Preparation of a hexanuclear co cluster modified germanotungstate material and photocatalytic application thereof

Abstract

The application relates to a hexanuclear Co cluster modified germanotungstate material and an application thereof. The hexanuclear Co cluster modified germanotungstate material has the chemical formula (GeW 12 O 40 )[Co II 6(pzta)6(H2O)4(OH)2], wherein pzta is 3-(pyridin-4-yl)pyrazole. A preparation method is as follows: germanotungstate, cobalt chloride, ammonium metavanadate and 3-(pyridin-4-yl)pyrazole are dissolved into deionized water to obtain a reaction solution, the pH value is adjusted, the temperature is 180 DEG C in a polytetrafluoroethylene reaction kettle for 72 h, then the temperature is programmed to 80 DEG C, and then the oven is directly turned off, so that brown red rhombic block crystals are obtained. The hexanuclear Co cluster modified germanotungstate material can be obtained.

Description

Technical Field

[0001] This invention relates to the preparation of hexanuclear Co cluster-modified germanium tungstate materials and their photocatalytic applications. Background Technology

[0002] Hydrogen energy is considered a promising clean energy source. Due to its advantages such as high efficiency, ubiquity, recyclability, and high energy density, photocatalytic water splitting to produce hydrogen is an effective way to alleviate environmental and energy pressures.

[0003] Polyacids are inorganic metal-oxygen clusters formed from transition metals, possessing excellent photosensitivity, capable of rapid and reversible multi-electron transfer, and maintaining structural stability. Therefore, polyacids are considered ideal photocatalysts. However, in the field of photocatalysis, traditional polyacid catalysts typically exhibit photocatalytic activity only under ultraviolet light irradiation, and their activity is relatively low. To overcome the shortcomings of traditional polyacid catalysts, a feasible approach is to combine polyacids with appropriate organometallic complexes to prepare novel photocatalysts, thereby improving their utilization of visible light.

[0004] Polyacids are composed of transition metal-oxygen anion clusters of d 0 The electronic structure of polyacids exhibits diversity in composition and structure, allowing for molecular-level regulation. Most classical polyacids are only active under ultraviolet light irradiation when heteropolyblue is not produced, and few exhibit activity under visible light irradiation, with relatively poor activity. Modification with transition metals can yield reduced polyacids, thereby enhancing photocatalytic activity. However, due to the differences in the electronic structures of different metals, the catalytic activity of metal ions varies. Among them, high-dimensional crystalline materials modified with high-nuclear polymetallic metals exhibit better catalytic activity while also possessing good stability.

[0005] Such materials typically have the following advantages:

[0006] 1. High-dimensional polyacid-based metal-organic frameworks provide organic ligands with regular porous structures, increasing the overall structural stability while possessing high porosity. This allows H+ in water to pass through. + It enters and achieves direct contact with the metal active sites inside the framework, thereby promoting the photocatalytic reaction.

[0007] 2. High-core metal clusters possess unique synergistic catalytic effects. Multiple metal atoms within a cluster can work together, exhibiting unique catalytic properties that a single metal atom cannot achieve. The synergistic effect between different metal atoms can enhance the rate and selectivity of catalytic reactions.

[0008] 3. Germanium tungstic acid (GTA) is a wide-bandgap semiconductor, typically around 3.5 eV. This gives it excellent light absorption performance in the ultraviolet region, enabling it to effectively utilize high-energy ultraviolet light for photocatalytic reactions. GTA exhibits good catalytic activity in the photocatalytic degradation of organic pollutants. Its unique crystal structure and electronic properties allow it to effectively separate photogenerated electron-hole pairs, improving photocatalytic efficiency. However, research on GTA photocatalysts remains limited, indicating significant potential for future development.

[0009] In summary, designing and preparing germanium tungstate materials modified with high-nuclear-metal clusters and applying them to photocatalytic water splitting for hydrogen production is a meaningful endeavor. Summary of the Invention

[0010] The purpose of this invention is to provide a hexanuclear Co cluster-modified germanium tungstate material and its photocatalytic application, overcoming the shortcomings of traditional polyacids, such as easy solubility, poor stability, and the need for noble metals as co-catalysts.

[0011] The chemical formula of a hexanuclear Co cluster-modified germanium tungstate material is (GeW 12 O 40 )[Co II [6(pzta)6(H2O)4(OH)2], where pzta is 3-(pyridin-4-yl)pyrazole; the crystal system is trigonal triclinic; the space group is R-3; and the unit cell parameters are α = 90°, β = 90°, and γ = 120°. z = 3.

[0012] A method for preparing a hexanuclear Co cluster-modified germanic tungstate material is carried out according to the following steps:

[0013] (1) Preparation of reaction solution with pH value of 2.0: Weigh 0.33g of K4 (GeW) containing germanotungstic acid. 12 O 40 0.16 g of cobalt chloride CoCl2·4H2O, 0.35 g of ammonium metavanadate NH4VO3 and 0.45 g of pzta were dissolved in 36 mL of deionized water. The pH of the solution was adjusted to 6.8 with 1 M HCl / NaOH solution and stirred at room temperature for 2 h.

[0014] (2) The above solution was divided into three equal portions and transferred to 20 mL polytetrafluoroethylene reactors. The reactors were kept at 180 °C for 72 h, then the temperature was lowered to 80 °C and the oven was directly shut off, yielding brownish-red rhombic block crystals. The product was separated by water washing and dried to obtain a hexanuclear Co cluster-modified germanium tungstate material (GeW). 12 O 40 )[Co II[6(pzta)6(H2O)4(OH)2], abbreviated as Co6-GeW 12 .

[0015] Compared with the prior art, the present invention has the following characteristics:

[0016] A novel polyacid-based metal-organic framework (MOF) material with the same structure was successfully prepared via a one-pot hydrothermal synthesis using germanotungstic acid, transition metal elements Co and PzTa as raw materials. This method overcomes the drawbacks of traditional polyacids, which are readily soluble in water and require noble metal co-catalysts. The conduction band potential was improved by UV-Vis diffuse reflectance analysis. Co6-GeW 12 The superior performance is attributed to the high-spin state of Co 2+ The ions, which form a metal-organic framework, expose the {2-10}, {303}, and {306} crystal planes during catalysis, allowing the locked polyacids to come into effective contact with the sacrificial agent to form a reduced state, thereby enhancing the absorption of polyacids in visible light.

[0017] Photocatalytic water splitting for hydrogen production experiments showed that Co6-GeW 12 The catalytic hydrogen production rate is 4742 μmol·g -1 h -1 This is 79 times the hydrogen production rate of the parent compound, germanotungstate. (Co6-GeW) 12 The photocurrent intensity is greater than that of the parent polyacid germanium tungstate, indicating that the recombination rate of photogenerated electrons and holes is significantly improved after the introduction of the hexanuclear transition metal cluster.

[0018] This invention provides a hexanuclear Co cluster-modified germanium tungstate material. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the structure of a hexanuclear Co cluster modified germanium tungstate material prepared in Example 1.

[0020] Figure 2 The image shows the X-ray powder diffraction pattern of a hexanuclear Co cluster-modified germanium tungstate material prepared in Example 1.

[0021] Figure 3 The X-ray photoelectron spectroscopy of a hexanuclear Co cluster-modified germanium tungstate material prepared in Example 1 is shown.

[0022] Figure 4 This is a schematic diagram illustrating the temperature-dependent magnetic susceptibility of a hexanuclear Co cluster-modified germanium tungstate material prepared in Example 1.

[0023] Figure 5 The photocatalytic cycle stability diagram and X-ray diffraction patterns before and after illumination are shown for a hexanuclear Co cluster modified germanium tungstate material prepared in Example 1.

[0024] Figure 6 The cyclic voltammogram of a hexanuclear Co cluster-modified germanotungstate material prepared in Example 1 was recorded in a 0.5 mol / L H2SO4 solution at a scan rate of 0.1 V / s. Detailed Implementation

[0025] The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0026] Example 1: A hexanuclear Co cluster-modified germanic tungstate material, comprising the following preparation steps:

[0027] (1) Preparation of reaction solution with pH value of 2.0: Weigh 0.33g of K4 (GeW) containing germanotungstic acid. 12 O 40 0.16 g of cobalt chloride CoCl2·4H2O, 0.35 g of ammonium metavanadate NH4VO3 and 0.45 g of pzta were dissolved in 36 mL of deionized water. The pH of the solution was adjusted to 6.8 with 1 M HCl / NaOH solution and stirred at room temperature for 2 h.

[0028] (2) The above solution was divided into three equal portions and transferred to 20 mL polytetrafluoroethylene reactors. The reactors were kept at 180 °C for 72 h, then the temperature was lowered to 80 °C and the oven was directly shut off, yielding brownish-red rhombic block crystals. The product was separated by water washing and dried to obtain a hexanuclear Co cluster-modified germanium tungstate material (GeW). 12 O 40 )[Co II [6(pzta)6(H2O)4(OH)2], abbreviated as Co6-GeW 12 .

[0029] The X-ray single-crystal diffraction structure analysis data of the hexanuclear Co cluster modified germanate tungstate material prepared in Example 1 are shown in Table 1.

[0030] Table 1: Co6-GeW 12 Crystallographic Data Sheet

[0031]

[0032] a R1=∑║F o │─│F c ║ / ∑│F o │, b wR2={∑[w(F o 2 —F c 2 ) 2 ] / ∑[w(F o 2 )2 ]} 1 / 2

[0033] X-ray single-crystal diffraction analysis showed that the chemical formula of the hexanuclear Co cluster-modified germanitic tungstate material prepared in Example 1 (GeW 12 O 40 )[Co II [6(pzta)6(H2O)4(OH)2], with the molecular formula C 36 H 34 GeN 30 Co6O 46 W 12 .

[0034] The present invention will be further described below with reference to the accompanying drawings and Embodiment 1:

[0035] Figure 1 This is a schematic diagram of the structure of a hexanuclear Co cluster-modified germanium tungstate material prepared in Example 1. The basic structural unit consists of a multi-acid anion GeW 12 It consists of 6 Co ions and 6 pzta organic ligands. The structure contains a polyacid anion cluster, GeW. 12 By coordinating terminal oxygen with hexanuclear metal ions, photogenerated electrons can be rapidly transferred through chemical bonds, which is beneficial for photocatalytic water splitting to produce hydrogen.

[0036] Figure 2 The image shows the X-ray powder diffraction pattern of a hexanuclear Co cluster-modified germanitic tungstate material prepared in Example 1. The powder XRD diffraction peak positions of the compounds match well with the simulated XRD diffraction peak positions obtained from single-crystal analysis, indicating that the compounds all have high purity.

[0037] Figure 3 The X-ray photoelectron spectroscopy (XPS) of a hexanuclear Co cluster-modified germanitic tungstate material prepared in Example 1 is shown. All Ge atoms in the crystal are +4 valence, and all W atoms are +6 valence, forming a Co6-GeW... 12 The Co in the compound has a +2 oxidation state. X-ray photoelectron spectroscopy was used to further determine the elemental composition and oxidation state of the compound. The figure shows Co6-GeW. 12 XPS spectra, high-resolution XPS spectra of Co 2p are shown ( Figure 3 -a) The characteristic peaks at 780.62 eV and 796.52 eV belong to Co. 2+ Co at 786.0 eV and 801.88 eV 2+ The satellite characteristic peaks are consistent with the coordination mode of the basic units in the crystal structure. Furthermore, the spin-orbit splitting energy (ΔE) 2p The value of Co increases with the number of unpaired electrons in the Co center. 2+△E 2p The value is 15.9 eV, and there is a clear satellite characteristic peak, proving that Co 2+ It exists in a high-spin state. The XPS high-resolution spectrum of W 4f shows characteristic peaks at 34.35 eV and 36.49 eV, indicating that W exists in the highest oxidation state, +6. Figure 3 -b).

[0038] Figure 4 This is a schematic diagram illustrating the temperature-dependent magnetic susceptibility of a hexanuclear Co cluster-modified germanitic tungstate material prepared in Example 1. (Co6-GeW) 12 χ at room temperature m T value is close to 18cm 3 K mol -1 Approximately six Co II The expected values ​​of the ions (S = 3 / 2, g = 2). The magnetism is antiferromagnetic. A fitting model can be obtained using the following parameters: a = -0.89(1), l = -74(2) cm. -1 |D|=6.9(3)cm -1 J = -0.37(4)cm -1 and zj=-0.29(1)cm -1 Furthermore, the low-spin state Co ion χ m T value is usually between 0 and 1 cm. 3 K mol -1 It is much smaller than 18cm. 3 K mol -1 This indicates that the Co ion is in a high-spin state.

[0039] Figure 5 The image shows the photocatalytic cycling stability of a hexanuclear Co cluster-modified germanitonic acid material prepared in Example 1, along with its X-ray diffraction patterns before and after illumination. Under typical photocatalytic hydrogen evolution test conditions, after 30 hours of illumination (5 cycles), the photocatalytic hydrogen evolution activity of the crystalline material was close to its initial value, indicating good photocatalytic cycling stability. Subsequently, the photocatalyst after the photocatalytic reaction was collected by centrifugation, and X-ray powder diffraction was used to verify the structural stability. X-ray powder diffraction showed that the peak positions of the compound did not change, indicating that the compound's structure was not destroyed and it has good structural stability. (Co6-GeW) 12 In the study, the proportion of the {1 0 1} crystal plane decreased, while the proportions of the {2 -1 0}, {3 0 3}, and {3 0 6} crystal planes increased significantly, indicating that the MOF structure acting as a "lock" was etched to a certain extent in the organic alkaline solution. Meanwhile, the 30-hour cycle stability test showed that the photocatalyst could be recycled at least 5 times.

[0040] Figure 6 Cyclic voltammetry of a hexanuclear Co cluster-modified germanitic tungstate material prepared in Example 1, recorded in 0.5 mol / L H₂SO₄ solution at a scan rate of 0.1 V / s. Co₆-GeW 12 Six pairs of redox peaks were observed (I-I', II-II', III-III', IV-IV', V-V', and VI-VI'), of which the half-wave potentials of -0.623V (I-I'), -0.548V (II-II'), -0.418V (III-III'), 0.066V (IV-IV'), and 0.230V (V-V') were assigned to W. VI / W V The redox process, with a half-wave potential of +0.468V (VI-VI'), is attributed to Co. 2+ The redox process at the W center. The curves show that the reduction potential difference between the W center and the metal ion allows photogenerated electrons to transfer from the W center to the metal ion. More importantly, Co6-GeW 12 The reduction peaks are sufficiently negative, especially I-I', which allows for the generation of H2.

[0041] In summary, the hexanuclear Co-cluster modified germanium-tungstate material prepared in Example 1 overcomes the drawbacks of traditional polyacids, which are readily soluble in water and require noble metals as co-catalysts. This work can provide a reference for the design and preparation of high-performance water splitting photocatalysts for hydrogen production.

Claims

1. A hexanuclear Co cluster-modified germanic tungstate material, characterized in that... The chemical formula of a hexanuclear Co cluster-modified germanium tungstate material is (GeW 12 O 40 )[Co II 6(pzta)6(H2O)4(OH)2], where pzta is 3-(pyridin-4-yl)pyrazole; the crystal system is trigonal triclinic; the space group is R-3; the unit cell parameters are α=90°, β=90°, γ=120°, a=18.0929(9) Å, b=18.0929(9) Å, c=20.815(2) Å, z=3.

2. The preparation method of the hexanuclear Co cluster modified germanate tungstate material as described in claim 1 is carried out according to the following steps: (1) Preparation of reaction solution with pH value of 2.0: Weigh 0.33g of K4 (GeW) containing germanotungstic acid. 12 O 40 0.16 g of cobalt chloride CoCl2·4H2O, 0.35 g of ammonium metavanadate NH4VO3 and 0.45 g of pzta were dissolved in 36 mL of deionized water, and the pH of the solution was adjusted to 6.8 with 1 M HCl / NaOH solution. The solution was stirred at room temperature for 2 h. (2) The above solution was divided into three equal portions and transferred to a 20 mL polytetrafluoroethylene reactor. The reactor was kept at 180 °C for 72 h, then the temperature was lowered to 80 °C and the oven was directly shut off to obtain brownish-red rhombic blocky crystals. The product was separated by water washing and dried to obtain a hexanuclear Co cluster-modified germanium tungstate material (GeW). 12 O 40 )[Co II [6(pzta)6(H2O)4(OH)2], abbreviated as Co6-GeW 12 .

3. The application of the hexanuclear Co cluster-modified germanitic tungstate material according to claim 1, characterized in that... A hexanuclear Co cluster-modified germanium tungstate material was used for photocatalytic water splitting to produce hydrogen; experiments on photocatalytic water splitting to produce hydrogen showed that Co6-GeW 12 The catalytic hydrogen production rate was 4742 μmol·g -1 h -1 It is 79 times the rate of hydrogen production catalyzed by the parent polyacid germanium tungstate.

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