL of methanol solution and disperse them by ultrasonic treatment;

[0075] (3) Subsequently, add 0.2 mmol of copper acetate, place the well-mixed solution at 0 °C and stir vigorously for 24 h, then slowly dropwise add 8 mL of sodium borohydride solution with a concentration of 0.2 mol / L (at 0 °C), and let it age for 2 h;

[0076] (4) Finally, after centrifugation, washing, and freeze-drying for multiple times, a bifunctional catalyst is obtained.

[0077] The prepared bifunctional catalyst was hot-pressed and loaded on the proton membrane as the working cathode, a standard solid-state Ag / AgCl electrode was used as the reference electrode, and a commercial iridium-plated titanium mesh was used as the counter electrode; 1 M sulfuric acid solution was used as the electrolyte for the anode, and 1 M KHCO3 solution was used as the electrolyte for the cathode; simulated natural gas with a CO2 volume content of 15% was used as the raw material gas, and the electroreduction of CO2 test was carried out at a constant potential. Gas chromatography was used to detect the gas-phase products, and nuclear magnetic resonance hydrogen spectroscopy was used to detect the liquid-phase products.

[0078] Comparative Example 1

[0079] The difference from Example 1 is that only SWCNTs were used for the carbon nanotubes.

[0080] (1) The carbon nanotubes were pretreated with a hydrochloric acid solution with a concentration of 0.6 mol / L for 24 h;

[0081] (2) 20 mg of the pretreated carbon nanotubes (single-walled carbon nanotubes (SWCNTs)) and 0.5 mmol of the amino small molecule (glutathione) were added to 40 mL of methanol solution and ultrasonically dispersed;

[0082] (3) Subsequently, 0.2 mmol of copper acetate was added, and the uniformly mixed solution was placed at 0 °C and stirred vigorously for 24 h, then 8 mL of a sodium borohydride solution with a concentration of 0.2 mol / L (at 0 °C) was slowly added dropwise, and it was left to age for 2 h;

[0083] (4) Finally, after multiple centrifugation, washing, and freeze-drying treatments, a bifunctional catalyst was obtained.

[0084] The prepared bifunctional catalyst was hot-pressed and loaded on the proton membrane as the working cathode, a standard solid-state Ag / AgCl electrode was used as the reference electrode, and a commercial iridium-plated titanium mesh was used as the counter electrode; 1 M sulfuric acid solution was used as the electrolyte for the anode, and 1 M KHCO3 solution was used as the electrolyte for the cathode; simulated natural gas with a CO2 volume content of 15% was used as the raw material gas, and the electroreduction of CO2 test was carried out at a constant potential. Gas chromatography was used to detect the gas-phase products, and nuclear magnetic resonance hydrogen spectroscopy was used to detect the liquid-phase products.

[0085] Comparative Example 2

[0086] The difference from Example 1 is that only MWCNTs were used for the carbon nanotubes.

[0087] (1) The carbon nanotubes were pretreated with a hydrochloric acid solution with a concentration of 0.6 mol / L for 24 h;

[0088] (2) 20 mg of the pretreated carbon nanotubes (multi-walled carbon nanotubes (MWCNTs)) and 0.5 mmol of the amino small molecule (glutathione) were added to 40 mL of methanol solution and ultrasonically dispersed;

[0089] (3) Subsequently, 0.2 mmol of copper acetate was added, and the uniformly mixed solution was placed at 0 °C and vigorously stirred for 24 h. Then, 8 mL of a sodium borohydride solution with a concentration of 0.2 mol / L (at 0 °C) was slowly added dropwise, and the solution was allowed to age for 2 h;

[0090] (4) Finally, after multiple centrifugations, washings, and freeze-drying treatments, a bifunctional catalyst was obtained.

[0091] The prepared bifunctional catalyst was hot-pressed and loaded on a proton membrane as the working cathode, a standard solid-state Ag / AgCl electrode was used as the reference electrode, and a commercial iridium-coated titanium mesh was used as the counter electrode; 1 M sulfuric acid solution was used as the electrolyte for the anode, and 1 M KHCO3 solution was used as the electrolyte for the cathode; a simulated natural gas with a CO2 volume content of 15% was used as the raw material gas, and the electroreduction of CO2 was tested at a constant potential. Gas chromatography was used to detect the gas-phase products, and nuclear magnetic resonance hydrogen spectroscopy was used to detect the liquid-phase products.

[0092] Comparative Example 3

[0093] The difference from Example 1 was that the amino small molecule was not added.

[0094] (1) The carbon nanotubes were pretreated with a hydrochloric acid solution with a concentration of 0.6 mol / L for 24 h;

[0095] (2) 20 mg of the pretreated carbon nanotubes (a mixture of 5 mg of single-walled carbon nanotubes (SWCNTs) and 15 mg of multi-walled carbon nanotubes (MWCNTs), SWCNTs:MWCNTs = 1:3) were added to 40 mL of methanol solution and ultrasonically dispersed;

[0096] (3) Subsequently, 0.2 mmol of copper acetate was added, and the uniformly mixed solution was placed at 0 °C and vigorously stirred for 24 h. Then, 8 mL of a sodium borohydride solution with a concentration of 0.2 mol / L (at 0 °C) was slowly added dropwise, and the solution was allowed to age for 2 h;

[0097] (4) Finally, after multiple centrifugations, washings, and freeze-drying treatments, a bifunctional catalyst was obtained.

[0098] The prepared bifunctional catalyst was hot-pressed and loaded on a proton membrane as the working cathode, a standard solid-state Ag / AgCl electrode was used as the reference electrode, and a commercial iridium-coated titanium mesh was used as the counter electrode; 1 M sulfuric acid solution was used as the electrolyte for the anode, and 1 M KHCO₃ solution was used as the electrolyte for the cathode; simulated natural gas with a CO₂ volume content of 15% was used as the raw material gas, and the electroreduction of CO₂ test was carried out under a constant potential. Gas chromatography was used to detect the gas-phase products, and nuclear magnetic resonance hydrogen spectroscopy was used to detect the liquid-phase products.

[0099] Comparative Example 4

[0100] The difference from Example 1 is that the amino small molecule is proline.

[0101] (1) The carbon nanotubes were pretreated with a hydrochloric acid solution with a concentration of 0.6 mol / L for 24 h;

[0102] (2) 20 mg of the pretreated carbon nanotubes (a mixture of 5 mg of single-walled carbon nanotubes (SWCNTs) and 15 mg of multi-walled carbon nanotubes (MWCNTs), SWCNTs:MWCNTs = 1:3) and 0.5 mmol of the amino small molecule (proline) were added to 40 mL of methanol solution and ultrasonicated for dispersion;

[0103] (3) Subsequently, 0.2 mmol of copper acetate was added, and the well-mixed solution was placed under vigorous stirring at 0 °C for 24 h, then 8 mL of a sodium borohydride solution with a concentration of 0.2 mol / L (at 0 °C) was slowly added dropwise, and it was left to age for 2 h;

[0104] (4) Finally, after multiple centrifugation, washing, and freeze-drying treatments, a bifunctional catalyst was obtained.

[0105] The prepared bifunctional catalyst was hot-pressed and loaded on a proton membrane as the working cathode, a standard solid-state Ag / AgCl electrode was used as the reference electrode, and a commercial iridium-coated titanium mesh was used as the counter electrode; 1 M sulfuric acid solution was used as the electrolyte for the anode, and 1 M KHCO₃ solution was used as the electrolyte for the cathode; simulated natural gas with a CO₂ volume content of 15% was used as the raw material gas, and the electroreduction of CO₂ test was carried out under a constant potential. Gas chromatography was used to detect the gas-phase products, and nuclear magnetic resonance hydrogen spectroscopy was used to detect the liquid-phase products.

[0106] Comparative Example 5

[0107] The difference from Example 5 is that the dosages of the amino small molecule and the Cu salt are excessive.

[0108] (1) The carbon nanotubes were pretreated with a hydrochloric acid solution with a concentration of 0.6 mol / L for 24 h;

[0109] (2) 20 mg of the pretreated carbon nanotubes (a mixture of 15 mg of single-walled carbon nanotubes (SWCNTs) and 5 mg of multi-walled carbon nanotubes (MWCNTs), SWCNTs:MWCNTs = 3:1) and 1.5 mmol of the amino small molecule (glutathione) were added to 40 mL of methanol solution and ultrasonically dispersed;

[0110] (3) Subsequently, 0.8 mmol of copper acetate was added, and the well-mixed solution was placed under vigorous stirring at 0 °C for 24 h, then 8 mL of a sodium borohydride solution with a concentration of 0.2 mol / L (at 0 °C) was slowly added dropwise, and it was left to age for 2 h;

[0111] (4) Finally, after multiple centrifugation, washing, and freeze-drying treatments, a bifunctional catalyst was obtained.

[0112] The prepared bifunctional catalyst was hot-pressed and loaded on a proton membrane as the working cathode, a standard solid-state Ag / AgCl electrode was used as the reference electrode, and a commercial iridium-coated titanium mesh was used as the counter electrode; 1 M sulfuric acid solution was used as the electrolyte for the anode, and 1 M KHCO3 solution was used as the electrolyte for the cathode; a simulated natural gas with a CO2 volume content of 15% was used as the raw material gas, and the electroreduction of CO2 was tested at a constant potential. Gas chromatography was used to detect the gas-phase products, and nuclear magnetic resonance hydrogen spectroscopy was used to detect the liquid-phase products.

[0113] Table 1 Catalytic activities of the catalysts in different examples

[0114]

[0115] Note: The "C 2+ " products include ethanol, ethylene, isopropanol, and acetic acid; the sum of the product selectivities is less than 100% due to certain systematic errors in the detection process.

[0116] As Figure 1 shown is the scanning electron microscope image of the carbon nanotubes (including single-walled carbon nanotubes and multi-walled carbon nanotubes), and there are no Cu nanoclusters loaded on its surface. As Figure 2 shown is the scanning electron microscope image of the carbon nanotubes after loading Cu nanoclusters, indicating that the Cu nanocluster catalyst loaded on the carbon nanotubes was successfully prepared by the method of Example 1 of the present invention.

[0117] As shown in Table 1, the Cu nanocluster bifunctional catalyst supported on carbon nanotubes prepared in Examples 1-5 can directly catalyze the conversion of low-concentration CO2 after adsorption and capture, showing high activity and methanol selectivity in the reaction of catalytic conversion to methanol. Specifically, the present invention proposes a synergistic strategy for carrier regulation and catalyst design: the carbon nanotube carrier synergistically composed of SWCNTs and MWCNTs can adsorb low-concentration CO2 and improve the dispersion and conductivity of Cu nanoclusters; amino small molecules can regulate the size and electronic structure of Cu nanoclusters, making the adsorption of intermediates moderate, thus improving methanol selectivity.

[0118] In Comparative Example 1, only SWCNTs were used as the carbon nanotube carrier. Due to the insufficient CO2 adsorption sites on SWCNTs, the supply of reactants was insufficient, resulting in a decrease in the selectivity of methanol products and a serious hydrogen evolution side reaction. In Comparative Example 2, only MWCNTs were used as the carbon nanotube carrier. Since Cu nanoclusters were highly dispersed on the surface of MWCNTs, the carrier carbon sites catalyzed the reduction of CO2 to CO, resulting in a decrease in methanol selectivity.

[0119] In Comparative Example 3, no amino small molecules were added. Due to the absence of amino small molecules as capping agents, Cu atoms aggregated, resulting in the reduction of CO2 to multi-carbon (C 2+ ) products and a decrease in methanol selectivity. In Comparative Example 4, the molecular chain steric hindrance of the amino small molecule used was too large. Proline contains a five-membered ring. Compared with the linear configurations of glutathione, cysteine, and lysine, it has a larger steric hindrance when coordinating with Cu atoms, resulting in too large or aggregated Cu nanoclusters, which also leads to the reduction of CO2 to multi-carbon (C 2+ ) products and a decrease in methanol selectivity.

[0120] In Comparative Example 5, the dosages of amino small molecules and Cu salts exceeded the scope defined in the claims. Due to the insufficient amino small molecules coordinating with Cu atoms and the aggregation of some uncoordinated Cu atoms, part of CO2 was reduced to multi-carbon (C 2+ ) products, resulting in a decrease in methanol selectivity.

[0121] The above are only the preferred embodiments of the present invention, and do not limit the patent scope of the present invention accordingly. Any equivalent structural or equivalent process transformations made using the description of the present invention, or directly or indirectly applied in other related technical fields, are similarly included in the patent protection scope of the present invention.

Claims

1. A preparation method of a bifunctional catalyst for direct carbon dioxide conversion, characterized in that, It includes the following steps: (1) Dispersing carbon nanotubes and amino small molecules in an organic solvent, where the mixing mass ratio of SWCNTs and MWCNTs in the carbon nanotubes is 1:3 to 3:1, and the amino small molecules are one or more of glutathione, cysteine, and lysine; (2) Then adding a copper salt, and the addition ratio of carbon nanotubes, amino small molecules, and copper salt is 20 mg: 0.0625 - 1.0 mmol: 0.1 - 0.3 mmol. Stir at -5 to 5 °C for 24 h or more; subsequently add an aqueous sodium borohydride solution at -5 to 5 °C, let it stand for aging for 2 - 4 h, and finally obtain a bifunctional catalyst through post-treatment.

2. The preparation method of the bifunctional catalyst for direct conversion of carbon dioxide according to claim 1, wherein, The SWCNTs are single-walled carbon nanotubes with a diameter of 0.4 - 2 nm; the MWCNTs are multi-walled carbon nanotubes with a diameter of 2 - 50 nm.

3. The preparation method of the bifunctional catalyst for direct conversion of carbon dioxide according to claim 1, characterized in that, The copper salt is a nitrate, chloride, carbonate, acetylacetonate or acetate containing Cu 2+ ions.

4. The preparation method of the bifunctional catalyst for direct conversion of carbon dioxide according to claim 1, characterized in that, The addition ratio of the carbon nanotubes and the organic solvent is 20 mg: 30 - 50 mL.

5. The preparation method of the bifunctional catalyst for direct conversion of carbon dioxide according to claim 1, characterized in that, The volume ratio of the aqueous sodium borohydride solution and the organic solvent is 1:4 - 8.

6. The preparation method of the bifunctional catalyst for direct carbon dioxide conversion according to claim 1, wherein, The concentration of the aqueous sodium borohydride solution is 0.1 - 1 mol / L.

7. The preparation method of the bifunctional catalyst for direct conversion of carbon dioxide according to claim 1, characterized in that, The organic solvent includes one or more of methanol, ethanol, ethylene glycol, acetone, and N,N-dimethylformamide.

8. The preparation method of the bifunctional catalyst for direct carbon dioxide conversion according to claim 1, characterized in that, The post-treatment includes centrifugation, washing, and freeze-drying in sequence.

9. Application of a bifunctional catalyst prepared by the preparation method according to any one of claims 1 - 8 in the catalytic conversion of carbon dioxide to methanol.

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