A catalyst for electroreduction of CO2 to syngas, its preparation method and its application

Abstract

This invention belongs to the field of CO / H2 syngas production, and discloses a catalyst for electroreduction of CO2 to produce syngas, its preparation method, and its application. This invention uses a precipitation method, followed by washing, drying, and calcination steps to synthesize a metal-doped zinc-based catalyst, constructing a highly efficient CO2RR syngas production system with a syngas ratio that can meet most industrial production needs. The M-ZnO catalyst prepared by this invention not only possesses excellent CO2RR syngas production performance, but also features a simple synthesis method, and the entire reaction process is safe and non-toxic, making it suitable for large-scale synthesis. This invention allows for the adjustment of the syngas ratio by regulating the amount of metal doping and the applied potential, which is beneficial for the widespread application of CO2RR syngas production technology.

Description

Technical Field

[0001] This invention belongs to the field of CO / H2 syngas preparation, and relates to a catalyst for electroreduction of CO2 to syngas and its preparation method. Background Technology

[0002] With China's rapid economic development and accelerated industrialization, the consumption of fossil fuels has increased dramatically. Over the past 20 years, atmospheric CO2 concentration has risen significantly, currently approaching 420 ppm, with an annual growth rate of 0.5%. Global average temperatures have also risen by 0.5°C between 2001 and 2024. To better address the major global challenge of climate change, CO2 emissions can be controlled and minimized through electrocatalysis, photocatalysis, and thermocatalysis, converting CO2 into various valuable compounds. Electrocatalysis not only enables the resource utilization of CO2 and promotes a better carbon cycle but also effectively utilizes renewable energy, and is considered one of the most promising CO2 conversion and utilization technologies of the 21st century.

[0003] Electrocatalysis is a promising strategy for CO2 utilization, enabling the direct conversion of CO2 into high-value-added chemicals and fuels under mild reaction conditions. Specifically, the product CO and byproduct H2 can combine in a water-based reaction medium via an electrocatalytic CO2 reduction reaction (CO2RR) to produce syngas (CO / H2). Syngas with varying CO / H2 molar ratios can be precisely tailored to downstream chemical processes for the production of different chemicals. For example, syngas fermentation requires a CO / H2 ratio of 1 to 3.3; Fischer-Tropsch synthesis requires a ratio of 0.4 to 2.0; alcohol production requires a ratio of 0.5 to 1.0; methanol production requires a ratio of 1.0; and methanation of syngas requires a ratio of 0.3. Mitra Bagheri et al. synthesized porous ZIF-8 and ZIF-67 catalysts for electroreduction of CO2 using a green steam-assisted dry gel method. By controlling the morphology of the catalysts, they produced syngas with an adjustable CO / H2 ratio. However, the preparation method of metal-organic framework (MOF) catalysts is relatively cumbersome, and the highest CO / H2 ratio produced within the test range is only about 2.0. In addition, the ZIF-8 catalyst produced 10-15% formate byproducts under the test conditions, which is not suitable for large-scale industrial applications (ChemSusChem, 2024:e202400684.). Mao et al. prepared Cu2O / In(OH)3 with a heterojunction structure as an electrocatalyst for syngas production. Within a voltage range of -0.665V to -1.235V (vs. RHE), the syngas ratio could be adjusted from 1:2 to 1:1 by adjusting the Cu / In feed ratio, while exhibiting high stability. However, the small range of syngas ratio limited the production of various chemicals (Applied Surface Science. 2024, 160114). Summary of the Invention

[0004] The purpose of this invention is to address the problems of complex preparation methods and small syngas ratio ranges in existing electrocatalysts for CO2 reduction to syngas (CO / H2), which prevent precise adaptation to downstream chemical production. This invention provides a catalyst for electrocatalytic CO2 reduction reaction (CO2RR) to syngas and its preparation method. The synthesized M-ZnO catalyst not only possesses excellent CO2RR syngas production performance, but also features a simple preparation method, and the entire reaction process is safe and non-toxic, making it suitable for large-scale synthesis.

[0005] The technical solution of the present invention:

[0006] A catalyst for the electroreduction of CO2 to produce syngas has the general chemical formula M-ZnO, where M is one or more of the following metallic elements: gallium (Ga), cerium (Ce), lanthanum (La), zirconium (Zr), iron (Fe), nickel (Ni), palladium (Pd), rhodium (Rh), and platinum (Pt).

[0007] A method for preparing a catalyst for the electroreduction of CO2 to syngas comprises the following steps:

[0008] Step 1: Dissolve the metal source in alcohol and mix ultrasonically until a homogeneous precursor solution is formed;

[0009] Step 2: Slowly add the precipitant to the precursor solution, heat and stir, let stand, and centrifuge to obtain a solid precipitate;

[0010] Step 3: After washing the solid precipitate, vacuum dry it, grind it thoroughly, and then heat and calcine it in a muffle furnace. After the calcination is completed, allow it to cool naturally to room temperature and grind it thoroughly to obtain the M-ZnO catalyst.

[0011] Further specifying, the metal source in step 1 comprises two parts: a Zn source and a doped metal source. The Zn source is one or more of a mixture of Zn-containing compounds such as zinc nitrate and zinc chloride; the doped metal source is one or more of a mixture of compounds containing the metal element such as nitrates and chlorides, wherein the molar proportion of the doped metal element in the total metal (M / (M+Zn)) ranges from 0.1% to 30%.

[0012] Further specifying, the doped metal source includes, but is not limited to, one or more of the following metal elements: gallium (Ga), cerium (Ce), lanthanum (La), zirconium (Zr), iron (Fe), nickel (Ni), palladium (Pd), rhodium (Rh), and platinum (Pt).

[0013] Further specifying, the alcohol in step 1 includes commonly used alcohol solvents such as ethanol, isopropanol, n-butanol, ethylene glycol, and n-hexanol, with ethanol being preferred. The ratio of the metal source to the alcohol is (0.01–5) g : (150–250) mL.

[0014] Further specifying, the precipitant in step 2 can be any of various precipitants such as oxalic acid, ammonia, and urea. Specifically, when using a liquid precipitant (such as oxalic acid or ammonia), the volume ratio of the precipitant to the precursor solution in step 1 is 1:1 to 1:5, preferably 1:2.5; when using a solid precipitant (such as urea), the molar ratio of the precipitant to the total amount of metal elements in step 1 is 1:1 to 3:1.

[0015] Further specifying, in step 2, stirring is carried out by water bath heating or oil bath heating, and the reaction is carried out at a temperature of 30 to 130°C, preferably 70°C, for a stirring time of 0.3 to 3 hours, preferably 1 hour.

[0016] Further specifying that the solid precipitate in step 3 must undergo at least one alcohol wash and at least one deionized water wash.

[0017] Further specifying, the drying temperature in step 3 is 30-100℃, preferably 80℃, and the drying time is 1-48h.

[0018] Further specifying, the calcination temperature in step 3 is 400–600℃, preferably 600℃, and the heating time is 3–5h, preferably 4h.

[0019] A method for preparing an M-ZnO electrode, the method comprising the following steps:

[0020] The M-ZnO catalyst was dissolved in a mixture of alcohol and Nafion, ultrasonically treated, and then uniformly coated onto the surface of carbon paper. After drying, the M-ZnO electrode was obtained.

[0021] Further specifying, the loading of the M-ZnO catalyst is 1–4 mg / cm³. 2 Preferably 2mg / cm 2 .

[0022] Further specifying, alcohols include commonly used volatile alcohol solvents such as ethanol and isopropanol, with ethanol being preferred.

[0023] Further specifying, the volume ratio of alcohol to Nafion solution is (80-120):1, preferably 100:1.

[0024] The application of an M-ZnO electrode in the preparation of syngas (CO / H2) is carried out according to the following steps:

[0025] Step 1: Using the M-ZnO electrode as the working electrode, Ag / AgCl as the reference electrode, and Pt electrode as the counter electrode, add electrolyte and assemble the electrolytic cell device.

[0026] Step 2: CO2 is continuously introduced into the electrolyte at a constant rate to perform constant potential electrolysis and obtain syngas with a suitable industrial synthesis ratio range.

[0027] Further specifying, the electrolyte in step 1 may contain one or more of KHCO3, KCl, etc., with a concentration of 0.05 to 1.0 M and a volume of 40 to 70 mL.

[0028] Further specifying, the electrolytic cell device in step 1 includes H-type electrolytic cells, flow electrolytic cells, and membrane electrolytic cells, etc.

[0029] Further specifying, the flow rate of CO2 introduced into the electrolytic cell device is 5 to 50 mL / min, preferably 20 mL / min.

[0030] The beneficial effects of this invention are:

[0031] (1) M-ZnO catalyst was synthesized by precipitation method. The synthesis method is simple, the reaction conditions are mild, and it is suitable for large-scale synthesis. The obtained catalyst is in powder form, has stable properties, and is easy to store.

[0032] (2) The M-ZnO catalyst prepared by the present invention can reduce CO2 to syngas through CO2RR, and the syngas ratio can be adjusted by the amount of metal doping and the applied potential. The obtained syngas meets the needs of most industrial production processes such as fermentation and Fischer-Tropsch synthesis.

[0033] (3) This invention constructs a series of M-ZnO catalysts with rich defect structures from a microscopic perspective, and uses them to prepare syngas with adjustable ratios for CO2RR. The entire reaction process is safe and non-toxic. Attached Figure Description

[0034] Figure 1 X-ray diffraction patterns of the 2% Ga-ZnO, 5% Ga-ZnO, 10% Ga-ZnO, 15% Ga-ZnO, and ZnO catalysts in Examples 1-4 and Comparative Example 1;

[0035] Figure 2 The images show the EPMA diagram and corresponding EDS elemental distribution diagram of the 5% Ga-ZnO catalyst in Example 2; where (a) is the EPMA diagram of the 5% Ga-ZnO catalyst, (b) is the distribution of Zn element in the 5% Ga-ZnO catalyst, (c) is the distribution of Ga element in the 5% Ga-ZnO catalyst, (d) is the distribution of O element in the 5% Ga-ZnO catalyst, and (e) is the distribution of Zn, Ga, and O elements in the 5% Ga-ZnO catalyst.

[0036] Figure 3 The results show the stability test of the 5% Ga-ZnO catalyst prepared in Example 2, where (a) shows the changes in the Faradaic efficiency and proportion of the syngas products, and (b) shows the changes in current density throughout the test range. Detailed Implementation

[0037] The specific embodiments of the present invention will be further described below with reference to the technical solutions and accompanying drawings.

[0038] The Zn-based catalyst described in this invention is a catalyst synthesized by precipitation method. Here, we will take the Zn-based catalyst prepared by oxalic acid as a precipitant as an example.

[0039] Raw materials were weighed according to a metal source and zinc source molar ratio of 1:9, mixed and dissolved in ethanol, and ultrasonically treated at room temperature to form a precursor solution. Excess oxalic acid solution was added dropwise as a precipitant to the precursor solution and stirred. The mixture was stirred continuously for 1 hour in a 70°C water bath, and then aged at room temperature for 2 hours. Subsequently, the mixture was washed alternately by centrifugation with ethanol and deionized water. The resulting white solid precipitate was dried at 80°C for 12 hours, thoroughly ground, and then calcined in a muffle furnace at 600°C for 4 hours to obtain the catalyst.

[0040] The structure of Zn-based catalysts can be optimized by changing the type and ratio of the metal source, zinc source, and precipitant in the raw materials, as well as the synthesis conditions. The synthesis of partially metal-doped Zn-based catalysts is shown in Table 1 below.

[0041] Table 1. Preparation of metal-doped Zn-based catalysts

[0042]

[0043]

[0044] Undoped ZnO was selected as a comparative example, and its preparation process was the same as above, except that only zinc source was added as the metal source, without any other metal sources.

[0045] Based on the catalyst synthesized in the table above, prepare an electrode for producing syngas. The specific steps are as follows:

[0046] After ultrasonically mixing 2 mL of ethanol and 20 μL of 5% Nafion solution, 2 mg of catalyst powder was added. The mixture was then ultrasonically treated and subsequently evenly coated onto carbon paper (1 × 1 cm). 2 After drying for 30 minutes, an M-ZnO electrode was obtained.

[0047] Application of the prepared M-ZnO electrode in the preparation of CO / H2 syngas:

[0048] (1) Use 55 mL of 0.1 M KHCO3 solution as the cathode electrolyte.

[0049] (2) Use 55 mL of 0.1 M KHCO3 solution as the anolyte.

[0050] (3) An H-type double-chamber electrolytic cell was assembled with an M-ZnO electrode as the cathode working electrode, an Ag / AgCl electrode as the reference electrode, and a Pt electrode as the counter electrode. The cathode and anode chambers were separated by a proton exchange membrane.

[0051] (4) High-purity CO2 gas was continuously and stably introduced into the cathode and anode chambers at a flow rate of 20 mL / min for 30 min to remove other impurity gases in the apparatus. Subsequently, voltages ranging from -0.79 V to -1.29 V (vs. RHE) were applied sequentially to test the electroreduction performance of CO2. Each catalyst required pretreatment at -0.69 V (vs. RHE) for 30 min before the formal test. The electrochemical performance of the catalysts was tested and analyzed using a CHI630E electrochemical workstation. Gas-phase products were sampled directly from the sealed electrolysis system and then tested by gas chromatography. CO was detected by a flame ionization detector (FID), and H2 was detected by a thermal conductivity detector (TCD). The products and their Faraday efficiency were qualitatively and quantitatively analyzed based on the gas chromatography results.

[0052] For electrocatalytic reaction examples with different catalysts, Table 2 lists the specific applications and effect data of Examples 1-8 for electrodes prepared according to the above method.

[0053] Table 2. Specific applications and effect data of some catalysts

[0054]

[0055] The Faraday efficiency of the gaseous products obtained by electrolysis in the above-described embodiments and comparative examples within a potential range of -0.79 to -1.29 V (vs. RHE) was determined.

[0056] For the 2% Ga-ZnO catalyst in Example 1, the Faradaic efficiency of CO can reach up to 58.56%, the total Faradaic efficiency of the syngas is close to 90%, and the ratio of the Faradaic efficiency of CO to H2 in the syngas (CO / H2) is in the range of 0.27 to 1.90, which is a good syngas ratio and is suitable for the industrial production of alcohol chemicals such as methane and methanol.

[0057] For the 5% Ga-ZnO catalyst in Example 2, the Faradaic efficiency of CO can reach up to 64.59%, the total Faradaic efficiency of syngas is close to 90%, and the ratio of the Faradaic efficiencies of CO and H2 in syngas (CO / H2) is in the range of 0.21 to 3.07, which is a good syngas ratio and suitable for syngas fermentation.

[0058] For the 10% Ga-ZnO catalyst in Example 3, the Faradaic efficiency of CO can reach up to 70.66%, the total Faradaic efficiency of syngas is close to 90%, and the ratio of the Faradaic efficiency of CO to H2 in syngas (CO / H2) is in the range of 0.30 to 3.98, which is a good syngas ratio and is suitable for Fischer-Tropsch synthesis of various high-value-added hydrocarbon chemicals.

[0059] For the 15% Ga-ZnO catalyst in Example 4, the Faradaic efficiency of CO can reach up to 69.19%, the total Faradaic efficiency of syngas is close to 90%, and the ratio of the Faradaic efficiencies of CO and H2 in syngas (CO / H2) is in the range of 0.27 to 3.64, which is a good syngas ratio and is suitable for Fischer-Tropsch synthesis of various high-value-added alcohol chemicals.

[0060] For Examples 5-8, by changing the type and proportion of the doped metal elements, a wide range of changes in the syngas ratio can be achieved, which can be precisely adapted to the industrial production of a variety of downstream chemicals.

[0061] For the ZnO catalyst in Comparative Example 1, the highest Faradaic efficiency for CO was only 49.87%, and the highest total Faradaic efficiency for syngas was only 82.84%, which decreased significantly with increasing applied potential. This indicates that a large number of byproducts are generated during the reaction, which not only reduces the purity of the syngas but also increases the complexity and cost of subsequent separation and purification steps. Furthermore, the Faradaic efficiency ratio of CO to H2 in the syngas (CO / H2) is relatively small, ranging from 0.63 to 1.25, limiting the industrial production potential of the syngas.

[0062] Characterization tests and analyses were performed on some of the embodiments.

[0063] X-ray diffraction was used to analyze the 2% Ga-ZnO, 5% Ga-ZnO, 10% Ga-ZnO, 15% Ga-ZnO, and ZnO catalysts obtained in the first step of Examples 1-4 and Comparative Example 1. The results are as follows: Figure 1 As shown, the five synthesized catalysts correspond to the main peak of ZnO in the standard PDF card (PDF#36-1451). Meanwhile, Ga, due to its highly dispersed state on the ZnO surface, did not exhibit obvious diffraction peaks.

[0064] The EPMA image of the 5% Ga-ZnO catalyst obtained in Example 2 was obtained, and the surface composition of the catalyst was analyzed by energy-dispersive X-ray spectroscopy (EDS), such as... Figure 2 As shown, the material is composed of Zn, Ga, and O elements, which are uniformly distributed and highly dispersed on the ZnO surface.

[0065] The stability of the electrocatalytic reduction system in Example 2 was determined by testing the stability of the synthesized 5% Ga-ZnO catalyst. Figure 3 As shown, the current density and CO / H2 syngas ratio remained relatively stable during long-term stability tests, indicating that the catalyst has good electrochemical stability, is not easily deactivated, and is expected to be promoted for industrial application.

[0066] In summary, compared with existing methods for preparing materials for the electrocatalytic reduction of CO2 to syngas, this invention has the following advantages: it possesses excellent CO2RR syngas production performance, the catalyst synthesis method is simple, the entire reaction process is safe and non-toxic, and it is suitable for large-scale synthesis. Furthermore, this invention allows for the adjustment of the syngas ratio by regulating the amount of metal doping and the applied potential, which is beneficial for the widespread application of CO2RR syngas production technology.

[0067] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited to the embodiments described herein. Many equivalent substitutions and modifications made without departing from the spirit and principle of the present invention should be included within the scope of the present invention.

Claims

1. An application of an M-ZnO electrode in the preparation of syngas, characterized in that, Follow these steps: Step 1: Using the M-ZnO electrode as the working electrode, Ag / AgCl as the reference electrode, and Pt electrode as the counter electrode, add electrolyte and assemble the electrolytic cell device; Step 2: CO2 is continuously introduced into the electrolyte at a constant rate to perform constant potential electrolysis and obtain syngas with a suitable industrial synthesis ratio range; The general chemical formula of the M-ZnO catalyst is M-ZnO, where M is Ga; The preparation steps of the M-ZnO catalyst are as follows: Step 1: Dissolve the metal source in alcohol and mix ultrasonically until a homogeneous precursor solution is formed; Step 2: Slowly add the precipitant to the precursor solution, heat and stir, let stand, and centrifuge to obtain a solid precipitate; Step 3: After washing the solid precipitate, vacuum dry it, grind it thoroughly, and then heat and calcine it in a muffle furnace. After the calcination is completed, allow it to cool naturally to room temperature and grind it thoroughly to obtain the M-ZnO catalyst. The metal source consists of two parts: a Zn source and a doped metal source, with a molar ratio of 0.1 to 30% between the doped metal source and the metal source.

2. The application according to claim 1, characterized in that, The electrolyte is KHCO3 and / or KCl, with a concentration of 0.05 ~ 1.0 M; The electrolytic cell device can be an H-type electrolytic cell, a flow electrolytic cell, or a membrane electrolytic cell; The flow rate of CO2 introduced into the electrolytic cell is 5 ~ 50 mL / min.

3. The application of the M-ZnO electrode according to claim 1 in the preparation of syngas, characterized in that, The method for preparing the M-ZnO electrode is as follows: The M-ZnO catalyst was dissolved in a mixture of alcohol and Nafion, ultrasonically treated, and then uniformly coated onto the surface of carbon paper. After drying, the M-ZnO electrode was obtained. Alcohols include ethanol and isopropanol; The volume ratio of alcohol to Nafion solution is (80 ~ 120):

1.

4. The application of the M-ZnO electrode according to claim 3 in the preparation of syngas, characterized in that, The loading of M-ZnO catalyst was 1 ~ 4 mg / cm³. 2 .

5. The application of the M-ZnO electrode according to claim 1 in the preparation of syngas, characterized in that, In step 1, The Zn source is zinc nitrate and / or zinc chloride; the doped metal source is one or more of nitrates and chlorides; Alcohols include ethanol, isopropanol, n-butanol, ethylene glycol, and n-hexanol, with a metal source to alcohol ratio of 0.01 ~ 5 g: 150 ~ 250 mL.

6. The application of the M-ZnO electrode according to claim 1 in the preparation of syngas, characterized in that, In step 2, Oxalic acid, ammonia, or urea are selected as precipitants. When using a liquid precipitant, the volume ratio of the precipitant to the precursor solution is 1:1 to 1:

5. When using a solid precipitant, the molar ratio of the precipitant to the total amount of metal elements in step 1 is 1:1 to 3:

1. Stirring is performed using water bath heating or oil bath heating, and the reaction is carried out at a temperature of 30 ~ 130 ℃ for 0.3 ~ 3 h.

7. The application of the M-ZnO electrode according to claim 1 in the preparation of syngas, characterized in that, In step 3, Solid precipitates must be washed with alcohol at least once and with deionized water at least once. The drying temperature is 30 ~ 100 ℃, and the drying time is 1 ~ 48 h; The calcination temperature is 400 ~ 600 ℃, and the calcination time is 3 ~ 5 h.

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