A highly stable graphene-coated Pd-Zn alloy catalyst and its preparation method

Abstract

The present invention provides a highly stable graphene-coated Pd-Zn alloy catalyst and a preparation method thereof. The present invention first utilizes a simple method to synthesize a carbon precursor with a three-dimensional structure; after loading a palladium salt on its surface, acetate is added as a small molecule modifier, and finally a graphene-coated Pd-Zn alloy catalyst is obtained by a one-step heat treatment, wherein the alloy particles are uniformly dispersed on the carbon support and have a particle size of less than 10 nm. The present invention is simple to operate, low in cost, and can be mass-produced, and the obtained catalyst has a unique graphene-coated structure, the alloy particles are bound by the graphite layer, which plays a role of physical isolation, reduces the shedding and aggregation of the catalyst in practical applications, prevents the disappearance of active sites, and improves the electrochemical performance of the catalyst as a whole. After 60,000 cycles of accelerated aging experiments, the mass activity loss rate of the catalyst is less than 5%, and has excellent stability.

Description

Technical Field

[0001] The present invention belongs to the field of catalysts and mainly relates to a highly stable graphene-coated Pd-Zn alloy catalyst and a preparation method thereof. Background Art

[0002] The electrocatalytic oxygen reduction reaction (O2R) is an important cathode reaction in various energy conversion and storage systems, including fuel cells and metal-air batteries. Currently, platinum-based catalysts from the platinum group metals and palladium-based catalysts, which have similar electronic structures to platinum, are common catalyst materials for accelerating the O2R. However, in practical applications, long-term operation can lead to performance degradation of alloy catalysts. This is because nanoparticles with a high surface area-to-volume ratio are thermodynamically metastable. After long reaction times, the nanoparticles tend to grow significantly during the catalytic process through physical aggregation or Oswald ripening, which results in a gradual loss of active surface area and mass activity, leading to reduced stability. Reducing the deactivation of active sites in practical applications and achieving O2R catalysts with low noble metal loading and high long-term stability remain significant challenges.

[0003] Carbon coating is an effective method for improving catalyst stability. This carbon coating acts as a physical barrier, stabilizing metal nanoparticles by hindering their migration and / or Ozwald ripening. Encapsulating the particles with materials such as zeolites not only protects them from shedding, but also effectively hinders their migration through the physical barrier. Furthermore, the porous structure effectively exposes active sites, protecting the catalyst's catalytic activity and significantly improving its activity and stability.

[0004] For example, Chinese invention patent application number CN201810466348.8 discloses a catalyst composed of N-doped porous carbon-coated Fe and Co bimetallic nanoparticles and its preparation method. The FeCo@NC catalyst, composed of N-doped porous carbon-coated Fe and Co, is prepared using a high-temperature, step-by-step calcination method. The catalyst exhibits a three-dimensional, porous, disordered stacked structure. The metal phase is encapsulated within the N-doped porous carbon, effectively improving the catalyst's stability and methanol resistance. Chinese invention patent application number CN201910758555.5 discloses a method for preparing and applying a composite material composed of carbon nanotubes and elemental cobalt. The resulting composite material (CNT@Co) exhibits excellent catalytic activity when used as an electrocatalyst for oxygen reduction, approaching that of platinum-carbon. This indicates that various new carbon-coated alloy / metal catalysts have been studied and applied, all exhibiting excellent catalytic activity and stability.

[0005] The present invention selects a precursor with a three-dimensional structure as the carbon source. During the high-temperature heat treatment process, the added acetate is converted into graphene under high-temperature treatment and coated on the alloy surface, forming a porous, multi-defective graphite carbon layer. Although the carbon layer is coated on the alloy surface, it does not cover the active sites, so that the catalyst has both high catalytic activity and stability. Summary of the Invention

[0006] The present invention aims to provide a graphene-coated Pd-Zn alloy catalyst that can be mass-produced, is low-cost, and has high stability. The present invention first utilizes a simple method to synthesize a carbon precursor with a three-dimensional structure, which is then compounded with a palladium salt, using acetate as a small molecule modifier; a graphene-coated Pd-Zn alloy catalyst is obtained through a one-step heat treatment. This unique graphene-coated structure binds the alloy, providing physical isolation, reducing catalyst shedding and aggregation in practical applications, preventing the disappearance of active sites, and improving the overall stability of the catalyst. The technical steps provided by the present invention are as follows:

[0007] Step 1: Precursor synthesis

[0008] The zinc salt is dissolved in a methanol solution as solution 1; the dimethylimidazole is dissolved in a methanol solution as solution 2; the concentration of solution 1 is 0.5 to 1.0 mol / L; the concentration of solution 2 is 0.5 to 1.0 mol / L, and solution 1 and solution 2 are mixed in a volume ratio of 1:1 to 5, stirred for 6 to 24 hours, and then centrifuged and dried to obtain a precursor;

[0009] Step 2: Alloy catalyst preparation

[0010] Weigh 100-300 mg of the precursor, redisperse it in a methanol solution to form a dispersion, and ultrasonically disperse it for 30 minutes; add palladium salt to the dispersion; continue to add acetate as a modifier, stir for 10-12 hours, and then centrifuge and dry to obtain an alloy catalyst raw material; the amount of palladium salt added is 5-10 mg; the amount of acetate added is 30-150 mg;

[0011] Step 3: Graphene coating formation

[0012] The powder obtained above is ground uniformly, and annealed for 1 to 3 hours at 800 to 1000° C. and a heating rate of 1 to 5° C. / min in a nitrogen or argon atmosphere to obtain a catalyst.

[0013] The zinc salt described in step 1 includes zinc nitrate or zinc acetate.

[0014] The palladium salt in step 2 is palladium chloride, potassium tetrachloropalladate or palladium acetylacetonate, and the acetate is potassium acetate or sodium acetate;

[0015] The prepared highly stable graphene-coated Pd-Zn alloy catalyst is used as an oxygen reduction catalyst in hydrogen fuel cells.

[0016] Based on the above production scheme, a highly stable graphene-coated Pd-Zn alloy catalyst was obtained. The catalyst should have the following characteristics: its structure is characterized by graphene-coated Pd-Zn alloy grains uniformly dispersed on a carbon support. After 60,000 cycles of accelerated aging, its mass activity loss rate is less than 5%. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 XRD pattern of the synthesized graphene-coated Pd-Zn alloy catalyst;

[0018] Figure 2 Low-magnification TEM image of the synthesized graphene-coated Pd-Zn alloy catalyst;

[0019] Figure 3 High-magnification TEM image of the synthesized graphene-coated Pd-Zn alloy catalyst;

[0020] Figure 4 LSV curves of the synthesized graphene-coated Pd-Zn alloy catalyst before and after 10,000 cycles; DETAILED DESCRIPTION

[0021] In order to explain the production process and principle of the present invention in more detail, embodiments are given. The embodiments are for explanation and illustration only and are not intended to limit the scope of the present invention.

[0022] Example 1

[0023] Operation 1: Precursor synthesis

[0024] Zinc acetate was dissolved in a methanol solution as solution 1, and the concentration of solution 1 was 1.0 mol / L; dimethylimidazole was dissolved in a methanol solution as solution 2, and the concentration of solution 2 was 0.5 mol / L; solution 1 and solution 2 were mixed in a volume ratio of 1:2, and stirred for 12 hours to obtain a precursor dispersion;

[0025] Operation 2: Alloy catalyst preparation

[0026] 150 mg of the precursor was weighed and redispersed in a methanol solution to form a dispersion, and ultrasonically dispersed for 30 minutes; 5 mg of potassium tetrachloropalladate was added to the dispersion; 150 mg of potassium acetate was added as a modifier, and the mixture was stirred for 10 hours and then centrifuged and dried to obtain an alloy catalyst raw material;

[0027] Operation 3: Graphene encapsulation formation

[0028] The powder obtained above was ground uniformly, and then treated at 950° C. for 3 h at a heating rate of 5° C. / min under a nitrogen atmosphere to obtain the target product of the present invention.

[0029] Based on the above production scheme, a highly stable graphene-coated Pd-Zn alloy catalyst is obtained. The obtained catalyst should have the following characteristics:

[0030] Its structural features are graphene-wrapped Pd-Zn alloy grains with a particle size of 6.3nm and uniform dispersion. After 60,000 cycles of accelerated aging, its mass activity loss rate was less than 5%.

[0031] Example 2

[0032] Operation 1: Precursor synthesis

[0033] Zinc nitrate was dissolved in a methanol solution as solution 1, and the concentration of solution 1 was 1.0 mol / L; dimethylimidazole was dissolved in a methanol solution as solution 2, and the concentration of solution 2 was 1.0 mol / L; solution 1 and solution 2 were mixed in a volume ratio of 1:3, and stirred for 24 hours to obtain a precursor dispersion;

[0034] Operation 2: Alloy catalyst preparation

[0035] 200 mg of the precursor was weighed and redispersed in a methanol solution to form a dispersion, and ultrasonically dispersed for 30 minutes; 5 mg of potassium tetrachloropalladate was added to the dispersion; 50 mg of potassium acetate was added as a modifier, and the mixture was stirred for 10 hours and then centrifuged and dried to obtain an alloy catalyst raw material;

[0036] Operation 3: Graphene encapsulation formation

[0037] The powder obtained above was ground uniformly, and then treated at 950°C for 3 h at a heating rate of 3°C / min under a nitrogen atmosphere to obtain the target compound of the present invention.

[0038] Based on the above production scheme, a highly stable graphene-coated Pd-Zn alloy catalyst is obtained. The obtained catalyst should have the following characteristics:

[0039] Its structural features include graphene-encapsulated Pd-Zn alloy grains with a particle size of 7.5 nm and uniform dispersion. After 60,000 cycles of accelerated aging, its mass activity loss rate was less than 5%.

[0040] Example 3

[0041] Operation 1: Precursor synthesis

[0042] Zinc nitrate was dissolved in a methanol solution as solution 1, and the concentration of solution 1 was 0.5 mol / L; dimethylimidazole was dissolved in a methanol solution as solution 2, and the concentration of solution 2 was 0.5 mol / L; solution 1 and solution 2 were mixed in a volume ratio of 1:5, and stirred for 12 hours to obtain a precursor dispersion;

[0043] Operation 2: Alloy catalyst preparation

[0044] 150 mg of the precursor was weighed and redispersed in a methanol solution to form a dispersion, which was then ultrasonically dispersed for 30 minutes. 5 mg of palladium chloride was added to the dispersion. 50 mg of potassium acetate was added as a modifier, and the mixture was stirred for 12 hours and then centrifuged and dried to obtain an alloy catalyst raw material.

[0045] Operation 3: Graphene encapsulation formation

[0046] The powder obtained above was ground uniformly, and then treated at 850° C. for 1 h at a heating rate of 5° C. / min under a nitrogen atmosphere to obtain the target product of the present invention.

[0047] Based on the above production scheme, a highly stable graphene-coated Pd-Zn alloy catalyst is obtained. The obtained catalyst should have the following characteristics:

[0048] Its structural features include graphene-encapsulated Pd-Zn alloy grains with a particle size of 5.8 nm and uniform dispersion. After 60,000 cycles of accelerated aging, its mass activity loss rate was less than 5%.

[0049] Example 4

[0050] Operation 1: Precursor synthesis

[0051] Zinc nitrate was dissolved in a methanol solution as solution 1, and the concentration of solution 1 was 0.5 mol / L; dimethylimidazole was dissolved in a methanol solution as solution 2, and the concentration of solution 2 was 0.5 mol / L; solution 1 and solution 2 were mixed in a volume ratio of 1:2, and stirred for 12 hours to obtain a precursor dispersion;

[0052] Operation 2: Alloy catalyst preparation

[0053] 150 mg of the precursor was weighed and redispersed in a methanol solution to form a dispersion, and ultrasonically dispersed for 30 minutes; 5 mg of potassium tetrachloropalladate was added to the dispersion; 150 mg of potassium acetate was added as a modifier, and the mixture was stirred for 10 hours and then centrifuged and dried to obtain an alloy catalyst raw material;

[0054] Operation 3: Graphene encapsulation formation

[0055] The powder obtained above was ground uniformly, and then treated at 800° C. for 3 h at a heating rate of 3° C. / min under a nitrogen atmosphere to obtain the target product of the present invention.

[0056] Based on the above production scheme, a highly stable graphene-coated Pd-Zn alloy catalyst is obtained. The obtained catalyst should have the following characteristics:

[0057] Its structural features include graphene-encapsulated Pd-Zn alloy grains with a particle size of 8.3 nm and uniform dispersion. After 60,000 cycles of accelerated aging, its mass activity loss rate was less than 5%.

[0058] Example 5

[0059] Operation 1: Precursor synthesis

[0060] Zinc nitrate is dissolved in a methanol solution as solution 1, and the concentration of solution 1 is 0.5 mol / L; dimethylimidazole is dissolved in a methanol solution as solution 2, and the concentration of solution 2 is 0.5 mol / L; solution 1 and solution 2 are mixed in a volume ratio of 1:2, and stirred for 24 hours to obtain a precursor dispersion;

[0061] Operation 2: Alloy catalyst preparation

[0062] 300 mg of the precursor was weighed and redispersed in a methanol solution to form a dispersion, and ultrasonically dispersed for 30 minutes; 5 mg of potassium tetrachloropalladate was added to the dispersion; 100 mg of potassium acetate was added as a modifier, and the mixture was stirred for 24 hours and then centrifuged and dried to obtain an alloy catalyst raw material;

[0063] Operation 3: Graphene encapsulation formation

[0064] The powder obtained above was ground uniformly, and then treated at 800° C. for 2 h at a heating rate of 5° C. / min under a nitrogen atmosphere to obtain the target product of the present invention.

[0065] Based on the above production scheme, a highly stable graphene-coated Pd-Zn alloy catalyst is obtained. The obtained catalyst should have the following characteristics:

[0066] Its structure features graphene-wrapped Pd-Zn alloy grains with a particle size of 6.7nm and uniform dispersion. After 60,000 cycles of accelerated aging, its mass activity loss rate was less than 5%.

[0067] Example 6

[0068] Operation 1: Precursor synthesis

[0069] Zinc acetate was dissolved in a methanol solution as solution 1, and the concentration of solution 1 was 1.0 mol / L; dimethylimidazole was dissolved in a methanol solution as solution 2, and the concentration of solution 2 was 1.0 mol / L; solution 1 and solution 2 were mixed in a volume ratio of 1:5, and stirred for 12 hours to obtain a precursor dispersion;

[0070] Operation 2: Alloy catalyst preparation

[0071] 150 mg of the precursor was weighed and redispersed in a methanol solution to form a dispersion, and ultrasonically dispersed for 30 minutes; 5 mg of potassium tetrachloropalladate was added to the dispersion; 100 mg of potassium acetate was added as a modifier, and the mixture was stirred for 10 hours and then centrifuged and dried to obtain an alloy catalyst raw material;

[0072] Operation 3: Graphene encapsulation formation

[0073] The powder obtained above was ground uniformly, and then treated at 950° C. for 2 h at a heating rate of 3° C. / min under a nitrogen atmosphere to obtain the target product of the present invention.

[0074] Based on the above production scheme, a highly stable graphene-coated Pd-Zn alloy catalyst is obtained. The obtained catalyst should have the following characteristics:

[0075] Its structural features are graphene-wrapped Pd-Zn alloy grains with a particle size of 6.8nm and uniform dispersion. After 60,000 cycles of accelerated aging, its mass activity loss rate was less than 5%.

Claims

1. A method for preparing a highly stable graphene-coated Pd-Zn alloy catalyst, characterized in that: The following steps are involved: Step 1: Precursor synthesis Zinc salt is dissolved in methanol solution as solution 1; Dimethylimidazole is dissolved in a methanol solution as solution 2; the concentration of solution 1 is 0.5-1.0 mol / L; the concentration of solution 2 is 0.5-1.0 mol / L, and solution 1 and solution 2 are mixed in a volume ratio of 1:1-5, stirred for 6-24 hours, and then centrifuged and dried to obtain a precursor; Step 2: Alloy catalyst preparation Weigh 100-300 mg of the precursor and redisperse it in a methanol solution to form a dispersion, which is then ultrasonically dispersed for 30 minutes. Add palladium salt to the dispersion. Continue to add acetate as a modifier, stir for 10-12 hours, and then centrifuge and dry to obtain an alloy catalyst raw material. The amount of palladium salt added is 5-10 mg; the amount of acetate added is 30-150 mg. Step 3: Graphene coating formation The alloy catalyst raw material obtained above was ground uniformly, and annealed at 800-1000°C, a heating rate of 1-5°C / min, for 1-3 hours under a nitrogen or argon atmosphere to obtain a catalyst.

2. The preparation method according to claim 1, wherein: The zinc salt in step 1 is zinc nitrate or zinc acetate.

3. The preparation method according to claim 1, wherein: The palladium salt in step 2 is palladium chloride, potassium tetrachloropalladate or palladium acetylacetonate, and the acetate is potassium acetate or sodium acetate.

4. A highly stable graphene-coated Pd-Zn alloy catalyst prepared by the method of claim 1.

5. Use of the highly stable graphene-coated Pd-Zn alloy catalyst according to claim 4 as an oxygen reduction catalyst for a hydrogen fuel cell.

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