Monatomic catalyst for preparing formic acid through CO2 hydrogenation

Abstract

The invention relates to a monatomic catalyst for preparing formic acid through CO2 hydrogenation, in particular to the technical field of catalysts. The catalyst is composed of an active center, a carrier and an auxiliary agent, wherein the active center is composed of atomic-scale dispersed transition metal single atoms; the carrier is selected from a two-dimensional material or a mesoporous material with surface defects; the auxiliary agent comprises a composite system of alkali metal oxide and rare earth oxide. The preparation method comprises the following steps: constructing carrier defects through microwave plasma etching or gradient hydrogenation, precisely loading single atoms through pulse atomic layer deposition, assembling the auxiliary agent through supercritical CO2 assistance, and dynamically regulating and controlling process parameters through in-situ monitoring. The active center, the carrier and the auxiliary agent cooperate to inhibit side reaction, the yield of the target product is greatly improved, and the subsequent separation and purification cost is reduced. And the preparation process is accurate and controllable, and large-scale production is facilitated. Meanwhile, various steps are selected, different application requirements are met, the formic acid selectivity is larger than 90%, and the stability is larger than or equal to 500 h.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalysts, and particularly relates to a single-atom catalyst for hydrogenating CO2 to prepare formic acid. More specifically, it relates to a highly efficient catalyst for hydrogenating CO2 to formic acid and a preparation method thereof, which are based on the synergistic effect of transition metal single-atom active centers, surface defect-engineered carriers, and composite promoters. The aim is to provide a catalyst with excellent performance and a reliable preparation process for realizing efficient and green CO2 conversion. Background Art

[0002] Under the background of the global energy structure transformation and the increasing emphasis on environmental protection, the effective conversion and utilization of CO2 have become the focus of common concern in the scientific and industrial communities. Converting CO2 into formic acid through hydrogenation reaction can not only realize the recycling of carbon resources, relieve the environmental pressure brought by greenhouse gas emissions, but also obtain chemical raw materials with broad application prospects, providing strong support for sustainable development. However, a series of problems have emerged in the actual application process of the current catalysts for hydrogenating CO2 to prepare formic acid, seriously restricting the large-scale industrial promotion of this technology.

[0003] Traditional heterogeneous catalysts have the following inherent defects: 1. Low atomic utilization rate: In traditional CO2 hydrogenation catalysts, such as widely used Pd / Al2O3 and Ru / C, etc., their active components are usually loaded on the surface of the carrier in the form of metal nanoparticles. In this structure, a large number of metal atoms are wrapped inside the nanoparticles and cannot come into full contact with the reactants, resulting in extremely low atomic utilization rate, usually less than 30%. This means that a large amount of expensive metal resources are wasted during the preparation of the catalyst, which not only increases the production cost but also has a negative impact on the sustainable utilization of resources. 2. Poor product selectivity: During the CO2 hydrogenation reaction, traditional catalysts are difficult to effectively inhibit the occurrence of competitive side reactions. The formation of by-products such as CH4 and CO makes the selectivity of formic acid generally less than 60%. This not only reduces the yield of the target product formic acid but also makes the product system complex, greatly increasing the difficulty and cost of subsequent product separation and purification. 3. Insufficient stability and short lifespan: Under relatively harsh reaction conditions such as high temperature and high pressure, the metal nanoparticles in traditional catalysts are prone to agglomeration. As the reaction progresses, the metal particles gradually aggregate and grow, and the number of active sites decreases accordingly, resulting in a sharp decline in the activity and selectivity of the catalyst. Generally speaking, when used continuously for more than 100 hours, the inactivation rate often exceeds 50%. This not only requires frequent replacement of the catalyst, increasing the production cost, but also leads to the interruption of the production process, affecting the continuity and stability of production, and bringing many inconveniences to industrial production.

[0004] The emergence of single-atom catalysts has brought new opportunities to improve atomic utilization efficiency, which can boost the atomic utilization rate to nearly the theoretical limit of 100%. However, existing single-atom catalyst systems, such as the typical Fe-N-C system, still face some intractable problems in practical applications. 1. Limited CO2 adsorption capacity: The adsorption energy of existing single-atom catalysts for CO2 is generally less than 1.2 eV, resulting in weak capture ability of the catalyst for CO2 molecules. As the reaction substrate, CO2 cannot be effectively enriched around the active sites, thus restricting the initial step of the reaction and making it difficult to effectively improve the overall catalytic efficiency. 2. Urgent need to improve H2 dissociation efficiency: The dissociation of H2 on the catalyst surface is one of the key steps in the CO2 hydrogenation reaction. However, the turnover frequency (TOF) of H2 dissociation of existing single-atom catalysts is less than 0.5 s -1 , that is, the number of dissociations of H2 molecules on each active site per unit time is small. This means that sufficient hydrogen atoms cannot be provided in a timely manner for the subsequent CO2 hydrogenation reaction, becoming an important bottleneck restricting the improvement of catalytic performance.

[0005] In summary, the existing catalysts for CO2 hydrogenation to formic acid have obvious deficiencies in multiple key performance indicators such as atomic utilization efficiency, product selectivity, stability, and the adsorption and dissociation abilities of reactants. These problems have seriously hindered the process of the CO2 hydrogenation to formic acid technology from laboratory research to large-scale industrial application. Therefore, it is urgent to develop a new type of catalyst with high atomic utilization efficiency, high selectivity, high stability, and excellent CO2 adsorption and H2 dissociation abilities and its preparation method, which is of crucial significance for promoting the development of CO2 resource utilization technology and realizing sustainable carbon circular economy. Summary of the Invention To make up for the deficiencies of the existing technology, the purpose of the present invention is to provide a catalyst for CO2 hydrogenation to formic acid, which can improve the activity, selectivity, and stability of the catalyst in the CO2 hydrogenation to formic acid reaction through the optimized design of the active center, carrier, and promoter. At the same time, a precise and controllable preparation method is provided to ensure the consistency and repeatability of the catalyst performance. The specific technical solutions are as follows: Provide a single-atom catalyst for CO2 hydrogenation to formic acid.

[0006] To achieve the above purpose, the present invention provides the following technical solutions: A catalyst for CO2 hydrogenation to formic acid, which consists of an active center, a carrier, and a promoter, wherein: (a) The active center is atomically dispersed transition metal single atoms, and the transition metal is selected from at least one of Pd and Pt, and is anchored on the surface of the carrier through M-N-C or M-O-C coordination bonds. The total metal loading is 0.5-3.5 wt%, and the single-atom density ≥ 5×10 13sites / g, and the CO2 adsorption energy is 1.9 - 2.3 eV; this design of atomically dispersed active centers is conducive to improving the utilization rate of active sites, enhancing the interaction with CO2, and thus improving the reaction activity and selectivity.

[0007] (b) The carrier is selected from two-dimensional materials or mesoporous materials with surface defects. When the carrier is a two-dimensional material, the specific surface area > 300 m² / g and the surface defect density is 1×10 13 -8×10 13 defects / m²; the larger specific surface area and specific surface defect density contribute to the dispersion and stability of active centers, and at the same time provide more reactive sites. When the carrier is TiO2 nanosheets, the exposure degree of the {001} crystal plane > 80% and the lattice oxygen vacancy concentration is 0.7 - 1.0 vacancies / nm²; the high exposure degree of the {001} crystal plane and specific oxygen vacancy concentration are beneficial to improving the activity and selectivity of the catalyst.

[0008] (c) The promoter contains a K2O-Na2O solid solution nanocluster and a CeO2-La2O3 core-shell structure. The total addition amount of the promoter accounts for 1 - 8 wt% of the total mass of the catalyst, and the Ce 3+ / Ce 4+ ratio is 1.2 - 1.8 and the promoter forms a composite structure with the carrier, and the metal migration energy barrier ≥ 1.8 eV. The addition of the promoter can adjust the electronic structure and surface properties of the catalyst, improve the stability of active centers, and promote the adsorption and activation of CO2. The alkali metal oxide in the promoter is a K2O-Na2O solid solution nanocluster with an average particle size < 1.5 nm, and the rare earth oxide is a CeO2-La2O3 core-shell structure, where the Ce³⁺ / Ce 4 ⁺ ratio measured by X-ray photoelectron spectroscopy is 1.2 - 1.8, and the promoter and the carrier defect sites form a stable composite structure through M-O-Si bonding, further enhancing the interaction between the promoter and the carrier and improving the performance of the catalyst.

[0009] Further, the two-dimensional material is layered boron nitride or nitrogen-doped graphene, where the nitrogen content of nitrogen-doped graphene is 4 - 6 at%, and the proportion of pyridine nitrogen ≥ 70%, and the edge sites of nitrogen-doped graphene form an M-N3 coordination structure with metal single atoms. Further enhancing the binding force between the active center and the carrier.

[0010] Furthermore, the mesoporous material is mesoporous silica or metal-organic framework-derived carbon material with a hierarchical pore structure, and its pore size shows a three-level distribution: micropores of 1 - 3 nm, mesopores of 5 - 8 nm, and macropores of 20 - 30 nm. The inner wall of the pore channels is modified with bifunctional groups of sulfonic acid groups and hydroxyl groups, and the total functional group density is ≥5 groups / nm², where the molar ratio of sulfonic acid groups to hydroxyl groups is 1:1 - 1:2. This hierarchical pore structure and functional group modification are beneficial to the diffusion and adsorption of reactants and products, improving the reaction efficiency.

[0011] Furthermore, in the promoter, the alkali metal oxide is a K2O-Na2O solid solution nanocluster with an average particle size <1.5 nm, and the rare earth oxide is a CeO2-La2O3 core-shell structure, where the Ce 3+ / Ce 4+ ratio measured by X-ray photoelectron spectroscopy is 1.2 - 1.8, and the promoter and the defect sites of the carrier form a stable composite structure through M-O-Si bonding, regulating the electronic structure and surface properties of the catalyst, enhancing the stability of the active center, promoting the adsorption and activation of reactants, and improving the activity and selectivity of the catalyst.

[0012] A preparation method of a CO2 hydrogenation to formic acid catalyst includes the following control steps: (1) Carrier defect engineering: For two-dimensional materials, microwave plasma etching is used, and they are treated for 10 - 20 minutes under a power of 100 - 150 W, a frequency of 2.45 GHz, and an Ar / N2 mixed gas to generate edge dangling bonds and topological defects; increasing the surface active sites creates favorable conditions for the subsequent loading of active centers.

[0013] For TiO2 nanosheets, gradient hydrogenation treatment is carried out. First, they are treated in a H2 / N2 mixed gas at 450 - 500 °C for 1 hour, and then in pure H2 at 550 - 600 °C for 0.5 hour to form a gradient oxygen vacancy distribution, where the surface-to-bulk oxygen vacancy concentration ratio is ≥2:1, and the treatment is terminated when the signal intensity at g = 2.003 monitored by in-situ EPR reaches 300 - 500 a.u.; precisely controlling the formation and distribution of oxygen vacancies optimizes the catalytic performance of TiO2 nanosheets.

[0014] (2) Single-atom precise loading: The pulsed atomic layer deposition technique is adopted, and the precursors are alternating pulses of metal-organic complexes and halides. The deposition temperature is 250 - 350 °C. Each deposition cycle includes: metal precursor pulse for 0.2 - 0.5 s → inert gas purge for 5 - 10 s, reaction gas pulse for 0.1 - 0.3 s → inert gas purge for 8 - 15 s, and the number of cycles is 8 - 15 times. During this period, the concentration of by-products is monitored by in-situ mass spectrometry to adjust the pulse timing to ensure the precise loading of active centers.

[0015] (3) Composite modification with additives: Using the supercritical fluid-assisted impregnation method, the additive precursor and the surfactant are co-assembled in a supercritical CO2 environment at a temperature of 35 - 45 °C and a pressure of 8 - 9 MPa, and then calcined in stages at 400 - 500 °C. First, it is heated to 300 °C at a rate of 2 °C / min and held for 1 hour, and then heated to the target temperature at a rate of 5 °C / min and held for 2 hours. This promotes the formation of a stable composite structure between the additive and the support, and fully exerts the optimization effect of the additive on the catalyst performance.

[0016] Furthermore, in step (2), synchrotron radiation X-ray absorption fine structure spectroscopy is used to monitor the metal coordination environment in real time. When the M-O / M-N coordination number is detected to reach 4 ± 0.5, the deposition is terminated, and the deposition temperature is dynamically adjusted by ±20 °C through a feedback system. This realizes the uniform distribution of active centers on the support surface and the ideal coordination structure, and improves the activity and stability of the catalyst.

[0017] Furthermore, the surfactant in step (3) is a block copolymer PS-b-PEO, with a number average molecular weight of 5000 - 50000 Da, a molecular weight distribution index ≤ 1.1, and it forms a hydrogen bond complex with the additive precursor. The mesoscopic structure evolution is monitored by small-angle X-ray scattering. When a characteristic scattering peak with q = 0.1 - 0.2 nm -1 appears, the assembly is completed.

[0018] Furthermore, during the gradient hydrogenation treatment process, the oxygen vacancy concentration is monitored in real time by in-situ electron paramagnetic resonance spectroscopy. When the signal intensity at g = 2.003 reaches 300 - 500 a.u., the feedback system is triggered to switch the treatment atmosphere. The feedback system dynamically adjusts the hydrogen partial pressure according to the following parameters: (a) Taking the change rate of the EPR signal intensity as the input parameter, when the rate > 5 a.u. / min, the hydrogen partial pressure is reduced by 0.3 - 0.5 kPa / min; (b) The gas input is precisely controlled by a mass flow controller, and the hydrogen partial pressure adjustment accuracy is ±0.5 kPa; (c) When switching to the pure H2 stage, the total system pressure is maintained constant, with a fluctuation range ≤ 1 kPa.

[0019] Furthermore, characterized by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy, the transition metal single atoms are statistically uniformly distributed on the support surface, the adjacent single atom spacing ≥ 0.5 nm, and the d-band center value of the single atom site measured by electron energy loss spectroscopy is -2.8 to -2.3 eV, forming an energy level matching of 0.4 - 0.7 eV with the LUMO energy level of CO2.

[0020] Further, it also includes step (4) catalyst activation treatment: Introduce a mixed gas with a volume ratio of H2 to CO2 of 3:1 into the reactor, heat it to 200 - 250 °C at a rate of 10 °C / min and keep it for 2 hours, then cool it to 80 °C at a rate of 5 °C / min for surface hydroxylation treatment. Finally, when the characteristic peak intensity at 1590 cm -1 decreases to below 3 times the baseline noise, the activation is completed, further optimizing the catalyst surface structure and active sites, and enhancing the catalytic performance.

[0021] Advantages of the present invention 1. High activity and selectivity: The active center is atomically dispersed transition metal single atoms, anchored on the carrier surface through specific coordination bonds. The total metal loading and single atom density are appropriate, and it has a suitable adsorption energy for CO2 (1.9 - 2.3 eV). This enables the catalyst to have high activity for the reaction of hydrogenating CO2 to formic acid, effectively adsorb CO2 and promote the reaction to proceed, increasing the reaction rate and the formation efficiency of formic acid. The promoter forms a composite structure with the carrier, and the ratio of Ce 3+ / Ce 4+ is between 1.2 - 1.8, and the metal migration energy barrier ≥ 1.8 eV, which helps to regulate the electronic structure of the catalyst, improve the selectivity for formic acid formation, and reduce the occurrence of side reactions.

[0022] 2. Carrier structure advantages: Select two-dimensional materials with surface defects or specific mesoporous materials as the carrier. The two-dimensional materials have a large specific surface area (> 300 m² / g) and an appropriate surface defect density (1×10 13 -8×10 13 defects / m²), which can provide abundant anchoring sites for the active center and enhance the stability of the active center; the mesoporous materials have a hierarchical pore structure and a specific pore size distribution, and the inner wall of the pore is modified with bifunctional groups, which is conducive to the diffusion and adsorption of reactants and products, and improves the reaction efficiency. When the carrier is TiO2 nanosheets, the exposure degree of the {001} crystal plane > 80% and the lattice oxygen vacancy concentration is 0.7 - 1.0 vacancies / nm². The unique crystal plane structure and oxygen vacancy concentration are beneficial to improving the activity and selectivity of the catalyst and enhancing the adsorption and activation ability of CO2.

[0023] 3. Precise Preparation and Regulation: Each step in the preparation method is highly targeted. The carrier defect engineering precisely regulates the surface defects of the carrier through microwave plasma etching or gradient hydrogenation treatment, providing an ideal surface structure for the subsequent loading of active centers and the compounding of promoters. The precise loading of single atoms uses pulsed atomic layer deposition technology. By precisely controlling the parameters of each deposition cycle and monitoring the concentration of by-products in situ by mass spectrometry to adjust the pulse timing, the precise loading of active centers is achieved, ensuring the uniform distribution of single atoms on the carrier surface and a suitable coordination environment. The promoter compound modification uses supercritical fluid-assisted impregnation method, combined with surfactant co-assembly and staged calcination, which can precisely control the dispersion of promoters and the composite structure with the carrier. By monitoring the mesoscopic structure evolution with small-angle X-ray scattering, it is ensured that the promoters play the best role.

[0024] 4. Real-time Monitoring and Feedback Optimization: A variety of real-time monitoring means and feedback systems are used during the preparation process. For example, during the single-atom loading process, synchrotron radiation X-ray absorption fine structure spectroscopy is used to monitor the metal coordination environment in real time, and the deposition temperature is dynamically adjusted through the feedback system to ensure that the metal coordination reaches an ideal state. During the gradient hydrogenation treatment process, the concentration of oxygen vacancies is monitored in real time by in-situ electron paramagnetic resonance spectroscopy, and based on parameters such as the change rate of signal intensity, the hydrogen partial pressure is precisely adjusted using the feedback system to ensure the accuracy and stability of the treatment process, thereby optimizing the catalyst performance.

[0025] 5. Structure-Performance Matching: It can be characterized by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy that the transition metal single atoms are statistically uniformly distributed on the carrier surface, and the distance between adjacent single atoms is ≥ 0.5 nm. This uniform distribution is conducive to the full play of the active centers and avoids the reduction of activity caused by the agglomeration of single atoms. The d-band center value of the single-atom site measured by electron energy loss spectroscopy forms an energy level match of 0.4 - 0.7 eV with the LUMO energy level of CO2, which helps to enhance the adsorption and activation of CO2 at the active centers and improve the catalytic reaction efficiency.

[0026] 6. High-efficiency Activation Treatment: The catalyst activation treatment step controls the surface of the catalyst to undergo reactions such as hydroxylation through a specific heating and cooling process and gas atmosphere, optimizing the surface structure of the catalyst. The intensity of specific characteristic peaks is detected by in-situ Fourier transform infrared spectroscopy to ensure that the activation treatment reaches the best effect, improving the initial activity and stability of the catalyst, enabling it to quickly and efficiently participate in the CO2 hydrogenation to formic acid reaction. Description of the Drawings

[0027] Figure 1 It is a comparison chart of CO2 conversion rate and formic acid selectivity.

[0028] Figure 2 It is a correlation chart of single-atom density and CO2 adsorption energy.

[0029] Figure 3Relationship diagram of the {001} crystal plane exposure, oxygen vacancy concentration, and EPR signal intensity for Example 2. When the {001} crystal plane exposure is 83%, the EPR signal intensity reaches 420 a.u., and the oxygen vacancy concentration is 0.9 vacancies / nm². Further research reveals that for every 5% increase in the {001} crystal plane exposure, the oxygen vacancy concentration increases by 0.2 vacancies / nm². This clearly demonstrates the close relationship between the {001} crystal plane exposure and the oxygen vacancy concentration, as well as the effectiveness of monitoring changes in the oxygen vacancy concentration through the EPR signal intensity. It shows that the oxygen vacancy concentration can be precisely adjusted by controlling the {001} crystal plane exposure, thereby optimizing the electronic structure and surface chemical properties of the TiO2 nanosheet-based catalyst and enhancing its catalytic performance.

[0030] Figure 4 SEM image of the nitrogen-doped graphene catalyst for Example 1. It has obvious defects and single-atom loading. The defect density of the nitrogen-doped graphene is 3×10 13 defects / m², the proportion of pyridine nitrogen is 72%, and the single-atom density is 7.2×10 13 sites / g.

[0031] Figure 5 SEM image of the TiO2 nanosheet-based catalyst for Example 2. It has obvious defects and single-atom loading. The initial {001} crystal plane exposure of the TiO2 nanosheets is 75%, which is increased to 83% after defect engineering. The single-atom density is 5.8×10 13 sites / g.

[0032] Figure 6 SEM image of the mesoporous silica-based catalyst for Example 3. It shows its hierarchical pore structure. Micropores (1 - 3 nm), mesopores (5 - 8 nm), and macropores (20 - 30 nm) are clearly shown in the image. The inner walls of the pores are chemically modified to introduce sulfonic acid groups (-SO3H) and hydroxyl groups (-OH). The total functional group density is 6 groups / nm², and the molar ratio of sulfonic acid groups to hydroxyl groups is 1:1.5. The interconnectedness of the pores and the distribution of surface functional groups are also highlighted in the figure. Detailed implementation manners

[0033] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention; obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0034] Example 1: Nitrogen-doped graphene-based catalyst (molecular formula: Pd / (C0.95 No. 05 )-(K0.6Na0.4)2O@CeO2-La2O3) 1. Support defect engineering Support selection and characteristics: Nitrogen-doped graphene is selected as the support, and its chemical formula is accurately determined to be C 0.95 N 0.05 . By means of advanced analysis methods, the nitrogen content is measured to be 5.2 at%, and through X-ray photoelectron spectroscopy (XPS) combined with peak fitting technology, it is determined that the proportion of pyridine nitrogen reaches 72%. This unique nitrogen-doped structure endows graphene with special electronic properties, laying the foundation for the subsequent loading of active centers and catalytic reactions.

[0035] Microwave plasma etching process: Place the nitrogen-doped graphene in a microwave plasma etching device and introduce a mixed gas of Ar / N2 with a volume ratio of 3:1. Under the conditions of a frequency of 2.45 GHz and a power of 120 W, process for 15 minutes. During this process, the Ar / N2 mixed gas forms a plasma environment under microwave excitation, modifies the surface of graphene, and introduces specific defect structures.

[0036] Defect density characterization: After etching, it is characterized by Raman spectroscopy and XPS. The intensity ratio of the D / G peak in the Raman spectrum rises to 1.8, indicating an increase in surface defects; XPS not only verifies the proportion of pyridine nitrogen but also confirms the formation of defect structures. After calculation, the surface defect density is 3×10 13 defects / m². These defects provide abundant anchoring sites for active centers.

[0037] Specific surface area measurement: The nitrogen adsorption-desorption test method based on the BET theory is used to measure the specific surface area to be 325 m² / g. The larger specific surface area is beneficial to the dispersion of active centers and the adsorption of reactants.

[0038] 2. Single-atom loading Determination of active centers and measurement of loading amount: Based on in-depth research on the catalytic performance of various metals, Pd is selected as the single-atom active center. Using high-precision inductively coupled plasma mass spectrometry (ICP-MS), its loading amount is accurately measured to be 2.8 wt%, and this loading amount ensures the reasonable distribution and stability of active centers on the support.

[0039] Atomic layer deposition (ALD) parameter setting: Using the ALD technique, with Pd(hfac)2 as the metal precursor and NH3 as the reaction gas, single-atom loading is carried out at a deposition temperature of 300 °C. Each deposition cycle includes: a Pd(hfac)2 pulse of 0.3 s, followed by an inert gas purge of 8 s; an NH3 pulse of 0.2 s, and then an inert gas purge of 12 s, and the cycle is repeated 12 times to achieve uniform and stable loading of Pd single atoms on the support surface.

[0040] Characterization of single-atom density and coordination structure: By means of aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observation and statistics, the single-atom density is 7.2×10 13 sites / g. Synchrotron radiation X-ray absorption fine structure spectroscopy (XAFS) shows the formation of a Pd-N3 coordination structure with a coordination number of 4.2. This structure enhances the interaction between the active center and the support, which is crucial for catalytic performance. The SEM image of the nitrogen-doped graphene catalyst is shown in Figure 4 .

[0041] Calculation of CO2 adsorption energy: Based on density functional theory (DFT), using professional quantum chemistry calculation software, the CO2 adsorption energy is obtained as 2.1 eV, indicating that the catalyst has good adsorption capacity for CO2, which is beneficial to the initiation of the reaction.

[0042] 3. Composite modification with promoters Composition and ratio of promoters: The promoters consist of a K2O-Na2O solid solution (K:Na = 1:0.5, particle size 1.3 nm) and a CeO2-La2O3 core-shell structure (Ce:La = 0.8:1). This specific composition and structure design aims to achieve the synergistic effect between the promoters and improve the overall performance of the catalyst.

[0043] Determination of the total promoter loading: By X-ray fluorescence spectroscopy (XRF) analysis, it is determined that the total promoter loading accounts for 6 wt% of the total mass of the catalyst, ensuring the best synergistic effect of the promoters in the catalyst system.

[0044] Ce³⁺ / Ce 4 ⁺ ratio determination: Using the XPS peak fitting method, the Ce³⁺ / Ce 4 ⁺ ratio is determined to be 1.5. This ratio has a key impact on the redox performance of the catalyst and thus affects the catalytic reaction process.

[0045] Calculation of metal migration energy barrier: By means of a DFT-based calculation method, the simulated metal migration energy barrier is obtained as 2.0 eV, effectively ensuring the stability of the active components of the catalyst during the reaction process.

[0046] 4. Catalyst activation treatment In a reactor with precise temperature and gas flow control, a mixed gas of H2 and CO2 with a volume ratio of 3:1 is introduced. It is heated to 220 °C at a rate of 10 °C / min and held for 2 hours, and then cooled to 80 °C at a rate of 5 °C / min for surface hydroxylation treatment. Through in-situ Fourier transform infrared spectroscopy real-time monitoring, when the intensity of the characteristic peak at 1590 cm -1 decreases to less than 3 times the baseline noise, the catalyst activation is completed. This activation process optimizes the surface structure and active sites of the catalyst and improves the catalytic performance.

[0047] 5. Performance Testing Reaction condition setting: The prepared catalyst was placed in a fixed-bed reactor, and the reaction temperature was set at 200 °C and the pressure at 3 MPa H2 to simulate the actual industrial reaction conditions for evaluating the actual application performance of the catalyst.

[0048] Determination of CO2 conversion and formic acid selectivity: Gas chromatography analysis was used, and the CO2 conversion reached 89%; high-performance liquid chromatography (HPLC) was used for detection, and the formic acid selectivity was 97%, indicating that the catalyst has high activity and high selectivity in the reaction of hydrogenating CO2 to formic acid.

[0049] Stability test: After 500 h of stability test, the catalyst activity decreased by less than 3%, and no agglomeration of Pd single atoms was found by HAADF-STEM observation, proving its good stability.

[0050] 6. Microstructure and Energy Level Characterization Through HAADF-STEM characterization, the transition metal Pd single atoms were statistically uniformly distributed on the surface of the nitrogen-doped graphene support, and the adjacent single atom spacing was ≥ 0.5 nm. The d-band center value of the single atom site was measured to be -2.5 eV by electron energy loss spectroscopy, forming an energy level matching of 0.5 eV with the lowest unoccupied molecular orbital (LUMO) energy level of CO2, and this energy level matching plays a key role in the efficient progress of the catalytic reaction.

[0051] Example 2: TiO2 Nanosheet-Based Catalyst (Molecular Formula: Pt / TiO2{001}-(K 0.7 La 0.3 )O@SiO2) 1. Support Defect Engineering Support selection and initial characteristics: TiO2 nanosheets were selected as the support. Through precise determination by X-ray diffraction (XRD), the initial exposure of the {001} crystal plane was 75%, and this crystal plane has unique activity in the reaction of hydrogenating CO2.

[0052] Gradient hydrogenation treatment process: The TiO2 nanosheets were placed in a tube furnace, first treated at 500 °C for 1 hour under H2 / N2 (volume ratio 1:4), and then switched to pure H2 and treated at 580 °C for 0.5 hour. Through this gradient hydrogenation treatment, the defect structure of the support was precisely regulated.

[0053] Characterization of lattice oxygen vacancy distribution and EPR signal intensity: Using XPS depth profiling technology, it was revealed that the surface-to-bulk oxygen vacancy concentration ratio was 2.5:1. At the same time, the oxygen vacancy concentration was monitored in real time by in-situ electron paramagnetic resonance spectroscopy (EPR). The treatment was terminated when the signal intensity reached 420 a.u. at g = 2.003, and the oxygen vacancy concentration was 0.9 vacancies / nm² by XPS quantitative analysis.

[0054] {001} Facet Exposure Variation: Through XRD analysis again, the exposure of the {001} facet of the treated TiO2 nanosheets is increased to 83%, effectively increasing the active sites of the catalyst.

[0055] 2. Single-Atom Loading Determination of Active Center and Loading Amount: Based on the evaluation of the catalytic performance of the Pt element, Pt is selected as the single-atom active center, and its loading amount is determined to be 1.5 wt% by ICP-MS.

[0056] Setting of Atomic Layer Deposition (ALD) Parameters: Using the ALD technique, with Pt(acac)2 as the metal precursor and Cl2 as the reaction gas, at a deposition temperature of 280 °C, each deposition cycle is as follows: Pt(acac)2 pulse for 0.4 s, inert gas purge for 10 s; Cl2 pulse for 0.2 s, inert gas purge for 15 s, and cycle 10 times to achieve uniform loading of Pt single atoms on the surface of the support.

[0057] Characterization of Single-Atom Density and Coordination Structure: By observing and counting with HAADF-STEM, the single-atom density is 5.8×10¹³ sites / g. The coordination structure is determined to be Pt-O4 with a coordination number of 3.8 by XAFS technology, and this structure is crucial for the stability and catalytic activity of Pt single atoms.

[0058] Calculation of CO2 Adsorption Energy: Based on DFT theory, the CO2 adsorption energy is calculated to be 2.3 eV by quantum chemistry calculation, indicating that the catalyst has a strong adsorption capacity for CO2, which is beneficial to the reaction.

[0059] 3. Promoter Composite Modification Characterization of Promoter Composition and Structure: The promoter is a K2O-La2O3 / SiO2 composite structure, which is prepared by the supercritical fluid-assisted impregnation method. The formation of M-O-Si bonding is verified by Fourier transform infrared spectroscopy (FTIR), and the characteristic peak position is at 1080 - 1120 cm -1 , proving the successful construction of the composite structure.

[0060] Determination of Total Promoter Loading: Using XRF analysis to determine that the total promoter loading accounts for 8 wt% of the total mass of the catalyst, ensuring the best performance of the promoter in the catalyst system.

[0061] Calculation of Metal Migration Energy Barrier: By means of DFT calculation method, the metal migration energy barrier is obtained as 1.9 eV, ensuring the stability of the active components of the catalyst during the reaction process.

[0062] 4. Catalyst Activation Treatment A mixed gas of H2 and CO2 with a volume ratio of 3:1 was introduced into the reactor, heated to 230 °C at a rate of 10 °C / min, held for 2 hours, and then cooled to 80 °C at a rate of 5 °C / min for surface hydroxylation treatment. Monitored by in-situ Fourier transform infrared spectroscopy, when the intensity of the characteristic peak at 1590 cm -1 dropped below 3 times the baseline noise, the catalyst activation was completed.

[0063] 5. Performance Testing Reaction conditions setting: In a fixed-bed reactor, the reaction temperature was set at 200 °C and the pressure was 3 MPa H2 to provide stable conditions for the reaction.

[0064] Determination of CO2 conversion and formic acid selectivity: Through gas chromatography analysis, the CO2 conversion reached 91%; using HPLC analysis, the formic acid selectivity was 95%, demonstrating the excellent performance of the catalyst in the target reaction.

[0065] Determination of H2 dissociation TOF: The turnover frequency (TOF) of H2 dissociation was accurately determined to be 2.1 s -1 through kinetic tests, reflecting the high-efficiency dissociation ability of the catalyst for H2.

[0066] Stability test: After 200 h of stability test, it was found by EPR monitoring that the oxygen vacancies remained at 0.8 vacancies / nm², proving the structural stability of the catalyst during the long-term reaction process. The relationship between the {001} crystal plane exposure, oxygen vacancy concentration, and EPR signal intensity in this example is shown in Figure 3 . The SEM image of the TiO2 nanosheet-based catalyst is shown in Figure 5 .

[0067] 6. Microstructure and Energy Level Characterization Through HAADF-STEM characterization, single atoms of transition metal Pt were statistically uniformly distributed on the surface of the TiO2 nanosheet support, and the adjacent single-atom spacing was ≥0.5 nm. The d-band center value of the single-atom site was measured to be -2.4 eV by electron energy loss spectroscopy, forming an energy level matching of 0.6 eV with the LUMO energy level of CO2, providing favorable electronic structure conditions for CO2 adsorption and activation.

[0068] Example 3: Mesoporous silica-based catalyst (molecular formula Pd / (mSiO 2- SO3H-OH)-(K 0.5 Na 0.5 )2O@CeO2-L a2 O3) 1. Support Selection and Treatment Mesoporous silica (mSiO2) with a hierarchical pore structure is selected as the carrier. Its pore sizes are distributed in three levels: micropores of 1 - 3 nm, mesopores of 5 - 8 nm, and macropores of 20 - 30 nm, which is beneficial to the diffusion of reactants and products. Sulfonic acid groups (-SO3H) and hydroxyl groups (-OH) are introduced into the inner wall of the pores through chemical modification. The total functional group density is 6 groups / nm², and the molar ratio of sulfonic acid groups to hydroxyl groups is 1:1.5 to adjust the surface chemical properties of the pores.

[0069] 2. Carrier defect engineering The mesoporous silica is pretreated and heat-treated under specific temperature and atmosphere to increase the surface active sites, creating favorable conditions for the subsequent bonding reaction between the promoter and the carrier.

[0070] 3. Single-atom loading Determination of active center and loading amount: Based on the analysis of the activity and selectivity of Pd element in the catalytic reaction, Pd is selected as the single-atom active center, and its loading amount is accurately determined to be 1.8 wt% by ICP-MS.

[0071] Setting of atomic layer deposition (ALD) parameters: Using the ALD technique, with Pd(hfac)2 as the metal precursor and NH3 as the reaction gas, at a deposition temperature of 320 °C, each deposition cycle is: Pd(hfac)2 pulse for 0.4 s, inert gas purge for 9 s; NH3 pulse for 0.2 s, inert gas purge for 13 s, and cycle 13 times to achieve uniform loading of Pd single atoms on the inner surface of the mesoporous silica pores.

[0072] Characterization of single-atom density and coordination structure: Through HAADF-STEM observation and statistics, the single-atom density is 6.5×10 13 sites / g. Synchrotron radiation XAFS shows the formation of a Pd-O-C coordination structure with a coordination number of 4.0, which helps to improve the activity and stability of the catalyst.

[0073] Calculation of CO2 adsorption energy: The CO2 adsorption energy calculated by DFT is 2.0 eV, indicating that the catalyst has good CO2 adsorption performance and is beneficial to the CO2 hydrogenation reaction.

[0074] 4. Promoter composite modification Composition and ratio of promoters: The promoter consists of a K2O-Na2O solid solution (K:Na = 1:1, particle size 1.2 nm) and a CeO2-La2O3 core-shell structure (Ce:La = 1:1) to achieve the synergistic effect between the promoters and improve the catalyst performance.

[0075] Determination of the total promoter loading amount: XRF analysis is used to determine that the total promoter loading amount accounts for 5 wt% of the total mass of the catalyst to ensure the best performance of the promoter.

[0076] Ce 3+ / Ce 4+ Ratio determination: The Ce 3+ / Ce 4+ ratio is 1.4, which affects the redox performance of the catalyst.

[0077] Calculation of metal migration energy barrier: The metal migration energy barrier is calculated to be 1.85 eV by means of DFT calculation, ensuring the stability of the active components of the catalyst.

[0078] Characterization of M-O-Si bonding: Through FTIR characterization, an obvious characteristic peak appears at 1080 - 1120 cm -1 −1, indicating that the promoter and the support form a stable composite structure through M-O-Si bonding.

[0079] 5. Catalyst activation treatment A mixed gas of H2 and CO2 with a volume ratio of 3:1 is introduced into the reactor, heated to 210 °C at a rate of 10 °C / min, held for 2 hours, and then cooled to 80 °C at a rate of 5 °C / min for surface hydroxylation treatment. Through in-situ Fourier transform infrared spectroscopy detection, when the intensity of the characteristic peak at 1590 cm -1 −1 drops below 3 times the baseline noise, the activation is completed, and the catalyst reaches the optimal active state. The SEM image of the mesoporous silica-based catalyst is shown in Figure 5 .

[0080] 6. Performance test Reaction condition setting: In a fixed-bed reactor, the reaction temperature is set at 200 °C and the pressure is 3 MPa H2, providing suitable reaction conditions.

[0081] Determination of CO2 conversion rate and formic acid selectivity: Gas chromatography analysis shows that the CO2 conversion rate is 86%, and HPLC analysis indicates that the formic acid selectivity is 93%, reflecting the good performance of this catalyst in the reaction of CO2 hydrogenation to formic acid.

[0082] Stability test: After 400 h of stability test, HAADF-STEM verifies that the single-atom distribution remains basically uniform, and the activity decline is less than 5%, proving its good stability.

[0083] 7. Microstructure and energy level characterization Through HAADF-STEM characterization, the transition metal Pd single atoms are statistically uniformly distributed on the surface of the mesoporous silica support, and the distance between adjacent single atoms is ≥0.5 nm. The d-band center value of the single-atom site is measured to be -2.6 eV by electron energy loss spectroscopy, forming an energy level matching of 0.45 eV with the LUMO energy level of CO2, providing a favorable electronic structure basis for the catalytic reaction.

[0084] Comparative example 1 1. Catalyst preparation: The nitrogen-doped graphene supported Pd catalyst was prepared by a conventional method without fine carrier defect engineering, promoter composite modification, and precise single-atom loading regulation. Specifically, the nitrogen-doped graphene was not treated by microwave plasma etching and directly loaded with Pd single atoms with a loading amount of 4 wt%, and no K2O-Na2O solid solution and CeO2-La2O3 core-shell structure promoters were added.

[0085] 2. Performance test: In the same fixed-bed reactor as in the example, the CO2 hydrogenation to formic acid reaction was carried out at a reaction temperature of 200 °C and a pressure of 3 MPa H2. Gas chromatography analysis showed that the CO2 conversion rate was 65%, and HPLC analysis showed that the formic acid selectivity was 82%. Compared with Example 1, the CO2 conversion rate and formic acid selectivity were significantly lower, indicating that the catalyst performance was poor due to the lack of fine regulation.

[0086] Comparative Example 2 1. Catalyst preparation: Using TiO2 nanosheets as the carrier, the oxygen vacancies and the exposure degree of the {001} crystal plane were not regulated by gradient hydrogenation treatment. Pt single atoms were directly loaded by the ordinary impregnation method with a loading amount of 0.3 wt%, and the promoter was a single K2O with a too high loading amount, accounting for 12 wt% of the total catalyst mass.

[0087] 2. Performance test: Under the same reaction conditions, the CO2 conversion rate was 45% and the formic acid selectivity was 78%. At the same time, the single-atom density was only 2×10¹³ sites / g, and the CO2 adsorption energy was 1.5 eV. Compared with Example 2, all performance indicators decreased significantly, indicating that improper carrier pretreatment, unreasonable loading of active centers, and inappropriate use of promoters had a serious impact on the catalyst performance.

[0088] Comparative Example 3 1. Catalyst preparation: Mesoporous silica was selected as the carrier, but its pore size grading and surface functional group modification were not carried out. Pd single atoms were loaded by the traditional physical mixing method with a loading amount of 0.2 wt%. Only the CeO2-La2O3 core-shell structure was used in the promoter part, and the ratio was unbalanced (Ce:La = 3:1), and the total loading amount was 10 wt%, and no M-O-Si bonding structure was formed.

[0089] 2. Performance test: In the CO2 hydrogenation to formic acid reaction under the same reaction conditions as in the example, that is, in a fixed-bed reactor at a reaction temperature of 200 °C and a pressure of 3 MPa H2, gas chromatography analysis showed that the CO2 conversion rate was 42% and HPLC analysis showed that the formic acid selectivity was 70%. The stability test found that the activity decreased significantly by more than 10% after 100 h of reaction. Compared with Example 3, due to the unoptimized carrier structure, insufficient loading of active centers, unbalanced promoter ratio, and failure to form an effective bonding structure, the catalyst performance was greatly reduced and the stability was poor.

[0090] Among them, the comparison chart of CO2 conversion rate and formic acid selectivity is shown in Figure 1 , and the correlation chart of single-atom density and CO2 adsorption energy is shown in Figure 2 .

[0091] Comparison charts of the data of the examples and comparative examples Sample <![CDATA[CO2 conversion rate (%)]]> Formic acid selectivity (%) Example 1 89 97 Example 2 91 95 Example 3 86 93 Comparative Example 1 65 82 Comparative Example 2 45 78 Comparative Example 3 42 70 Table 1 Comparison of CO2 conversion rate and formic acid selectivity It can be intuitively seen from Table 1 that the catalysts of the examples are significantly superior to the comparative examples in terms of CO2 conversion rate and formic acid selectivity. This indicates that through steps such as carefully designed carrier treatment, active center loading, and promoter composite modification, the performance of the catalyst in the reaction of hydrogenating CO2 to formic acid can be significantly improved.

[0092]

[0093] Table 2 Correlation between single-atom density and CO2 adsorption energy It can be seen from Table 2 that the single-atom density is positively correlated with the CO2 adsorption energy (R² = 0.94). The higher single-atom density in the examples corresponds to a higher CO2 adsorption energy, while the lower single-atom density in Comparative Example 2 results in a lower CO2 adsorption energy. This shows that within a certain range, increasing the single-atom density helps to enhance the CO2 adsorption capacity of the catalyst, and thus may improve the catalytic reaction efficiency, providing an important basis for optimizing the catalyst design.

[0094] Matters not covered by this invention are well-known technologies.

[0095] The above examples are only for illustrating the technical concept and features of the present invention, and their purpose is to enable those familiar with this technology to understand the content of the present invention and implement it accordingly, and shall not be used to limit the protection scope of the present invention. All equivalent changes or modifications made according to the spirit and essence of the present invention should be covered within the protection scope of the present invention.

Claims

1. A catalyst for hydrogenating CO2 to formic acid, characterized in that: It consists of an active center, a carrier, and a promoter, where: (a) The active center is an atomically dispersed transition metal single atom, and the transition metal is selected from at least one of Pd and Pt, and is anchored on the surface of the carrier through M-N-C or M-O-C coordination bonds. The total metal loading is 0.5-3.5 wt%, the single atom density ≥ 5×10 13 sites / g, and the CO2 adsorption energy is 1.9-2.3 eV; (b)The carrier is selected from two-dimensional materials or mesoporous materials with surface defects. When the carrier is a two-dimensional material, the specific surface area > 300 m² / g and the surface defect density is 1×10 13 -8×10 13 defects / m²; when the carrier is a TiO2 nanosheet, the exposure degree of the {001} crystal plane > 80% and the lattice oxygen vacancy concentration is 0.7 - 1.0 vacancies / nm²; (c) The promoter contains a K2O-Na2O solid solution nanocluster and a CeO2-La2O3 core-shell structure. The total addition amount of the promoter accounts for 1-8 wt% of the total mass of the catalyst, and the Ce 3+ / Ce 4+ ratio is 1.2-1.8, and the promoter forms a composite structure with the carrier, and the metal migration energy barrier ≥ 1.8 eV.

2. The CO2 hydrogenation to formic acid catalyst according to claim 1, characterized in that: The two-dimensional material is layered boron nitride or nitrogen-doped graphene, where the nitrogen-doped graphene has a nitrogen content of 4-6 at%, and the proportion of pyridine nitrogen is ≥70%. The edge sites of the nitrogen-doped graphene form an M-N3 coordination structure with metal single atoms.

3. The CO2 hydrogenation to formic acid catalyst according to claim 1, characterized in that: The mesoporous material is mesoporous silica or metal-organic framework-derived carbon material with a hierarchical pore structure. Its pore size shows a three-stage distribution: micropores of 1-3 nm, mesopores of 5-8 nm, and macropores of 20-30 nm. The inner wall of the pore channels is modified with a bifunctional group of sulfonic acid groups and hydroxyl groups, and the total functional group density is ≥5 groups / nm², where the molar ratio of sulfonic acid groups to hydroxyl groups is 1:1 - 1:

2.

4. The CO2 hydrogenation to formic acid catalyst according to claim 1, characterized in that: In the said auxiliary agent, the alkali metal oxide is a K2O-Na2O solid solution nanocluster with an average particle size < 1.5 nm, and the rare earth oxide is a CeO2-La2O3 core-shell structure, where Ce 3+ / Ce 4+ The ratio is determined by X-ray photoelectron spectroscopy to be 1.2 - 1.8, and the auxiliary agent and the carrier defect sites form a stable composite structure through M-O-Si bonding.

5. A method for preparing a CO2 hydrogenation to formic acid catalyst according to any one of claims 1-4, characterized in that, It includes the following control steps: (1) Carrier defect engineering: The two-dimensional material is treated by microwave plasma etching at a power of 100-150 W, a frequency of 2.45 GHz, and an Ar / N2 mixed gas for 10-20 minutes to generate edge dangling bonds and topological defects; The TiO2 nanosheets are subjected to gradient hydrogenation treatment. First, they are treated in a H2 / N2 mixed gas at 450-500 °C for one hour, and then in pure H2 at 550-600 °C for 0.5 hour to form a gradient oxygen vacancy distribution, where the surface-to-bulk oxygen vacancy concentration ratio is ≥2:1, and the treatment is terminated when the signal intensity at g = 2.003 is monitored by in-situ EPR to reach 300-500 a.u.; (2) Precise single-atom loading: The pulsed atomic layer deposition technique is used, with the precursors being alternating pulses of metal organic complexes and halides. The deposition temperature is 250-350 °C. Each deposition cycle includes: metal precursor pulse for 0.2-0.5 s → inert gas purge for 5-10 s, reaction gas pulse for 0.1-0.3 s → inert gas purge for 8-15 s, and the number of cycles is 8-15 times. During this period, the by-product concentration is monitored by in-situ mass spectrometry to adjust the pulse timing; (3) Promoter composite modification:

6. The preparation method of the CO2 hydrogenation to formic acid catalyst according to claim 5, characterized in that: The supercritical fluid-assisted impregnation method is used to co-assemble the promoter precursor and the surfactant in a supercritical CO2 environment at a temperature of 35-45 °C and a pressure of 8-9 MPa, and then calcined in stages at 400-500 °C. First, it is heated to 300 °C at a rate of 2 °C / min and held for 1 hour, and then heated to the target temperature at a rate of 5 °C / min and held for 2 hours.

7. The preparation method of the CO2 hydrogenation to formic acid catalyst according to claim 5, characterized in that: The surfactant described in step (3) is a block copolymer PS-b-PEO with a number average molecular weight of 5000-50000 Da, a molecular weight distribution index ≤ 1.1, and forms a hydrogen bond complex with the precursor of the additive. The mesoscopic structure evolution is monitored by small-angle X-ray scattering. When a characteristic scattering peak with q = 0.1-0.2 nm -1 appears, the assembly is completed.

8. The preparation method of the CO2 hydrogenation to formic acid catalyst according to claim 5, characterized in that: In step (2), synchrotron radiation X-ray absorption fine structure spectroscopy is used to monitor the metal coordination environment in real time. When the M-O / M-N coordination number is detected to reach 4±0.5, the deposition is terminated, and the deposition temperature is dynamically adjusted by ±20 °C through a feedback system. During the gradient hydrogenation treatment process, the oxygen vacancy concentration is monitored in real time by in-situ electron paramagnetic resonance spectroscopy. When the signal intensity at g = 2.003 reaches 300-500 a.u., the feedback system is triggered to switch the treatment atmosphere. The feedback system dynamically adjusts the hydrogen partial pressure according to the following parameters: (a) Taking the change rate of the EPR signal intensity as the input parameter, when the rate > 5 a.u. / min, the hydrogen partial pressure is reduced by 0.3-0.5 kPa / min; (b) The gas input is precisely controlled by a mass flow controller, and the adjustment accuracy of the hydrogen partial pressure is ±0.5 kPa; (c)When switching to the pure H2 stage, keep the total system pressure constant, with a fluctuation range ≤ 1 kPa.

9. The CO2 hydrogenation to formic acid catalyst according to any one of claims 1-4, characterized in that: Characterized by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy, the transition metal single atoms are statistically uniformly distributed on the surface of the support, the distance between adjacent single atoms is ≥ 0.5 nm, and the d-band center value of the single-atom sites measured by electron energy loss spectroscopy is -2.8 to -2.3 eV, forming an energy level matching of 0.4 - 0.7 eV with the LUMO energy level of CO2.

10. The preparation method of the CO2 hydrogenation to formic acid catalyst according to any one of claims 5-8, characterized in that: It also includes step (4) catalyst activation treatment: Introduce a mixed gas with a volume ratio of H2 to CO2 of 3:1 into the reactor, heat it to 200 - 250 °C at a rate of 10 °C / min and hold for 2 hours, then cool it to 80 °C at a rate of 5 °C / min for surface hydroxylation treatment. Finally, when the characteristic peak intensity at 1590 cm -1 decreases to below 3 times the baseline noise as detected by in-situ Fourier transform infrared spectroscopy, the activation is completed.

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