Polyoxometallate cluster, preparation method and application of polyoxometallate cluster in CO2 hydro-conversion process

Abstract

The invention relates to a polyoxometallate cluster, a preparation method and application of the polyoxometallate cluster in a CO2 hydro-conversion process, and belongs to the field of catalytic materials. The invention provides an Anderson type PtMo6O24 cluster which is positioned in a stable metal organic framework, has a clear molecular structure and has a catalytic effect on low-temperature CO2 hydrogenation. After the performance of the cluster is operated for a long time, the activity or methanol selectivity of the cluster does not show an attenuation sign, and the one-way yield of the cluster also exceeds that of the most advanced heterogeneous catalyst. In combination with in-situ spectrum and DFT (Discrete Fourier Transform) calculation, the formation of CH3OH is mainly dominated by reverse water-gas shift and a subsequent CO hydrogenation path, and an HCOO path possibly serves as an auxiliary path. The cluster structure with a clear structure is used for CO2 hydrogenation, and has the characteristics of high activity and low-temperature catalysis.

Description

Technical Field

[0001] This invention relates to a polyoxometalate cluster, its preparation method, and its use in the CO2 hydrogenation conversion process, belonging to the field of catalytic materials. Background Technology

[0002] Converting the primary greenhouse gas CO2 into high-value chemicals is a promising solution for establishing a sustainable closed-loop system to address the challenges of climate change. Methanol is one of the most valuable products from CO2 conversion because it is an important and versatile feedstock in chemical production and the energy sector. The hydrogenation of CO2 to methanol is an exothermic process.

[0003] ;

[0004] Higher temperatures are thermodynamically unfavorable for methanol synthesis. Therefore, developing catalysts capable of low-temperature operation has been a research hotspot. Benchmark Cu / ZnO / Al2O3 catalysts typically operate at high temperatures (≥ 250 °C), leading to high energy consumption and low CH3OH selectivity. Introducing H2-activating components (such as Pd and Ni) into metal oxide catalysts (such as In2O3 / ZrO2 and ZnO / ZrO2) can improve catalytic activity at relatively lower temperatures (200-250 °C). However, this often comes at the cost of reduced methanol selectivity, as CO2 is over-hydrogenated to methane or reduced to CO via reverse water-gas shift (RWGS). The complexity of the properties of such heterogeneous catalysts, including multiple active sites located on metal particles, metal oxides, supports, and their interfaces, makes establishing a clear relationship between catalyst structure and performance challenging. Summary of the Invention

[0005] To address the aforementioned problems, this invention proposes an Anderson-type PtMo6O with a well-defined molecular structure located within a stable metal-organic framework. 24 The cluster exhibits catalytic activity for low-temperature CO2 hydrogenation. After prolonged operation, the cluster showed no signs of performance degradation in either activity or methanol selectivity, and its single-pass yield surpassed that of state-of-the-art heterogeneous catalysts.

[0006] A composite catalytic material comprises platinum and molybdenum-containing polyoxometalate clusters and a porous metal-organic framework; wherein the polyoxometalate clusters are confined within the channels of the metal-organic framework, the metal-organic framework having mesoporous and microporous structures, and the polyoxometalate clusters are specifically confined within the c-pores of the metal-organic framework, rather than located in the ab-plane channels; the active metal atoms of the polyoxometalate clusters can contact from the mesopores and micropores, and the catalytic material is subjected to hydrogen reduction treatment to form an active structure containing platinum-molybdenum bonds; the polyoxometalate clusters are distributed in a monodisperse or isolated state within the channels of the metal-organic framework.

[0007] The polyoxometalate clusters are Anderson-type polyoxometalate clusters, and their chemical formula is preferably [PtMo6O]. 24 ] n- , where n is an integer; the metal-organic framework is a zirconium (Zr)-based or hafnium (Hf)-based metal-organic framework; preferably, the metal-organic framework is NU1K.

[0008] The central platinum atom in the polyoxometalate cluster can be replaced by other noble metals from Groups 8-10; the skeletal molybdenum atom in the polyoxometalate cluster can be partially or completely replaced by tungsten (W).

[0009] A method for preparing the composite catalytic material includes the following steps:

[0010] a) Platinum and molybdenum-containing polyoxometalate clusters are loaded into the pores of a porous metal-organic framework by impregnation to obtain an intermediate material;

[0011] b) The intermediate material obtained in step a) is reduced in a hydrogen-containing atmosphere to form the composite catalyst material.

[0012] During the reduction process described in step b), the temperature is 150-250℃, preferably 180-220℃; the time is 0.5-4h, preferably 1-2h; and the volume fraction of hydrogen in the hydrogen-containing atmosphere is 5-25%.

[0013] In step a), the mass ratio of the platinum and molybdenum-containing polyoxometalate clusters to the porous metal-organic framework is 1:1-5:1.

[0014] The platinum and molybdenum-containing polyoxometalate clusters are PtMo6O 24 The synthesis steps include: mixing an aqueous solution containing a molybdenum precursor with an aqueous solution containing a platinum precursor, then adding acid to adjust the pH of the mixed solution for crystallization, and separating to obtain the PtMo6O. 24Preferably, the molybdenum-containing precursor is sodium molybdate, and the platinum-containing precursor is sodium hexahydroxyplatinate; the molar ratio of Mo to Pt in the molybdenum-containing precursor and the platinum-containing precursor is 5:1-7:1; the pH of the mixed solution is adjusted to 1.5-2.0 with acid.

[0015] The porous metal-organic framework is NU1K, and its synthesis steps include: mixing a zirconium source and benzoic acid in an organic solvent and heating to obtain a solution containing zirconium nodes; adding a 1,3,6,8-tetra(4-carboxyphenyl)pyrene solution and trifluoroacetic acid to the solution containing zirconium nodes, mixing evenly, and heating to react; after the reaction is completed, washing and drying are performed to obtain NU1K; preferably, the zirconium source is zirconium oxychloride octahydrate, and the organic solvent is N,N-dimethylformamide; preferably, the heating temperature is 100-140℃; and the reaction time is 12-24h.

[0016] The application of the composite catalytic material in the catalytic hydrogenation of carbon dioxide to methanol.

[0017] In the carbon dioxide hydrogenation reaction, the reaction temperature is 100-200℃; the reaction pressure is 1.5-8.0 MPa, preferably 4.0-6.0 MPa; and the molar ratio of hydrogen to carbon dioxide in the reaction feed gas is 1:1-5:1, preferably 3:1.

[0018] The reaction temperature is 160-200℃, preferably 180℃.

[0019] Under reaction conditions of 180℃ and 5.0 MPa, after 5000 hours of continuous operation, the catalytic material showed no significant decrease in its conversion rate of carbon dioxide and its selectivity for methanol.

[0020] Within the reaction temperature range of 100-200℃, the single-pass yield of methanol is 1.2-13.1%; the selectivity for methanol is 64-81%.

[0021] The beneficial effects of this invention are: This invention utilizes PtMo6O 24Clusters are precisely confined within the c-pores of the MOF, forming a highly active Pt-Mo bond structure upon hydrogen activation. This catalyst efficiently catalyzes the hydrogenation of CO2 to methanol at relatively low temperatures (100-200℃), achieving methanol single-pass yields of 1.2-13.1% and selectivity as high as 64-81%, significantly outperforming existing benchmark catalysts, and even initiating the reaction at room temperature. Thanks to the strong physical confinement of polyoxometalate monoclusters within the MOF pores, aggregation and loss of active components during the reaction are effectively prevented. After 5000 hours of continuous operation at 180℃ and 5.0 MPa, neither catalytic activity nor methanol selectivity showed significant decline. The mesoporous / microporous hierarchical structure of the MOF promotes rapid mass transfer of gas molecules, and the isolated, dispersed clusters maximize the exposure of active sites, achieving quasi-homogeneous and highly efficient catalysis. Attached Figure Description

[0022] Figure 1 Figure 1 shows the catalytic performance test results of different catalysts in the hydrogenation of CO2 to methanol. Among them, (a) PtMo6O 24 Flowchart of @NU1K preparation. (b) At 1500 ml·g cat -1 ·h -1 Under GHSV conditions, Cu / ZnO / Al2O3 (CuZnAl) and PtMo6O 24 @NU1K catalyst's net space-time yield of methanol in the 100-200℃ range (STY) methanol (c) Pt@NU1K, PtMo6O 24 / ZrO2, PtV9O 28 @NU1K, PtW6O 24 @NU1K,PtMo6O 24 SC, PtMo6O 24 @NU901, PtMo6O 24 @NU1008, PtMo6O 24 @Hf-NU1K and PtMo6O 24 @NU1K catalyst CO2 conversion and product selectivity at 160℃, 180℃ and 200℃, respectively. (d) PtMo6O 24 @NU1K at 1500ml·g cat -1 ·h -1 Stability tests were conducted on CO2 hydrogenation under specific conditions, and selectivity (S) and conversion (C) were monitored over 5000 h. Activity tests were performed in a tubular fixed-bed reactor at a pressure of 50 bar and an H2 / CO2 ratio of 3:1.

[0023] Figure 2 PtMo6O under H2 activation and reaction conditions 24 The in-situ X-ray scattering characterization results of @NU1K are shown in the figure. (a) shows the PtMo6O under H2 activation and (b) reaction conditions, as monitored by in-situ XRD. 24 @NU1K crystallinity and topological evolution during temperature cycling (room temperature → 200℃ → room temperature). (cf) PtMo6O revealed by DED 24 Location of clusters within the NU1K framework: (c) fresh sample, (d) H2 activation, (e) reaction conditions at 200 °C, and (f) post-reaction. Isolated PtMo6O confined in the c-channels of NU1K by in-situ dPDF tracking during (g) H2 activation and (h) reaction. 24 The local structural evolution of the cluster, the temperature rises from room temperature to 200°C and then falls back.

[0024] Figure 3 : Native and reduced PtMo6O 24 The microstructure characterization results of the @NU1K catalyst are shown in the figure. (a) X-ray absorption near-edge structure spectrum (Pt) indicates the oxidation state; the amplified portion shows PtMo6O. 24 @NU1K-R fitting curves and parameters on the EXAFS data of Pt L3-edge. (b) Pearson correlation matrices of simulated PDFs and experimental dPDFs identified the most promising candidate models for further analysis. (c) PtMo6O was determined based on in-situ DED, dPDF and XAFS analyses. 24 The location and atomic structure of the @NU1K-R cluster. (d) CO-DRIFTS of the two samples preserved the CO adsorption pattern observed in the DRIFTS. (e) PtMo6O recorded at different temperatures. 24 In-situ DRIFTS activated by H2 on @NU1K. (f) Activated PtMo6O 24 @NU1K catalyst, NU1K, Mo7O 24 EPR studies of @NU1K and fresh samples. Magnified portion shows... Figure 3 A magnified view of the oxygen vacancy region in f to highlight microscopic features. (gh) PtMo6O observed along the c-axis. 24 @NU1K Cs-corrected HAADF-STEM images at different magnifications, showing PtMo6O 24 The clusters are precisely encapsulated within the c-channels of NU1K, and (i) shows the atomic model corresponding to the red dashed box in h. (jm)PtMo6O 24 The EDS elemental distribution map of @NU1K shows a uniform distribution of zirconium, platinum, and molybdenum. Detailed Implementation

[0025] The geometric or electronic confinement of metal heteroatoms within precisely molecular-level polyoxometalate clusters, combined with the confinement of these clusters by a porous crystal framework, transforms heterogeneous gas-solid catalysis into quasi-homogeneous catalysis. Here, the present invention relates to PtMo6O 24 @NU1K, where PtMo6O 24 Clusters were encapsulated in the c-pores of zirconium-based MOFNU1K using a simple impregnation process. Figure 1 (a) PtMo6O is used as a highly efficient and stable catalyst for the low-temperature CO2 hydrogenation to methanol. Compared with previously reported Pt-based catalysts, PtMo6O 24 The @NU1K catalyst exhibits high CH3OH activity, thus enabling continuous methanol synthesis at low temperatures.

[0026] Example 1 Na2[H6PtMo6O] 24 ·29H2O (abbreviated as PtMo6O) 24 Synthesis of )

[0027] Pale yellow platinum crystals were obtained from a mixed aqueous solution containing Na₂Pt(OH)₆ and Na₂MoO₄·2H₂O. 0.7328 g of Na₂MoO₄·2H₂O was dissolved in 5 mL of H₂O, and 0.1732 g of Na₂Pt(OH)₆ was dissolved in 10 mL of H₂O. The two solutions were then mixed and stirred for 30 min. Crystallization began after adjusting the pH to 1.8 with 1 mol / L HNO₃. The crystals were collected by vacuum filtration.

[0028] Example 2: Synthesis of NU1000 (abbreviated as NU1K)

[0029] 4.85 g ZrOCl2·8H2O and 100 g benzoic acid were mixed in 300 mL DMF in a glass bottle and dissolved by sonication. The clear solution was kept in an oven at 100 °C for 1 h. Simultaneously, 2 g H4TBAPy was added to 100 mL DMF and heated to 100 °C for 1 h. After cooling to room temperature, 1,3,6,8-tetra(4-carboxyphenyl)pyrene H4TBAPy solution and 2 mL LTF were added to the pre-prepared solution containing Zr nodes and sonicated for 20 min. The yellow suspension was placed in an oven preheated to 120 °C overnight (18 h). After cooling to room temperature, the yellow powder material was washed three times with DMF, soaking for about 1 h between washes. To remove the coordinating modifier (benzoic acid) from the nodes, HCl washing was performed as follows: The resulting yellow powder was suspended in 650 mL DMF in a glass bottle and 25 mL of 8 M HCl aqueous solution was added. The mixture was heated overnight in an oven at 100°C. After cooling to room temperature, the powder was washed three times with DMF, three times with acetone (soaking for about 1 hour between washes), and soaked in acetone overnight. The NU1K powder was collected by centrifugation, dried in a vacuum dryer, and activated. (Yield: approximately 2.8 g of activated NU1K).

[0030] Example 3 PtMo6O 24 @NU1K's synthesis

[0031] In a centrifuge tube, 137 mg of PtMo6O 24 Dissolve in 10 mL of deionized water. Add 50 mg of NU1K to the solution and suspend by sonication for about 1 min. Shake or stir the suspension periodically. After 3 days, wash the solid twice with water. Then wash the solid three times with acetone. [The text abruptly ends here, likely due to an incomplete translation or missing information.] 24 @NU1K was soaked in acetone overnight to ensure thorough removal of moisture. After drying in a vacuum desiccator prior to activation, a pale yellow target product was obtained. ICP-OES analysis of the solid material was performed to determine the final POM loading.

[0032] Comparative Example 1: PtMo6O 24 Synthesis of ZrO2

[0033] Prepared by equal-volume impregnation method, and determined by ICP-OES, PtMo6O 24 Pt loading in ZrO2 and PtMo6O 24 @NU1K is quite similar.

[0034] Comparative Example 2: Synthesis of Pt@NU1K

[0035] H₂PtCl₆ solution (6.0 mM) was prepared by dissolving H₂PtCl₆·6H₂O in ultrapure water. 16.6 mg of PVP (Mw = 55,000) was dissolved in 45 mL of ethanol, and 5.0 mL of the 6.0 mM H₂PtCl₆ solution was added dropwise. After stirring at room temperature for approximately 10 min, the mixture was refluxed under air for 3 h to form PVP-stabilized Pt NPs. Next, 80 mg of NU₁K powder was added to the solution with vigorous stirring at room temperature and stirred for 3 h. The resulting Pt@NU₁K was then collected by centrifugation at 5,017 x g for 10 min and washed three times with ethanol. Finally, the Pt@NU₁K powder was dried at room temperature in a vacuum oven.

[0036] In-situ diffuse reflectance Fourier transform infrared spectroscopy characterization of CO2 hydrogenation to methanol at room temperature start-up:

[0037] Measurement method:

[0038] DRIFTS CO chemisorption assay: After loading 20 mg of sample into the sample cell, the sample was purged with Ar and the background spectrum was collected. Subsequently, the flow rate was increased to 20 mL / min. - ¹ A 10% CO / Ar mixed gas was introduced into the sample cell for 20 minutes to saturate the sample. Finally, the gas was injected at a rate of 20 mL / min. - ¹ Ar gas purging for 30 minutes to remove gaseous CO, with 16 scans at 4 cm⁻¹ - The spectrum before reduction was acquired at a resolution of ¹. The sample was then purged with Ar and reduced at 200°C in a 10% H₂ / Ar atmosphere. After cooling the sample to room temperature under Ar atmosphere, the background spectrum was acquired. Subsequently, the sample was reduced at a flow rate of 20 mL / min. - ¹ A 10% CO / Ar mixed gas was introduced into the sample cell for 20 minutes to saturate the sample. Finally, the gas was injected at a rate of 20 mL / min. - ¹ Ar gas purging for 30 minutes to remove gaseous CO, with 16 scans at 4 cm⁻¹ - The reconstructed spectrum was acquired at a resolution of ¹.

[0039] DRIFTS H2 dissociation test: After loading fresh sample into the sample cell, the sample was purged with Ar and the background spectrum was acquired. Then, the flow rate was increased to 20 mL / min. - ¹ A 10% H₂ / Ar mixture was introduced into the sample cell, and the temperature was gradually increased to 30°C, 50°C, 100°C, 150°C, and 200°C. During this process, 16 scans were performed at each temperature point, with a scanning distance of 4 cm⁻¹. - ¹ Infrared spectra were continuously acquired at a resolution of [resolution value missing]. A series of spectra were continuously acquired until the signal stabilized at 200°C. Spectra were continuously acquired at each temperature point during the cooling process from 200°C to 150°C, 100°C, 50°C, and 30°C.

[0040] In-situ DRIFTS testing under a CO2+H2 reaction atmosphere: Measurements were performed on H2-activated samples under different temperature and pressure conditions. The samples were first reduced for 1 hour at 200°C and 0.1 MPa in a 10% H2 / Ar atmosphere. Background spectra were acquired after the reduction step. Subsequently, the flow rate was 20 mL / min. - ¹ A reaction gas (24% CO2, 72% H2, and 4% Ar) at a pressure of 0.1 MPa was introduced into the sample, and the temperature was gradually increased to 30°C, 60°C, 100°C, 150°C, and 200°C. During this process, 16 scans were performed at each temperature point, with a scanning depth of 4 cm⁻¹. - Infrared spectra were continuously acquired at a resolution of ¹. A series of spectra were continuously acquired until the signal stabilized at 200°C. Subsequently, the pressure was gradually increased to 0.5 MPa, 1.0 MPa, and 1.5 MPa, during which infrared spectra were continuously acquired at each pressure point until the signal stabilized.

[0041] PtMo6O reduced in H2 24 @NU1K(PtMo6O 24 On the @NU1K-R, CO2 hydrogenation to methanol can even be initiated at room temperature, supported by in-situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS), which shows CO and CH3O peaks when a mixture of CO2 and H2 passes through the catalyst at room temperature. In contrast, at the same gas hourly space velocity (GHSV), a commercial Cu / ZnO / Al2O3 catalyst used as a benchmark for this reaction only begins to produce methanol at a temperature of 140°C. Figure 1 (b). Under the same reaction conditions, PtMo6O changes with temperature. 24 @NU1K's net space-time yield of methanol is consistently higher than that of Cu / ZnO / Al2O3. Figure 1 (b). Within a temperature range of 100-200℃, PtMo6O 24 The @NU1K catalyst exhibited conversions ranging from 1.6% to 20.4% and selectivity ranging from 81% to 64% in the CO2 hydrogenation to methanol reaction. Within the 100-200℃ range, its single-pass methanol yield reached 1.2-13.1%. Long-term stability tests of the MOF-confined catalyst (i.e., 5000 h at 180℃) showed no signs of activity degradation (average conversion 16.5%) or selectivity degradation (average CH3OH selectivity 78.6%). Scanning transmission electron microscopy (STEM) images showed that PtMo6O after the reaction... 24 The size and morphology of the @NU1K microcrystals remained well maintained. No aggregated nanoparticles or clusters were observed on the outer surface. This indicates that PtMo6O 24@NU1K exhibits stability for the low-temperature CO2 hydrogenation to methanol production.

[0042] PtMo6O with a similar Pt loading was specifically selected. 24 ZrO2 and Pt@NU1K were used as control samples for systematic comparison. Since NU1K is a Zr-based MOF, ZrO2 was chosen as a conventional oxide support to assess whether the MOF confinement effect (rather than just Zr-related interactions) was critical. The synthesized PtMo6O 24 / ZrO2 exhibited low CO2 conversion (3.9%) and low CH3OH selectivity (3.3%) at 180℃. Figure 1 (c). TEM images show that, without MOF nanoconfining, the catalyzed PtMo6O 24 Clusters aggregated into large nanoparticles of approximately 10 nm on the ZrO2 support, resulting in fewer exposed active sites and a significant decrease in activity. This highlights the indispensable role of the porous structure of NU1K in stabilizing and dispersing the clusters. Pt@NU1K, composed of 2 nm Pt nanoparticles uniformly dispersed on NU1K, exhibits negligible activity, showing only 0.6% CO2 conversion and 9.3% CH3OH selectivity at 180 °C. Figure 1 c). MoO in Pt@NU1K x The absence of [something] allows us to decouple the contribution of the cooperative Pt-Mo interaction. The poor performance of Pt@NU1K highlights the [something] of MoO. x The importance of Pt in modulating electronic states and catalytic behavior. Under the same reaction conditions, these Pt-based reference samples exhibited significantly lower activity, with CO as the major product. In contrast, PtMo6O 24 @NU1K has a yield two orders of magnitude higher, and methanol is the main product.

[0043] Testing of the catalytic performance of CO2 hydrogenation:

[0044] Test Method: The catalytic performance of CO2 hydrogenation was evaluated using a tubular fixed-bed continuous flow reactor (Hefei In-situ Technology Co., Ltd.). In a typical reaction, 200 mg of catalyst was mixed and diluted with 1.0 g of quartz sand and then charged into the reactor, and incubated at 200 °C at 10% H2 / N2 (0.1 MPa, 30 mL·min). -1Reduction was performed in an atmosphere for 1 h. After reduction, the reaction gas (24% CO2, 72% H2, and 4% Ar) was introduced into the reactor and pressurized to 5.0 MPa at room temperature. The temperature was raised to 200 °C and then lowered to room temperature, with data measurements taken every 20 °C for 4 h at each temperature. The flow rate was adjusted to achieve the desired mass hourly space velocity (WHSV). The outlet gas was analyzed using an online gas chromatograph equipped with a thermal conductivity (TCD) and flame ionization (FID) detector. A 5A packed column connected to the TCD detector was used to analyze CO, Ar, and CO2 products; while a PLOTQ capillary column connected to the FID detector was used to analyze methane, methanol (MeOH), dimethyl ether (DME), and other products. CO2 conversion, selectivity, space-time yield (STY), and methanol yield were expressed as C(CO2), S(CH3OH), STY(CH3OH), and Y(CH3OH), respectively, and calculated using formulas (2)-(5).

[0045]

[0046] F is the molar flow rate of the product and reactants; W is the weight of the catalyst sample.

[0047] The STY of MeOH is calculated according to formula (4) and normalized to catalyst mass (g). MeOH ·kg catalyst -1 ·h -1 ),in This is the molar mass of methanol (32 g / mol). Note that all data points were collected 4 h after the reaction to ensure the catalyst reached steady state at each temperature.

[0048] To demonstrate the unique role of the synergistic effect between Pt and Mo in the selective production of methanol, we will examine two other Pt-based polyoxometalates (POMs), namely PtV9O. 28 and PtW6O 24 PtV9O was successfully confined within the c-pores of NU1K as a comparative catalyst for CO2 hydrogenation. At 180 °C, PtV9O... 28 @NU1K and PtW6O 24 The CO2 conversion rates of @NU1K were 2.2% and 2.3%, respectively, and the methanol selectivity was 31.3% and 41.0%, respectively. Figure 1 (c). Under the same reaction conditions, the activity and methanol selectivity of these catalysts were significantly lower than those of PtMo6O. 24 @NU1K, this highlights PtMo6O 24Clusters offer unique advantages in the selective formation of methanol. Furthermore, we synthesized Anderson-type polyoxometalate clusters containing other transition metals or main group metals, namely ZnMo6O. 24 and AlMo6O 24 The clusters were encapsulated in c-pores of NU1K for comparative CO2 hydrogenation tests. The results showed that these clusters exhibited almost no catalytic activity at low temperatures, with CO2 conversion remaining below 1.0% even at 200 °C. The observed negligible CO2 hydrogenation activity (conversion <1.0% at 200 °C) can likely be attributed to the inherently weak hydrogen dissociation capabilities of these base metals. These results collectively confirm the irreplaceable role of Pt in promoting catalytic cycling, while the MOF matrix enhances stability and site accessibility.

[0049] To elucidate the role of MOF-NU1K in the catalytic reaction, we used PtMo6O 24 @NU1K and unloaded PtMo6O 24 Clusters supported by three structurally similar MOFs (NU901, NU1008, and Hf-NU1K) were compared. At 180 °C, pristine PtMo6O without NU1K confinement... 24 The CO2 conversion rate was only 3.6%, significantly lower than that of the single-cluster PtMo6O confined in NU1K. 24 The percentage reached was 15.7% Figure 1 c). This is mainly attributed to the original PtMo6O. 24 The extremely low specific surface area and lack of porosity (less than 10 m² / g) hinder the contact of reactant gas molecules with the vast majority of clusters, thus limiting catalytic performance. Figure 1 (a) NU901 is an allotrope of NU1K, possessing the same metal oxide nodes and organic ligands, but with a different topology. NU901 contains 12 Å rhombic micropores, while NU1K has 31 Å hexagonal mesopores and 12 Å triangular micropores. However, the c-pores of NU901 are identical to those of NU1K, thus providing the same sites for immobilizing catalytic clusters. PtMo6O 24 The CO2 conversion rate and methanol selectivity of @NU901 were 9.0% and 55.0%, respectively, both lower than those of PtMo6O. 24 @NU1K Figure 1 (c). This performance difference may be attributed to the less favorable mass transfer of reactant and product molecules within micropores compared to mesopores. NU1008 and NU1K have the same topological structure, but different organic ligands: NU1K uses a four-linked 1,3,6,8-(terebenzoic acid)pyrene (TBAPy). 4- ) ligands, while NU1008 uses 1,2,4,5-tetra(4-carboxyphenyl)-3,6-dibromobenzene (TCPB)4- The compositional change resulted in a smaller c-pore size (4 Å × 10 Å) in NU1008 compared to NU1K (8 Å × 10 Å), and forced POMs to be localized in a more confined manner. Although NU1008 possesses mesoporous structures that facilitate molecular diffusion, PtMo6O... 24 The CO2 conversion rate and methanol selectivity of @NU1008 were 12.1% and 61.5%, respectively, slightly lower than those of PtMo6O. 24 @NU1K Figure 1 These results suggest that narrower c-pores may restrict gas molecules from interacting with PtMo6O. 24 Effective contact between cluster reaction centers. Hf-NU1K has the same topology and essentially the same pore size as NU1K, but the chemical composition of the metal oxide nodes is different, being Hf6(μ3-O)4(μ3-OH)4(H2O)4(OH)4. 8+ and Zr6(μ3-O)4(μ3-OH)4(H2O)4(OH)4 8+ PtMo6O 24 The CO2 conversion rate and methanol selectivity of @Hf-NU1K were 15.7% and 75.9%, respectively, compared with PtMo6O 24 @NU1K is quite ( Figure 1 These results suggest that the main active sites for the catalytic reaction are likely PtMo6O3 located within the c-pores. 24 Clusters, rather than zirconia nodes, are present. NU1K materials possess a mesoporous structure that facilitates efficient mass transfer between reactant and product molecules. Its c-type pores not only stably confine PtMo6O 24 The single clusters (8.6 Å × 7.8 Å × 3.1 Å) prevent aggregation during the reaction and provide an accessible active interface for reactant molecules.

[0050] Powder X-ray diffraction (PXRD) and distribution function (PDF) analysis confirmed that PtMo6O 24 It has been successfully introduced into the c-hole of NU1K without disrupting the long-range order of the framework. Figure 2 (a and c). During H2 activation and heating, the relative peak intensities in PXRD change, but the space group (P6 / mmm) of NU1K remains unchanged. Figure 2 a). The differential envelope density (DED) plot shows that PtMo6O 24 The position of the cluster within the c-pore remains unchanged. Figure 2 (d). By from PtMo6O at the corresponding temperature 24 The difference PDF (dPDF) obtained by subtracting the NU1K PDF from the @NU1K PDF reveals the relationship with PtMo6O.24 Related local structural changes. Attributable to PtMo6O 24 The intensity of the strong peak at -3.3 Å of the internal metal decreases, while a new peak (PtMo6O) appears at -2.7 Å. 24 -R). This shorter metal-oxygen distance is shorter than the known bond lengths of metals Pt or Mo. Simultaneously, the peak intensity associated with the metal-oxygen distance decreases, indicating partial reduction of the metal-oxygen bond. During the holding period, the peak intensity at 2.7 Å in dPDF increases ( Figure 2 (g). During CO2 hydrogenation, PXRD, DED, and dPDF results consistently indicate that PtMo6O 24 The structures of both the -R clusters and the NU1K framework remained stable. No phase transitions or different crystal phases were observed in PXRD. Figure 2 (b). DED shows PtMo6O 24 -R remains within the c-hole ( Figure 2 (e and f), no evidence of cluster migration / leakage or aggregation was found under catalytic reaction conditions. dPDF showed no significant changes except for an increase in peak intensity at 2.7 Å. Figure 2 The variation from 3.0 to 3.7 Å is attributed to reversible structural changes in the NU1K reference PDF at different temperatures.

[0051] In-situ X-ray absorption spectroscopy (XAS) was used to track the structural changes of the catalyst during H2 activation. The normalized X-ray absorption near-edge structure (XANES) plot shows that PtMo6O 24 The white line intensity of @NU1K-R is between that of Pt foil and PtO2. Figure 3 (a), which indicates that PtMo6O 24 In @NU1K-R, the valence state of Pt is below +4. Extended X-ray absorption fine structure (EXAFS) analysis of the central Pt shows a significant decrease in the coordination number of the first Pt-O shell, and the disappearance of the Pt···Mo / O peak at -3.3 Å. EXAFS curve fitting shows that the coordination number of Pt-O significantly decreases from 6.0 to 2.0 after H2 reduction. PtMo6O 24 @NU1K-R in the high k range (-8 to 12 Å) -1 The more rapid decrease in the k²-weighted oscillation amplitude also suggests a significant reduction in the Pt coordination number. Notably, a peak corresponding to the Pt-Mo bond appears at -2.74 Å. DFT calculations were performed using Gaussian 16 by applying a PtMo6O bond to PtMo6O. 24 Hydrogen atoms were added and the relaxation geometry was calculated to simulate the structural transition. This was applied to PtMo6O. 24@NU1K-R's experiments screened 115 computational simulation models for the distribution function (PDF) and XAS data. Figure 3 (b) was used to assess structural changes during the reaction. The model with the highest matching degree was used to evaluate the reaction network. In different models, a consistently shorter Pt-Mo distance was observed when μ3-O was removed. In these models, the coordination of Pt changed from a six-coordinate octahedral PtO6 to a two-coordinate PtO2, a geometry that provides open sites for binding gas molecules. Early spectroscopic characterization and DFT calculations confirmed that when activated in a reducing atmosphere, the central Pt site protrudes from the Mo6 ring, making it more exposed and resulting in a lower coordination number. Combining comprehensive in-situ synchrotron radiation characterization with theoretical modeling, we successfully reconstructed the PtMo6O confined in the NU1K c-pores under reaction conditions. 24 Atomic structure of clusters Figure 3 (c), which is crucial for elucidating the reaction mechanism and establishing the structure-activity relationship.

[0052] PtMo6O 24 @NU1K CO adsorption drifts at 2004 cm⁻¹ -1 A single peak was observed nearby. Figure 3 The peak (d) is attributed to CO linearly adsorbed on isolated Pt cations. After reduction, the peak shifts to a lower wavenumber, down to 1999 cm⁻¹. -1 And the intensity increased significantly ( Figure 3 (d). CO-DRIFTS results confirmed the retention of isolated Pt sites, while exposed sites promoted CO adsorption, consistent with XANES and EXAFS results. To investigate the H2 activation process, in-situ DRIFTS were collected in an ambient pressure atmosphere with a 10% H2 / Ar gas flow, ranging from 30°C to 200°C. After H2 activation at 200°C, CO adsorption was observed at 3600 cm⁻¹. -1 A series of infrared bands corresponding to different hydroxyl stretching vibrations were observed, confirming the bonding of protons with O atoms. Simultaneously, a band corresponding to the Pt-hydride vibration at 2031 cm⁻¹ was captured. -1 The single peak at Pt provides strong evidence for the heterolytic dissociation of H2. That is, H2 in Pt δ+ Induced heterolytic dissociation occurs, and the ion overflows into PtMo6O. 24 The surrounding μ3-O atoms generate OH and Pt-hydride species ( Figure 3 (e). After heating the sample in H2 at 200 °C for one hour, the vibrational intensity of the Pt-hydride decreased (but was still observable) as the temperature decreased to room temperature. The decrease in intensity indicates that H2 in PtMo6O 24 Activation on the @NU1K catalyst is reversible. Electron paramagnetic resonance (EPR) spectroscopy is used to study the active sites of CO2 activation. Figure 3 As shown in f, PtMo6O 24 Two types of EPR signals appeared on the @NU1K-R. One belongs to Mo 5+ Ions, in g ⊥ =1.9378 and g ∥ A broad axial resonance is observed at g=1.8758; another sharp and nearly symmetrical resonance is attributed to free electrons trapped in oxygen vacancies (OVs) at g=1.9973. Based on the net extraction of neutral oxygen from H2 to form water, each extracted atom leaves two electrons, which is expected to create vacancies. The role of oxygen vacancies is likely to trap CO2 molecules. We hypothesize that CO2 molecules are trapped by electrons from Pt. δ+ and Mo 5+ Electron transfer is activated. If Mo7O 24 The central Mo in the cluster is replaced by Pt, and it is similar to Mo7O. 24 Compared to @NU1K, PtMo6O 24 @NU1K oxygen vacancies and Mo 5+ The concentration of Pt increased significantly. This increase is attributed to Pt. δ+ Accelerated hydrogen dissociation and hydrogen overflow to the nearest μ3-O and peripheral Mo6O x Nanorings. To observe the internal structure, microcrystals were sliced ​​into thin slices (-70 nm) using ultrathin slicing to facilitate alignment of the structure along the

[001] direction. Given the high reactivity of the reduced and reacted samples in air (sparking or burning phenomena observed), and the unavoidable exposure to moisture during the ultrathin slicing process, these factors could potentially alter the sample structure. To accurately analyze the location of clusters within NU1K, we used fresh samples for slicing and analysis. PtMo6O obtained along the

[001] direction. 24 Typical STEM and magnified images of @NU1K are as follows: Figure 3 As shown in g and h. The high-resolution image clearly reveals the ordered structure of NU1K, revealing its characteristic hexagonal channels (31 Å) and smaller triangular channels (12 Å) in some regions. Notably, Figure 3 The hollow hexagonal and triangular channels in the h-shape confirm that the clusters are located only within the c-pores, not in the ab-face channels. The Pt atoms in the disk-shaped clusters can contact each other through mesopores and micropores, which is consistent with... Figure 2 The DED data in c is consistent. Figure 3 i was built Figure 3 PtMo6O within the red dashed box 24 The corresponding atomic model of @NU1K-R. Energy-dispersive spectroscopy (EDS) mapping detected signals of platinum and molybdenum in the imaging region, and their spatial distribution matched the distribution of zirconium very well. Figure 3 (j and m). This indicates that PtMo6O24 The clusters are uniformly dispersed and isolated throughout the c-pores of the crystal.

Claims

1. A composite catalytic material, characterized in that, The catalytic material comprises platinum and molybdenum-containing polyoxometalate clusters and a porous metal-organic framework. The polyoxometalate clusters are confined within the pores of the metal-organic framework, which has mesoporous and microporous structures. The polyoxometalate clusters are specifically confined within the c-pores of the metal-organic framework, rather than within the ab-plane channels. The catalytic material is subjected to hydrogen reduction treatment to form an active structure containing platinum-molybdenum bonds. The polyoxometalate clusters are distributed in a monodisperse or isolated state within the pores of the metal-organic framework.

2. The composite catalytic material according to claim 1, characterized in that, The polyoxometalate clusters are Anderson-type polyoxometalate clusters with the chemical formula [PtMo6O]. 24 ] n- , where n is an integer; the metal-organic framework is a zirconium-based or hafnium-based metal-organic framework; the metal-organic framework is NU1K.

3. The composite catalytic material according to claim 1, characterized in that, The central platinum atom in the polyoxometalate cluster is replaced by other noble metals from groups 8-10; the skeletal molybdenum atom in the polyoxometalate cluster is partially or completely replaced by tungsten.

4. A method for preparing the composite catalytic material according to claim 1, characterized in that, Includes the following steps: a) Platinum and molybdenum-containing polyoxometalate clusters are loaded into the pores of a porous metal-organic framework by impregnation to obtain an intermediate material; b) The intermediate material obtained in step a) is reduced in a hydrogen-containing atmosphere to form a composite catalyst material.

5. The method according to claim 4, characterized in that, During the reduction process described in step b), the temperature is 150-250℃ and the time is 0.5-4h. The volume fraction of hydrogen in the hydrogen-containing atmosphere is 5-25%; in step a), the mass ratio of platinum and molybdenum-containing polyoxometalate clusters to porous metal-organic frameworks is 1:1-5:

1.

6. The method according to claim 4, characterized in that, The platinum and molybdenum-containing polyoxometalate clusters are PtMo6O 24 The synthesis steps include: mixing an aqueous solution containing a molybdenum precursor with an aqueous solution containing a platinum precursor, then adding acid to adjust the pH of the mixed solution for crystallization, and separating to obtain PtMo6O. 24 The molybdenum-containing precursor is sodium molybdate, and the platinum-containing precursor is sodium hexahydroxyplatinate; the molar ratio of Mo to Pt in the molybdenum-containing precursor and the platinum-containing precursor is 5:1-7:1; the pH of the mixed solution is adjusted to 1.5-2.0 with acid.

7. The method according to claim 4, characterized in that, The porous metal-organic framework is NU1K, and its synthesis steps include: mixing a zirconium source and benzoic acid in an organic solvent and heating to obtain a solution containing zirconium nodes; adding a 1,3,6,8-tetra(4-carboxyphenyl)pyrene solution and trifluoroacetic acid to the solution containing zirconium nodes, mixing evenly and heating to react; after the reaction is completed, washing and drying to obtain NU1K; the zirconium source is zirconium oxychloride octahydrate.

8. The application of the composite catalytic material according to claim 1 in the catalytic hydrogenation of carbon dioxide to methanol.

9. The application according to claim 8, characterized in that, In the carbon dioxide hydrogenation reaction, the reaction temperature is 100-200℃; the reaction pressure is 1.5-8.0 MPa; the molar ratio of hydrogen to carbon dioxide in the reaction feed gas is 1:1 - 5:1; and the reaction temperature is 160-200℃.

10. The application according to claim 8, characterized in that, Within the reaction temperature range, the single-pass yield of methanol is 1-15%; the selectivity for methanol is 60-85%.

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