In single-atom catalyst, preparation method and application thereof

Abstract

The present invention provides an In single-atom catalyst, its preparation method, and application, relating to the field of electrocatalytic carbon dioxide reduction technology. Metallic In has high formic acid selectivity, and using a doped carbon substrate can increase the rate of electron transfer in the CO2RR reaction. The carbon substrate of the present invention has a porous two-dimensional sheet structure with a larger specific surface area, which facilitates the exposure of more In active sites, thereby improving the catalytic activity of the catalyst. Furthermore, the doped N element (or N and S elements) promotes the dispersion of In single atoms and firmly anchors them, forming uniform, highly coordinated catalytic active centers, thereby improving the activity and stability of the catalyst.

Description

Technical Field

[0001] The present invention relates to the technical field of electrocatalytic carbon dioxide reduction, and in particular to an In single-atom catalyst and a preparation method and application thereof. Background Art

[0002] The overconsumption of traditional fossil fuels has led to a rapid increase in total carbon dioxide emissions, causing various global challenges, including the greenhouse effect, rising sea levels, and extreme weather. Therefore, converting carbon dioxide into high-value-added products to alleviate energy and environmental pressures is crucial. Electrocatalytic carbon dioxide reduction (CO2RR) is a technology that can reduce and recover the greenhouse gas CO2 under relatively mild reaction conditions, converting it into other carbon products. It is an effective way to achieve a carbon-neutral cycle and address environmental pollution and energy crises. In recent years, main-group metal In-based CO2RR electrocatalysts have been discovered to reduce CO2 to formic acid (HCOOH) while offering the advantage of low preparation costs. Furthermore, Indium metal boasts low toxicity and environmental friendliness, making it a rapidly emerging research focus in the CO2RR field. However, Indium-based CO2RR catalysts often suffer from disadvantages such as high overpotentials, poor stability, and low formic acid selectivity. Summary of the Invention

[0003] The purpose of the present invention is to provide an In single-atom catalyst and its preparation method and application. The indium single-atom catalyst of the present invention is used for electrocatalytic carbon dioxide reduction and has high catalytic activity, stability and formic acid selectivity.

[0004] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0005] The present invention provides an In single-atom catalyst, comprising a doped carbon substrate and In single atoms distributed on the doped carbon substrate; the doped carbon substrate is a porous two-dimensional sheet structure; the doped carbon substrate is an N-doped carbon substrate or an N and S co-doped carbon substrate; the In single atoms are bonded to the doping elements in the doped carbon substrate through coordination bonds.

[0006] Preferably, the content of In single atoms in the In single atom catalyst is 1 to 2 wt%.

[0007] Preferably, when the doped carbon substrate is an N-doped carbon substrate, the N content in the In single-atom catalyst is 12 to 14 at %;

[0008] When the doped carbon substrate is a N and S co-doped carbon substrate, the content of N in the In single atom catalyst is 12 to 14 at %, and the content of S is 0.1 to 0.3 at %.

[0009] Preferably, the average pore size of the In single atom catalyst is 5 to 11 nm, and the specific surface area is 250 to 600 m 2 / g.

[0010] The present invention provides a method for preparing the In single-atom catalyst described in the above scheme, comprising the following steps:

[0011] Ball milling the raw materials to obtain ball mill material; the raw materials include melamine, indium acetylacetonate and trimesic acid, or include melamine, indium acetylacetonate, trimesic acid and a sulfur source;

[0012] The ball mill material is subjected to a first calcination, a temperature increase, and a second calcination in sequence under a protective atmosphere to obtain the In single-atom catalyst; the temperature of the first calcination is 550-650° C.; the temperature of the second calcination is 800-1000° C.

[0013] Preferably, the holding time of the first calcination is 1.5 to 2.5 hours.

[0014] Preferably, the holding time of the second calcination is 0.5 to 1.5 hours.

[0015] Preferably, the mass ratio of melamine to indium acetylacetonate is 60:(1-5); the mass ratio of melamine to trimesic acid is (5-15):1; when the preparation raw materials include a sulfur source, the mass ratio of melamine to the sulfur source is 6:(1-2.5).

[0016] Preferably, after the second calcination, the calcined product is washed with HCl solution and HNO3 solution in sequence; the concentrations of the HCl solution and the HNO3 solution are both 1M.

[0017] The present invention provides the use of the In single-atom catalyst described in the above scheme or the In single-atom catalyst prepared by the preparation method described in the above scheme in electrocatalytic carbon dioxide reduction.

[0018] The present invention provides an In single-atom catalyst, comprising a doped carbon substrate and In single atoms distributed on the doped carbon substrate; the doped carbon substrate is a porous two-dimensional sheet structure; the doped carbon substrate is an N-doped carbon substrate or an N and S co-doped carbon substrate; the In single atoms are bound to the doping element in the doped carbon substrate via coordination bonds. Metallic In has high formic acid selectivity, and using the doped carbon substrate as a substrate can increase the rate of electron transfer in the CO2RR reaction. The carbon substrate of the present invention is a porous two-dimensional sheet structure with a larger specific surface area, which is conducive to exposing more In active sites, thereby improving the catalytic activity of the catalyst. In addition, the doped N element (or N and S elements) promotes the dispersion of the In single atoms and firmly anchors the In single atoms to form uniform, highly coordinated catalytic active centers, thereby improving the activity and stability of the catalyst.

[0019] The results of the embodiment show that the N-doped carbon In single-atom catalyst synthesized by the present invention has much higher CO2RR electrocatalytic activity than the In nanoparticle catalyst. The overpotential of the N-doped carbon In single-atom catalyst reaction is 0.61 V. At a reduction potential of -1.03 V (vs RHE), the formic acid Faradaic efficiency is 86.86%. After 12.5 hours of continuous operation, the average Faradaic efficiency of formic acid is 80.2%, and the catalytic activity is maintained at 92%.

[0020] Compared to single N-doping, the performance of the In single-atom catalyst co-doped with N and S carbon is further improved. At a reduction potential of -1.03V (vs RHE), the Faraday efficiency of formic acid reaches a maximum of 94.48%, an increase of 7.6% compared to the single-atom catalyst without S doping. It also has a wider electrochemical range, a Tafel slope of 162.2mv / dec, and better kinetic characteristics. The durability of the material is also enhanced. It can continue to work for 15 hours at the optimal performance potential and maintain a formic acid selectivity of more than 92.4%. The S element plays a role in regulating the electronegativity of the electrocatalytic active center, giving the catalyst better CO2RR electrocatalytic performance.

[0021] The present invention synthesizes In single-atom catalysts, thereby greatly reducing the amount of metal used and saving production costs while improving performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 AC-HAADF-STEM images of In SAs / NC and corresponding EELS mapping images of In, N, and C;

[0023] Figure 2 AFM images of In SAs / NC;

[0024] Figure 3Thermogravimetric curves of In SAs / NC, In NPs / C and NC;

[0025] Figure 4 TEM images of In SAs / NC, In NPs / C, and NC;

[0026] Figure 5 is the particle size statistics of In NPs / C;

[0027] Figure 6 XRD and Raman patterns of In SAs / NC and NC;

[0028] Figure 7 XRD comparison chart and Raman comparison chart of In SAs / NC and In NPs / C;

[0029] Figure 8 N2 adsorption-desorption isotherms of In SAs / NC, In NPs / C and NC;

[0030] Figure 9 are the pore size distribution curves of In SAs / NC, In NPs / C, and NC;

[0031] Figure 10 The XPS full spectrum and fine spectrum of In SAs / NC;

[0032] Figure 11 The liquid product of In SAs / SNC at the reduction potential of -1.03 V (vs RHE) 1 HNMR spectra and formic acid concentration in the product 1 HNMR standard curve;

[0033] Figure 12 Comparison of the CO2RR electrocatalytic performance of In SAs / NC, In NPs / C and NC;

[0034] Figure 13 Linear cyclic voltammetry test results of In SAs / NC, In NPs / C and NC;

[0035] Figure 14 The Tafel slope plots of In SAs / NC, InNPs / C, and NC were determined based on the current density of the main product, formic acid;

[0036] Figure 15 LSV curve of In SAs / NC in 0.5M KHCO3 solution saturated with N2 and CO2 and SCN of In SAs / NC - Toxicity test result diagram;

[0037] Figure 16 Figure 2 is the long-term stability test results of In SAs / NC, In NPs / C and NC;

[0038] Figure 17 Thermogravimetric analysis diagrams of In SAs / SNC-2 and SNC;

[0039] Figure 18 TEM images of In SAs / SNC and SNC with different sulfur contents;

[0040] Figure 19 XRD patterns and Raman patterns of In SAs / SNC with different sulfur contents;

[0041] Figure 20 N2 adsorption-desorption isotherms of In SAs / SNC with different sulfur contents;

[0042] Figure 21 is the pore size distribution curve of In SAs / SNC with different sulfur contents;

[0043] Figure 22 The XPS full spectrum and fine spectrum of In SAs / SNC-2;

[0044] Figure 23 The XPS full spectrum and fine spectrum of SNC;

[0045] Figure 24 AC-HAADF-STEM analysis results of In SAs / SNC-2;

[0046] Figure 25 AFM images of In SAs / SNC-2;

[0047] Figure 26 The X-ray near-edge absorption spectra of In K-edge and the extended X-ray absorption spectra of In K-edge Fourier transform of In SAs / SNC-2, In2O3 and indium foil are shown.

[0048] Figure 27 Figures 2 and 3 are the formic acid Faraday efficiency diagram and hydrogen Faraday efficiency diagram of In SAs / SNC with different S contents;

[0049] Figure 28 Comparison of the CO2RR electrocatalytic performance of In SAs / SNC-2 and blank control sample SNC;

[0050] Figure 29 LSV curves and formic acid current density diagrams of In SAs / SNC-2, In SAs / NC and SCN;

[0051] Figure 30are the Tafel slope curves of In SAs / SNC-2, In SAs / NC, and SNC;

[0052] Figure 31 LSV diagram of In SAs / SNC-2 in 0.5M N2 and CO2 saturated KHCO3 solution and SCN of In SAs / SNC-2 - Toxicity test result diagram;

[0053] Figure 32 The long-term stability test results of In SAs / SNC-2 and SNC are shown. DETAILED DESCRIPTION

[0054] The present invention provides an In single-atom catalyst, comprising a doped carbon substrate and In single atoms distributed on the doped carbon substrate; the doped carbon substrate is a porous two-dimensional sheet structure; the doped carbon substrate is an N-doped carbon substrate or an N and S co-doped carbon substrate; the In single atoms are bonded to the doping elements in the doped carbon substrate through coordination bonds.

[0055] In the present invention, the content of In single atoms in the In single atom catalyst is preferably 1 to 2 wt %, and in the embodiment of the present invention, specifically 1.06 wt % or 1.21 wt %.

[0056] In the present invention, when the doped carbon substrate is an N-doped carbon substrate, the N content in the In single-atom catalyst is preferably 12 to 14 at %, more preferably 12.5 to 13.5 at %. In the embodiment of the present invention, it is specifically 12.96 at %.

[0057] When the carbon substrate is a N and S co-doped carbon substrate, the N content in the In single-atom catalyst is preferably 12 to 14 at%, more preferably 12.5 to 13.6 at%. The S content is preferably 0.1 to 0.3 at%, more preferably 0.15 to 0.25 at%. In an embodiment of the present invention, the N content is specifically 13.53%, and the S content is 0.21 at%.

[0058] In the present invention, the In single-atom catalyst inevitably further contains O element.

[0059] In the present invention, the average pore size of the In single atom catalyst is preferably 5 to 11 nm, more preferably 6 to 8 nm; the specific surface area is preferably 250 to 600 m 2 / g, more preferably 400 to 580 m 2 / g, more preferably 520 to 570 m 2 / g.

[0060] In the present invention, the thickness of the doped carbon substrate is preferably 0.2 to 1 nm. Furthermore, the thickness of the N-doped carbon substrate is preferably 0.5 to 1 nm, more preferably 0.8 nm. The thickness of the N and S co-doped carbon substrate is preferably 0.2 to 0.5 nm, more preferably 0.43 nm.

[0061] In the present invention, the nitrogen is preferably present in the form of pyridinic nitrogen (398.2 eV), In-N (400.2 eV), pyrrolic nitrogen (400.5 eV), graphitic nitrogen (401.3 eV), and nitrogen oxide (403.0 eV). When the doped carbon substrate is an N-doped carbon substrate, the nitrogen is mainly present in the form of pyrrolic nitrogen; when the doped carbon substrate is a N and S co-doped carbon substrate, the nitrogen is mainly present in the form of pyridinic nitrogen. Pyridinic nitrogen has stronger activity and alkalinity than pyrrolic nitrogen and promotes the CO2RR electrocatalytic reaction.

[0062] In the present invention, the S is preferably -CS-, -C=S- and C-SO X It exists in the form of (x=2,3,4).

[0063] Metallic In has high formic acid selectivity, and using doped carbon as a substrate can increase the rate of electron transfer in the CO2RR reaction. The carbon substrate of the present invention has a porous two-dimensional sheet structure with a larger specific surface area, which is conducive to exposing more In active sites, thereby improving the catalytic activity of the catalyst. In addition, the doped N element (or N and S elements) promotes the dispersion of In single atoms and firmly anchors In single atoms to form uniform, highly coordinated catalytic active centers, thereby improving the activity and stability of the catalyst.

[0064] The present invention provides a method for preparing the In single-atom catalyst described in the above scheme, comprising the following steps:

[0065] Ball milling the raw materials to obtain ball mill material; the raw materials include melamine, indium acetylacetonate and trimesic acid, or include melamine, indium acetylacetonate, trimesic acid and a sulfur source;

[0066] The ball mill material is subjected to a first calcination, a temperature increase, and a second calcination in sequence under a protective atmosphere to obtain the In single-atom catalyst; the temperature of the first calcination is 600° C.; the temperature of the second calcination is 900° C.

[0067] In the present invention, unless otherwise specified, all raw materials used are commercially available products well known in the art.

[0068] The invention ball-mills the prepared raw materials to obtain ball-milled materials.

[0069] In the present invention, when the carbon substrate is N-doped, the raw materials include melamine, indium acetylacetonate, and trimesic acid. When the carbon substrate is N- and S-co-doped, the raw materials include melamine, indium acetylacetonate, trimesic acid, and a sulfur source.

[0070] In the present invention, the mass ratio of melamine to indium acetylacetonate is preferably 60:(1-5), more preferably 60:(2-3); the mass ratio of melamine to trimesic acid is preferably (5-15):1, more preferably 10:1; when the preparation raw materials include a sulfur source, the mass ratio of melamine to sulfur source is preferably 6:(1-2.5), more preferably 6:1, 6:1.5, 6:2 or 6:2.5. In the present invention, the sulfur source preferably includes L-cysteine. In the present invention, the trimesic acid acts as a plasticizer and crosslinking agent to promote the condensation of melamine to form carbon nitride (C3N4).

[0071] In the present invention, the ball milling is preferably wet ball milling; the grinding balls used in the wet ball milling are preferably zirconium beads; the ball milling medium is preferably anhydrous ethanol; and the wet ball milling time is preferably 0.5 to 3 hours. The present invention has no particular requirements for the ball milling speed; any ball milling speed known in the art will suffice. The present invention utilizes ball milling to achieve uniform mixing of the raw materials for preparation.

[0072] After the wet ball milling is completed, the present invention preferably dries the wet material to obtain a ball milled material. In the present invention, the drying temperature is preferably 80° C. and the drying time is preferably 3 hours.

[0073] After the ball mill material is obtained, the present invention sequentially performs a first calcination, a temperature increase, and a second calcination on the ball mill material under a protective atmosphere to obtain an In single-atom catalyst.

[0074] In the present invention, the protective atmosphere is preferably an Ar atmosphere. The temperature of the first calcination is 550-650°C, preferably 600°C, and the holding time is preferably 1.5-2.5h, more preferably 2h; the temperature of the second calcination is 800-1000°C, preferably 900°C, and the holding time is preferably 0.5-1.5h, more preferably 1h. In the present invention, the heating rate for heating to the temperature of the first calcination is 2-3°C / min; the heating rate for heating from the temperature of the first calcination to the temperature of the second calcination is preferably 2-3°C / min. In the present invention, the first calcination, heating and second calcination are preferably carried out in a tube furnace.

[0075] In the present invention, during the first calcination process, melamine decomposes into carbon nitride, and during the second calcination process, the carbon nitride decomposes to form a nitrogen-doped carbon substrate; when the preparation raw materials include a sulfur source, during the second calcination process, the sulfur source decomposes to form sulfur doping.

[0076] After the second calcination, the present invention sequentially washes the calcined product with an HCl solution and an HNO3 solution. Prior to washing, the present invention preferably grinds the calcined product. In the present invention, washing with an HCl solution preferably comprises placing the calcined product in an HCl solution and stirring at 80°C for 12 hours. After the HCl solution wash, the present invention preferably separates the solid sample from the hydrochloric acid solution by centrifugation and continues washing with an HNO3 solution. In the present invention, washing with an HNO3 solution preferably comprises placing the sample washed with the HCl solution in an HNO3 solution and stirring at 80°C for 24 hours. In the present invention, the concentrations of both the HCl solution and the HNO3 solution are preferably 1M. The present invention utilizes HCl and HNO3 solutions for washing to dissolve indium oxide and other unstable components that may be present in the calcined product.

[0077] After the HNO3 solution is cooled to room temperature, the present invention preferably separates the sample from the nitric acid solution by centrifugation, and then washes it several times with a large amount of pure water until the sample solution is neutral. The sample is placed in a vacuum oven at 80°C and dried for 24 hours to obtain the In single-atom catalyst.

[0078] The present invention provides the use of the In single-atom catalyst described in the above scheme or the In single-atom catalyst prepared by the preparation method described in the above scheme in electrocatalytic carbon dioxide reduction.

[0079] The In single-atom catalyst provided by the present invention, its preparation method and application are described in detail below with reference to the examples, but they should not be understood as limiting the scope of protection of the present invention.

[0080] Example 1

[0081] The preparation method of In SAs / NC single atom catalyst adopts the following steps:

[0082] (1) Take 6g of melamine (C3H6N6), 0.2484g of indium acetylacetonate (C 15 H 21 lnO6), 0.6g of trimesic acid (BTC) were placed in a ball mill, an appropriate amount of zirconium beads and 10mL of anhydrous ethanol were added, and after ball milling for 1 hour, the uniform milky white solution was poured out and placed in an 80℃ oven, kept warm for 3 hours and dried for use.

[0083] (2) The solid obtained in the above steps was placed in a porcelain boat, placed in a tube furnace, and carbonized by staged calcination in an Ar atmosphere: the temperature was increased from room temperature to 600°C at a heating rate of 3°C / min and retained for 2 h; then the temperature was increased to 900°C at a heating rate of 2°C / min and retained for 1 h; finally, the mixture was naturally cooled to room temperature.

[0084] (3) The calcined sample was ground and placed in a round-bottom flask. 20 mL of 1 M HCl solution was added and stirred at 80 °C for 12 h. The solid sample was separated from the hydrochloric acid solution by centrifugation. The sample and 20 mL of 1 M HNO3 were placed in a round-bottom flask again, heated to 80 °C, and stirred for 24 h.

[0085] (4) After the pickling solution has cooled to room temperature, separate the sample from the nitric acid solution by centrifugation and then wash it several times with plenty of pure water until the sample solution becomes neutral.

[0086] (5) The sample was placed in a vacuum oven at 80 °C and dried for 24 h to obtain an In single-atom catalyst, which was denoted as In SAs / NC.

[0087] Comparative Example 1

[0088] Preparation of catalyst InNPs / C:

[0089] In a ball mill, add 0.6 g of trimesic acid, 0.244 g of InCl3, and 5 mL of anhydrous ethanol, and ball mill for 2 hours. The mixture obtained after ball milling is placed in an 80°C oven to dry. Then, it is placed in a tube furnace and heated to 900°C at a rate of 5°C / min in an Ar atmosphere for carbonization for 2 hours. The mixture is cooled to room temperature to obtain an In nanoparticle catalyst, which is recorded as InNPs / C.

[0090] Comparative Example 2

[0091] Preparation of blank control NC:

[0092] 6 g of melamine and 0.6 g of trimesic acid were mixed uniformly by ball milling, and then heated to 900° C. at a rate of 5° C. / min in a tube furnace under an Ar atmosphere for carbonization for 2 h, and then cooled to room temperature to obtain a blank control sample NC.

[0093] The structure and performance characterization of Example 1 and Comparative Examples 1-2:

[0094] (1) Spherical aberration corrected scanning transmission electron microscopy (AC-HAADF-STEM)

[0095] The In SAs / NC of Example 1 was observed by scanning transmission electron microscopy with spherical aberration correction. Figure 1 As shown, (ab) are AC-HAADF-STEM images of In SAs / NC; (cd) are the corresponding EELS mapping images of In, N and C. Figure 1 It can be seen that In exists in the form of a single atom.

[0096] (2) Atomic force microscopy (AFM)

[0097] The In SAs / NC of Example 1 was observed by atomic force microscopy. Figure 2 As shown. Figure 2 It can be seen that InSAs / NC is a two-dimensional layered structure of a single atomic layer with a thickness of 0.8 nm.

[0098] (3) Thermogravimetric analysis (TG)

[0099] Thermogravimetric analysis of In SAs / NC, InNPs / C and NC was performed, and the results were as follows: Figure 3 As shown. Figure 3 As can be seen, the yield of NC is 0% at around 800°C, indicating that the carbon and nitrogen elements in the material have been completely lost at high temperatures. In SAs / NC, on the other hand, maintains a yield of approximately 10%, due to the presence of metallic In and the weight of the material produced by oxidation reactions. The final yield of In NPs / C is approximately 30%, three times that of In SAs / NC. The TG curves show that between 0 and 100°C, all three materials experience slight mass loss, a process characterized by water loss. Between 450 and 800°C, all three materials experience significant weight loss, attributed to the rapid decomposition of nitrogen and carbon elements.

[0100] (4) Transmission electron microscopy (TEM) characterization

[0101] Transmission electron microscopy observations of In SAs / NC, InNPs / C and NC were performed. Figure 4 shown. Figure 4 In the figure, (ac) are TEM images of InSAs / NC; (df) are TEM images of NC; (gi) are TEM images of InNPs / C. Figure 4 It can be seen that all three materials show the morphology of amorphous carbon because the carbonization temperature is not high enough to form an ordered graphite structure. Both In SAs / NC and NC materials have obvious porous structures. This is because the precursor melamine is rich in nitrogen. The porous morphology formed by the release of nitrogen during the calcination process is conducive to increasing the specific surface area of ​​the catalyst and exposing more active sites. InNPs / C is a carbon substrate loaded with In metal nanoparticles. Figure 5 The particle size statistics show that the average particle size of the In nanoparticles is 47.5 nm. Notably, the In SAs / NC catalyst shows no signs of metal clusters or particles, indicating that the metallic In content in the material is very low or that the metal exists in atomic form. Because the minimum size that a transmission electron microscope can capture is 2 nm, single atoms cannot be observed. To further confirm the presence of single atoms, spherical aberration-corrected electron microscopy (AC-HAADF-STEM) was performed. The presence of In in single atomic form has been previously confirmed.

[0102] (5) X-ray powder diffraction (XRD) and Raman spectroscopy (Raman) characterization

[0103] X-ray powder diffraction (XRD) and Raman spectroscopy were used to characterize In SAs / NC and NC. Figure 6 shown. Figure 6 In the figure, (a) is the XRD pattern of In SAs / NC and NC, and (b) is the Raman pattern of In SAs / NC and NC. In the XRD diffraction patterns of In SAs / NC and NC, only the 2θ angle has peak responses at around 25° and 44°, which are Bragg broad peaks. These are the diffraction peaks of the (100) and (002) planes of graphite phase carbon, corresponding to the peak positions of the standard card JCPDS NO.34-0567. It is worth noting that there are no diffraction peaks related to metal In in the XRD diffraction pattern of In SAs / NC, indicating that only a trace amount of metal exists or the metal is dispersed in the form of single atoms, which is consistent with the results of the TEM image. Observe the Raman spectra of In SAs / NC and N / C, where the D peak (1340cm -1 ) and G peak (1575cm -1 ) has the strongest response intensity. The D peak is the Raman resonance caused by scattering of defects in the material. The size of the defects and the degree of disorder of the material can be reflected by the D peak. The carbon atom sp 2 The in-plane vibration of the orbital produces the G peak, which is a reflection of the order of the catalyst graphitization. The ratio of the D peak to the G peak intensity is usually D / I G As a factor to measure the degree of graphitization of materials, the two are negatively correlated. D / I G I than N / C D / I G The value is slightly higher, indicating that the addition of metal In increases the surface defects of the material and reduces the degree of graphitization. This is consistent with the results of TEM images, which show that the morphology of In SAs / NC shows porous two-dimensional flake characteristics.

[0104] The XRD patterns of In SAs / NC and InNPs / C were compared. Figure 7 As shown in (a), it can be seen that In SAs / NC only has a peak signal of graphite phase carbon, and no metal peak is detected. In NPs / C, on the other hand, shows a very sharp signal peak, which matches the standard card PDF#06-041 for metal In, further confirming the presence of In metal in InNPs / C. The Raman patterns of InSAs / NC and InNPs / C are compared, and the results are shown in Figure 2. Figure 7 As shown in (b), the I of InNPs / C D / IG The value is 1.08, slightly larger than the I of In SAs / N D / I G The ratio indicates that In metal nanoparticles reduce the degree of graphitization of the material.

[0105] (6) N2 adsorption-desorption curve (BET)

[0106] The N2 adsorption-desorption curve (BET) reflects the specific surface area and pore size of the material. BET tests were performed on In SAs / NC, InNPs / C and NC, and the results are shown in Figure 8 . Figure 8 In the middle, (ac) are the N2 adsorption-desorption isotherms of In SAs / NC, InNPs / C and NC, respectively; Figure 8 The N2 adsorption and desorption curves of the three materials are all type IV, which is the characteristic linearity of mesoporous structure and slit structure. The specific surface area of ​​InSAs / NC is 554.6927m 2 / g, which is larger than the specific surface area of ​​InNPs / C (502.0633 m 2 / g, indicating that nitrogen doping will increase the specific surface area of ​​the material. The escape of N elements under high temperature carbonization leaves pores and defects, which leads to an increase in the specific surface area of ​​the material, which is consistent with the Raman results. A large specific surface area is conducive to the exposure of active sites and improves the performance of the material. The specific surface area of ​​the metal-free blank control sample NC is 78.14m 2 / g, which is much lower than the two materials with added metal In, which shows that the addition of metal greatly improves the specific surface area of ​​the material.

[0107] Figure 9 is the pore size distribution curve of In SAs / NC, InNPs / C and NC; Figure 9 The pore size distribution diagram shows that the average pore sizes of InSAs / NC, InNPs / C, and NC are 7.46 nm, 12.17 nm, and 14.73 nm, respectively. InSAs / NC has the smallest pore size and a more uniform pore distribution, which provides favorable conditions for electron and product transfer, increasing the reaction rate and enhancing the catalytic activity of CO₂RR.

[0108] (7) X-ray photoelectron spectroscopy analysis

[0109] X-ray photoelectron spectroscopy was used to further characterize In SAs / NC. Figure 10 shown. Figure 10In the figure, (a) XPS full spectrum of In SAs / NC; (b) N1s fine spectrum of In SAs / NC; (c) In 3d fine spectrum of In SAs / NC; (d) C1s fine spectrum of In SAs / NC. It can be seen that there are signal peaks of C, N, In, and O in the XPS full spectrum of In SAs / NC. The signal peak of O may come from the inevitable interference brought by moisture and oxygen in the air. The signal peak of metal In is weak, indicating that its content is very low, which is consistent with the results of XRD. From the fine spectrum of N1s, it can be seen that there are 5 types of N: pyridinic nitrogen (398.2eV), In-N (400.2eV), pyrrolic nitrogen (400.5eV), graphitic nitrogen (401.3eV), and oxidized nitrogen (403.0eV). The peak height of pyrrolic nitrogen is much higher than that of pyridinic nitrogen, indicating that the high-temperature carbonization process is conducive to the formation of pyrrolic nitrogen. As shown in the figure, Figure 10 As shown in (c), In 3d 5 / 2 The peak is located at 445.0eV, between In 0 (443.8 eV) and In 3+ (445.7eV), showing the valence state of metallic indium in In SAs / NC. δ+ (0<δ<3).

[0110] (8) Elemental analysis by inductively coupled plasma-mass spectrometry (ICP-MS)

[0111] The In elemental content of In SAs / NC, InNPs / C, and NC was precisely determined using ICP-MS. The atomic contents of In, C, and N were determined by X-ray photoelectron spectroscopy (XPS). The results are shown in Table 1. As shown in Table 1, the weight percent In content of In SAs / NC is 1.21 wt%, and the weight percent In content of In NPs / C is 5.9 wt%.

[0112] Table 1 Elemental content statistics of In SAs / NC, In NPs / C, and NC (oxygen is not listed)

[0113]

[0114] (9) CO2 reduction performance of In SAs / NC

[0115] Test method introduction:

[0116] All electrochemical measurements were performed at room temperature and atmospheric pressure using an electrochemical workstation CHI660. The tests were conducted in a double-channel H-type sealed electrolytic cell, with the anode and cathode separated by a Nafion-117 proton exchange membrane. A metal Pt sheet was used as the counter electrode, and a saturated calomel electrode was used as the reference electrode (SCE). The working electrode was a carbon fiber paper loaded with catalyst ink, with a catalyst loading area of ​​1×1 cm. 2 square.

[0117] Preparation of catalyst ink: Weigh 2.0 mg of catalyst into a 5 mL glass vial, add 300 μL of deionized water, 300 μL of anhydrous ethanol, and 10 μL of 5% Nafion solution. Place the mixed solution in an ultrasonic cleaner for 30 minutes to mix until homogenized. Apply the prepared catalyst ink dropwise onto a pre-cut 1×2 cm 2 The carbon paper was dried at 50 °C and the loading was 2 mg cm -2 .

[0118] The gaseous products (H2, CO) obtained after the carbon dioxide reduction reaction were quantified by gas chromatography. The gas chromatograph was equipped with a flame ionization detector (FID) for detecting and quantifying CO and a thermal conductivity detector (TCD) for detecting and quantifying H2. Ultrapure helium (He, 99.9999%) was used as the carrier gas for the gas chromatography. The average flow rate of carbon dioxide was controlled at 20cc min by a flow meter. -1 The gas chromatograph was calibrated using a standard gas mixture under standard conditions (1 atmosphere, 298K); the liquid product (HCOO - ) was added with dimethyl sulfoxide (DMSO) as an internal standard and detected by nuclear magnetic resonance (NMR) 1 HnmR) for detection, analysis and quantification.

[0119] The liquid products of the electrocatalytic CO2RR in this experiment are all formic acid. Figure 11 (a) shows the liquid product of In SAs / SNC at a reduction potential of -1.03 V (vs RHE) 1 HNMR graph. The water peak at the chemical shift of 4.69 ppm has been suppressed, and 8.29 ppm is HCOO - The chemical shift of the internal standard DMSO is at 2.57 ppm. Figure 11 (b) is a gradient of sodium formate solution and DMSO as internal standard, which is used to determine the concentration of formic acid in the product. 1 The HNMR standard curve has an accuracy of 0.9991, indicating that the test error is very small.

[0120] Carbon dioxide reduction performance test:

[0121] At room temperature and pressure, the electrochemical test was carried out in a 0.5M KHCO3 solution as the electrolyte. The electrolyte was injected into both chambers of the H-type electrolytic cell, and the liquid needed to cover the proton exchange membrane. Before each test, CO2 gas was continuously passed into the electrolyte for 30 minutes to saturate the electrolyte solution with CO2. Before the CO2RR test, the working electrode was first activated by cyclic voltammetry under CO2 saturation conditions. The activation time was 1 hour, the voltage range was set to -0.8 to -2.0 V (vs. SCE), and the scan rate was 50 mV / s. The scan rate of the linear sweep voltammetry test (LSV) was 1 MVs. -1 Throughout the electrochemical test, CO2 gas was introduced into the electrolyte at a steady flow rate to provide sufficient CO2. The pH value of the system was measured to be 7.2 using a pH meter. At 25°C, all working potentials can be converted to reversible hydrogen electrode (RHE) potentials using the Nernst equation:

[0122] The Faraday efficiency of the gas phase product is calculated based on the gas concentration (ppm) measured by the gas chromatograph (GC). The calculation formula is:

[0123]

[0124] Where ν is the gas flow rate measured at room temperature at the outlet using a flow meter, y is the measured product volume concentration, N = 2 is the number of electrons required to form a CO or H2 molecule, and F is the Faraday constant (96,500 C·mol -1 ), j is the corresponding current density.

[0125] The concentration of the liquid phase product was determined by NMR spectrometry ( 1 The solution was measured with DMSO as the internal standard and the standard curve was calculated with a series of gradient concentrations of potassium bicarbonate solution as the standard solution. The Faraday efficiency of the liquid product was calculated as follows:

[0126]

[0127] where n HCOO - is the formic acid content measured in the cathode chamber, and t is the reaction time. By multiplying the overall geometric current density with its corresponding Faradaic efficiency, HCOO at different potentials can be calculated. - The current density of .

[0128] The conversion frequency (TOF) of carbon dioxide reduction to formic acid can be calculated by the following formula:

[0129]

[0130] is the formic acid fraction, and F is the Faraday constant (96,500 C mol -1 ). N is the number of electrons transferred to form the product. ω is the metal loading in the catalyst. is the mass of the catalyst in the electrode. M metal = atomic mass of In (114.8 gmol -1 ).

[0131] Figure 12 Comparison of the electrocatalytic performance of CO2RR of In SAs / NC, InNPs / C and NC, including (a) the Faraday efficiency of In SAs / NC, InNPs / C and NC for formic acid; (b) the Faraday efficiency of In SAs / NC, InNPs / C and NC for hydrogen. Figure 12 (a) The Faraday efficiency of formic acid shows that the formic acid Faraday efficiency of the single-atom catalyst In SAs / NC is generally greater than that of the metal nanoparticle catalyst InNPs / C. At low potentials, the formic acid Faraday efficiency of both increases with the negative shift of the applied potential. At -1.13V (vs RHE), InNPs / C has the highest formic acid Faraday efficiency of only 36.2%, while In SAs / NC has the highest formic acid Faraday efficiency of 86.86% at -1.03V, which is more than twice that of InNPs / C. Figure 12 The hydrogen Faradaic efficiency plot in (b) also shows that the overall Faradaic efficiency of hydrogen production from In SAs / NC is lower than that from InNPs / C, and increases more slowly. With In NPs / C, the HER reaction gradually becomes dominant as the voltage increases, while the yield of formic acid decreases. The metal-free blank control, NC, produces almost no formic acid, and even no formic acid is detected in the liquid product at low potentials. The NC product is primarily H2.

[0132] In summary, In SAs / NC performs best, followed by InNPs / C, and finally NC. The addition of metallic In is responsible for the material's formic acid selectivity and effective suppression of the HER reaction. The size of the metallic In significantly influences the catalyst's performance. When the size of metallic In is minimized, existing as a single atom, its performance significantly improves compared to metal nanoparticles, while also providing strong suppression of the HER reaction.

[0133] The reaction overpotential (η) is a key indicator of CO2RR catalytic performance. Under ideal conditions, the potential required for a catalyst to undergo a reduction reaction is the theoretical reduction potential. However, in practice, due to kinetic obstacles, the potential required for the actual reaction is often more negative than the theoretical value. The difference between the actual working potential required for the reaction and the calculated theoretical potential is called the overpotential, which is used to measure the kinetic difficulty of the reaction. The smaller the overpotential, the better the catalytic activity of the catalyst.

[0134] Figure 13 The linear cyclic voltammetry test results of In SAs / NC, InNPs / C and NC, including (a) LSV diagram of In SAs / NC, InNPs / C and NC in 0.5M saturated KHCO3 solution, (b) current density diagram of In SAs / NC, In NPs / C and NC at different potentials. The main product obtained by the In-based electrocatalyst synthesized in the present invention after reducing CO2 is HCOOH, and the theoretical standard electrode potential is -0.61V (vs. SHE), corresponding to -0.37V (vs. RHE). Figure 13 From the LSV curve in (a), we can see that when the current density reaches 1mA / cm -2 When the voltage required for In SAs / NC is -0.26 V (vs RHE), the voltage required for InNPs / C is -0.31 V (vs RHE), and the voltage required for NC is -0.44 V (vs RHE). In other words, In SAs / NC has the lowest catalytic starting point and higher catalytic activity, followed by InNPs / C, and the voltage required for NC is the most negative. When the current density continues to increase to 10 mA / cm -2 When the reduction potential of In SAs / NC is -0.98 V (vs RHE), the reduction potential of InNPs / C is -1.03 V (vs RHE), and the reduction potential of NC is -1.07 V (vs RHE). The overpotentials (η) of the reactions are 0.61 V, 0.66 V, and 0.7 V, respectively. The overpotential of In SAs / NC is the smallest. Figure 13 The partial current density plot for formic acid in (b) shows that the current density for formic acid production using In SAs / NC increases significantly after the potential exceeds -0.6 V (vs RHE), far exceeding that of InNPs / C and approximately twice that of InNPs / C. The Faradaic efficiency of NC for formic acid production is extremely low, resulting in a partial current density close to zero. This demonstrates that during the CO₂RR electrocatalytic reaction, the majority of electron transfer on the In SAs / NC catalyst is used to produce formic acid, demonstrating high formic acid selectivity.

[0135] Figure 14 Figure 3 shows the Tafel slopes of In SAs / NC, InNPs / C, and NC, determined by the current density of the main product, formic acid. The values ​​are 199.9 mv / dec, 230.4 mv / dec, and 322.4 mv / dec, respectively. As can be seen, In SAs / NC has the smallest Tafel slope, indicating a higher reaction rate when catalyzing CO₂ to HCOOH.

[0136] Figure 15LSV curves of In SAs / NC in 0.5 M KHCO3 solution saturated with N2 and CO2 (a) and SCN of In SAs / NC - Poisoning test results (b). Figure 15 As shown in (a), under N2 saturation conditions, the current density decayed, indicating that the CO2RR electrocatalytic reaction occurred in the system. - It is easy to bind tightly to the metal center, causing the metal active site to be deactivated, and is used to determine the reaction active site of the catalyst. As shown in (b), when SCN is added to the electrolyte, - After that, the current density of the reaction decayed significantly, indicating that the active sites of the catalyst In SAs / NC originated from the In metal center.

[0137] Long stability test:

[0138] In addition to performance evaluation criteria such as Faraday efficiency and electrical density, long-term stability is also an important indicator for measuring catalyst performance for CO2RR electrocatalysts. Especially in industrial applications, a catalyst with strong stability can effectively reduce production costs and shorten production cycles, which is of great significance in practical applications. The peak Faraday efficiency of formic acid of In SAs / NC is at an applied potential of -1.03V (vs RHE), the efficiency of InNPs / C in producing formic acid is the highest at -1.13V (vs RHE), and the Faraday efficiency of NC in producing formic acid is the highest at -1.23V (vs RHE). Therefore, these three potentials were selected to test the stability of CO2RR electrocatalysis of these three catalysts, and the liquid products were collected at the end to calculate the Faraday efficiency of formic acid. The results are as follows: Figure 16 As shown. The long-term stability test of In SAs / NC was carried out for 12.5 hours, and the average Faradaic efficiency of formic acid was 80.2%, and 92% of the catalytic activity was maintained. The long-term stability test of InNPs / C was carried out for 6 hours, and the average Faradaic efficiency of formic acid was 29.6%, and 81% of the catalytic activity was maintained. The long-term stability test of NC was carried out for 5.5 hours, and the Faradaic efficiency of formic acid was very low and could be ignored. In summary, the stability of In single-atom catalysts In SAs / NC is higher than that of In nanoparticles InNPs / C. The active sites of single atoms are relatively firm and stable. After catalytic reduction of CO2 to produce HCOOH for up to 12.5 hours, the Faradaic efficiency of formic acid only slightly decayed, which shows that the material has high applicability.

[0139] Examples 2 to 5

[0140] (1) Take different masses (1g, 1.5g, 2g, 2.5g) of L-cysteine ​​(C3H7NO2S), 6g of melamine, 0.2484g of indium acetylacetonate (C 15H 21 lnO6), 0.6g of trimesic acid were placed in a ball mill, an appropriate amount of zirconium beads and 10mL of anhydrous ethanol were added, and after ball milling for 1 hour, the uniform milky white solution was poured out and placed in an 80℃ oven, kept warm for 3 hours and dried for use.

[0141] (2) The solid obtained in the above steps was placed in a porcelain boat, placed in a tube furnace, and carbonized by staged calcination in an Ar atmosphere: the temperature was increased from room temperature to 600°C at a heating rate of 3°C / min, retained for 2 h, then increased to 900°C at a heating rate of 2°C / min, retained for 1 h; finally, it was naturally cooled to room temperature.

[0142] (3) The calcined sample was ground and placed in a round-bottom flask. 20 mL of 1 M HCl solution was added and stirred at 80 °C for 12 h. The solid sample was separated from the hydrochloric acid solution by centrifugation. The sample and 20 mL of 1 M HNO3 were placed in a round-bottom flask again, heated to 80 °C, and stirred for 24 h.

[0143] (4) After the pickling solution has cooled to room temperature, separate the sample from the nitric acid solution by centrifugation and then wash it several times with plenty of pure water until the sample solution becomes neutral.

[0144] (5) The samples were placed in a vacuum oven at 80°C and dried for 24 h to obtain In SAs / SNC-1, In SAs / SNC-2, In SAs / SNC-3, and In SAs / SNC-4, respectively.

[0145] Comparative Example 3

[0146] Preparation of blank control SNC:

[0147] 1.5 g of L-cysteine, 6 g of melamine, and 0.6 g of trimesic acid were mixed uniformly by ball milling. The mixture was then heated to 900 °C at a rate of 5 °C / min in an Ar atmosphere in a tube furnace and carbonized for 2 h. The mixture was then cooled to room temperature to obtain a blank control sample SNC.

[0148] The structures and performance characteristics of Examples 2 to 5 and Comparative Example 3 are as follows:

[0149] (1) Elemental analysis by inductively coupled plasma-mass spectrometry (ICP-MS)

[0150] Table 2 summarizes the elemental content of In SAs / SNC-2 and SNC. The indium metal content of In SAs / SNC-2 was accurately measured by ICP-MS, yielding a weight fraction of 1.06 wt%. Combined with Table 1, the indium content of In SAs / NC is slightly higher than that of In SAs / SNC-2. The atomic percentages of sulfur, carbon, nitrogen, and indium were determined by X-ray photoelectron spectroscopy (XPS). The indium atomic ratio of In SAs / SNC-2 is 0.26%, consistent with the ICP-MS results. The sulfur content of In SAs / SNC-2 is 0.21%, while the SNC blank control sample has a low sulfur content of 0.14%. During high-temperature calcination, a large amount of sulfur evaporates with the gas.

[0151] Table 2 Elemental content statistics of In SAs / SNC-2 and SNC (O element is not listed)

[0152]

[0153] (2) Thermogravimetric (TG) analysis

[0154] Figure 17 Thermogravimetric (TG) analysis of In SAs / SNC-2 and SNC reveals that the yield of SNC is 0% at around 800°C due to the complete release of S, C, and N at high temperatures. In SAs / SNC-2, however, maintains a yield of 4% due to the presence of metals. The TG curves show that both materials experience water and weight loss between 0 and 100°C. Between 450 and 800°C, both In SAs / SNC-2 and SNC undergo rapid decomposition, with significant release of S, N, and C, leading to a sharp drop in weight.

[0155] (3) Transmission electron microscopy (TEM) characterization

[0156] Figure 18 Figure 1 shows TEM images of In SAs / SNC and SNC with varying sulfur contents. (ac)(df)(gi)(jl)(mo) are TEM images of In SAs / SNC-1, In SAs / SNC-2, In SAs / SNC-3, In SAs / SNC-4, and SNC, respectively. Due to insufficient carbonization temperature, the catalysts exhibit a low degree of graphitization, exhibiting a two-dimensional, flake-like morphology. The catalyst surfaces are porous and fragmented, a result of defects formed by the continuous release of heteroatoms such as nitrogen and sulfur during the calcination process. This results in a large surface area, exposing more metal active sites and significantly enhancing the material's catalytic performance.

[0157] It is worth noting that the In SAs / SNC catalyst only has the morphology of amorphous carbon, and no metal particles are found at the highest magnification, indicating that the content of metallic In is very low or exists in the form of single atoms.

[0158] (4) X-ray powder diffraction (XRD) and Raman spectroscopy characterization

[0159] Figure 19 (a) shows the XRD diffraction patterns of In SAs / SNCs with varying sulfur contents. Only the diffraction peaks of the carbon 100 and 002 planes are visible, with no other peaks present. As previously mentioned, ICP-MS analysis revealed a metallic In content of only 1.06 wt% in the In SAs / SNCs, and TEM images show no metallic particles, consistent with the XRD results.

[0160] Figure 19 (b) is the Raman spectra of In SAs / SNC and SNC. With the increase of sulfur content, the I D / I G The value is also increasing, and the I D / I G =1.01Ratio of In SAs / NC D / I G The value is slightly higher, indicating that the addition of S reduces the graphitization degree of the catalyst, which is consistent with the broken and porous morphology of the material surface in the TEM image. It is worth noting that the I D / I G The value is higher than that of NC, indicating that S and N co-doping reduces the graphitization degree of the material.

[0161] (5) BET characterization

[0162] BET characterization of In SAs / SNC with different sulfur contents is shown in Figure 20 .from Figure 20 It can be seen that the specific surface areas of In SAs / SNC with different sulfur contents are larger than that of SNC, among which In SAs / SNC-2 has the largest BET specific surface area, which is 552.3781 m 2 / g, much larger than the specific surface area of ​​SNC 160.987m 2 / g, indicating that the addition of metal In can greatly increase the specific surface area of ​​the catalyst. Figure 21Figure 2 shows the average pore size on the surface of In SAs / SNCs with different sulfur contents. It can be seen that the pore size decreases with increasing sulfur content. The pore size of In SAs / SNC-2 is 6.5 nm, approximately half that of SNCs. Metallic In catalyzes the formation of fine pores in graphitic carbon, creating a fragmented morphology. This facilitates electron transport during the reduction reaction and enhances the catalytic activity of the material.

[0163] (6) X-ray photoelectron spectroscopy (XPS) characterization

[0164] X-ray photoelectron spectroscopy (XPS) was performed on In SAs / SNC-2, and the results were as follows: Figure 22 As shown, among them, (a) XPS full spectrum of InSAs / SNC-2; (b) N1s fine spectrum of In SAs / SNC-2; (c) In 3d fine spectrum of In SAs / SNC-2; (d) S 2p fine spectrum of In SAs / SNC-2; (e) C1s fine spectrum of In SAs / SNC-2. From (a) the full XPS spectrum of In SAs / SNC-2, we can see that there are signal peaks of C, N, O, S, and In, among which O is the inevitable pollution brought by air and moisture in the environment. (b) From the fine spectrum of N, we can see that N exists in five forms, namely nitrogen oxide, graphitic nitrogen, pyrrolic nitrogen, In-N, and pyridinic nitrogen. (c) is the fine spectrum of In, In 3d 5 / 2 The peak position is between In 0 and In 3+ (d) is the fine spectrum of S, which shows that the coordination environment of S is relatively complex. There are three types of sulfur coordination compounds. The peaks at 163-165eV are generated by the covalent bonds of -CS- and -C=S-, while the peak at around 168eV corresponds to sulfates at the edge of graphene (C-SO X , x=2,3,4); the peak of In-S bond at 162.0 eV indicates that metal In is coordinated with N and S at the same time.

[0165] The SNC was characterized by X-ray photoelectron spectroscopy (XPS). Figure 23, including: (a) XPS full spectrum of SNC; (b) N1s fine spectrum of SNC; (c) S 2p fine spectrum of SNC; (d) C1s fine spectrum of SNC. From the XPS analysis spectrum of SNC, it can be seen that only C, N, and O can be seen in the full spectrum, while the S content is too low, and the peak of the S element is almost invisible in the full spectrum. Quantitative analysis shows that the S content in SNC is 0.14at% (Table 2). From the N1s fine spectrum of SNC, it can be seen that there are four types of nitrogen, namely oxidized nitrogen, graphitic nitrogen, pyrrolic nitrogen, and pyridinic nitrogen. It is worth noting that in the N1s spectrum of In SAs / SNC, the peak intensity of pyridinic nitrogen is much higher than that of pyrrolic nitrogen. The pyrrolic nitrogen peak of SNC is higher than that of pyridinic nitrogen, which indicates that the introduction of metal In promotes the formation of more pyridinic nitrogen. Pyridinic nitrogen has stronger activity and alkalinity than pyrrolic nitrogen, and has a promoting effect on the CO2RR electrocatalytic reaction.

[0166] (8) Spherical aberration corrected scanning transmission electron microscopy (AC-HAADF-STEM) analysis

[0167] AC-HAADF-STEM was used to further confirm the morphology, atomic state and distribution of In SAs / SNC-2. Figure 24 , where (ab) AC-HAADF-STEM images of In SAs / SNC-2; (cd) corresponding EELS mapping images of In, N, and S; (f) intensity distribution of atomic signals along the XY line in (e). Figure 24 The bright spots circled in red in (ab) are metal In. It can be clearly seen that the metal In is evenly dispersed in the material. Figure 24 Figures (cd) show the elemental distribution of In, N, and S, obtained by electron energy loss spectroscopy (EELS) analysis of In SAs / NCs. This figure demonstrates uniform distribution of In. The aforementioned XRD and TEM analysis results both indicate the absence of metal or metal oxides in the catalyst. Combined with the AC-HAADF-STEM image, it is clear that metallic In exists in a single atomic form. Figure 24 (f) is based on Figure 24 (e) Atomic intensity distribution along the XY direction shows that In atoms are distributed at a distance of at least 0.303, which is larger than the effective diameter between atoms, further confirming that metallic In exists in the form of single atoms.

[0168] (9) Atomic force microscopy was used to further determine the surface morphology and thickness of In SAs / SNC-2. Figure 25 .Depend on Figure 25 It was found that In SAs / SNC-2 was in flake form, which was consistent with the results of TEM test, and its thickness was 0.43 nm.

[0169] (10) X-ray absorption fine structure spectroscopy (XAFS) was used to study the electronic structure and coordination of metal In in single-atom indium catalysts. Figure 26 . Figure 26 (a) depicts the X-Near absorption spectrum (XANES) of InSAs / SNC-2, In2O3, and indium foil. The absorption edge of In in SAs / SNC is between that of indium foil and In2O3, indicating that In has a unique electronic structure. δ+ (0<δ<3), which may be due to the unique coordination environment of the catalyst. Figure 26 (b) is the extended X-ray absorption spectrum of In K-edge Fourier transform. There is a strong absorption peak at , indicating that most of the In in In SAs / NC is coordinated with N and exists in the form of In-N. In addition, there is no In-In bond (located at The presence of ions (approximately 100 nm) in the single-atom catalysts confirms the monodisperse presence of In in both samples, with N providing anchoring sites and an In-N coordination structure. To further confirm the dispersed sites of metal atoms in the single-atom catalysts, the present invention employed wavelet transform (high resolution in both k-space and R-space) for analysis, detecting extremely strong In-N signals in both In SAs / SNCs and In SAs / NCs, consistent with the results discussed above.

[0170] (11) CO2 reduction performance of In SAs / SNC

[0171] Referring to the carbon dioxide reduction performance test method of In SAs / NC, the CO2RR electrocatalytic performance of In SAs / SNC with different S contents and blank control sample SNC was tested. The results are shown in Figure 27 The amount of sulfur introduced into the In SAs / SNC-2 catalyst is optimal, which can maximize the formic acid selectivity and catalytic activity of the In single atom catalyst.

[0172] In SAs / SNC-2 has the best CO2RR electrocatalytic performance. Figure 28(a) shows a comparison of the CO₂RR electrocatalytic performance of In SAs / SNC-2 and the blank control (SNC). It can be seen that the product of In SAs / SNC-2 is primarily formic acid. With increasing negative potential, the Faradaic efficiency of formic acid increases, reaching a peak of 94.48% at -1.03 V (vs RHE). The formic acid yield remains above 85% between -1.03 V and 1.23 V (vs RHE). In contrast, the blank control (SNC) exhibits almost zero Faradaic efficiency for formic acid, lacking metallic In. Figure 28 As shown in (b), the primary product of SNC is hydrogen. The Faradaic efficiency of hydrogen increases with increasing applied voltage, reaching 66.7% at -1.23 V (vs RHE). This indicates that the catalytic activity of In SAs / SNC-2 originates from the In metal sites, which contribute to the material's formic acid selectivity.

[0173] In order to compare the CO2RR catalytic activity of the catalysts, the linear voltammetric scan was performed in 0.5 mM KHCO3 solution at a scan rate of 1 mV / s to measure the catalytic line voltage (LSV). Figure 29 As shown in (a), the sulfur-doped In single-atom catalyst In SAs / SNC-2 has the lowest starting point, with a current density of 1 mA cm at a potential of -0.33 V (vs RHE). -2 , followed by In SAs / NC, which can only reach the same current density at a reduction potential of -0.45V (vs RHE), while SNC has the highest starting point, reaching -0.62V (vs RHE). This shows that the addition of metal In can significantly increase the reaction starting potential of the catalyst, and sulfur doping also enhances the catalytic activity of the material. As the applied potential shifts negatively, the current density reaches 10mAcm -2 When the reduction potentials of the catalysts In SAs / SNC-2, In SAs / NC and SCN were -0.95V (vs RHE), -0.98V (vs RHE) and -1.09V (vs RHE), the catalytic overpotentials of the three were 0.58V, 0.61V and 0.72V respectively. That is, the blank control sample SNC had the highest overpotential and the lowest catalytic activity, indicating that metal In can significantly increase the current density of the catalyst and improve the catalytic activity. In addition, the addition of sulfur reduces the catalytic overpotential of the material and can further enhance the catalytic activity of the In single atom catalyst. Figure 29 From the formic acid partial current density diagram of the catalyst in (b), it can be seen that the formic acid selectivity and partial current density of In SAs / SNC-2 increase with the negative shift of the reduction potential. When the potential is greater than -0.95 V (vs RHE), the partial current density of formic acid far exceeds that of InSAs / NC.

[0174] Figure 30 is the Tafel slope curve of In SAs / SNC-2, In SAs / NC and SNC, Figure 30 It can be seen that the Tafel slopes of the three are 162.2mv / dec, 199.9mv / dec and 374.3mv / dec, respectively. That is, the sulfur-modified In single-atom catalyst In SAs / SNC-2 has the lowest Tafel slope compared with the others, indicating that its CO2RR electrocatalytic reaction rate is faster and has better reaction kinetic characteristics.

[0175] Figure 31 (a) is the LSV diagram of In SAs / SNC-2 in 0.5M N2 and CO2 saturated KHCO3 solution. It can be seen that under CO2 saturated conditions, the current density of the catalyst is greater than that under N2 saturated conditions, indicating that In SAs / SNC-2 as an electrocatalyst working system undergoes CO2RR reaction. As we all know, SCN - It has a strong ability to bind to metals and can easily poison metal-based catalysts, causing catalyst deactivation. Figure 31 (b) shows that SCN is added to the electrolyte - After that, the current density of the catalyst decreased significantly, indicating that the catalytic activity of In SAs / SNC-2 was mainly contributed by metal In.

[0176] Long-term stability tests were conducted on In SAs / SNC-2 and SNC to evaluate the catalysts more comprehensively. The above experiments show that the formic acid Faradaic efficiency of In SAs / SNC-2 reaches a peak of 94.48% at a reduction potential of -1.03 V (vs RHE), which shows the best performance. Therefore, this potential was selected to conduct long-term stability tests in 0.5M KHCO3 solution. The results are shown in Figure 2. Figure 32 .from Figure 32 As shown in (a), the In SAs / SNC-2 system maintained stable operation for approximately 15 hours. The liquid product was collected and calculated to have an average Faradaic efficiency of 87.3% for formic acid, maintaining a selectivity of 92.4%. The SNC system maintained long-term stability for approximately 6 hours, less than half the time of the In SAs / SNC-2 system.

[0177] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. An In single-atom catalyst, characterized in that It comprises a doped carbon substrate and In single atoms distributed on the doped carbon substrate; the doped carbon substrate is a porous two-dimensional sheet structure; the doped carbon substrate is an N-doped carbon substrate or an N and S co-doped carbon substrate; the In single atoms are bonded to the doping elements in the doped carbon substrate through coordination bonds; The preparation method of the In single-atom catalyst comprises the following steps: Ball milling the raw materials to obtain ball mill material; the raw materials include melamine, indium acetylacetonate and trimesic acid, or include melamine, indium acetylacetonate, trimesic acid and a sulfur source; The ball mill material is subjected to a first calcination, a temperature increase, and a second calcination in sequence under a protective atmosphere to obtain the In single-atom catalyst; the temperature of the first calcination is 550-650°C; the temperature of the second calcination is 800-1000°C; the holding time of the first calcination is 1.5-2.5 hours; the holding time of the second calcination is 0.5-1.5 hours; the mass ratio of melamine to indium acetylacetonate is 60:(1-5); the mass ratio of melamine to trimesic acid is (5-15):1; when the preparation raw materials include a sulfur source, the mass ratio of melamine to sulfur source is 6:(1-2.5).

2. The In single-atom catalyst according to claim 1, characterized in that The content of In single atoms in the In single atom catalyst is 1-2 wt %.

3. The In single-atom catalyst according to claim 1 or 2, characterized in that When the doped carbon substrate is an N-doped carbon substrate, the N content in the In single-atom catalyst is 12 to 14 at %; When the doped carbon substrate is a N and S co-doped carbon substrate, the content of N in the In single atom catalyst is 12 to 14 at %, and the content of S is 0.1 to 0.3 at %.

4. The In single-atom catalyst according to claim 1, characterized in that The average pore size of the In single-atom catalyst is 5 to 11 nm, and the specific surface area is 250 to 600 m 2 / g.

5. The method for preparing the In single-atom catalyst according to any one of claims 1 to 4, comprising the following steps: Ball milling the raw materials to obtain ball mill material; the raw materials include melamine, indium acetylacetonate and trimesic acid, or include melamine, indium acetylacetonate, trimesic acid and a sulfur source; The ball mill material is subjected to a first calcination, a temperature increase, and a second calcination in sequence under a protective atmosphere to obtain the In single-atom catalyst; the temperature of the first calcination is 550-650°C; the temperature of the second calcination is 800-1000°C; the holding time of the first calcination is 1.5-2.5 hours; the holding time of the second calcination is 0.5-1.5 hours; the mass ratio of melamine to indium acetylacetonate is 60:(1-5); the mass ratio of melamine to trimesic acid is (5-15):1; when the preparation raw materials include a sulfur source, the mass ratio of melamine to sulfur source is 6:(1-2.5).

6. The preparation method according to claim 5, characterized in that After the second calcination, the calcined product is washed with HCl solution and HNO3 solution in sequence; the concentrations of the HCl solution and the HNO3 solution are both 1M.

7. Use of the In single-atom catalyst according to any one of claims 1 to 4 or the In single-atom catalyst prepared by the preparation method according to any one of claims 5 to 6 in electrocatalytic carbon dioxide reduction.

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