CN114939436A - Preparation method of Pd nano-particle porous composite material and application of Pd nano-particle porous composite material in low-temperature and normal-temperature hydrogen storage - Google Patents

Abstract

The invention belongs to the technical field of functional materials, relates to the technical field of hydrogen storage materials, and in particular relates to a preparation method of a Pd nanoparticle porous composite material and its low-temperature and normal-temperature hydrogen storage applications. The invention synthesizes Pd@UiO-66 composite materials with or without cerium doping, the materials have different morphologies, and can simultaneously have both normal temperature hydrogen storage performance and low temperature hydrogen storage performance. The mass hydrogen storage density of cerium-doped Pd@Ce‑H‑UiO‑66 composites is as high as 11.6 wt% under low temperature and high pressure conditions (77K, 80bar H ₂ ), and the mass density of this material at room temperature is still higher than that of the original MOF. It is excellent, and it also has excellent recycling performance. In addition, the material can also be used to catalyze the hydrogenation of styrene, and can achieve high-performance catalysis under normal temperature and pressure, and the TOF at room temperature is as high as 2383h ^-1 , which fully proves that the material developed by the present invention has wide application prospects.

Description

A kind of preparation method of Pd nanoparticle porous composite material and its low temperature and normal temperature hydrogen storage application

technical field

The invention belongs to the technical field of functional materials, relates to the technical field of hydrogen storage materials, and in particular relates to a preparation method of a Pd nanoparticle porous composite material and its low-temperature and normal-temperature hydrogen storage applications.

Background technique

As energy consumption increases year by year and environmental problems become increasingly prominent, finding clean, safe and renewable energy is the best way to solve the energy crisis and achieve the goals of "carbon peaking" and "carbon neutrality". Hydrogen is considered to be the most promising alternative to fossil fuels in the 21st century. First of all, hydrogen is a clean energy. The product of its reaction with oxygen is water without other by-products, and water is recognized as a non-toxic and drinkable liquid; at the same time, hydrogen is a renewable energy. It is a relatively mature technology. If the excess solar energy can also be utilized, the environmental benefits will be huge. Secondly, the high calorific value of hydrogen combustion can fully meet the energy requirements. The development of hydrogen energy can effectively promote the realization of the "dual carbon" goal.

However, the storage and transportation of hydrogen is a recognized bottleneck for hydrogen energy technology. Among them, gaseous hydrogen storage requires ultra-high pressure-resistant steel cylinders, and the hydrogen storage density is low. Liquid hydrogen storage requires ultra-low temperature conditions to avoid the loss of hydrogen gasification, so the energy consumption and cost are high, and the safety is poor. Solid-state hydrogen storage has the advantages of safety, high energy density and low cost, and is a good hydrogen storage method. Solid-state hydrogen storage can be divided into physical hydrogen storage and chemical hydrogen storage. Physical hydrogen storage refers to the adsorption of hydrogen molecules on some lightweight, large specific surface area, porous nanostructured materials (such as activated carbon, carbon nanotubes, zeolites, etc.) through physical action. Chemical hydrogen storage refers to the splitting of hydrogen molecules into hydrogen atoms, which chemically react with elements or alloys of transition metals, alkali metals or alkaline earth metals to form metal hydrides. However, the current field of solid-state hydrogen storage is still lacking. For example, physical hydrogen storage materials such as porous carbon require harsh operating conditions, while chemical hydrogen storage materials such as alloys have low hydrogen storage capacity and slow hydrogen absorption/desorption kinetics at room temperature. And other shortcomings, it is difficult to meet the needs of industrialization. Therefore, the importance of developing cheap, safe and high-density hydrogen storage materials for the development of hydrogen energy is self-evident.

Metal-organic frameworks (MOFs) have ultra-high specific surface areas, tunable framework structures and unsaturated metal sites, and noble metals (Pd, Pt, etc.) have excellent hydrogen storage capacity. Combining the advantages, noble metal@MOF composites (Pd@MOF, Pt@MOF, etc.) utilizing host-guest synergy are theoretically promising for hydrogen storage. A study has synthesized MIL-100(Al)/Pd material, which can absorb almost twice as much hydrogen as the pristine MOF material at room temperature, mainly due to the formation of Pd hydrides and the promotion of hydrogen molecules by hydrogen overflow. The dissociation on the surface of metal nanoparticles and the diffusion of single atomic hydrogen into the MOF; but at 77K, the hydrogen absorption of the MIL-100(Al)/Pd composite is less than that of the original MOF material due to the blockage of the MOF pores by Pd. half of . Some studies have also applied Pd@HKUST-1 to hydrogen storage at room temperature. The HKUST-1 in the outer layer can significantly enhance the kinetic behavior of hydrogen storage and improve the absorption/desorption rate of hydrogen, which increases the hydrogen storage capacity of Pd by 74%. , but its hydrogen storage performance at low temperature and high pressure has not been verified. Another study has synthesized Pt@CYCU-3 material, using the hydrogen overflow ability of Pt and the high specific surface area of CYCU-3, the mass density of hydrogen storage at 298K, 100bar has a mass density of 0.36wt%, but its hydrogen storage under low temperature and high pressure. Performance has also not been verified. It can be seen that for noble metal @MOF materials, it is very difficult to achieve high-pressure hydrogen storage performance under two different conditions at low temperature and normal temperature.

In general, there is no material that can simultaneously store hydrogen at different temperatures, and the current MOF materials all have problems such as low hydrogen storage capacity. MOF materials mainly store hydrogen through physical adsorption, and they have high hydrogen storage capacity at low temperature, but due to poor activation ability at room temperature, the hydrogen storage capacity at room temperature is extremely low; precious metals such as Pd and Pt store hydrogen through chemical action, but High hydrogen storage capacity only at room temperature. Although noble metals such as Pd and Pt are combined with MOF, the hydrogen storage capacity of the material at room temperature can be improved again, but the low-temperature hydrogen storage capacity of the MOF is worse than that of the original MOF due to the blockage of the pores of the MOF caused by the noble metal loading. Therefore, it is necessary to develop noble metal@MOF materials with high hydrogen storage performance under two different conditions at low temperature and normal temperature.

SUMMARY OF THE INVENTION

In order to overcome the above-mentioned deficiencies of the prior art, the primary purpose of the present invention is to provide a preparation method of a Pd nanoparticle porous composite material. On the premise of not losing the high specific surface area of MOF itself, by avoiding pore blockage caused by the process of MOF loading noble metals such as Pd, a noble metal@MOF composite with high specific surface area was obtained. It is used for high pressure hydrogen storage at low temperature and room temperature.

The second object of the present invention is to provide the application of the Pd nanoparticle porous composite material prepared by the above preparation method. The Pd nanoparticle porous composite material synthesized by the invention can have both excellent hydrogen storage performance at different temperatures, and can also be used as an efficient heterogeneous catalyst, and has wide application.

The above-mentioned first purpose of the present invention is achieved through the following technical solutions:

A preparation method of a Pd nanoparticle porous composite material, specifically: dispersing a MOF sample in n-hexane to prepare a n-hexane solution, dissolving a palladium metal salt in water, dropwise into the above-mentioned n-hexane solution, and stirring to form After the solid is precipitated, the supernatant is removed, and the solid precipitate is dried and then dispersed in water. After reduction with sodium borohydride, the Pd nanoparticle porous composite material is obtained by centrifugation, washing and drying. The MOF sample is doped with cerium metal during the synthesis process. And Ce-H-UiO-66 with acid added or Ce-UiO-66 doped with cerium metal but not acid added during synthesis or H-UiO-66 without cerium metal doped but acid added during synthesis. Therefore, the synthesized Pd nanoparticle porous composites are also called Pd@UiO-66 composites.

Preferably, the preparation method of Ce-H-UiO-66 doped with cerium metal and adding acid in the synthesis process is as follows: 1-1.5g zirconium tetrachloride and 0.6-1.0g terephthalic acid are dissolved in 125-175mL N ,N-dimethylformamide, then dissolve 0-1.86g cerium chloride hydrate in 25-50mL N,N-dimethylformamide, when the solid is completely dissolved, mix the two solutions, then add 25-50mL of N,N-dimethylformamide and 15-25mL of acid solution, heated to 120-150°C for 12-24h after stirring, and finally obtained by centrifugation, washing and drying.

Preferably, the preparation method of Ce-UiO-66 doped with cerium metal but without adding acid in the synthesis process is: dissolving 1-1.5g zirconium tetrachloride and 0.6-1.0g terephthalic acid in 125-175mL N, N-dimethylformamide, then dissolve 0-1.86g cerium chloride hydrate in 25-50mL N,N-dimethylformamide, when the solid is completely dissolved, mix the two solutions, then add 25 -50mL N,N-dimethylformamide, heated to 120-150°C after stirring for 12-24h, and finally obtained by centrifugation, washing and drying.

Preferably, the preparation method of H-UiO-66 without doping cerium metal but adding acid in the synthesis process is: dissolving 1-1.5g zirconium tetrachloride and 0.6-1.0g terephthalic acid in 125-175mL N, In N-dimethylformamide, when the solid is completely dissolved, add 25-50 mL of N,N-dimethylformamide and 15-25 mL of acid solution, stir and heat to 120-150 °C for 12-24 h, Finally obtained by centrifugation, washing and drying.

The present invention firstly obtains three MOF precursor materials by controlling the variables, which are Ce-H-UiO-66 doped with cerium metal and acid added in the synthesis process, and Ce doped with cerium metal but no acid added in the synthesis process. -UiO-66 and H-UiO-66 undoped with cerium metal but acid added. After that, by cleverly using the dual-solvent impregnation method, Pd particles with a size of about 15 nm were successfully combined with different UiO-66 precursors, and three kinds of Pd nanoparticle porous composite materials were obtained, namely Pd@Ce-H-UiO-66, Pd@Ce-H-UiO-66, Pd@Ce-UiO-66 and Pd@H-UiO-66. The specific synthesis schematic diagram is shown in Figure 1.

The current precious metal@MOF composites will cause problems such as pore blockage and greatly reduced specific surface after loading precious metals, so it is difficult to maintain the low-temperature hydrogen storage performance. In contrast, the method for synthesizing Pd nanoparticle porous composite materials of the present invention can obtain Pd particles with uniform size distribution, and the three composite materials can still maintain the high specific surface area of the MOF itself, and the pores are not blocked, so they still have excellent performance. The low-temperature hydrogen storage performance, especially the hydrogen storage mass density of Pd@Ce-H-UiO-66 material at a low temperature of 77 K and a high pressure of 80 barH ₂ is as high as 11.6 wt%, which is in the leading position among the current Pd-based MOF composites. At the same time, the test results of hydrogen storage at room temperature show that the performance of cerium-doped composites is better than that of cerium-free composites, which is due to the hydrogen overflow effect and charge regulation effect brought by doped cerium to the composites. In addition, the experimental results of catalytic hydrogenation also demonstrate the efficient catalytic performance of the cerium-doped Pd@UiO-66 composites.

In general, compared with the hydrogen storage performance of the existing similar materials, the synthesized material of the present invention can maintain its own low-temperature hydrogen storage performance, has extremely high room temperature hydrogen storage performance, and can also be used as an efficient heterogeneous catalyst. , has a wide range of applications.

Further, in the preparation process of the above three MOF samples, the stirring temperature is 20-30° C. and the speed is 400-600 rpm.

Further, in the preparation process of the above three MOF samples, after stirring, the samples were heated to 120° C. and continuously stirred for 24 h.

Preferably, the palladium metal salt is selected from any one of chloride salt, acetate salt and nitrate salt, and the acid is any one of formic acid, acetic acid and hydrochloric acid.

Preferably, the mass fraction of the added palladium metal salt is 0.1wt%-50wt% of the MOF sample.

Preferably, the stirring temperature during the synthesis of the Pd nanoparticle porous composite material is 20-30° C., and the stirring speed is 400-600 rpm.

Preferably, the molar ratio of palladium ions in the sodium borohydride and the palladium metal salt is 1:1-10:1. Specifically, the molar ratio of palladium ions in the sodium borohydride and the palladium metal salt is 1:1.15.

Preferably, the volume ratio of n-hexane to water is 10:1-100:1. Specifically, the volume ratio of n-hexane to water is 20:1.

The above-mentioned second purpose of the present invention is achieved through the following technical solutions:

The present invention also provides the Pd nanoparticle porous composite material prepared by the above preparation method.

The present invention also provides the application of the above-mentioned Pd nanoparticle porous composite material in low temperature and/or normal temperature hydrogen storage.

The present invention also provides the application of the above-mentioned Pd nanoparticle porous composite material in catalytic hydrogenation.

Preferably, the catalytic hydrogenation is a catalytic hydrogenation reaction of styrene.

The Pd@UiO-66 composite material synthesized by the invention can simultaneously have both normal temperature hydrogen storage performance and low temperature hydrogen storage performance under normal temperature and normal pressure. Among them, the cerium-doped Pd@UiO-66 material can utilize hydrogen overflow and charge effect, and has excellent high-pressure hydrogen storage capacity at low temperature and room temperature, especially at low temperature and high pressure (77K, 80bar H ₂ ) The cerium-doped material The mass hydrogen storage density of Pd@Ce-H-UiO-66 composites is as high as 11.6 wt%, which is in the leading position in the current Pd-based MOF composites, and the room temperature hydrogen storage of cerium-doped Pd@UiO-66 composites Compared with the original MOF, the mass density is greatly improved. Meanwhile, the three synthesized Pd@UiO-66 composites also have excellent recycling performance. In addition, the synthesized three Pd@UiO-66 composites can also be used as efficient heterogeneous catalysts for catalytic hydrogenation reactions (such as catalytic styrene hydrogenation), proving the materials have a wide range of applications and uses.

Compared with the prior art, the beneficial effects of the present invention are:

The invention discloses a preparation method of a Pd nanoparticle porous composite material. Acidic Ce-UiO-66 or H-UiO-66 with no cerium metal doping but acid addition during the synthesis process) was used as the precursor material. Through an ingenious two-solvent method, cerium doping and different shapes were successfully synthesized. Pd@UiO-66 composites with appearance. Among them, the distribution of cerium element in the cerium-doped composite material is uniform, but the particle size and dispersion of the supported Pd particles are not different, which proves that this method can accurately control the growth of Pd particles in MOF. After loading Pd particles, the specific surface area of the material hardly changed, indicating that the pores of the MOF were not blocked. And after loading Pd, the material basically maintains the low-temperature hydrogen storage performance of the original MOF material, especially the hydrogen storage mass density of Pd@Ce-H-UiO-66 material at a low temperature of 77K and a high pressure of 80bar _H2 is as high as 11.6wt%. In the current Pd-based MOF composites in a leading position. At the same time, due to the hydrogen overflow effect and charge regulation brought by cerium doping, the performance of cerium doped materials after Pd loading has been greatly improved compared with the original MOF. The performance of the composites is better than that of the cerium-free composites. In addition, the composite material synthesized by the present invention can also be used as an efficient heterogeneous catalyst in catalytic hydrogenation reaction. To sum up, it can be seen that the hydrogen storage material synthesized by the present invention can have both excellent hydrogen storage performance at different temperatures, and can also be used as an efficient heterogeneous catalyst, which has a wide application prospect.

Description of drawings

Fig. 1 is the synthesis schematic diagram of three kinds of UiO-66 precursors and subsequent loading Pd composite materials;

Figure 2 shows the XRD patterns of the original MOF and Pd composites of Ce-H-UiO-66, Ce-UiO-66 and H-UiO-66;

Figure 3 is the TEM image of Pd@Ce-H-UiO-66;

Figure 4 is the TEM image of Pd@Ce-UiO-66;

Figure 5 is the TEM image of Pd@H-UIO-66;

Figure ₆ is the N adsorption curve of the Ce-H-UiO-66 sample and its composites loaded with Pd particles;

Figure ₇ is the N adsorption curve of the Ce-UiO-66 sample and its composites loaded with Pd particles;

Figure ₈ is the N adsorption curve of the H-UiO-66 sample and its loaded Pd particles;

Figure ₉ shows the H adsorption curves of three Pd@UiO-66 composites at room temperature of 298K and 1bar _H2 ;

Figure 10 shows the cycle performance of H adsorption of _three Pd@UiO _- 66 composites at room temperature of 298K and 1bar H;

Figure 11 shows the H ₂ adsorption curves of three UiO-66 samples and three Pd@UiO-66 composites at low temperature 77K and 1 bar H ₂ ;

Figure 12 shows the H adsorption curves of Pd@Ce-H _- UiO-66 composites at low temperature 77K and 0-90 bar _H2 .

Detailed ways

The specific embodiments of the present invention will be further described below. It should be noted here that the descriptions of these embodiments are used to help the understanding of the present invention, but do not constitute a limitation of the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

The experimental methods in the following examples are conventional methods unless otherwise specified, and the experimental materials used in the following examples can be purchased through conventional commercial channels unless otherwise specified.

Terminology Explanation:

MOF: metal-organic framework metal organic framework.

Hydrogen storage: The hydrogen storage system is an energy storage unit for hydrogen fuel cell vehicles. There are three main technical paths: high-pressure hydrogen storage (by increasing the pressure of hydrogen to reduce the volume, hydrogen is in a gaseous state); low-temperature liquefied hydrogen storage (hydrogen is in liquid state) and liquid Or solid hydrogen storage (using the physical adsorption or chemical reaction of liquid or solid to hydrogen to store hydrogen in solid materials, mainly including benzene, alloy hydrogen storage, nano hydrogen storage).

Hydrogen storage capacity: Hydrogen storage capacity refers to the available amount of hydrogen that can be delivered to the fuel cell system divided by the total mass/volume of the entire storage system, which includes all stored hydrogen, media, reactants (such as those in the hydrolysis system). water) and system components. Generally, two parameters, mass hydrogen storage density and volume hydrogen storage density, are used to evaluate the hydrogen storage capacity of the hydrogen storage system.

Hydrogen spillover effect: Hydrogen spillover (Spillover) means that the active center (original active center) on the surface of the solid catalyst generates an ionic or free radical active species by adsorption, and they migrate to other active centers ( secondary active centers).

Charge regulation: By adjusting the charge properties at the surface interface, the energy band bending and the built-in electric field strength of the material surface and interface are regulated.

TEM: Transmission Electron Microscopy.

SEM: Field emission scanning electron microscope image.

TOF: mass activity per unit time.

Specific surface area: Specific surface area refers to the total area per unit mass of material, divided into two categories: external surface area and internal surface area. Specific surface area is used to evaluate catalysts, adsorbents and other porous substances (such as asbestos, mineral wool, diatomite and clays Minerals) is one of the important indicators of industrial utilization.

Example 1 Preparation of Pd@Ce-H-UiO-66 composites

(1) Preparation of Ce-H-UiO-66: 5 mmol (1.17 g) of zirconium tetrachloride and 5 mmol (0.83 g) of terephthalic acid were weighed and dissolved in 125 mL of N,N-dimethylformamide. Another 5 mmol (1.86 g) of CeCl ₃ ·7H ₂ O was dissolved in 25 mL of N,N-dimethylformamide. When the solid was completely dissolved, the two solutions were mixed, and 0.5 mol (18.8 mL) of formic acid and 25 mL of N,N-dimethylformamide were added. After stirring at room temperature for 30 min, the solution was heated to 120 °C and continued to stir for 24 h. The white solid obtained after centrifugation was washed twice with N,N-dimethylformamide, once with methanol, and dried under vacuum at 60°C for 12 h, and the obtained sample was named Ce-H-UiO-66.

(2) Preparation of Pd@Ce-H-UiO-66: Weigh 0.5 g of Ce-H-UiO-66 and ultrasonically disperse it in 40 mL of n-hexane to prepare a n-hexane solution. Weigh 88.3 mg of Na ₂ PdCl ₄ and dissolve it in 2 mL of deionized water, drop it into the above n-hexane solution dropwise, and stir at room temperature for 3 h. During stirring, solids settled at the bottom of the flask and were tan. The supernatant was poured out, and the solid was put into a vacuum oven together with the flask, and dried at 60° C. for 6-12 h (dry to almost zero water content). Take out the flask, disperse the solid in 30mL deionized water, add 1.5mL NaBH4 solution dropwise under ice-water bath and vigorous stirring ₍ prepared by dissolving _14.1mg NaBH4 in 1.5mL deionized water, prepared and used now), It can be observed that the color of the solid becomes darker rapidly, and the mixture is vigorously stirred for 30 minutes after the dropwise addition. After centrifugation, it was washed twice with methanol, and vacuum-dried at 60 °C for 12 h to obtain a dark gray solid product, denoted as Pd@Ce-H-UiO-66.

Example 2 Preparation of Pd@Ce-UiO-66 composites

(1) Preparation of Ce-UiO-66: 5 mmol (1.17 g) of zirconium tetrachloride and 5 mmol (0.83 g) of terephthalic acid were weighed and dissolved in 125 mL of N,N-dimethylformamide. Another 5 mmol (1.86 g) of CeCl ₃ ·7H ₂ O was dissolved in 25 mL of N,N-dimethylformamide. When the solid was completely dissolved, the two solutions were mixed, and 25 mL of N,N-dimethylformamide was added, and after stirring at room temperature for 30 min, heated to 120° C. and continued stirring for 24 h. The white solid obtained after centrifugation was washed twice with N,N-dimethylformamide, twice with methanol, and dried under vacuum at 60 °C for 12 h, and the obtained sample was named Ce-UiO-66.

(2) Preparation of Pd@Ce-UiO-66: The preparation method is the same as that in Example 1, except that Ce-H-UiO-66 is replaced by Ce-UiO-66, and the obtained product is denoted as Pd@Ce-UiO-66.

Example 3 Preparation of Pd@H-UiO-66 composites

(1) Preparation of H-UiO-66: 5 mmol (1.17 g) of zirconium tetrachloride and 5 mmol (0.83 g) of terephthalic acid were weighed and dissolved in 175 mL of N,N-dimethylformamide. When the solid was completely dissolved, 0.5 mol (18.8 mL) of formic acid and 25 mL of N,N-dimethylformamide were added, and after stirring at room temperature for 30 min, the mixture was heated to 120° C. and stirred continuously for 24 h. The white solid obtained by centrifugation was washed twice with N,N-dimethylformamide, twice with methanol, and dried under vacuum at 60°C for 12 h, and the obtained sample was named H-UiO-66.

(2) Preparation of Pd@Ce-UiO-66: The preparation method is the same as that in Example 1, except that Ce-H-UiO-66 is replaced by H-UiO-66, and the obtained product is denoted as Pd@H-UiO-66.

Experimental Example 1 Characterization and Performance Testing

Six materials (Pd@Ce-H-UiO-66, Ce-H-UiO-66, Pd@Ce-UiO-66, Ce-UiO-66, Pd@H-UiO-66, Pd@H-UiO-66, Ce-UiO-66, -66, H-UiO-66) as the sample, carry out the following characteristic characterization or performance test:

(1) XRD characterization

For the synthesized six materials (Pd@Ce-H-UiO-66, Ce-H-UiO-66, Pd@Ce-UiO-66, Ce-UiO-66, Pd@H-UiO-66, H-UiO -66) XRD characterization. As shown in Fig. 2, it can be seen that the three MOF precursors obtained correspond to the standard spectra of UiO-66, and the bulging peaks of Pd species also appear after loading Pd respectively, which proves the complexation of Pd@UiO-66. Successful synthesis of materials.

(2) TEM characterization

Three synthesized Pd@UiO-66 materials (Pd@Ce-H-UiO-66, Pd@Ce-UiO-66, Pd@H-UiO-66) were characterized by TEM. As shown in Figure 3, Figure 4 and Figure 5, it can be seen that among the three Pd@UiO-66 materials obtained, the Pd particles are uniformly distributed in the MOF, and the particle size is about 15 nm, and the two materials ( The morphologies of Figures 3 and 5) are all regular octahedrons, while the morphology of the material without formic acid (Figure 4) is a random block. Therefore, combined with the previous analysis, it can be seen that the present invention successfully prepared Pd@UiO-66 composite materials with or without cerium doping, the materials have different morphologies, and the distribution of Pd particles is uniform.

(3) Characterization of nitrogen adsorption and desorption

For the synthesized six materials (Pd@Ce-H-UiO-66, Ce-H-UiO-66, Pd@Ce-UiO-66, Ce-UiO-66, Pd@H-UiO-66, H-UiO -66) Carry out nitrogen adsorption and desorption characterization to analyze and compare the pore properties of the materials. The results are shown in Figure 6, Figure 7, Figure 8 and Table 1, it can be seen that the three materials (Pd@Ce-H-UiO-66, Pd@Ce-UiO-66, Pd@H-UiO-66) The adsorption amount and pore size distribution did not change significantly before and after loading Pd; further from the specific surface area data obtained according to the BET calculation method (as shown in Table 1), it can be seen that the specific surface areas of the three materials before and after loading Pd are only very little reduction. From the above overall results, it can be seen that the pores of the MOF precursor are not blocked after Pd loading. Thanks to this, the prepared Pd@UiO-66 materials can maintain high low-temperature hydrogen storage performance.

Table 1 Summary of BET specific surface area of six materials

(4) Hydrogen storage performance test

The hydrogen storage properties of three Pd@UiO-66 materials (Pd@Ce-H-UiO-66, Pd@Ce-UiO-66, Pd@H-UiO-66) were mainly tested, and the test conditions were low temperature 77K, 1bar H ₂ and normal temperature 298K, 1bar H ₂ and low temperature 77K, 0-90barH ₂ . The specific test method is as follows: First, place the sample in the sample tube and weigh it and record the mass. The mass of the empty tube is recorded as M1. Then, the samples were activated under high temperature vacuum degassing (150 °C for 24 h) to remove the guest molecules in the MOF. After activation, after the sample is cooled down, test the quality again, which is recorded as the mass after activation M2, so the mass of the sample is M=M2-M1. After that, the activated sample tube containing the sample was placed in a liquid nitrogen bath (77K) for nitrogen adsorption test. After the test, high-temperature vacuum activation was performed again (activation at 150 °C for 24 h), and finally the hydrogen adsorption test was performed at 298K.

The results are shown in Figure 9, Figure 10, Figure 11, Figure 12 and Table 2. In terms of hydrogen storage performance under the test conditions of 298K at room temperature and 1bar _H2 , as shown in Figure 9, it can be clearly seen that the cerium-doped Pd@ The UiO-66 material has an excellent mass hydrogen storage density, and its performance far exceeds that of the undoped cerium material, which is benefited from the hydrogen overflow effect and charge regulation effect of cerium doping on the material. At the same time, all three materials have excellent cycle performance. As shown in Figure 10, the performance of the materials changes little after 3 hydrogen storage cycles. In terms of hydrogen storage performance under the low temperature 77K, 1bar H ₂ test conditions, as shown in Figure 11, thanks to the unblocked pores, the low temperature hydrogen storage performance of the three materials is close to that of the original MOF precursor, and all maintain high hydrogen storage performance mass density. Especially the Pd@Ce-H _- UiO-66 material has a hydrogen storage mass density of up to 11.6 wt% at a low temperature of 77 K and a high pressure of 80 bar H, as shown in Fig. 12, which is in the leading position among the current Pd-based MOF composites .

It can be seen from the above overall results that the present invention, without losing the high specific surface area of the MOF itself, avoids pore blockage caused by the process of loading the MOF with precious metals such as Pd, and utilizes the hydrogen overflow effect and the charge control effect, thereby preparing the High-pressure Pd@MOF hydrogen storage materials that can be applied to both low temperature and room temperature.

Table ₂ Summary of H adsorption data for six materials at 1 bar and different temperatures

(5) Analysis of catalytic hydrogenation performance

The synthesized Pd-based composite material was used as a heterogeneous catalyst, and styrene was selected as the catalytic substrate. The hydrogen pressure of the catalytic condition and the hydrogen pressure of the hydrogen storage test were the same as 1 atmosphere pressure (1 bar) to compare the cerium-doped and non-doped cerium. The catalytic activity of the hydrogen storage material is high and low, thereby verifying the effect of cerium doping on the ability of the material to activate hydrogen. The specific reaction conditions are as follows: the dosage of the catalyst is 10 mg, and the dosage of styrene is 10 mmol, all of which are added to 10 mL of absolute ethanol, ultrasonicated for 10 min to obtain a uniform mixed solution, and the mixed solution is transferred to a 50 mL round-bottomed flask, covered with rubber After plugging, use a vacuum pump to evacuate for 3 minutes, and then quickly insert a self-made hydrogen balloon to make the hydrogen pressure consistent with the atmospheric pressure, stir at room temperature for 1 hour, take samples every 15 minutes, and analyze the catalytic results with a gas chromatography-mass spectrometer (GC-MS). .

The experimental results in Table 3 show that the cerium-doped Pd@Ce-H-UiO-66 has better catalytic activity than the undoped cerium-doped Pd@H-UiO-66, and the Pd@Ce-H-UiO-66 at room temperature The hydrogenation TOF (conversion frequency, ie the number of substrates converted per hour per catalyst) of 66 is as high as 2383h ^-1 , and the TOF at 50°C is as high as 2770h ^-1 . This reflects the effect of cerium doping on the improvement of the hydrogen activation capacity of the composite material from the side, thereby laying a good foundation for the improvement of its hydrogen storage capacity, which proves that the Pd-based composite material developed by the present invention has a wide application prospect.

Table 3 Summary of styrene hydrogenation properties of materials at room temperature and 50 °C

To sum up, the method for preparing the Pd nanoparticle porous composite material of the present invention is simple to operate, and can obtain Pd particles with uniform size distribution, and the obtained composite material can still retain the high specific surface area of the original MOF after loading a high load of precious metals. , and after loading Pd particles, the three composites can still maintain the high specific surface area of the MOF itself, and the pores are not blocked. In terms of hydrogen storage performance, the hydrogen storage mass density of the composite material prepared by the present invention can be at a relatively high level under normal temperature and normal pressure and at low temperature and high pressure, which makes up for the fact that there is currently no material with high hydrogen storage performance at different temperatures. defects, and the mass hydrogen storage density of cerium-doped Pd@Ce-H-UiO-66 composites is as high as 11.6 wt% under low temperature and high pressure conditions (77K, 80bar H ₂ ), which is among the highest among the current Pd-based MOF composites. leading position. In terms of heterogeneous catalysis, the material prepared by the present invention can have a very high TOF value in the styrene hydrogenation reaction, which proves that the material has a wide range of applications and uses. In addition, the present invention has expansibility and is applicable to different MOF carriers, and provides a new idea for developing more effective hydrogen storage materials in the future.

The embodiments of the present invention have been described above in detail, but the present invention is not limited to the described embodiments. For those skilled in the art, without departing from the principle and spirit of the present invention, various changes, modifications, substitutions and alterations to these embodiments still fall within the protection scope of the present invention.

Claims (10)

1. a preparation method of a Pd nanoparticle porous composite material, is characterized in that, MOF sample is dispersed in n-hexane and is prepared into n-hexane solution, after palladium metal salt is dissolved in water, dropwise in above-mentioned n-hexane solution, After stirring to form a solid precipitate, remove the supernatant, dry the solid precipitate and then disperse it in water. After reduction with sodium borohydride, the Pd nanoparticle porous composite material is obtained by centrifugation, washing and drying. The MOF sample is doped in the synthesis process. Ce-H-UiO-66 doped with cerium metal and acid added or Ce-UiO-66 doped with cerium metal but not acid added during synthesis or H-UiO-66 without cerium metal doped but acid added during synthesis . 2. the preparation method of a kind of Pd nanoparticle porous composite material according to claim 1, is characterized in that, the preparation method of Ce-H-UiO-66 that is doped with cerium metal and adds acid in the synthesis process is: -1.5g zirconium tetrachloride and 0.6-1.0g terephthalic acid are dissolved in 125-175mL N,N-dimethylformamide, then 0-1.86g cerium chloride hydrate is dissolved in 25-50mL N,N- In dimethylformamide, when the solid is completely dissolved, mix the two solutions, add 25-50 mL of N,N-dimethylformamide and 15-25 mL of acid solution, stir and heat to 120-150°C and continue to stir 12-24h, and finally obtained by centrifugation, washing and drying. 3. the preparation method of a kind of Pd nanoparticle porous composite material according to claim 1, is characterized in that, the preparation method of Ce-UiO-66 doped with cerium metal but without acid in the synthesis process is: 1- 1.5g zirconium tetrachloride and 0.6-1.0g terephthalic acid are dissolved in 125-175mL N,N-dimethylformamide, and then 0-1.86g cerium chloride hydrate is dissolved in 25-50mL N,N- In dimethylformamide, when the solid is completely dissolved, mix the two solutions, add 25-50 mL of N,N-dimethylformamide, stir and heat to 120-150 °C for 12-24 hours, and finally Obtained by centrifugation, washing and drying. 4. the preparation method of a kind of Pd nanoparticle porous composite material according to claim 1, is characterized in that, the preparation method of H-UiO-66 that does not dope cerium metal but adds acid in the synthesis process is: 1.5g of zirconium tetrachloride and 0.6-1.0g of terephthalic acid were dissolved in 125-175mL of N,N-dimethylformamide. When the solid was completely dissolved, 25-50mL of N,N-dimethylformamide was added. The amide and 15-25mL acid solution are heated to 120-150°C after stirring for 12-24h, and finally obtained by centrifugation, washing and drying. 5. the preparation method of a kind of Pd nanoparticle porous composite material according to claim 1, is characterized in that, described palladium metal salt is selected from any one in chloride salt, acetate and nitrate, described The acid is any one of formic acid, acetic acid, and hydrochloric acid. 6. The preparation method of a Pd nanoparticle porous composite material according to claim 1, wherein the molar ratio of palladium ions in the sodium borohydride and the palladium metal salt is 1:1-10:1. 7 . The method for preparing a Pd nanoparticle porous composite material according to claim 1 , wherein the volume ratio of the n-hexane to water is 10:1-100:1. 8 . 8. The Pd nanoparticle porous composite material prepared by the preparation method according to any one of claims 1-7. 9. Application of the Pd nanoparticle porous composite material according to claim 8 in hydrogen storage at low temperature and/or normal temperature. 10. The application of the Pd nanoparticle porous composite material of claim 8 in catalytic hydrogenation.

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