A method for preparing supported palladium nanocatalysts and their applications

Abstract

This invention discloses a method for preparing supported palladium nanocatalysts and their applications. The method includes: Step 1, adding an aqueous precursor solution containing sulfite ions to an aqueous precursor solution containing a soluble Pd salt to carry out a complexation reaction, thereby obtaining [Pd(SO3)4]. 6‑ Step 1: Add an alkaline aqueous solution to the reaction system of Step 1, causing the complex to precipitate as a white precipitate. Step 2: Wash the white precipitate and react it with H2O2 to obtain a palladium oxide colloidal dispersion. Step 4: Add a support to the colloidal dispersion for impregnation and adsorption, followed by centrifugation, washing, drying, and calcination in a reducing atmosphere to obtain the supported palladium nanocatalyst. The catalyst prepared by this invention avoids the adsorption and poisoning of organic impurities on the catalyst surface, exhibits high catalyst activity, can operate for a long time without agglomeration or decay, and demonstrates good catalyst stability.

Description

Technical Field

[0001] This invention belongs to the field of palladium nanocatalyst technology, specifically relating to a method for preparing and applying a supported palladium nanocatalyst. Background Technology

[0002] Palladium metal is one of the most commonly used catalyst components in the chemical and chemical industries, and it can be applied in fuel cells, selective oxidation, hydrogenation / hydrogenolysis, reforming, and biomass conversion. For example, in the formic acid dehydrogenation reaction, palladium nanocatalysts exhibit higher catalytic activity than other precious metal components. However, compared with other precious metals, palladium's relative reactivity results in a higher surface energy for its nanocatalysts, making them prone to particle agglomeration or component loss during preparation or reaction. Currently, palladium nanocatalysts exhibit relatively low catalytic activity and short lifespan. Therefore, it is crucial to develop highly active and stable palladium nanocatalysts.

[0003] Many factors influence the activity and stability of palladium nanocatalysts, including particle size, dispersibility on the support surface, interaction between palladium and the support, chemical valence state of the palladium active phase, microstructure, and dominant exposed crystal faces. These factors are closely related to the preparation method, process parameters, and post-treatment strategies of palladium nanocatalysts. Traditional palladium nanocatalysts are mostly obtained using a liquid-phase impregnation-reduction method. Commonly used reducing agents include sodium borohydride, ascorbic acid, and glucose. During the preparation process, to inhibit the growth and aggregation of palladium nanoparticles, organic molecules such as polyols, PVP, and oleylamine / oleic acid are often used as dispersants and stabilizers to protect the palladium nanoparticles. For example, Chinese patent CN100402144C discloses a method for preparing a Pd / C catalyst, which uses a palladium mixture prepared by mixing 8-hydroxyquinoline and water to impregnate or spray onto activated carbon. Based on the strong interaction between the 8-hydroxyquinoline complexing agent and Pd, a stable complex is generated to reduce the particle size of the Pd nanocatalyst and improve the dispersibility of Pd on the carbon support surface. CN110038561A discloses a method for synthesizing highly active palladium nanocatalysts by increasing the concentration of chloropalladium acid precursor. The applicant utilizes the adsorption stabilizing effect of polybenzoxazine resin on the palladium precursor. The polybenzoxazine resin support is immersed in an aqueous solution of chloropalladium acid for a prolonged period (6 hours at room temperature), followed by solid-liquid separation to obtain polybenzoxazine resin with a large amount of chloropalladium acid adsorbed on its surface. After drying, the polybenzoxazine resin-supported palladium nanocatalyst is obtained by heat treatment at 500℃ under an argon atmosphere. CN106816605B discloses a method for preparing palladium nanocatalysts supported on titanium plates. The applicant uses a titanium plate as a support, adds oleic acid or oleate to form a gel with a palladium salt precursor, then uniformly coats the gel onto the titanium plate, and treats it at 180-350℃ under gas protection for 0.5-2 hours to obtain the palladium nanocatalyst supported on the titanium plate. CN106935872B discloses a method for loading palladium nanocatalyst particles via liquid-phase reduction, using CeO2 and activated carbon as a composite support and chloropalladium acid as a noble metal precursor. The applicant discovered that the composite support can significantly improve the dispersibility of Pd particles, thereby enhancing the catalytic activity and stability of the catalyst for alcohols.

[0004] In summary, all the above-mentioned processes for synthesizing palladium nanocatalysts utilize organic solvents or polymers as protective agents or stabilizers. However, existing strategies struggle to effectively remove organic impurities from the catalyst surface without affecting the microstructure / composition of the palladium catalyst, severely impacting its catalytic performance and limiting its practical applications. Therefore, it is necessary to improve existing palladium nanocatalyst preparation methods and explore a novel, efficient, and green palladium nanocatalyst synthesis technology that does not require the addition of polymeric activators. Summary of the Invention

[0005] The purpose of this invention is to overcome the above-mentioned shortcomings of existing palladium nanocatalyst preparation technologies and provide a novel, green, efficient, and surface-free method for preparing palladium nanocatalysts without the addition of polymeric surfactants.

[0006] This invention uses inorganic ionic complexes instead of polymeric surfactants as stabilizers for nanoparticles, avoiding the adsorption and poisoning of organic impurities on the catalyst surface. The inorganic complexing agent is easily removed through washing, resulting in a clean catalyst surface and high catalytic performance. Secondly, this invention uses nano-palladium oxide as an intermediate species. The oxygen atoms in palladium oxide can interact with various types of supports through multiple forces, resulting in strong interactions between palladium and the support, good dispersibility, and the catalyst can operate for extended periods without aggregation or degradation, exhibiting good stability. Furthermore, this invention is an aqueous synthesis with low reaction temperatures and simple steps, solving the problems of complex preparation processes, harsh conditions, and difficulty in mass production of existing supported palladium nanocatalysts.

[0007] This invention is achieved through the following technical solution:

[0008] A method for preparing a supported palladium nanocatalyst, the method comprising the following steps:

[0009] Step 1: Add the aqueous precursor solution containing sulfite to the aqueous precursor solution containing soluble Pd salt to carry out a complexation reaction, thus preparing [Pd(SO3)4]. 6- Complexes;

[0010] Step 2: Add an alkaline aqueous solution to the reaction system of Step 1, causing the complex to precipitate out as a white precipitate;

[0011] Step 3: After washing the white precipitate, react it with H2O2 to obtain a palladium oxide colloidal dispersion;

[0012] Step 4: After adding the carrier to the colloidal dispersion for impregnation and adsorption, the mixture is centrifuged, washed, dried, and calcined in a reducing atmosphere to obtain the supported palladium nanocatalyst.

[0013] Based on the above scheme, preferably, a method for preparing a novel, green, efficient, polymeric surfactant-free, and surface-impurity-free supported palladium nanocatalyst specifically includes the following steps:

[0014] Step 1: Add the precursor aqueous solution containing sulfite ions dropwise to the precursor aqueous solution containing soluble Pd salt at room temperature. Stop adding when the color of the palladium precursor solution becomes significantly lighter. Continue magnetic stirring at room temperature for 1-2 hours to ensure that the complexation reaction between Pd and sulfite ions is complete.

[0015] Step 2: Slowly add the alkaline aqueous solution dropwise to the reaction solution obtained in Step 1 while stirring. Adjust the pH of the solution to 9-11. Stop adding the alkaline solution when a small amount of red precipitate is produced in the reaction system.

[0016] Step 3: Keep the pH of the reaction system constant and continue to add the precursor aqueous solution containing sulfite to the reaction system of Step 2 under stirring. The small amount of red precipitate produced above will gradually disappear and a large amount of white precipitate will be produced at the same time. Stop adding the white precipitate when it stops precipitating. After the addition is complete, continue the reaction for 1-2 hours to ensure that the white precipitate is completely formed.

[0017] Step 4: Wash the white precipitate generated in Step 3 with a large amount of deionized water, centrifuge and dry it to obtain a white powder;

[0018] Step 5: Dissolve the white powder from Step 4 completely in a dilute acid aqueous solution to form a colorless solution. Under ice-water bath conditions, slowly add hydrogen peroxide aqueous solution to the solution, control the reaction temperature and pH, and continue stirring for 1-2 hours to form a water-soluble PdO dispersion. This dispersion has a significant Tyndall effect and does not settle after a long time.

[0019] Step 6: Add the carrier to the palladium oxide colloidal dispersion obtained in step 5, disperse it by ultrasonication, and continue stirring at room temperature overnight to ensure complete deposition. Then, perform solid-liquid separation, washing, and drying to obtain solid powder.

[0020] Step 7: Transfer the solid powder obtained in Step 6 to a tube furnace and heat it in a protective atmosphere at 50-300℃ for 10 min-5 h to obtain a supported palladium nanocatalyst.

[0021] Preferably, in step 1, the soluble Pd salt is one or more of the following Pd salt precursor aqueous solutions: chloropalladium acid, sodium chloropalladium, potassium chloropalladium, and palladium dichloride. The Pd concentration in the precursor aqueous solution is 10-50 g / L. The sulfite-containing precursor aqueous solution is one or more of the following: sodium bisulfite, potassium bisulfite, sulfurous acid, sodium sulfite, and potassium sulfite aqueous solution. The sulfite concentration in the precursor aqueous solution is 50-100 g / L, and the dropping rate is 2-6 mL / min. The molar ratio of sulfite to Pd precursor (based on Pd) in the reaction system is 5:1-20:1.

[0022] Preferably, the alkaline aqueous solution in step 2 is one or more of sodium carbonate, sodium bicarbonate, sodium hydroxide, potassium hydroxide, or ammonia, and the slow dripping rate is 2-6 mL / min. The dripping process is accompanied by stirring, and the stirring speed is 200-1000 r / min.

[0023] Preferably, in step 3, the stirring speed is 200-1000 rpm and the reaction temperature is 0-30℃;

[0024] Preferably, in step 5, the dilute acid solution is an aqueous solution of dilute sulfuric acid, dilute hydrochloric acid, and acetic acid, with a concentration of 0.05-0.2 mol / L, and the pH of the reaction system is 2-5; the amount of H2O2 added is related to [Pd(SO3)4]. 6- The molar ratio of the complex is 1:1 to 5:1; the reaction temperature is 0 to 10℃;

[0025] Preferably, the carrier in step 6 comprises one or a mixture of several of activated carbon, conductive carbon, mesoporous carbon, carbon aerosol, aluminum oxide, silicon dioxide, zirconium dioxide, and cerium dioxide; the solid-liquid separation method is centrifugation or filtration; and the drying temperature is 30-60℃.

[0026] Preferably, the protective atmosphere in step 7 is hydrogen, ammonia, or a hydrogen / argon mixture (H2 content 5 vol%).

[0027] The palladium nanoparticles in the catalyst prepared by the above method of the present invention have a particle size of 2-4 nm.

[0028] The supported palladium nanocatalyst prepared by the above method of the present invention can be used for oxygen reduction electrocatalytic reaction.

[0029] The reason for using water-soluble Pd precursors in step 1 is that water-soluble Pd precursors are easy to prepare and relatively inexpensive, and aqueous synthesis helps with subsequent batch scale-up, has less environmental pollution, and lower preparation costs.

[0030] Step 1 involves slowly adding the sulfite precursor solution to the Pd precursor aqueous solution, utilizing the strong coordination of the sulfite ion to complex with Pd, thus obtaining [Pd(SO3)4]. 6- The complex, sulfite ions, are inorganic compound stabilizers that protect the subsequently formed palladium oxide nanoparticles and prevent aggregation. The purpose of slowly adding sulfite is to control the amount of sulfite ions added, because sulfite ions have reducing properties; adding them too quickly can easily cause the Pd precursor to be directly reduced, preventing the formation of [Pd(SO3)4]. 6- Complexes;

[0031] The alkaline aqueous solution is slowly added in step 2 to adjust the pH of the reaction system to be alkaline, because [Pd(SO3)4] 6- The complex exists in ionic form in both strongly acidic and strongly alkaline solutions, except when the pH is moderately alkaline (pH = 9-11) [Pd(SO3)4]. 6- Only complexes will precipitate out as a white precipitate;

[0032] The addition of sulfite solution in step 3 is to ensure complete precipitation, guaranteeing that 100% of the Pd species in the solution precipitate out of the solution and are completely converted into palladium sulfite.

[0033] The use of a large amount of deionized water for washing in step 4 is to ensure that chloride ions are completely removed and to ensure the high activity of the catalyst.

[0034] H2O2 is used as the oxidant in step 5 because the product of the reaction is water, which will not pollute the reaction system; dilute acid solution is added after the reaction to destroy the stability of palladium oxide colloid and make efficient deposition on the solid support in the future.

[0035] The drying temperature in step 6 is maintained at 30-60℃ to avoid the aggregation of palladium oxide nanoparticles caused by high-temperature drying.

[0036] The heat treatment temperature of 50-200℃ in step 7 is to ensure that palladium oxide is completely reduced to zero-valent Pd without causing particle growth and agglomeration.

[0037] As can be seen from the above description, the present invention has the following advantages:

[0038] 1. This invention uses inorganic sulfite ions instead of polymeric surfactants as stabilizers for nanoparticles, avoiding the adsorption and poisoning of organic impurities on the catalyst surface, resulting in high catalyst activity;

[0039] 2. This invention uses nano-palladium oxide as an intermediate species. The oxygen atoms in palladium oxide can interact with various types of supports through multiple forces. The interaction between palladium and the support is strong, the dispersion is good, the catalyst can operate for a long time without agglomeration or decay, and the catalyst has good stability.

[0040] 3. This invention is an aqueous synthesis with low reaction temperature and simple steps, which can solve the problems of complex preparation process, harsh conditions and difficulty in mass production of existing supported palladium nanocatalysts. Attached Figure Description

[0041] Figure 1 This is the XRD pattern of the Pd / C catalyst prepared in Comparative Example 1 of this invention.

[0042] Figure 2 This is a TEM image of the Pd / C catalyst prepared in Comparative Example 1 of this invention.

[0043] Figure 3 These are the CV and oxygen reduction polarization curves of the Pd / C catalyst prepared in Comparative Example 1 of this invention.

[0044] Figure 4 This is the XRD pattern of the Pd / C catalyst prepared in Example 1 of this invention.

[0045] Figure 5 This is a TEM image of the Pd / C catalyst prepared in Example 1 of the present invention.

[0046] Figure 6 These are the CV and oxygen reduction polarization curves of the Pd / C catalyst prepared in Example 1 of this invention.

[0047] Figure 7 This is a TEM image of the Pd / C catalyst prepared in Example 2 of the present invention. Detailed Implementation

[0048] The present invention will be specifically described below with reference to examples. Unless otherwise specified, the raw materials used in the following embodiments and comparative examples are all commercially available conventional raw materials.

[0049] Comparative Example 1

[0050] Preparation of Pd / C nanocatalysts (sodium borohydride reduction method, PVP as surfactant)

[0051] At room temperature, 100 mg of palladium chloride powder was ultrasonically dissolved in 100 mL of 0.1 mol / L hydrochloric acid aqueous solution. 10 mg of PVP powder (molecular weight 300,000) and 240 mg of XC-72R carbon powder were ultrasonically dispersed in 10 mL of deionized water. After ultrasonically mixing the three precursor aqueous solutions, 10 mL of freshly prepared NaBH4 aqueous solution (50 mM) was added dropwise to the reaction system under stirring at a rate of 2 mL / min. After the addition was complete, stirring was continued at room temperature for 3 hours to ensure complete Pd reduction. After the reaction was completed, a black solid powder was obtained after filtration and washing. The solid powder was placed in a tube furnace and heat-treated at 300°C for 1 hour to oxidize and remove the PVP protective agent. Then, a protective gas of H2 / Ar (5% vol H2 content) was passed through and the mixture was kept at 100°C for 1 hour to obtain a Pd / C catalyst, wherein the mass ratio of Pd to XC-72R was 3:7. Figure 1 and 2 The images show the XRD and TEM spectra of the prepared Pd / C catalyst, respectively. It can be seen that the Pd / C catalyst prepared by this method has a large metal particle size and severe agglomeration.

[0052] The electrochemical activity of the obtained catalyst was evaluated using a rotating disk electrode. The specific steps were as follows: Approximately 5 mg of the prepared Pd / C catalyst was accurately weighed and mixed with 20 μL of Nafion (5 wt%) solution and 5 mL of ethanol. The mixture was ultrasonically dissolved to obtain a uniformly dispersed catalyst slurry. Then, 10 μL of the catalyst slurry was transferred and coated onto a GC rotating disk electrode with an area of ​​0.19625 cm². After drying, the working electrode was obtained. The electrochemical active area (ECSA) of the catalyst was measured by recording the cyclic voltammetry (CV) curve of the catalyst in a 0.1 M perchloric acid aqueous solution purged with high-purity nitrogen gas. Specifically, the scan rate was 50 mV / s from 0 V to 1.2 V. The ECSA of the Pd / C catalyst could be calculated by obtaining the integrated area of ​​the hydrogen adsorption-desorption peak region (0-0.4 V) on the CV curve. This value is related to the particle size of the Pd nanoparticles and their dispersion on the carbon support. The smaller the particle size of the catalyst and the better its dispersion on the carbon support surface, the larger its ECSA. The oxygen reduction activity was tested by scanning from 0 V to 1 V at a scan rate of 10 mV / s in a 0.1 M perchloric acid aqueous solution saturated with oxygen to obtain the oxygen reduction curve. Figure 3 The CV and oxygen reduction polarization curves of the obtained Pd / C catalyst in a nitrogen- and oxygen-saturated 0.1M perchloric acid solution are shown.

[0053] Example 1

[0054] Step 1: At room temperature, 100 mg of palladium chloride powder was ultrasonically dissolved in 100 mL of hydrochloric acid aqueous solution (HCl concentration: 0.1 mol / L). A 50 g / L sodium bisulfite precursor aqueous solution was then added dropwise at a rate of 2 mL / min. When the reaction system changed from red to pale yellow, it indicated the presence of [Pd(SO3)4]. 6- To ensure the complex formation, sodium bisulfite aqueous solution can be added dropwise for 1-2 minutes to ensure the complexation reaction is complete. The molar ratio of sulfite solution to Pd precursor in the reaction system is 5:1, and the pH of the solution is approximately 5.

[0055] Step 2: Adjust the pH of the reaction system with sodium carbonate aqueous solution (20g / L). Stop adding the solution when a brick-red precipitate appears. At this point, the pH of the solution is about 11.

[0056] Step 3: Continue to add sodium bisulfite aqueous solution (10 g / L). The brick-red precipitate will gradually disappear, and at the same time, a large amount of white precipitate will be produced. During the addition of sodium bisulfite, the pH of the solution will drop slightly. During the reaction, sodium carbonate aqueous solution needs to be continuously added to maintain the pH of the reaction system above 9. After the white precipitate stops precipitating, stop adding sodium bisulfite solution and continue to stir the reaction at room temperature for 1 hour to ensure that the white precipitate is completely formed.

[0057] Step 4: Separate the white precipitate from the supernatant by centrifugation, and wash the white precipitate with a large amount of deionized water until no chloride ions are present in the precipitate, and then dry it in an air oven at 60°C.

[0058] Step 5: Dissolve the dried white precipitate in 0.2 mol / L dilute sulfuric acid solution to obtain a colorless and transparent liquid, and add excess H2O2 aqueous solution (30 wt% H2O2 aqueous solution, where the molar ratio of H2O2 to Pd is 2:1). Control the reaction temperature of the reaction system at 5 degrees Celsius, and stop the reaction after magnetic stirring for 3 hours; an aqueous colloidal solution of palladium oxide is obtained, and the solution exhibits a significant Tyndall effect.

[0059] Step 6: Weigh 240 mg of XC-72 activated carbon powder and ultrasonically disperse it in 100 mL of deionized water. Quickly add the aqueous dispersion of XC-72R activated carbon to the aqueous colloidal solution of palladium oxide. Continue to add 20 mL of 0.2 mol / L sulfuric acid solution to promote the deposition of palladium oxide colloid on the carrier surface. After magnetic stirring for 6 hours, centrifuge and wash with a large amount of deionized water. Place the solid obtained after centrifugation in a vacuum oven and dry it at 40 degrees Celsius for 4 hours to obtain PdO / C solid powder.

[0060] Step 7: Transfer the PdO / C solid powder to a tube furnace, and heat it at 100°C for 1 hour under H2 / Ar (5% vol H2 content) protective gas to obtain a supported Pd / C nanocatalyst, wherein the mass ratio of metallic Pd to XC-72R is 3:7.

[0061] Figure 4 and 5 The XRD and TEM images of the prepared Pd / C catalyst are shown. The broadening of the corresponding XRD diffraction peaks and the TEM images indicate that the prepared Pd nanoparticles are small. The average grain size is about 3.0 nm according to the Scherrer formula. The composition and elemental composition analysis of the catalyst show that the surface of the prepared catalyst is clean and there are no obvious organic / inorganic impurities.

[0062] The obtained Pd / C catalyst was evaluated for its oxygen reduction reaction activity using a rotating disk electrode. The specific steps were as follows: Approximately 5 mg of Pd / C catalyst was accurately weighed and mixed with 20 μL of Nafion (5 wt%) solution and 5 mL of ethanol. The mixture was ultrasonically sonicated to obtain a uniformly dispersed catalyst slurry. Then, 10 μL of the catalyst slurry was transferred and coated onto a GC rotating disk electrode with an area of ​​0.19625 cm². After drying, the working electrode was obtained. The electrochemical active area (ECSA) of the catalyst was measured by recording the cyclic voltammetry (CV) curve of the catalyst in a 0.1 M perchloric acid aqueous solution purged with high-purity nitrogen gas. Specifically, the scan rate was 50 mV / s from 0 V to 1.2 V. The ECSA of the Pd / C catalyst could be calculated by obtaining the integrated area of ​​the hydrogen adsorption-desorption peak region (0-0.4 V) on the CV curve. This value is related to the particle size of the Pd nanoparticles and their dispersion on the carbon support. Smaller particle size and better dispersion on the carbon support surface result in a larger ECSA. The oxygen reduction activity was tested by scanning from 0 V to 1 V at a scan rate of 10 mV / s in a 0.1 M perchloric acid aqueous solution saturated with oxygen to obtain the oxygen reduction curve. Figure 6 The CV and oxygen reduction polarization curves of the obtained Pd / C catalyst in a nitrogen- and oxygen-saturated 0.1M perchloric acid solution are shown. At the same polarization voltage, a larger absolute value of the current indicates a higher current value for the oxygen reduction reaction and thus higher catalyst activity. Compared to Comparative Example 1, at a polarization voltage of 0.8V (vs RHE), the oxygen reduction current corresponding to Example 1 is approximately 2.5 mA / cm². 2 It is 1.7 times that of the Pd / C prepared in the comparative ratio.

[0063] Example 2

[0064] The specific experimental steps in this embodiment are similar to those in Example 1, except that KB-300J conductive carbon black is used as the carbon support, and the amount of Pd precursor used is 20g. Pd The reaction system contains a 100 g / L potassium sulfite aqueous solution and a 100 g / L potassium sulfite aqueous solution as the sulfite precursor. The sulfite is added to the Pd precursor at a rate of 5 ml / min. When the solution changes from orange to pale yellow, the molar ratio of sulfite solution to Pd precursor in the reaction system is 10:1, and the pH is approximately 4. The stirring speed is maintained at 1000 rpm. The pH of the reaction system is adjusted to 12 using sodium hydroxide aqueous solution, at which point the addition is stopped. The solution then becomes colorless with a small amount of white precipitate forming. Potassium sulfite aqueous solution is added dropwise until the white precipitate no longer increases. Subsequent steps are the same as in Example 1. The Pd / C catalyst prepared in this example has a loading of 20 wt%, where the mass ratio of Pd to KB-300J is 2:8.

[0065] Electron microscopy characterization revealed that the Pd nanoparticles prepared using the above steps had an average particle size of approximately 3.0 nm and exhibited no obvious agglomeration. Figure 7 The obtained Pd / C catalyst was evaluated for its oxygen reduction reaction activity using a rotating disk electrode, following the same procedures as in Example 1. The results showed that at a polarization voltage of 0.8 V (vs RHE), the oxygen reduction current corresponding to the Pd / C prepared in Example 2 was approximately 3 mA / cm². 2 It is twice that of the Pd / C prepared in the comparative ratio.

[0066] Example 3

[0067] The specific experimental steps in this embodiment are similar to those in Example 1, except that carbon nanotubes are used as the carbon support, and the amount of Pd precursor used is 40g. Pd The reaction system contains a 100 g / L Pd nitrate aqueous solution and a 100 g / L sulfite precursor solution. The sulfite is added to the Pd precursor at a rate of 4 ml / min. When the solution changes from orange to pale yellow, the molar ratio of sulfite solution to Pd precursor in the reaction system is 20:1, and the pH is approximately 4. The stirring speed is maintained at 500 rpm. The pH of the reaction system is adjusted to 12 using sodium hydroxide aqueous solution, at which point the addition is stopped. The solution then becomes colorless with a small amount of white precipitate. Potassium sulfite aqueous solution is continued to be added until the white precipitate no longer increases. Subsequent steps are the same as in Example 1. The Pd / C catalyst prepared in this example has a loading of 40 wt%, where the mass ratio of Pd to KB-300J is 2:8.

[0068] Electron microscopy characterization revealed that the Pd nanoparticles prepared using the above steps had an average particle size of approximately 3.0 nm and exhibited no obvious agglomeration. Figure 7 The obtained Pd / C catalyst was evaluated for its oxygen reduction reaction activity using a rotating disk electrode, following the same procedures as in Example 1. The results showed that at a polarization voltage of 0.8 V (vs RHE), the oxygen reduction current corresponding to the Pd / C prepared in Example 3 was approximately 2.8 mA / cm². 2 It is 1.8 times that of the Pd / C prepared in the comparative ratio.

[0069] It is understood that the above detailed description of the present invention is for illustrative purposes only and is not intended to limit the technical solutions described in the embodiments of the present invention. Those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention to achieve the same technical effects; as long as the usage requirements are met, they are all within the protection scope of the present invention.

Claims

1. The application of a supported palladium nanocatalyst in the oxygen reduction electrocatalytic reaction, characterized in that, The preparation method of supported palladium nanocatalysts includes the following steps: Step 1: Add the precursor aqueous solution containing sulfite ions dropwise to the precursor aqueous solution containing soluble Pd salt at room temperature. Stop adding the palladium precursor aqueous solution when the color becomes significantly lighter. Continue stirring at room temperature for 1-2 hours. Step 2: Add the alkaline aqueous solution dropwise to the solution after the reaction in Step 1 while stirring. Adjust the pH of the solution to 9-11. Stop adding the alkaline solution when a red precipitate begins to form in the reaction system. Step 3: Keep the pH of the reaction system constant and continue to add the precursor aqueous solution containing sulfite to the reaction system of Step 2 under stirring. The red precipitate gradually disappears and a white precipitate is formed at the same time. Stop adding the white precipitate when it stops precipitating and continue the reaction for 1-2 hours to ensure that the white precipitate is completely formed. Step 4: Wash the white precipitate generated in Step 3 with deionized water, centrifuge and dry it to obtain a white powder; Step 5: Completely dissolve the white powder from Step 4 in an acidic aqueous solution, add hydrogen peroxide aqueous solution dropwise under ice-water bath conditions, and continue stirring the reaction for 1-2 hours to form a water-soluble palladium oxide colloidal dispersion. Step 6: Add the carrier to the palladium oxide colloidal dispersion obtained in step 5, disperse it by ultrasonication, stir it at room temperature overnight, and then perform solid-liquid separation, washing, and drying to obtain solid powder. Step 7, heat the solid powder obtained in step 6 at 50-300°C. o The supported palladium nanocatalyst was obtained by heating in a protective atmosphere of C for 10 min to 5 h.

2. The application according to claim 1, characterized in that, In step 1, the soluble Pd salt is one or more of chloropalladium acid, sodium chloropalladium, potassium chloropalladium, and palladium dichloride, the Pd concentration in the precursor aqueous solution of the Pd salt is 10-50 g / L, and the precursor aqueous solution containing sulfite is one or more of sulfurous acid, sodium sulfite, and potassium sulfite. In step 1, the dropping rate is 2-6 mL / min; the molar ratio of sulfite to Pd ions in the reaction system is 5:1-20:1; and the mass concentration of sulfite in the precursor aqueous solution is 50-100 g / L. In step 2, the dropping rate is 2-6 mL / min and the stirring speed is 200-1000 r / min; in step 3, the stirring speed is 200-1000 rpm. In step 3, the reaction temperature is 0-30 ℃; In step 5, the pH of the reaction system is 2-5; the reaction temperature is 0-10 ℃; and the concentration of the acidic aqueous solution is 0.05-0.2 mol / L. In step 6, the drying temperature is 30-60℃; The palladium nanoparticles have a particle size of 2-4 nm.

3. The application according to claim 1, characterized in that, In step 2, the alkaline aqueous solution is one or more of sodium carbonate, sodium bicarbonate, sodium hydroxide, potassium hydroxide, or ammonia solution.

4. The application according to claim 1, characterized in that, In step 5, the acidic aqueous solution is one or more of dilute sulfuric acid, dilute hydrochloric acid, and acetic acid aqueous solution.

5. The application according to claim 1, characterized in that, In step 6, the carrier is one or a mixture of several of the following: activated carbon, conductive carbon, mesoporous carbon, carbon aerosol, aluminum oxide, silicon dioxide, zirconium dioxide, and cerium dioxide; the solid-liquid separation method is centrifugation or filtration; in step 7, the protective gas is hydrogen or a hydrogen / argon mixture.

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