Catalyst for preparing high-purity methane by reducing high-selectivity carbon dioxide and preparation method of catalyst

Abstract

The invention discloses a catalyst for preparing high-purity methane through high-selectivity carbon dioxide reduction and a preparation method of the catalyst, and belongs to the technical field of photocatalysis. In order to solve the industrial common problems of low efficiency, poor selectivity and the like of a traditional cadmium sulfide (CdS) photocatalyst in the CO2 reduction process, the invention innovatively provides a defect anchoring strategy: CdS rich in sulfur vacancies is taken as a carrier, the vacancies are taken as anchoring sites, Ru atoms are anchored to positions adjacent to the sulfur vacancies, and a stable Ru-S4 coordination structure is formed. The structure optimizes the adsorption energy of a peer-to-peer key intermediate and promotes deep reduction of CO2. The catalyst prepared by the preparation method disclosed by the invention not only completely inhibits hydrogen evolution reaction (HER) in photocatalytic conversion reaction of CO2 and H2O, but also greatly inhibits generation of a shallow reduction product CO, and the selectivity of a target product CH4 is greatly improved to 99% from about 10% of a traditional CdS system. According to the invention, high-selectivity CO2 methanation is realized, and a brand new technical path is provided for preparation of high-purity solar fuel.

Description

Technical Field

[0001] This invention relates to the field of photocatalyst technology, and more particularly to a catalyst for the highly selective reduction of carbon dioxide to prepare high-purity methane and its preparation method, especially its application in photocatalytic carbon dioxide reduction. Background Technology

[0002] With the acceleration of global industrialization, atmospheric CO2 concentrations continue to rise, and the resulting greenhouse effect and climate change have become major environmental challenges facing humanity. Utilizing solar-driven photocatalysis technology to convert CO2 into high-value-added chemicals and fuels can not only effectively reduce carbon emissions but also achieve the chemical storage of solar energy.

[0003] Among the many CO2 reduction products, methane (CH4) is considered one of the most promising target products due to its high calorific value, ease of storage and transportation, and direct connection to existing natural gas pipeline networks. However, the CO2 molecule has high thermodynamic stability (C=O bond energy of approximately 750 kJ / mol), and its complete reduction to CH4 requires the transfer of 8 electrons and 8 protons, resulting in extremely slow reaction kinetics. Furthermore, the photocatalytic CO2 reduction process is often accompanied by the formation of various byproducts (such as CO, HCOOH, CH3OH, etc.), leading to a significantly low CH4 selectivity, which severely restricts the practical application of this technology.

[0004] Crucially, in a photocatalytic system using H2O as both electron and proton donors, the hydrogen evolution reaction (HER: 2H+) is significantly enhanced. + +2e - →H2), as a thermodynamically more favorable competing pathway, often consumes a large number of photogenerated electrons, severely weakening the efficiency and selectivity of CO2 reduction. Thermodynamic analysis shows that H... + The standard reduction potential (E) for H2 is reduced to H2. 0 =0 V, NHE, pH=0) is significantly higher than the potential for CO2 reduction to CH4 (E 0 =-0.24 V, NHE, pH=0), and the hydrogen evolution reaction involves only two electron transfers, giving it a more prominent kinetic advantage. Therefore, how to effectively suppress hydrogen evolution side reactions and achieve the directional transfer of photogenerated electrons to the CO2 reduction pathway has become the core challenge in the field of photocatalytic CO2 reduction.

[0005] Cadmium sulfide (CdS), as a typical visible-light-responsive semiconductor, is widely used in photocatalysis due to its suitable band gap (approximately 2.4 eV) and conduction band position (approximately -0.9 V, NHE). However, conventional CdS has significant drawbacks in CO2 photoreduction: severe recombination of photogenerated carriers leads to low quantum efficiency; limited surface active sites restrict CO2 adsorption and activation capabilities; extremely low selectivity for multi-electron reduction products (such as CH4), mainly generating the two-electron product CO; the hydrogen evolution side reaction violently consumes a large number of photogenerated electrons, and CdS itself is prone to photocorrosion, resulting in poor stability.

[0006] Single-atom catalysts have attracted widespread attention due to their 100% atomic utilization and uniform active site structure. However, single atoms, due to their high surface energy, tend to migrate and aggregate on the catalyst surface, forming thermodynamically more stable nanoparticles and losing their unique advantages. Therefore, how to stably anchor single atoms on the support surface has become a core technical challenge in the preparation of single-atom catalysts.

[0007] In existing technologies, single-atom anchoring typically relies on surface defect sites, coordination unsaturated sites, or functional groups on the support. For metal sulfide supports, sulfur vacancy defects not only modulate the electronic structure and optical properties of the support but also serve as natural anchoring sites, achieving stable loading through strong chemical bonds with metal single atoms. However, in existing studies on ruthenium supported on cadmium sulfide, it is often difficult to precisely control the concentration and distribution of sulfur vacancies, leading to the easy aggregation or loss of Ru species. Furthermore, existing technologies often rely on organic sacrificial agents (such as triethanolamine and lactic acid) to capture holes and maintain the reaction, which not only increases costs but also masks the true activity and hydrogen evolution inhibition capability of the catalyst in an all-aqueous system. Currently, few studies have been able to achieve complete inhibition of the hydrogen evolution reaction and obtain near-100% methane selectivity in a pure water system by precisely constructing a "sulfur vacancy-Ru single atom" structure.

[0008] Therefore, developing a preparation method based on a defect anchoring strategy to precisely fix Ru single atoms using sulfur vacancies and construct a novel photocatalyst with high activity, high CH4 selectivity, and complete inhibition of hydrogen evolution is of great significance for promoting the practical application of photocatalytic CO2 reduction technology. Summary of the Invention

[0009] This invention aims to solve the technical problems of severe hydrogen evolution side reactions, low methane selectivity, and easy aggregation and deactivation of single-atom active sites in existing cadmium sulfide photocatalysts during photocatalytic carbon dioxide reduction. This invention provides a catalyst for the highly selective reduction of carbon dioxide to prepare high-purity methane and its preparation method. By constructing abundant sulfur vacancy defects in situ on the cadmium sulfide surface, and utilizing these sulfur vacancies as traps to precisely anchor Ru single atoms, a unique Ru-S coordination structure is constructed, thereby altering the reaction pathway and achieving effective suppression of the hydrogen evolution reaction and highly selective generation of methane products in a pure water system.

[0010] To achieve the above objectives, the present invention provides the following technical solution: In a first aspect, the present invention provides a catalyst for the highly selective reduction of carbon dioxide to prepare high-purity methane, comprising a CdS nanostructure rich in sulfur vacancies as a support, and Ru single atoms supported on the surface of the support. The Ru is dispersed in single-atom form, anchored to sulfur vacancy sites or their vicinity on the surface of the CdS support, and forms a Ru-S coordination structure with the surrounding sulfur atoms; the catalyst does not contain Ru aggregates or nanoparticles.

[0011] Furthermore, the loading of Ru single atoms is 0.5% to 5.0% of the total mass of the photocatalyst.

[0012] Furthermore, the microstructure of the sulfur-vacancy-rich CdS support is one or more of nanorods, nanosheets, nanoparticles, or nanospheres; the Ru-S coordination structure was confirmed by X-ray absorption fine structure spectroscopy (EXAFS), and the coordination number of the Ru atom was 4.

[0013] Secondly, the present invention provides a method for preparing a catalyst for the highly selective reduction of carbon dioxide to produce high-purity methane, comprising the following steps: 4-1. Defect Support Preparation: Cadmium sulfide nanomaterials were prepared by a hydrothermal method, and sulfur vacancies were introduced on the surface of cadmium sulfide by controlling the molar ratio of sulfur source to cadmium source in the precursor, resulting in a CdS support rich in sulfur vacancies; further, step 4-1 can be specifically as follows: Preparation of defect-rich cadmium sulfide support: Cadmium source and sulfur source were dissolved in deionized water at a molar ratio of 1:(0.5~2.0) and stirred until homogeneous to obtain a mixed solution; the mixed solution was transferred to a reaction vessel and subjected to hydrothermal reaction at 180℃ for 12 hours; after the reaction was completed, the precipitate was collected, washed and dried to obtain cadmium sulfide (CdS) support rich in sulfur vacancy defects.

[0014] 4-2. Ru source introduction: The CdS support rich in sulfur vacancies obtained in step 4-1 is dispersed in a solvent, and a ruthenium precursor solution is added and mixed evenly to obtain a precursor mixture; specifically, it can be as follows: Ru single-atom impregnation loading: The defect-rich cadmium sulfide support obtained in step 4-1 is dispersed in a solvent, a certain amount of ruthenium source solution is added, and impregnation adsorption is carried out under stirring conditions. 4-3. Single-atom anchoring: The precursor mixture is heat-treated to induce coordination reactions of ruthenium ions near the vacancies by using sulfur vacancies as traps. After the reaction is completed, the catalyst is obtained by washing and drying.

[0015] Furthermore, step 4-3 can be specifically as follows: Post-treatment and activation: The solvent was removed from the mixture after impregnation in step 4-2 and dried. Then, heat treatment was performed under an inert atmosphere to make the adsorbed Ru species stably anchored on the sulfur vacancies on the cadmium sulfide surface, thus obtaining the defective cadmium sulfide-supported Ru single-atom photocatalyst.

[0016] Further, in step 4-1, the molar ratio of the cadmium source to the sulfur source is 2.5:1. The cadmium source is selected from cadmium acetate, cadmium nitrate, cadmium chloride, or cadmium sulfate; the sulfur source is selected from thiourea, thioacetamide, or L-cysteine.

[0017] Further, the ruthenium precursor mentioned in step 4-2 is selected from ruthenium chloride (RuCl3), ruthenium acetylacetonate, or ruthenium nitronitrosylate; the solvent is water, ethanol, or ethylene glycol. The amount of ruthenium source added is such that the mass percentage of Ru in the final catalyst is 0.5% to 5.0%.

[0018] Furthermore, in step 4-3, the heat treatment temperature is 300~500℃, and the time is 1~4 hours. This step aims to remove ligand residues and enhance the interaction between Ru atoms and the support.

[0019] Thirdly, the present invention provides the application of the above-mentioned defective cadmium sulfide-supported Ru single-atom photocatalyst in the photocatalytic reduction of carbon dioxide.

[0020] Furthermore, the application refers to the reduction of carbon dioxide to methane in a reaction system using water as an electron donor and proton source, under visible light or simulated sunlight irradiation.

[0021] Furthermore, the catalyst has the properties of inhibiting hydrogen evolution reaction and inhibiting carbon monoxide formation; in the reaction products, the selectivity of methane reaches more than 99%, and no additional organic sacrificial agent is used.

[0022] Compared with the prior art, the present invention achieves the following significant and practically valuable technical effects: 1. Precise Coupling of Defect Engineering and Single-Atom Technology: This invention employs a step-by-step strategy of "first constructing the defect support, then impregnating and anchoring." First, by controlling the precursor ratio and hydrothermal conditions, a CdS support rich in sulfur vacancies is directly synthesized. These pre-defined sulfur vacancies become ideal "trapping traps" for subsequent Ru atoms. Compared to the traditional one-pot method, this approach allows for more precise control of the defect concentration and single-atom loading of the support, ensuring that Ru atoms exist in a thermodynamically stable Ru-S bond form, achieving atomic-level dispersion and avoiding aggregation.

[0023] 2. Complete suppression of side reactions (suppression of hydrogen evolution and CO) to achieve highly selective methane formation: The unique Ru-S coordination environment alters the electronic structure of the active sites. The Ru single-atom sites significantly weaken the dependence on H. + The adsorption of CO2 enhances the adsorption of key intermediates for CO2 reduction (such as...). The adsorption and activation of ) were demonstrated by the catalyst. Experiments showed that the catalyst almost completely suppressed the hydrogen evolution reaction (HER) in a pure water system, and the selectivity of methane (CH4) was close to 100%, breaking through the bottleneck of severe hydrogen evolution and complex products that are usually found in cadmium sulfide-based materials.

[0024] 3. Excellent stability and charge separation efficiency: Sulfur vacancies not only act as anchoring sites but also as electron trapping centers, effectively promoting the transfer of photogenerated electrons from the CdS conduction band to the Ru active sites and suppressing carrier recombination. Simultaneously, the robust Ru-S bonding endows the catalyst with extremely high structural stability; during long-term photocatalytic cycling tests, the catalyst did not exhibit significant photocorrosion or activity degradation.

[0025] 4. Green and environmentally friendly artificial photosynthesis: The catalyst prepared by this invention does not rely on expensive and polluting organic sacrificial agents such as triethanolamine. It can directly use water as an electron source, realizing a green and economical process of converting CO2 into high-value-added fuel (methane), which has important application prospects. Attached Figure Description

[0026] Figure 1 X-ray diffraction (XRD) patterns of the defect-rich cadmium sulfide support and the photocatalyst loaded with Ru single atoms prepared in Example 1 of this invention. Figure 2 The image shows the photocatalyst prepared in Example 1 of this invention, where a is a transmission electron microscope (TEM) image, b is a high-resolution transmission electron microscope (HRTEM) image, and ce is the corresponding EDS elemental distribution map of Cd, S, and Ru.

[0027] Figure 3 This invention uses 1.0% Ru-CdS v Images of different regions of the catalyst under (ab) spherical aberration corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM) clearly show the dispersion state of Ru single atoms on the CdS surface; Figure 4 The electron paramagnetic resonance (EPR) spectrum of the photocatalyst prepared in Example 1 of this invention confirms the presence of sulfur vacancies and the changes in signal before and after loading. Figure 5The following are synchrotron X-ray absorption fine structure (XAFS) spectra of Ru in the photocatalyst prepared in Example 1 of this invention: a is the Ru K-edge XANES spectrum, b is the Ru K-edge EXAFS spectrum, c is the Fourier transform amplitude of the Ru K-edge EXAFS spectrum in R space, and d is the Fourier transform amplitude of the Ru K-edge EXAFS spectrum in K space. Figure 6 This is a bar chart showing the yield and selectivity of the photocatalyst for CO2 reduction in a pure water system according to an embodiment of the present invention; Figure 7 1.0% Ru-CdS prepared in Example 1 of this invention v Stability testing of photocatalysts. Detailed Implementation

[0028] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings and examples.

[0029] Example 1 Defective cadmium sulfide-supported Ru single-atom photocatalyst (denoted as Ru-CdS) v Preparation of ) (1) Defect-rich cadmium sulfide (CdS) v Preparation of the carrier: Weigh 2.5 mmol of cadmium acetate and 1 mmol of thiourea and dissolve them in 60 mL of deionized water. Stir magnetically for 30 minutes at room temperature until a homogeneous and transparent mixed solution is formed. Transfer the solution to a 100 mL stainless steel reactor lined with polytetrafluoroethylene, seal it, and place it in an oven at 180°C for 12 hours. After naturally cooling to room temperature, collect the resulting yellow precipitate by centrifugation, wash it three times with deionized water and anhydrous ethanol, dry it overnight in a vacuum drying oven at 60°C, and grind it to obtain defect-rich cadmium sulfide powder (CdS). v ).

[0030] (2) Impregnation loading of Ru single atoms: Weigh 100 mg of the CdS prepared above. v The powder was dispersed in 20 mL of deionized water and sonicated for 10 minutes to ensure uniform dispersion. Then, a certain volume of RuCl3 aqueous solution (concentration 10 g / L, calculated based on a Ru element mass fraction of 1.0 wt%) was slowly added dropwise to the suspension. The mixture was stirred continuously at room temperature in the dark for 4 hours to allow Ru ions to be fully adsorbed onto the defect sites on the CdS surface.

[0031] (3) Post-processing and activation: The solvent in the suspension obtained in step (2) was removed by rotary evaporation, and the resulting solid was vacuum dried at 60°C. Finally, the dried solid powder was placed in a tube furnace and heated to 300°C at a heating rate of 5°C / min under an argon (Ar) atmosphere. The temperature was held for 2 hours, and after natural cooling, the defective cadmium sulfide-supported Ru single-atom photocatalyst, denoted as 1.0%Ru-CdS, was obtained. v .

[0032] Example 2 Preparation of photocatalysts with different Ru loading The only difference from Example 1 is the amount of RuCl3 solution added in step (2), with the theoretical Ru loading controlled at 0.3%, 0.5 wt%, 3.0 wt%, and 5.0 wt%, respectively. The remaining steps are identical to Example 1. The resulting samples are denoted as 0.3% Ru-CdS. v 0.5% Ru-CdS v 3.0% Ru-CdS v and 5.0% Ru-CdS v .

[0033] Comparative Example 1 Defect-free CdS and Ru-CdS v Preparation The only difference from Example 1 is the amount of thiourea added in step (1). In this case, the molar ratio of cadmium acetate to thiourea was controlled to be 1:1.02 to obtain defect-free CdS, and the subsequent steps of loading Ru (1.0 wt%) were the same as in Example 1.

[0034] Photocatalytic carbon dioxide reduction performance test Test conditions: The photocatalytic reaction was carried out in a sealed quartz glass reactor (approximately 250 mL in volume). 10.0 mg of the photocatalyst powder prepared in the examples or comparative examples was weighed and uniformly dispersed in 30 mL of deionized water to form a suspension. High-purity carbon dioxide (CO2, 99.999%) gas was continuously bubbled into the reaction system for 30 minutes to completely remove air and saturate the solution with CO2. The reactor was sealed, and the circulating cooling water system was turned on to maintain a constant reaction temperature of 25 ± 1 °C. A 300 W xenon lamp (equipped with a 420 nm cutoff filter to provide visible light) was used as the light source for irradiation. Magnetic stirring was maintained during the reaction. Every hour, 0.5 mL of gas was extracted from the top space of the reactor using an automated sampling system and injected into a gas chromatograph (GC) equipped with a methanation conversion furnace, a thermal conductivity detector (TCD), and a flame ionization detector (FID) for qualitative and quantitative analysis. No hydrogen (H2) was detected in the product. The calculated yields and selectivity of the main products, methane (CH4) and carbon monoxide (CO), are shown in the table below. The analytical results are also shown in Table 1. Table 1 Analysis Results

[0035] Results analysis: 1. Catalyst structural characterization Crystal structure and Ru dispersion state (XRD and TEM analysis): such as Figure 1 As shown, all samples (CdS) v The X-ray diffraction (XRD) patterns of the catalysts (with different Ru loadings) were in perfect agreement with the CdS standard card (JCPDS No. 10-0454). Even in samples with a Ru loading as high as 5.0 wt%, no diffraction peaks belonging to metallic Ru or Ru oxide crystalline phases were observed. This phenomenon strongly suggests that Ru species do not form macroscopic crystals or nanoparticles, but exist in a highly dispersed form. Transmission electron microscopy (TEM) images ( Figure 2 This further corroborates the point, as no obvious particles or clusters were observed, and the mapping diagram showed that the Ru species were uniformly dispersed.

[0036] Direct observation of Ru single atoms and their coordination environment (AC-HAADF-STEM and XAFS analysis): To directly verify the morphology of Ru at the atomic scale, we employed aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (AC-HAADF-STEM). Figure 3 As shown, in 1.0% Ru-CdS vIn the atomic resolution image, isolated, significantly brighter bright spots (marked with yellow circles) can be clearly seen scattered against the CdS lattice background (where Cd and S atoms are darker). Since the contrast of the HAADF-STEM image is approximately proportional to the square of the atomic number (Z-contrast), these isolated bright spots correspond to Ru atoms with larger atomic numbers, intuitively demonstrating that Ru is highly dispersed in single-atom form on the CdS support surface.

[0037] To further elucidate the chemical bonding between these Ru single atoms and the support, we performed synchrotron X-ray absorption fine structure (XAFS) tests. Figure 5 It showed 1.0% Ru-CdS v The Fourier transform extended edge fine structure (FT-EXAFS) spectrum of the sample on the Ru K-side shows only a main peak at ~1.8 Å (uncorrected phase), while the typical Ru-Ru coordination peak in metallic Ru foil is absent at ~2.4 Å. Quantitative fitting confirmed that this main peak originates from Ru-S coordination, with a coordination number of approximately 4 and a bond length of approximately 2.6 Å. This confirms at the atomic coordination level that a stable chemical bond is formed between the Ru single atom and the sulfur support atom, constituting the structure designed in this invention. Coordination active center, and nonmetallic Ru clusters.

[0038] Confirmation and evolution of sulfur vacancy defects (EPR analysis): Electron paramagnetic resonance (EPR) is a sensitive technique for detecting unpaired electrons in materials. For example... Figure 4 As shown, the pure defect carrier CdS v A strong paramagnetic resonance signal was observed at g ≈ 2.003, which is widely recognized as a characteristic signal of defect states formed by sulfur vacancies (V) trapping single electrons in CdS, directly proving that we successfully introduced abundant sulfur vacancies through precursor modulation. Notably, with Ru loading, the sulfur vacancy rate increased at 1.0% Ru-CdS... v The signal in the middle is significantly reduced, which does not mean that sulfur vacancies have disappeared, but rather that the adsorbed Ru... 3+ The ions interact strongly with sulfur vacancy sites, forming Ru-S bonds and altering the local electronic spin state of the sulfur vacancy, thus "quenching" its EPR signal. This signal change provides direct spectroscopic evidence that the Ru species is precisely anchored at the sulfur vacancy defect site.

[0039] In summary, the characterization results from XRD, EPR, TEM, AC-HAADF-STEM, and XAFS form a complete chain of evidence, jointly demonstrating the success of this invention: First, a CdS-V support rich in sulfur vacancies was prepared; subsequently, Ru precursor ions were selectively adsorbed and anchored at these defect sites; after heat treatment, Ru exists stably in single-atom form and forms a unique Ru-S coordination structure with surrounding S atoms. This precisely constructed "defect-anchored single-atom" structure is the source of its superior catalytic performance. 2. A significant improvement in CH4 selectivity and yield: as shown in Table 1 and Figure 6 As shown, the core advantages of this invention are fully demonstrated. Pure defect CdS carrier (CdS... V The main product is CO, with a CH4 selectivity of only 8.3%. The introduction of a Ru single atom results in a significant leap in CH4 selectivity, reaching 99.01% at the optimal loading (1.0 wt%), almost completely directing the product to methane. Simultaneously, the CH4 yield increases from 0.30 μmol·g⁻¹ in the pure support. -1 ·h -1 Significantly increased to 27.04 μmol·g -1 ·h -1 The increase was more than 90 times. This clearly confirms that the Ru-S single-atom sites constructed through defect anchoring are the key active centers driving the deep reduction of CO2 and the efficient generation of CH4.

[0040] 3. Effective suppression of competing side reactions: No hydrogen (H2) was detected in the reaction products of all Ru-loaded samples (especially in the 0.5%–3.0% loading range), achieving complete suppression of the hydrogen evolution reaction (HER). Furthermore, compared to pure CdS… v The yield of CO also decreased significantly (from 3.32 to 0.27 μmol·g⁻¹). -1 ·h -1 This indicates that the Ru single-atom site not only activates CO2, but more importantly, changes the reaction pathway, enhancing the further hydrogenation of the CO intermediate and the breaking of the CO bond, rather than directly desorbing to generate CO, thereby suppressing the generation of the two main competing byproducts (H2 and CO) from the source.

[0041] 4. Loading optimization and volcano-shaped curve: The CH4 yield exhibits a "volcano-shaped" curve that first increases and then decreases with changes in Ru loading. Figure 6When the loading is too low (0.3%), the number of active sites is insufficient; when the loading reaches 1.0%, the number of active sites and the defect sites on the support achieve optimal matching, resulting in the best performance; when the loading is too high (3.0%, 5.0%), it may exceed the defect anchoring capacity, leading to weak aggregation of a small number of Ru species or the formation of suboptimal coordination environments, thus reducing performance. However, even at a loading of 5.0%, the CH4 selectivity is still as high as 96.27%, indicating that this defect-synergistic single-atom catalytic effect has a unique advantage in enhancing CH4 selectivity.

[0042] 5. The key role of defect anchoring: Comparative Example 1 with 1.0% Ru-CdS v Compared to Comparative Example 1, 1% Ru-CdS (with the same amount of Ru loaded on a near-defect-free support), the former's CH4 yield (27.04) was more than 3.5 times that of the latter (7.69). This strongly demonstrates that pre-constructed sulfur vacancy defects are crucial for achieving efficient, uniform single-atom-level dispersion of Ru and for forming a highly active Ru-S coordination structure. Ru on defect-free supports may be unevenly distributed or exhibit weak interactions, leading to significantly inferior catalytic performance.

[0043] 6. Catalyst stability: For the optimal catalyst 1.0% Ru-CdS v Four consecutive photocatalytic cycle tests were conducted, each lasting four hours. Figure 7 The results showed that the CH4 yield remained stable throughout the four cycles without significant decline, and the selectivity remained consistently around 99%. XRD and TEM characterization of the catalyst after the reaction revealed no significant phase changes or Ru species aggregation, confirming the catalyst's good photochemical stability and recyclability.

[0044] Significantly improved CH4 selectivity and yield: This is the core finding of this study. Compared to pure CdS, the Ru-loaded sample exhibited superior methanation capability.

[0045] A leap in selectivity: Pure defective CdS typically tends to generate CO or H2, but with the introduction of Ru, the selectivity for CH4 is remarkably increased to 99%. This indicates that Ru, as a co-catalyst, drastically alters the reaction pathway of photogenerated electrons, concentrating them highly on the process of deep reduction of CO2 to CH4.

[0046] Significantly enhanced activity: In terms of yield, the CH4 yield of Ru-CdS was more than 20-fold higher than that of pure CdS. This substantial improvement confirms the role of the Ru site in promoting the transformation of key intermediates (such as...). Protonization It plays a key role in the dynamics of multi-electron transport.

[0047] Effective suppression of byproducts (H2 and CO): While achieving high CH4 production, we observed that competing side reactions were significantly suppressed.

[0048] Suppression of hydrogen evolution reaction (HER): The hydrogen evolution reaction, which usually dominates in aqueous photocatalysis, is effectively suppressed, indicating that photogenerated electrons tend to flow to CO2 reduction sites rather than proton reduction sites.

[0049] Inhibition of CO formation: Unlike many sulfide catalysts that produce CO as the main product, the introduction of Ru prevents the desorption of CO, promoting its further hydrogenation to form CH4. This means that the Ru site not only activates CO2, but also promotes the breaking of CO bonds and the formation of CH bonds, thereby achieving a deep reduction from CO2 to CH4, rather than remaining at the CO stage.

[0050] The above description is merely a preferred embodiment of the present invention. However, the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention should be covered within the scope of protection of the present invention.

Claims

1. A catalyst for the highly selective reduction of carbon dioxide to prepare high-purity methane, characterized in that, It includes a CdS nanostructure rich in sulfur vacancies as a support, and Ru single atoms loaded on the surface of the support; The Ru is dispersed in single-atom form, anchored to sulfur vacancy sites or their vicinity on the surface of the CdS support, and forms a Ru-S coordination structure with the surrounding sulfur atoms; the catalyst does not contain Ru aggregates or nanoparticles.

2. The catalyst for the highly selective reduction of carbon dioxide to prepare high-purity methane according to claim 1, characterized in that, The loading of Ru single atoms is 0.5% to 5.0% of the total mass of the photocatalyst.

3. The catalyst for the highly selective reduction of carbon dioxide to prepare high-purity methane according to claim 1, characterized in that, The microstructure of the sulfur-vacancy-rich CdS support is one or more of nanorods, nanosheets, nanoparticles, or nanospheres; the Ru-S coordination structure was confirmed by X-ray absorption fine structure spectroscopy (EXAFS), and the coordination number of the Ru atom was 4.

4. A method for preparing a catalyst for the highly selective reduction of carbon dioxide to produce high-purity methane according to any one of claims 1-3, characterized in that, Includes the following steps: 4-1. Preparation of defect support: Cadmium sulfide nanomaterials were prepared by hydrothermal method, and sulfur vacancies were introduced on the surface of cadmium sulfide by controlling the molar ratio of sulfur source to cadmium source in the precursor to obtain a CdS support rich in sulfur vacancies. 4-2. Ru source introduction: The CdS support rich in sulfur vacancies obtained in step 4-1 is dispersed in a solvent, and a ruthenium precursor solution is added and mixed evenly to obtain a precursor mixture. 4-3. Single-atom anchoring: The precursor mixture is heat-treated to induce coordination reactions of ruthenium ions near the vacancies by using sulfur vacancies as traps. After the reaction is completed, the catalyst is obtained by washing and drying.

5. The preparation method according to claim 4, characterized in that, In step 4-1, the molar ratio of the cadmium source to the sulfur source is 2.5:

1.

6. The preparation method according to claim 4, characterized in that, The ruthenium precursor mentioned in step 4-2 is selected from ruthenium chloride, ruthenium acetylacetone, or ruthenium nitronitrosylate; the solvent is water, ethanol, or ethylene glycol.

7. The preparation method according to claim 4, characterized in that, In step 4-3, the heat treatment temperature is 300~500℃ and the time is 1~4 hours.

8. The application of a catalyst for the highly selective reduction of carbon dioxide to prepare high-purity methane according to any one of claims 1-3 in photocatalytic carbon dioxide reduction.

9. The application according to claim 8, characterized in that, The application refers to the reduction of carbon dioxide to methane in a reaction system using water as an electron donor and proton source, under visible light or simulated sunlight.

10. The application according to claim 9, characterized in that, The catalyst has the properties of inhibiting hydrogen evolution reaction and inhibiting carbon monoxide formation; the selectivity of methane in the reaction products reaches more than 99%, and no additional organic sacrificial agent is used.

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