A series catalyst, a preparation method thereof and application thereof in preparation of multi-carbon products by electrocatalytic carbon dioxide reduction reaction

Abstract

The application provides a tandem catalyst and a preparation method and application thereof in preparation of multi-carbon products in an electrocatalytic carbon dioxide reduction reaction, and relates to the technical field of heterogeneous catalyst materials.The tandem catalyst provided by the application comprises a multi-stage pore carbon carrier and monodisperse metal atoms and metal nanoparticles loaded on the surface of the multi-stage pore carbon carrier.The multi-stage pore carbon is used as the carrier in the application, which is beneficial to mass transfer in the process of electrocatalytic carbon dioxide;the monodisperse metal atom-metal nanoparticle composite active site is constructed on the multi-stage pore carbon carrier, which can act on different proton transfer-electron coupling steps in the process of electrocatalytic carbon dioxide reduction, and the selectivity of the multi-carbon product is improved;the monodisperse metal atom can provide a CO-rich local environment and oxygen-containing groups at the same time, the former improves the yield of the multi-carbon product, and the latter improves the selectivity of the multi-carbon product.The tandem catalyst provided by the application can electrocatalyze the conversion of carbon dioxide into multi-carbon products with high selectivity.

Description

Technical Field

[0001] This invention relates to the field of heterogeneous catalyst materials technology, and in particular to a tandem catalyst, its preparation method, and its application in the electrocatalytic reduction of carbon dioxide to prepare multi-carbon compounds. Background Technology

[0002] Carbon dioxide, as a major component of greenhouse gases, can be reduced primarily through two methods: capture and storage (CPS), which involves adsorbing and capturing CO2 using chemical / physical methods for underground / oceanic storage; and conversion into high-value chemicals, such as synthetic urea, cyclic carbonates, and polymers, under the action of catalysts. Due to the high activation energy barrier of linear CO2 molecules, the above methods often require harsh conditions such as high temperature and high pressure, resulting in high energy consumption (emission of new CO2) and low efficiency (wide product distribution). Electrocatalytic reduction of carbon dioxide (CO2) can convert CO2 into hydrocarbon fuels under mild conditions using clean electricity. This not only solves the problem of wasted renewable energy due to intermittent emissions but also alleviates the environmental problems caused by the greenhouse gas CO2 and yields high-value hydrocarbons.

[0003] The core of CO2 electrocatalytic reduction technology is the CO2 reduction reaction at the cathode. Using water and CO2 as raw materials, a multi-step coordinated proton transfer-electron coupling process is carried out at the reduction potential to convert CO2 into products such as carbon monoxide, formic acid, methanol, ethylene, and ethanol. However, CO2 possesses a highly stable chemical structure and is not easily reactive, necessitating the development of high-performance electrocatalysts to accelerate the reaction. In practical applications, this electrocatalyst must balance requirements related to catalyst cost, product selectivity, formation rate, and long-term durability.

[0004] Currently, the most studied electrocatalysts for carbon dioxide reduction include Cu, Au, Ag, Zn, and Pd. However, the types of reduction products, conversion rates, and current efficiencies vary among different catalyst types. Among them, noble metal catalysts such as Au, Ag, and Pd exhibit high activity and selectivity for C1 products (CO, formic acid, etc.), but low activity and poor selectivity for multi-carbon products such as C2 products (ethylene, ethanol, acetone, etc.). Catalysts with relatively high selectivity for C2 products are almost exclusively Cu-based catalysts, including metallic Cu and its oxides. However, the overall reactivity of C2 products is low, and the selectivity of specific products is difficult to control. For example, Chinese patents CN111229261A and CN113136599A aim to achieve the reduction of carbon dioxide to prepare multi-carbon products by modifying the surface of Cu-based electrodes (halogen modification) or forming ion vacancies on Cu electrodes. However, the multi-step CO2 reaction only occurs at a single active site on the Cu electrode surface, making selective control of the multi-step reduction process difficult. Summary of the Invention

[0005] In view of this, the present invention aims to provide a tandem catalyst, its preparation method, and its application in the electrocatalytic reduction of carbon dioxide to prepare multi-carbon products. The tandem catalyst provided by the present invention can electrocatalyze the conversion of carbon dioxide into multi-carbon products with high selectivity.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution: The present invention provides a tandem catalyst comprising a hierarchical porous carbon support and monodisperse metal atoms and metal nanoparticles supported on the surface of the hierarchical porous carbon support.

[0007] Preferably, the pore structure in the hierarchical porous carbon support includes micropores, mesopores, and macropores, wherein the pore size of the micropores is <2nm, the pore size of the mesopores is 2~50nm, and the pore size of the macropores is >1μm; the metal element in the monodisperse metal atoms includes one or more of iron, nickel, cobalt, copper, manganese, zinc, silver, chromium, molybdenum, and indium; and the metal element in the metal nanoparticles includes one or more of iron, nickel, cobalt, copper, manganese, zinc, silver, chromium, molybdenum, and indium.

[0008] Preferably, the mass content of monodisperse metal atoms in the tandem catalyst is 0.01~10%, and the mass content of metal nanoparticles is 0.5~20%.

[0009] This invention provides a method for preparing the tandem catalyst described above, comprising the following steps: A water-based polymer or water-based polymer monomer is mixed with a monodisperse metal precursor, water, and a dopant to obtain a polymeric gel pre-liquid; The polymeric gel pre-liquid was subjected to directional freezing and thawing sequentially to obtain a directional gel complex. The directional gel composite was subjected to a first pyrolysis to obtain a hierarchical porous carbon-supported monodisperse metal atom material. The multi-level porous carbon-supported monodisperse metal atom material and the metal nanoparticle precursor solution are combined, and the resulting composite material is subjected to a second pyrolysis to obtain the tandem catalyst.

[0010] Preferably, the aqueous polymer comprises natural aqueous polymers and / or synthetic aqueous polymers. The natural aqueous polymers include one or more of starch, cellulose, alginic acid, hyaluronic acid, chitosan, poly-L-lysine, and poly-L-glutamic acid. The synthetic aqueous polymers include one or more of polyvinyl alcohol, polyacrylic acid, polypyrrole, polymethacrylic acid, polyacrylamide, and polyvinylpyrrolidone. The aqueous polymer monomers include one or two of pyrrole and acrylamide. The monodisperse metal precursors include one or more of sulfates, chlorides, nitrates, porphyrins, and acetylacetones of monodisperse metal elements. The dopant includes one or more of nitrogen dopant, sulfur dopant, boron dopant, and phosphorus dopant.

[0011] Preferably, the mass ratio of the water-based polymer or water-based polymer monomer to the monodisperse metal precursor, water, and dopant is (5~a):(0.1~10):100:(1~20), where a is the solubility of the water-based polymer or water-based polymer monomer in water.

[0012] Preferably, the directional freezing uses liquid nitrogen to construct a temperature gradient; the directional freezing and thawing are repeated, with one directional freezing and thawing operation constituting one operation, and the operation is repeated no less than 3 times; the time for a single directional freezing is 1~30 minutes.

[0013] Preferably, the metal nanoparticle precursor includes one or more of the following: sulfate, chloride, nitrate, porphyrin, acetylacetone, oleate, and ionic liquid of the metal nanoparticle element; the composite method includes impregnation or electrochemical deposition.

[0014] Preferably, the first pyrolysis comprises: heating from room temperature to a first temperature at a first heating rate and holding at that temperature for 0.5 to 2.0 hours; then heating from the first temperature to a second temperature at a second heating rate and holding at that temperature for 0.5 to 2.0 hours; and then cooling to room temperature in the furnace; wherein the first temperature is 100 to 150°C and the second temperature is 500 to 1000°C; the first heating rate is 1 to 3°C / min and the second heating rate is 3 to 10°C / min; and the first pyrolysis is carried out in a protective atmosphere. The second pyrolysis includes: heating from room temperature to a third temperature at a third heating rate and holding at that temperature for 0.5 to 2.0 hours; then cooling the furnace to room temperature; the third heating rate is 1 to 5 °C / min, and the third temperature is 400 to 800 °C; the second pyrolysis is carried out in a protective atmosphere.

[0015] This invention provides the application of the tandem catalyst described in the above technical solutions or the tandem catalyst prepared by the above technical solutions in the electrocatalytic carbon dioxide reduction reaction to prepare multi-carbon products.

[0016] This invention provides a tandem catalyst comprising a hierarchical porous carbon support and monodisperse metal atoms and metal nanoparticles supported on the surface of the hierarchical porous carbon support. The use of hierarchical porous carbon as a support facilitates mass transfer during the electrocatalytic reduction of carbon dioxide. The invention constructs monodisperse metal atom-metal nanoparticle composite active sites on the surface of the hierarchical porous carbon support, which can act on different proton transfer-electron coupling steps in the electrocatalytic reduction of carbon dioxide, thereby improving the selectivity of multi-carbon products. The monodisperse metal atoms provide a CO-rich local environment and simultaneously provide oxygen-containing groups; the former increases the yield of multi-carbon products, while the latter improves the selectivity of multi-carbon products. Therefore, the tandem catalyst provided by this invention can electrocatalyze the conversion of carbon dioxide into multi-carbon products with high selectivity.

[0017] This invention provides a method for preparing the tandem catalyst described in the above technical solution. This invention uses a polymer gel as a hierarchical porous carbon support precursor, adsorbs / grafts monodisperse metal atom precursors onto the polymer carbon chain, and pins the monodisperse metal atoms by adding dopants. Using a directional freezing process, directional channels are constructed inside the gel by the directional growth of ice crystals. After pyrolysis, a hierarchical porous carbon-supported monodisperse metal atom material is obtained, and finally, metal nanoparticles are composited on its surface. This invention addresses the different requirements for catalytic active sites in different reaction steps of the electrocatalytic carbon dioxide reaction. By using a polymer gel as a hierarchical porous carbon support precursor, it loads monodisperse metal atom active sites and metal nanoparticle active sites in stages to improve the reaction activity and selectivity of specific electrocatalytic reaction pathways, achieving the goal of highly selective preparation of multi-carbon products. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the structure of the directional freezing single-sided heat transfer mold in this invention; Figure 1 1-Open container, 2-Metallic heat conductor, 3-Insulation material layer; Figure 2 This is a scanning electron microscope image of the gel obtained after directional freezing in Example 1; Figure 3 Transmission electron microscopy (TEM) image of the tandem catalyst prepared in Example 1; Figure 4 This is a BET pore analysis diagram of the hierarchical porous carbon-supported monodisperse Co atom material obtained from the first stage of pyrolysis in Example 2. Detailed Implementation

[0019] The present invention provides a tandem catalyst comprising a hierarchical porous carbon support and monodisperse metal atoms and metal nanoparticles supported on the surface of the hierarchical porous carbon support.

[0020] In this invention, the pore structure of the hierarchical porous carbon support preferably includes micropores, mesopores, and macropores, wherein the pore size of the micropores is <2 nm, the pore size of the mesopores is 2~50 nm, and the pore size of the macropores is >1 μm. This invention uses hierarchical porous carbon as a support, which is beneficial for the mass transfer process during the electrocatalytic carbon dioxide reaction. In this invention, the metal element in the monodisperse metal atoms preferably includes one or more of iron, nickel, cobalt, copper, manganese, zinc, silver, chromium, molybdenum, and indium, more preferably iron, nickel, cobalt, silver, or zinc; the metal element in the metal nanoparticles preferably includes one or more of iron, nickel, cobalt, copper, manganese, zinc, silver, chromium, molybdenum, and indium, more preferably copper. In this invention, the mass content of the monodisperse metal atoms in the tandem catalyst is preferably 0.01~10%, more preferably 0.2~5%, further preferably 0.5~3.5%, and the mass content of the metal nanoparticles is preferably 0.5~20%, more preferably 3~15%, further preferably 5~10.5%. This invention constructs monodisperse metal atom-metal nanoparticle composite active sites on a hierarchical porous carbon support. These sites can act on different proton transfer-electron coupling steps in the electrocatalytic reduction of carbon dioxide (CO2). Monodisperse metal atoms primarily promote the conversion of CO2 to CO, while metal nanoparticles primarily promote the reaction of CO with CO to obtain multi-carbon products, thus improving the selectivity of multi-carbon products. The monodisperse metal atoms provide a CO-rich local environment and simultaneously provide oxygen-containing groups; the former increases the yield of multi-carbon products, while the latter improves their selectivity. This invention can flexibly control the electrocatalytic reaction process and enhance the activity and selectivity of specific CO2 reaction pathways by adjusting the loading of the composite active sites, the relative positions of the monodisperse metal atoms and metal nanoparticles (relatively far apart when both are present, and closer together when both are present), and the metal valence state.

[0021] This invention provides a method for preparing the tandem catalyst described above, comprising the following steps: A water-based polymer or water-based polymer monomer is mixed with a monodisperse metal precursor, water, and a dopant to obtain a polymeric gel pre-liquid; The polymeric gel pre-liquid was subjected to directional freezing and thawing sequentially to obtain a directional gel complex. The directional gel composite was subjected to a first pyrolysis to obtain a hierarchical porous carbon-supported monodisperse metal atom material. The multi-level porous carbon-supported monodisperse metal atom material and the metal nanoparticle precursor solution are combined, and the resulting composite material is subjected to a second pyrolysis to obtain the tandem catalyst.

[0022] Unless otherwise specified, all raw materials involved in this invention are commercially available products well known to those skilled in the art.

[0023] This invention involves mixing an aqueous polymer or aqueous polymer monomer with a monodisperse metal precursor, water, and a dopant to obtain a polymeric gel pre-liquid. In this invention, the aqueous polymer preferably includes natural aqueous polymers and / or synthetic aqueous polymers. The natural aqueous polymer preferably includes one or more of starch, cellulose, alginate, hyaluronic acid, chitosan, poly-L-lysine, and poly-L-glutamic acid, more preferably cellulose, alginate, or chitosan, with the cellulose preferably being polymethylcellulose. The synthetic aqueous polymer preferably includes one or more of polyvinyl alcohol, polyacrylic acid, polypyrrole, polymethacrylic acid, polyacrylamide, and polyvinylpyrrolidone, more preferably polyvinyl alcohol, polypyrrole, or polyacrylamide. In this invention, the aqueous polymer monomer preferably includes one or two of pyrrole and acrylamide. In this invention, the monodisperse metal precursor preferably comprises one or more of the following: sulfate, chloride, nitrate, porphyrin, and acetylacetone salts of a monodisperse metal element, more preferably chloride or nitrate; the monodisperse metal element is one or more of the following: iron, nickel, cobalt, copper, manganese, zinc, silver, chromium, molybdenum, and indium. In this invention, the dopant preferably comprises one or more of the following: nitrogen atom dopant, sulfur atom dopant, boron atom dopant, and phosphorus atom dopant; the nitrogen atom dopant preferably comprises one or more of the following: melamine, urea, pyrrole, cyanamide, dicyandiamide, guanidine hydrochloride, and 2,3-diaminopyridine; the sulfur atom dopant preferably comprises one or more of the following: L-glutathione, dibenzyl disulfide, thioacetamide, and elemental sulfur; the boron atom dopant preferably comprises one or more of the following: boric acid and sodium borohydride; the phosphorus atom dopant preferably comprises phytic acid. In embodiments of the present invention, the dopant is preferably urea, cyanamide, dicyandiamide, L-glutathione, or guanidine hydrochloride. The present invention introduces the dopant into the gel system. During subsequent pyrolysis, the dopant forms coordination structures of CN, CS, CB, and CP with the carbon transformed from the gel. These structures can further pin the monodisperse metal atoms through, for example, CNM (M represents a metal atom) bonding structures, preventing their aggregation. In the present invention, the preferred mass ratio of the aqueous polymer or aqueous polymer monomer to the monodisperse metal precursor, water, and dopant is (5~a):(0.1~10):100:(1~20), more preferably (5~20):(0.1~2):100:(5~20), where a is the solubility of the aqueous polymer or aqueous polymer monomer in water. In this invention, the preferred method for mixing the aqueous polymer or aqueous polymer monomer with the monodisperse metal precursor, water, and dopant is to dissolve the aqueous polymer or aqueous polymer monomer, the monodisperse metal precursor, and the dopant in a portion of water, and then mix them. This invention does not have any particular requirements for the specific mixing method, as long as the components are mixed into a homogeneous liquid.When the aqueous polymer or aqueous polymer monomer cannot be mixed with the monodisperse metal precursor, water, and dopant to form a gel at room temperature, the present invention preferably heats the resulting mixture. The present invention does not have particular requirements for the heating temperature, as long as a solid or semi-solid gel can be formed. When the mixture still cannot form a gel under heating conditions, the present invention further preferably adds a crosslinking agent and / or surfactant to the mixture. The present invention does not have particular requirements for the type and amount of the crosslinking agent and surfactant; crosslinking agents and surfactants well known to those skilled in the art can be used to ensure the formation of a solid or semi-solid gel at room temperature or under heating conditions. After mixing, the present invention further preferably allows the resulting homogenized liquid to stand to eliminate air bubbles; the standing time is preferably 60-180 minutes.

[0024] After obtaining the pre-gel liquid of the polymer, the present invention sequentially performs directional freezing and thawing on the pre-gel liquid to obtain a directional gel complex. In the present invention, the directional freezing preferably uses liquid nitrogen to construct a temperature gradient. In an embodiment of the present invention, the directional freezing is preferably performed using a directional freezing single-sided heat transfer mold, the structure of which is as follows: Figure 1 As shown; the directional freezing single-sided heat transfer mold includes an open container 1, which is preferably a cylindrical or cuboid container with an open top and a flat bottom surface; the outer perimeter of the open container 1 is made of plastic, such as polypropylene or polyvinyl chloride, and the bottom surface is made of metal or glass, such as copper; the outer perimeter of the open container 1 is preferably wrapped with an insulating material layer 3, such as foam. During directional freezing, the polymer gel pre-liquid is slowly poured into the open container of the directional freezing single-sided heat transfer mold, and then the directional freezing single-sided heat transfer mold is placed on the surface of a metal heat conductor 2, which is partially immersed in liquid nitrogen at the same temperature as the liquid nitrogen. In this invention, the metal heat conductor is preferably a copper block or copper column, and the upper surface dimension of the metal heat conductor is larger than the lower surface dimension of the open container; the distance between the polymer gel pre-liquid and the liquid nitrogen cryogenic environment can be controlled by adjusting the height of the metal heat conductor.

[0025] In this invention, the directional freezing and thawing are preferably repeated, with one directional freezing and thawing cycle constituting one operation. The number of repetitions is preferably no less than 3 times, more preferably 3-6 times. The duration of a single directional freezing operation (i.e., the settling time of the pre-gel liquid on the surface of the metal heat conductor) is preferably 1-30 minutes, more preferably 5-20 minutes. The thawing temperature is preferably room temperature, and the thawing time is determined by ensuring complete thawing. During the directional freezing process, the "free water" within the polymer gel system grows upwards under a unidirectional temperature gradient, but its growth is simultaneously constrained by the polymer chain structure, thus forming directional ice crystal growth, with polymer chains interspersed between the ice crystals. This directional freezing process constructs directional channels within the polymer. The repeated directional freezing and thawing in this invention helps ensure complete ice crystal growth.

[0026] After the directional freezing and thawing are completed, the present invention preferably places the obtained gel composite at -5 to -20°C to obtain the directional gel composite. In the present invention, the settling temperature is preferably -10 to -20°C, and the settling time is preferably 8 to 24 hours, more preferably 12 to 24 hours; the settling specifically involves placing the gel composite in a refrigerator at the specified temperature. The present invention provides an environment and time for the cross-linking and curing of long polymer chains through the settling process.

[0027] After obtaining the oriented gel composite, the present invention performs a first pyrolysis on the oriented gel composite to obtain a hierarchical porous carbon-supported monodisperse metal atom material. In the present invention, the first pyrolysis preferably includes: heating from room temperature to a first temperature at a first heating rate and holding at that temperature for 0.5-2.0 h; then heating from the first temperature to a second temperature at a second heating rate and holding at that temperature for 0.5-2.0 h; and then cooling to room temperature in the furnace. In the present invention, the first temperature is 100-150°C, preferably 120-150°C; the second temperature is 500-1000°C, preferably 600-900°C; the first heating rate is 1-3°C / min, preferably 1-2°C / min; and the second heating rate is 3-10°C / min, preferably 5-10°C / min. In the present invention, the first pyrolysis is preferably carried out in a protective atmosphere, preferably nitrogen or argon, and the flow rate of the protective atmosphere is preferably 0.1-1.0 mL / min, more preferably 0.1-0.5 mL / min. During the first temperature holding process, the main focus is on removing free water or physically adsorbed water that is not tightly bound to the polymer components. During the second temperature holding process, the main processes are the carbonization of the polymer and the pinning of monodisperse metal atoms. The atomic-level active sites formed by the monodisperse metal atoms (M) are stably maintained through dopant atoms (X) introduced by the dopant on the carbon support (C) via CXM bonds. In this invention, the formation of the hierarchical pores is mainly due to: the formation of directional macropores in the hierarchical porous carbon structure through the above-mentioned directional freezing, and the formation of mesopores and small pores in the hierarchical porous structure through doping with dopant and pyrolysis of the polymer.

[0028] After the first pyrolysis, the present invention preferably subjectes the obtained first pyrolysis product to acid washing, water washing, and drying sequentially to obtain a hierarchical porous carbon-supported monodisperse metal atom material. In the present invention, the acidic reagent used for acid washing is preferably sulfuric acid, nitric acid, or hydrochloric acid, and the concentration of the acidic reagent is preferably 0.5~3 mol / L; the acid washing temperature is preferably room temperature to 60℃, and the acid washing is preferably carried out under mechanical stirring; the acid washing time is preferably 8~24 hours. In the present invention, the water washing method is preferably mechanical stirring, centrifugation, or dialysis filtration, and the water washing is preferably performed multiple times until neutral. During the acid washing and water washing processes, a small amount of metal nanoparticles and impurities formed on the surface of the carbon support are removed. In the present invention, the drying temperature is preferably below 70℃, and the drying time is preferably above 12 hours.

[0029] After obtaining a hierarchical porous carbon-supported monodisperse metal atom material, this invention combines the hierarchical porous carbon-supported monodisperse metal atom material with a metal nanoparticle precursor solution, and then performs a second pyrolysis on the resulting composite material to obtain the tandem catalyst. In this invention, the metal nanoparticle precursor preferably includes one or more of the following: sulfate, chloride, nitrate, porphyrin, acetylacetone, oleate, and ionic liquid of the metal nanoparticle element. The ionic liquid is preferably an imidazole ionic liquid, a pyridine ionic liquid, or a quaternary ammonium salt ionic liquid. In the embodiments of this invention, the metal nanoparticle precursor is more preferably a chloride, nitrate, or oleate of the metal nanoparticle element. The metal nanoparticle element is one or more of the following: iron, nickel, cobalt, copper, manganese, zinc, silver, chromium, molybdenum, and indium. In this invention, the solvent of the metal nanoparticle precursor solution is preferably water, an organic solvent, or an ionic liquid. The organic solvent is preferably n-hexane, and the ionic liquid is preferably an imidazole ionic liquid, a pyridine ionic liquid, or a quaternary ammonium salt ionic liquid. The concentration of the metal nanoparticle precursor solution is preferably 0.1 mol / L to b mol / L, where b mol / L is the saturation concentration of the metal nanoparticle precursor solution. In this invention, the composite method preferably includes impregnation or electrochemical deposition. The specific operation of the impregnation method is preferably as follows: the hierarchical porous carbon-supported monodisperse metal atom material is impregnated into the metal nanoparticle precursor solution under mechanical stirring. In this invention, the mechanical stirring rate is preferably not higher than 500 rpm; the impregnation time is preferably 5 to 30 min, more preferably 10 to 15 min; after impregnation, the impregnation system is preferably filtered to separate the solid product, which is the composite material.

[0030] In this invention, the preferred specific operation of the electrochemical deposition method is as follows: the hierarchical porous carbon-supported monodisperse metal atom material is coated onto the surface of a Pt or graphite electrode as the negative electrode, an inert electrode is used as the positive electrode, the metal nanoparticle precursor solution is used as the electrolyte, and a voltage is applied to perform electrochemical deposition. This invention does not have any particular requirements for the coating method; any coating method well-known to those skilled in the art can be used. In this invention, the inert electrode is preferably a Pt, stainless steel, or graphite electrode. 。 In this invention, the voltage is preferably 1-5V, more preferably 1.5-3V, and the electrochemical deposition time is preferably 1-5min, more preferably 1-3min. After the electrochemical deposition, the coated portion on the surface of the Pt or graphite electrode is preferably scraped off, and the resulting solid product is the composite material.

[0031] In this invention, the second pyrolysis preferably includes: heating from room temperature to a third temperature at a third heating rate and holding at that temperature for 0.5 to 2.0 hours; then cooling to room temperature in the furnace. In this invention, the third heating rate is preferably 1 to 5 °C / min, more preferably 3 to 5 °C / min; the third temperature is 400 to 800 °C, more preferably 500 to 600 °C; the second pyrolysis is preferably carried out in a protective atmosphere, preferably nitrogen or argon, and the flow rate of the protective atmosphere is preferably 0.1 to 1.0 mL / min, more preferably 0.1 mL / min. During the second pyrolysis, the metal ions in the composite material obtained by combining the hierarchical porous carbon-supported monodisperse metal atomic material and the metal nanoparticle precursor solution are mainly reduced to metal nanoparticles. Since no new doping atoms are introduced, the metal ions are mostly aggregated in the form of nanoparticles. After the second pyrolysis, the obtained second pyrolysis product is preferably washed with water and dried sequentially to obtain the tandem catalyst.

[0032] This invention provides the application of the tandem catalyst described in the above technical solutions or the tandem catalyst prepared by the above preparation methods in the electrocatalytic carbon dioxide reduction reaction to prepare multi-carbon products. In this invention, the multi-carbon products are preferably C2 products, which preferably include ethanol and / or ethylene; the reaction products of the electrocatalytic carbon dioxide reduction reaction also include C1 products, such as carbon monoxide, formic acid, formate, etc. In this invention, the electrocatalytic carbon dioxide reduction reaction uses an electrode formed by loading the tandem catalyst slurry onto the surface of a current collector as the cathode, an electrochemically inert metal as the anode, and a constant voltage direct current as the power source. In this invention, the slurry is preferably obtained by mixing the tandem catalyst with a conductive additive and a binder; the conductive additive preferably includes one or more of carbon black, graphene, and Ketjen black; the binder preferably includes Nafion solution and / or PTFE; the mass ratio of the tandem catalyst, conductive additive, and binder is preferably 8:1:1. This invention does not have special requirements for the current collector; current collectors well known to those skilled in the art can be used, specifically such as carbon fiber or carbon nanotubes. In this invention, the electrochemically inert metal specifically includes stainless steel, platinum electrodes, graphite electrodes, etc. In this invention, the reactor configuration for the electrocatalytic carbon dioxide reduction reaction is preferably an H-type electrolyzer, which is a type of flow electrolyzer. In this invention, the feed gas for the electrocatalytic carbon dioxide reduction reaction preferably includes oxygen in addition to carbon dioxide, with the oxygen acting as an initiator; the volume ratio of oxygen to carbon dioxide is preferably (0.05~5):100, more preferably (0.5~2.5):100. In this invention, the voltage of the power supply is preferably -0.6~-2.0V RHE. The tandem catalyst provided by this invention can electrocatalyze the conversion of carbon dioxide into multi-carbon products with high selectivity, and the selectivity of the multi-carbon products is preferably greater than 40%.

[0033] The following examples illustrate the tandem catalyst and its preparation method provided by the present invention, as well as its application in the electrocatalytic reduction of carbon dioxide to prepare multi-carbon products. However, these examples should not be construed as limiting the scope of protection of the present invention.

[0034] Example 1 Polyvinyl alcohol, deionized water, urea (dopant), and ferric chloride (monodisperse metal precursor) were mixed in a mass ratio of 5:100:10:0.5, heated to 60°C, and allowed to stand for 60 minutes to eliminate air bubbles, thus obtaining a polymeric pregel solution. A certain amount of the bubble-free pregel solution was slowly poured into a directional freezing single-sided heat transfer cylindrical mold (e.g., Figure 1As shown, the mold is open at the top, with glass at the bottom and surrounded by polypropylene. Liquid nitrogen is injected into a cold chamber, and the heat transfer copper block is immersed in the liquid nitrogen. After the temperature of the copper block drops to the temperature of the liquid nitrogen and the liquid nitrogen is below the surface of the copper block, the single-sided heat transfer mold containing the pre-gelling liquid is slowly placed on the surface of the copper block. After standing for 5 minutes, the initially oriented gel is placed at room temperature to thaw. This step is repeated 3 times to ensure complete ice crystal growth. After directional freezing, the gel is placed in a -10°C freezer for 12 hours.

[0035] The gel composite, after directional freezing, was placed in a heat treatment furnace for the first stage of pyrolysis, starting from room temperature and increasing to 120°C at a rate of 1°C / min, and held at this temperature for 1 hour. Then, the temperature was increased to 700°C at a rate of 5°C / min and held at this temperature for 1 hour. The furnace was then cooled to room temperature under an inert atmosphere of argon at a flow rate of 0.1 mL / min. The resulting solid product was washed with 1 mol / L hydrochloric acid for 12 hours, followed by washing with water until neutral, and then dried at 60°C for 12 hours to obtain a hierarchical porous carbon-supported monodisperse Fe atom material.

[0036] A 1 mol / L copper chloride solution was prepared, and the hierarchical porous carbon-supported monodisperse Fe atomic material was impregnated in the solution and mechanically stirred for 10 min. The mixture was then filtered to obtain a solid product. This product underwent a second-stage heat treatment: the temperature was increased to 500 °C at a rate of 5 °C / min and held at that temperature for 1 h. It was then cooled to room temperature in the furnace under an inert atmosphere of argon gas at a flow rate of 0.1 mL / min. After washing with deionized water and drying, the tandem catalyst was obtained.

[0037] Figure 2 This is a scanning electron microscope (SEM) image of the gel obtained after directional freezing in Example 1. Figure 2 It can be seen that the polymer gel forms oriented channels, and more small and medium pores are formed on the inner wall of the channels, which greatly enhances the specific surface area of ​​the carbon support precursor and provides more loading sites for monodisperse metal atoms and nano-metal particles.

[0038] Figure 3 This is a transmission electron microscope (TEM) image of the tandem catalyst prepared in Example 1. Figure 3 It can be seen that the porous carbon support surface is loaded with two different high-contrast metal species: monodisperse metal Fe atoms (0.7wt%) and Cu nanoparticles (3wt%).

[0039] The tandem catalyst was mixed with conductive carbon black and PTFE at a mass ratio of 8:1:1 and coated onto the surface of a current collector made of carbon fiber paper as the cathode. An electrochemically inert metal was used as the anode, and the carbon dioxide electrocatalytic reaction was carried out in an H-type electrolyzer. The reactant gas was a carbon dioxide / oxygen mixture containing 0.5% (by volume of carbon dioxide) oxygen. The catalyst detected multi-carbon products at a RHE voltage of -1.2V. Specifically, the total Faraday efficiency of the reduction products was approximately 70%, with C2 products (molar ratio: ethylene / ethanol = 1.2) accounting for 50% and C1 products (CO) accounting for 20%.

[0040] Example 2 Polymethyl cellulose (with acrylic acid as a crosslinking agent), deionized water, dicyandiamide (dopant), and cobalt nitrate (monodisperse metal precursor) were mixed in a mass ratio of 10:100:5:1, heated to 50°C, and allowed to stand for 120 minutes to eliminate air bubbles, thus obtaining a pre-gel liquid for polymeric gelation. A certain amount of the bubble-free pre-gel liquid was slowly poured into a directional freezing single-sided heat transfer cuboid mold (e.g., Figure 1 As shown, the mold is open at the top, with glass at the bottom and surrounded by polypropylene. Liquid nitrogen was injected into a cold chamber, and the heat transfer copper block was immersed in the liquid nitrogen. After the temperature of the copper block dropped to the temperature of the liquid nitrogen and the liquid nitrogen was below the surface of the copper block, the single-sided heat transfer mold containing the pre-gelling liquid was slowly placed on the surface of the copper block. After standing for 10 minutes, the initially oriented gel was placed at room temperature to thaw. This step was repeated 3 times to ensure complete ice crystal growth. After directional freezing, the gel was placed in a -20°C freezer for 8 hours.

[0041] The gel composite, after directional freezing, was placed in a heat treatment furnace for the first stage of pyrolysis, starting from room temperature and increasing to 150°C at a rate of 2°C / min, and held at this temperature for 2 hours. Then, the temperature was increased to 800°C at a rate of 5°C / min and held at this temperature for 1 hour. The furnace was then cooled to room temperature under an inert atmosphere of argon at a flow rate of 0.5 mL / min throughout the process. The resulting solid product was washed with 2 mol / L sulfuric acid for 8 hours, followed by washing with water until neutral, and then dried at 70°C for 24 hours to obtain a hierarchical porous carbon-supported monodisperse Co atom material.

[0042] A 0.5 mol / L copper nitrate solution was prepared, and the hierarchical porous carbon-supported monodisperse Co atom material was immersed in the solution and mechanically stirred for 30 min. The mixture was then filtered to obtain a solid product. This product underwent a second-stage heat treatment: the temperature was increased to 600 °C at a rate of 5 °C / min and held at this temperature for 2 h. It was then cooled to room temperature in the furnace under an inert atmosphere of argon at a flow rate of 0.1 mL / min. After washing with deionized water and drying, a tandem catalyst (monodisperse Co atom mass content of 1.2% and Cu nanoparticle mass content of 3.9%) was obtained.

[0043] Figure 4 The image shows the BET pore analysis of the hierarchical porous carbon-supported monodisperse Co atom material obtained from the first stage of pyrolysis in Example 2. It can be seen that the obtained material has micropores <2nm and mesopores 2~50nm.

[0044] The tandem catalyst was mixed with conductive carbon black and PTFE at a mass ratio of 8:1:1 and coated onto the surface of a current collector made of carbon fiber paper as the cathode. An electrochemically inert metal was used as the anode, and a carbon dioxide electrocatalytic reaction was carried out in an H-type electrolyzer. The reactant gas was a carbon dioxide / oxygen mixture containing 2% (by volume of carbon dioxide) oxygen. The catalyst detected multi-carbon products at a RHE voltage of -1.6 V. Specifically, the total Faraday efficiency of the reduction products was approximately 74%, with C2 products (molar ratio: ethylene / ethanol = 2.3) at 52% and C1 products (CO) at 22%.

[0045] Example 3 Sodium alginate (with calcium chloride as a crosslinking agent), deionized water, cyanamide (dopant), and silver chloride (monodisperse metal precursor) were mixed in a mass ratio of 15:100:10:1 and allowed to stand for 120 minutes to eliminate air bubbles, thus obtaining a polymeric gel pre-solution. A certain amount of the bubble-free gel pre-solution was slowly poured into a directional freezing single-sided heat transfer cylindrical mold (e.g., Figure 1 As shown, the mold is open at the top, with a copper sheet at the bottom and surrounded by polyvinyl chloride (PVC) material. Liquid nitrogen is injected into a cold chamber, and the heat transfer copper block is immersed in the liquid nitrogen. Once the copper block temperature drops to the liquid nitrogen temperature and the liquid nitrogen level is below the top surface of the copper block, the single-sided heat transfer mold containing the pre-gelling liquid is slowly placed on the top surface of the copper block. After standing for 20 minutes, the initially oriented gel is placed at room temperature to thaw. This step is repeated 5 times to ensure complete ice crystal growth. After directional freezing, the gel is placed in a -5°C freezer for 24 hours.

[0046] The gel composite, after directional freezing, was placed in a heat treatment furnace for the first stage of pyrolysis, starting from room temperature and increasing to 150°C at a rate of 3°C / min, and held at this temperature for 2 h. Then, the temperature was increased to 600°C at a rate of 5°C / min and held at this temperature for 2 h. The furnace was then cooled to room temperature under an inert atmosphere of argon gas at a flow rate of 0.1 mL / min throughout the process. The resulting solid product was washed with 2 mol / L hydrochloric acid for 12 h, then washed with water until neutral, and dried at 50°C for 24 h to obtain a hierarchical porous carbon-supported monodisperse Ag atom material.

[0047] A 1 mol / L copper oleate solution (hexane as solvent) was prepared, and the hierarchical porous carbon-supported monodisperse Ag atom material was impregnated in the solution. The mixture was mechanically stirred for 15 min, and then filtered to obtain a solid product. This product underwent a second-stage heat treatment: the temperature was increased to 600 °C at a rate of 2 °C / min and held at that temperature for 1 h. It was then cooled to room temperature in the furnace under an inert atmosphere of argon at a flow rate of 0.1 mL / min. After washing with deionized water and drying, a tandem catalyst (monodisperse Ag atom mass content 1.5%, Cu nanoparticle content 7.8%) was obtained.

[0048] The tandem catalyst was mixed with Ketjen black and PTFE in a mass ratio of 8:1:1 and coated onto the surface of a current collector made of carbon fiber paper as the cathode. An electrochemically inert metal was used as the anode, and the carbon dioxide electrocatalytic reaction was carried out in an H-type electrolyzer. The reactant gas was a carbon dioxide / oxygen mixture containing 1.0% (by volume of carbon dioxide) oxygen. The catalyst detected multi-carbon products at a RHE voltage of -1.4 V. Specifically, the total Faraday efficiency of the reduction products was approximately 65%, with C2 products (molar ratio: ethylene / ethanol = 3.1) at 42% and C1 products (CO) at 23%.

[0049] Example 4 Pyrrole (prepared with surfactant sodium dodecyl sulfate and crosslinking agent ammonium persulfate), deionized water, L-glutathione (dopant), and nickel chloride (monodisperse metal precursor) were mixed in a mass ratio of 20:100:20:0.1 and allowed to stand for 180 min to eliminate air bubbles, thus obtaining a polymer pregel solution. A certain amount of the bubble-free pregel solution was slowly poured into a directional freezing single-sided heat transfer cuboid mold (e.g., Figure 1 As shown, the mold is open at the top, with a copper sheet at the bottom and surrounded by polyvinyl chloride (PVC) material. Liquid nitrogen is injected into a cold chamber, and the heat transfer copper block is immersed in the liquid nitrogen. Once the copper block temperature drops to the liquid nitrogen temperature and the liquid nitrogen level is below the top surface of the copper block, the single-sided heat transfer mold containing the pre-gelling liquid is slowly placed on the top surface of the copper block. After standing for 30 minutes, the initially oriented gel is placed at room temperature to thaw. This step is repeated 5 times to ensure complete ice crystal growth. After directional freezing, the gel is placed in a -20°C freezer for 24 hours.

[0050] The gel composite, after directional freezing, was placed in a heat treatment furnace for the first stage of pyrolysis, starting from room temperature and increasing to 120°C at a rate of 2°C / min, and held at this temperature for 1 hour. Then, the temperature was increased to 700°C at a rate of 10°C / min and held at this temperature for 1 hour. The furnace was then cooled to room temperature under an inert atmosphere of argon at a flow rate of 0.1 mL / min throughout the process. The resulting solid product was washed with 1 mol / L nitric acid for 12 hours, followed by washing with water until neutral, and then dried at 60°C for 24 hours to obtain a hierarchical porous carbon-supported monodisperse Ni atom material.

[0051] A 1 mol / L copper chloride solution was prepared, and a hierarchical porous carbon-supported monodisperse Ni atom material was coated onto the surface of a Pt electrode as the negative electrode. An inert electrode was used as the positive electrode. Electrochemical deposition was performed at a constant potential of 1.5 V for 1 min. The coated portion on the Pt electrode surface was then scraped off to obtain a solid product. This product underwent a second-stage heat treatment, i.e., the temperature was increased to 550 °C at a rate of 5 °C / min and held at this temperature for 1 h. It was then cooled to room temperature in the furnace under an inert atmosphere of argon gas at a flow rate of 0.1 mL / min throughout the process. After washing with deionized water and drying, a tandem catalyst was obtained (monodisperse Ni atom mass content 0.3%, Cu nanoparticle content 10.5%).

[0052] The tandem catalyst was mixed with conductive carbon black and PTFE at a mass ratio of 8:1:1 and coated onto the surface of a current collector made of carbon fiber paper as the cathode. An electrochemically inert metal was used as the anode, and a carbon dioxide electrocatalytic reaction was carried out in an H-type electrolyzer. The reactant gas was a carbon dioxide / oxygen mixture containing 0.5% (by volume of carbon dioxide) oxygen. The catalyst detected multi-carbon products at a voltage of -1.8 VRHE. Specifically, the total Faraday efficiency of the reduction products was approximately 72%, with C2 products (molar ratio: ethylene / ethanol = 1.5) accounting for 51% and C1 products (CO) accounting for 21%.

[0053] Example 5 Acrylamide (with crosslinking agent N,N-bisacrylamide and ammonium persulfate), deionized water, guanidine hydrochloride (dopant), and zinc chloride (monodisperse metal precursor) were mixed at a mass ratio of 20:100:10:2, heated to 50°C, and allowed to stand for 150 min to eliminate air bubbles, thus obtaining a polymeric pregel solution. A certain amount of the bubble-free pregel solution was slowly poured into a directional freezing single-sided heat transfer cylindrical mold (e.g., Figure 1 As shown in the image, the mold is open at the top, with a copper sheet at the bottom and surrounded by polyvinyl chloride (PVC) material. Liquid nitrogen is injected into a cold chamber, and the heat transfer copper block is immersed in the liquid nitrogen. Once the copper block temperature drops to the liquid nitrogen temperature and the liquid nitrogen level is below the top surface of the copper block, the single-sided heat transfer mold containing the pre-gelling liquid is slowly placed on the top surface of the copper block. After standing for 30 minutes, the initially oriented gel is placed at room temperature to thaw. This step is repeated three times to ensure complete ice crystal growth. After directional freezing, the gel is placed in a -10°C freezer for 24 hours.

[0054] The gel composite, after directional freezing, was placed in a heat treatment furnace for the first stage of pyrolysis, starting from room temperature and increasing to 120°C at a rate of 2°C / min, and held at this temperature for 2 hours. Then, the temperature was increased to 900°C at a rate of 5°C / min and held at this temperature for 1 hour. The furnace was then cooled to room temperature under an inert atmosphere of argon gas at a flow rate of 0.1 mL / min throughout the process. The resulting solid product was washed with 1 mol / L hydrochloric acid for 8 hours, followed by washing with water until neutral, and then dried at 70°C for 24 hours to obtain a hierarchical porous carbon-supported monodisperse Zn atom material.

[0055] A 1 mol / L copper sulfate solution was prepared, and a hierarchical porous carbon-supported monodisperse Zn atom material was coated onto the surface of a Pt electrode as the negative electrode. An inert electrode was used as the positive electrode. Electrochemical deposition was performed at a constant potential of 2 V for 5 min. The coated portion on the Pt electrode surface was then scraped off to obtain a solid product. This product underwent a second-stage heat treatment, i.e., the temperature was increased to 600 °C at a rate of 5 °C / min and held at this temperature for 1 h. It was then cooled to room temperature in the furnace under an inert atmosphere of argon gas at a flow rate of 0.1 mL / min throughout the process. After washing with deionized water and drying, a tandem catalyst (monodisperse Zn atom mass content 3.4%, Cu nanoparticle content 9.2%) was obtained.

[0056] The tandem catalyst was mixed with conductive carbon black and PTFE in a mass ratio of 8:1:1, and then coated onto the surface of a current collector made of carbon fiber paper as the cathode. An electrochemically inert metal was used as the anode, and a carbon dioxide electrocatalytic reaction was carried out in an H-type electrolyzer. The reactant gas was a carbon dioxide / oxygen mixture containing 5.0% (by volume of carbon dioxide) oxygen. The catalyst detected multi-carbon products at a RHE voltage of -1.2 V. Specifically, the total Faraday efficiency of the reduction products was approximately 69%, with C2 products (molar ratio: ethylene / ethanol = 1.7) at 48% and C1 products (CO) at 21%.

[0057] Example 6 Chitosan (with acetic acid and glutaraldehyde as crosslinking agents), deionized water, urea (dopant), and ferric nitrate (monodisperse metal precursor) were mixed in a mass ratio of 15:100:15:0.5, heated to 45°C, and allowed to stand for 60 minutes to eliminate air bubbles, thus obtaining a pre-gel solution. A certain amount of the bubble-free pre-gel solution was slowly poured into a directional freezing single-sided heat transfer cylindrical mold (e.g., Figure 1As shown in the image, the mold is open at the top, with a glass bottom and a polypropylene surround. Liquid nitrogen was injected into a cold chamber, and the heat transfer copper block was immersed in the liquid nitrogen. Once the copper block's temperature dropped to the liquid nitrogen temperature and the liquid nitrogen was below the top surface of the copper block, the single-sided heat transfer mold containing the pre-gelling liquid was slowly placed on the top surface of the copper block. After standing for 15 minutes, the initially oriented gel was thawed at room temperature. This step was repeated 6 times to ensure complete ice crystal growth. After directional freezing, the gel was frozen at -5°C for 24 hours.

[0058] The gel composite, after directional freezing, was placed in a heat treatment furnace for the first stage of pyrolysis, starting from room temperature and increasing to 130°C at a rate of 1°C / min, and held at this temperature for 1 h. Then, the temperature was increased to 600°C at a rate of 5°C / min and held at this temperature for 2 h. The furnace was then cooled to room temperature under an inert atmosphere of argon gas at a flow rate of 0.1 mL / min throughout the process. The resulting solid product was washed with 3 mol / L hydrochloric acid for 12 h, followed by washing with water until neutral, and then dried at 60°C for 24 h to obtain a hierarchical porous carbon-supported monodisperse Fe atom material.

[0059] A 1 mol / L copper nitrate solution was prepared, and a hierarchical porous carbon-supported monodisperse Fe atom material was coated onto the surface of a Pt electrode as the negative electrode. An inert electrode was used as the positive electrode. Electrochemical deposition was performed at a constant potential of 3 V for 1 min. The coated portion on the Pt electrode surface was then scraped off to obtain a solid product. This product underwent a second-stage heat treatment, i.e., the temperature was increased to 500 °C at a rate of 3 °C / min and held at this temperature for 1 h. It was then cooled to room temperature in the furnace under an inert atmosphere of argon gas at a flow rate of 0.1 mL / min throughout the process. After washing with deionized water and drying, a tandem catalyst (monodisperse Fe atom mass content of 0.25% and Cu nanoparticle content of 9.1%) was obtained.

[0060] The tandem catalyst was mixed with conductive carbon black and PTFE at a mass ratio of 8:1:1 and coated onto the surface of a current collector made of carbon fiber paper as the cathode. An electrochemically inert metal was used as the anode, and the carbon dioxide electrocatalytic reaction was carried out in an H-type electrolyzer. The reactant gas was a carbon dioxide / oxygen mixture containing 2.5% (by volume of carbon dioxide) oxygen. The catalyst detected multi-carbon products at a voltage of -1.4 VRHE. Specifically, the total Faraday efficiency of the reduction products was approximately 62%, with C2 products (molar ratio: ethylene / ethanol = 3.4) at 45% and C1 products (CO) at 17%.

[0061] As can be seen from the above embodiments, the tandem catalyst provided by the present invention can electrocatalyze the conversion of carbon dioxide into multi-carbon products with high selectivity.

[0062] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A tandem catalyst comprising a hierarchical porous carbon support and monodisperse metal atoms and metal nanoparticles supported on the surface of the hierarchical porous carbon support; The method for preparing the tandem catalyst includes the following steps: A water-based polymer or water-based polymer monomer is mixed with a monodisperse metal precursor, water, and a dopant to obtain a polymeric gel pre-liquid; the dopant includes one or more of nitrogen atom dopant, sulfur atom dopant, boron atom dopant, and phosphorus atom dopant; The polymer gel pre-liquid is subjected to directional freezing and thawing sequentially to obtain a directional gel complex; the directional freezing and thawing are repeated, with one directional freezing and thawing operation constituting one operation, and the operation is repeated no less than 3 times; the time for a single directional freezing is 1~30 min; The directional gel composite was subjected to a first pyrolysis to obtain a hierarchical porous carbon-supported monodisperse metal atom material. The multi-level porous carbon-supported monodisperse metal atom material and the metal nanoparticle precursor solution are combined, and the resulting composite material is subjected to a second pyrolysis to obtain the tandem catalyst.

2. The tandem catalyst according to claim 1, characterized in that, The pore structure of the hierarchical porous carbon support includes micropores, mesopores, and macropores. The pore size of the micropores is <2 nm, the pore size of the mesopores is 2~50 nm, and the pore size of the macropores is >1 μm. The metal elements in the monodisperse metal atoms include one or more of iron, nickel, cobalt, copper, manganese, zinc, silver, chromium, molybdenum, and indium. The metal elements in the metal nanoparticles include one or more of iron, nickel, cobalt, copper, manganese, zinc, silver, chromium, molybdenum, and indium.

3. The tandem catalyst according to claim 1 or 2, characterized in that, The mass content of monodisperse metal atoms in the tandem catalyst is 0.01~10%, and the mass content of metal nanoparticles is 0.5~20%.

4. The method for preparing the tandem catalyst according to any one of claims 1 to 3, characterized in that, Includes the following steps: A water-based polymer or water-based polymer monomer is mixed with a monodisperse metal precursor, water, and a dopant to obtain a polymeric gel pre-liquid; the dopant includes one or more of nitrogen atom dopant, sulfur atom dopant, boron atom dopant, and phosphorus atom dopant; The polymer gel pre-liquid is subjected to directional freezing and thawing sequentially to obtain a directional gel complex; the directional freezing and thawing are repeated, with one directional freezing and thawing operation constituting one operation, and the operation is repeated no less than 3 times; the time for a single directional freezing is 1~30 min; The directional gel composite was subjected to a first pyrolysis to obtain a hierarchical porous carbon-supported monodisperse metal atom material. The multi-level porous carbon-supported monodisperse metal atom material and the metal nanoparticle precursor solution are combined, and the resulting composite material is subjected to a second pyrolysis to obtain the tandem catalyst.

5. The preparation method according to claim 4, characterized in that, The aqueous polymer includes natural aqueous polymers and / or synthetic aqueous polymers. The natural aqueous polymers include one or more of starch, cellulose, alginic acid, hyaluronic acid, chitosan, poly-L-lysine, and poly-L-glutamic acid. The synthetic aqueous polymers include one or more of polyvinyl alcohol, polyacrylic acid, polymethacrylic acid, polyacrylamide, and polyvinylpyrrolidone. The aqueous polymer monomers include one or two of pyrrole and acrylamide. The monodisperse metal precursors include one or more of sulfates, chlorides, nitrates, porphyrins, and acetylacetones of monodisperse metal elements.

6. The preparation method according to claim 4 or 5, characterized in that, The mass ratio of the aqueous polymer or aqueous polymer monomer to the monodisperse metal precursor, water and dopant is (5~a):(0.1~10):100:(1~20), where a is the solubility of the aqueous polymer or aqueous polymer monomer in water.

7. The preparation method according to claim 4, characterized in that, The directional freezing uses liquid nitrogen to create a temperature gradient.

8. The preparation method according to claim 4, characterized in that, The metal nanoparticle precursor includes one or more of the following: sulfates, chlorides, nitrates, porphyrins, acetylacetones, oleates, and ionic liquids of the metal nanoparticle element; the composite method includes impregnation or electrochemical deposition.

9. The preparation method according to claim 4, characterized in that, The first pyrolysis includes: heating from room temperature to a first temperature at a first heating rate and holding at that temperature for 0.5 to 2.0 hours; then heating from the first temperature to a second temperature at a second heating rate and holding at that temperature for 0.5 to 2.0 hours; and then cooling to room temperature in the furnace; wherein the first temperature is 100 to 150°C and the second temperature is 500 to 1000°C; the first heating rate is 1 to 3°C / min and the second heating rate is 3 to 10°C / min; and the first pyrolysis is carried out in a protective atmosphere. The second pyrolysis includes: heating from room temperature to a third temperature at a third heating rate and holding at that temperature for 0.5 to 2.0 hours; then cooling the furnace to room temperature; the third heating rate is 1 to 5 °C / min, and the third temperature is 400 to 800 °C; the second pyrolysis is carried out in a protective atmosphere.

10. The application of the tandem catalyst according to any one of claims 1 to 3 or the tandem catalyst prepared by the preparation method according to any one of claims 4 to 9 in the electrocatalytic reduction of carbon dioxide to prepare multi-carbon products.

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