A low specific surface area palladium powder having a bimodal particle size distribution and a method for its preparation

Abstract

The present application relates to the technical field of noble metal powder preparation, and particularly relates to a low specific surface area palladium powder with a bimodal particle size distribution and a preparation method thereof. The present application adopts a composite alkali solution system to precisely control nucleation-growth kinetics. Specifically, the complex alkali can form a stable complex with palladium ions, delaying the reduction process of palladium. The non-complex alkali only provides an alkaline environment without complexation. The combination of the two can precisely control the nucleation-growth kinetics of the system, realize the reasonable matching of the nucleation rate and the grain growth rate, and in-situ generate a particle system with a bimodal particle size distribution in a single reduction process. The structure effectively controls the specific surface area of the palladium powder below 2.0 m 2 / g, which not only retains the high activity characteristics of the main body fine particles (about 100 nm), but also significantly reduces the surface energy by introducing an appropriate amount of coarse particle component, fundamentally solving the problems of serious agglomeration and poor dispersibility of traditional nano-palladium powder caused by excessive specific surface area.

Description

Technical Field

[0001] This invention relates to the field of precious metal powder preparation technology, and in particular to a low specific surface area palladium powder with a bimodal particle size distribution and its preparation method. Background Technology

[0002] Silver-palladium paste has become an important choice for electronic pastes used in high-reliability multilayer ceramic capacitors (MLCCs) due to its excellent conductivity, sintering resistance, migration resistance, good solderability, and moderate cost, especially in key fields such as aerospace and defense. Palladium powder, as a crucial functional material in silver-palladium paste, directly determines the rheological properties, printability, and microstructure after sintering of the paste through its particle size, morphology, specific surface area, and dispersion state.

[0003] Currently, liquid-phase chemical reduction is widely used in industry to prepare palladium powder. While this method is mature and low-cost, it struggles to achieve a synergistic control of both low specific surface area and small particle size. Generally, palladium powder with smaller particle size is more prone to agglomeration and exhibits a significantly increased specific surface area. Experiments show that palladium powder with excessively large particle size or irregular morphology is easily coated by silver particles in silver-palladium composite pastes, failing to fully exert its key role in inhibiting silver migration, thus leading to a decrease in the paste's conductivity. Conversely, palladium powder with excessively small particle size, excessively high specific surface area, or severe agglomeration will result in poor printing density of the internal electrode paste, large sintering shrinkage, and defects such as macroscopic cracks and pores in the sintered layer. These problems directly restrict the large-scale application of high-performance palladium powder. Therefore, developing a novel chemical reduction process capable of simultaneously achieving submicron-sized palladium powder with small particle size and low specific surface area has become an urgent task to overcome the technological bottlenecks of high-end electronic pastes and meet the needs of next-generation microelectronics manufacturing. Summary of the Invention

[0004] In view of this, the purpose of this invention is to provide a palladium powder with a low specific surface area and a method for preparing the same, exhibiting a bimodal particle size distribution. The palladium powder prepared by this invention has a dual-scale structure, retaining the main fine particles while introducing an appropriate amount of coarse particles to reduce the specific surface area, thus overcoming the technical bottleneck of simultaneously achieving small particle size and low specific surface area.

[0005] To achieve the above-mentioned objectives, the present invention provides the following technical solution: This invention provides a method for preparing palladium powder with a low specific surface area and a bimodal particle size distribution, comprising the following steps: The pH of the palladium source solution is adjusted to 7-9 using a composite alkaline solution to obtain a palladium control solution. The alkaline in the composite alkaline solution includes complexed alkaline and non-complexed alkaline. The molar ratio of the complexed alkaline to the non-complexed alkaline is 1:(0.2-1). The complexed alkaline refers to an alkaline substance that can form a stable complex with palladium ions, and the opposite is a non-complexed alkaline. An aqueous solution of the dispersant is mixed with the palladium regulating solution to obtain a pre-reaction solution; A reducing agent aqueous solution is added dropwise to the pre-reaction solution to carry out a reduction reaction. After standing and aging, the solid and liquid are separated to obtain the low specific surface area palladium powder with bimodal particle size distribution.

[0006] Preferably, the complexing base includes ammonia.

[0007] Preferably, the non-complexing base includes one or more of sodium hydroxide, sodium carbonate, and sodium bicarbonate.

[0008] Preferably, the total concentration of complexed and non-complexed alkalis in the composite alkaline solution is 1~10 mol / L.

[0009] Preferably, the reduction reaction is carried out at a temperature of 20-35°C for a time of 30-60 min.

[0010] Preferably, the dispersant is one or two of polyvinylpyrrolidone, gum arabic, gelatin and polyvinyl alcohol; the mass ratio of the dispersant to palladium in the palladium control solution is (0.02~0.3):1.

[0011] Preferably, the reducing agent includes sodium borohydride; the concentration of the aqueous solution of the reducing agent is 0.5~1.0 mol / L; and the molar ratio of the reducing agent to palladium in the pre-reaction solution is (0.5~3):1.

[0012] Preferably, the concentration of palladium in the pre-reaction solution is 0.05~0.25 mol / L.

[0013] Preferably, the palladium source solution is a chloropalladium acid solution or a palladium nitrate solution.

[0014] This invention provides palladium powder with a bimodal particle size distribution and a low specific surface area, prepared by the method described above, with a specific surface area of ​​2.0 m². 2 / g or less; the low specific surface area palladium powder with bimodal particle size distribution is composed of coarse particles and fine particles, wherein the particle size of the coarse particles is 300~500 nm and the particle size of the fine particles is 50~200 nm.

[0015] This invention provides a method for preparing palladium powder with a bimodal particle size distribution and low specific surface area, comprising the following steps: adjusting the pH of a palladium source solution to 7-9 using a composite alkaline solution to obtain a palladium regulating solution; the alkaline in the composite alkaline solution includes complexed alkaline and non-complexed alkaline; the molar ratio of the complexed alkaline to the non-complexed alkaline is 1:(0.2-1); the complexed alkaline refers to an alkaline substance that can form a stable complex with palladium ions, and the opposite is a non-complexed alkaline; mixing an aqueous solution of a dispersant with the palladium regulating solution to obtain a pre-reaction solution; adding an aqueous solution of a reducing agent dropwise to the pre-reaction solution to carry out a reduction reaction, and after standing and aging, separating the solid and liquid to obtain the palladium powder with a bimodal particle size distribution and low specific surface area. This invention employs a composite alkaline solution system to precisely regulate nucleation-growth kinetics. Specifically, the complexing base forms a stable complex with palladium ions, slowing down the palladium reduction process, while the non-complexing base only provides an alkaline environment without any complexing effect. The combination of these two components allows for precise regulation of the system's nucleation-growth kinetics, achieving a reasonable match between the nucleation rate and grain growth rate. This enables the in-situ generation of palladium powder with a bimodal particle size distribution during a single reduction process. This bimodal size structure effectively controls the specific surface area of ​​the palladium powder to 2.0 μm. 2 Below / g, it retains the high activity characteristics imparted by the main fine particles (about 100 nm), and significantly reduces the surface energy by introducing an appropriate amount of coarse particle components, fundamentally solving the problems of severe agglomeration and poor dispersibility caused by the excessive specific surface area of ​​traditional nano-palladium powder.

[0016] Furthermore, the present invention has the following advantages: This invention significantly optimizes the rheological properties and sintering density of electronic pastes. The dual-scale particle size distribution of palladium powder obtained by this invention allows fine particles to fully fill the voids between coarse particles, forming a denser packing structure. In MLCC paste applications, the paste exhibits excellent rheological properties and uniform film formation. During sintering, the low specific surface area significantly reduces residual organic volatiles and severe shrinkage, effectively preventing the formation of film cracks and pores, and ensuring the high density and high conductivity of the internal electrodes.

[0017] The process is simple, highly controllable, and exhibits excellent batch repeatability and stability. This invention regulates palladium nucleation-growth kinetics through a composite alkaline solution system, requiring only a single reduction reaction to construct a dual-scale structure, eliminating the need for subsequent complex fractionation or compounding processes. The wide process parameter window demonstrates good tolerance to minor fluctuations in reaction conditions, ensuring consistency in key indicators such as particle size distribution, morphology, and specific surface area across different batches, providing a reliable guarantee for large-scale stable production. Attached Figure Description

[0018] Figure 1 This is a SEM image of the palladium powder prepared in Example 1 of the present invention; Figure 2 This is a particle size distribution diagram of the palladium powder prepared in Example 1 of the present invention; Figure 3 This is a SEM image of the palladium powder prepared in Example 2 of the present invention; Figure 4 This is a SEM image of the palladium powder prepared in Example 3 of the present invention; Figure 5 This is a SEM image of the palladium powder prepared in Example 4 of the present invention; Figure 6 This is a SEM image of the palladium powder prepared in Example 5 of the present invention; Figure 7 This is a SEM image of the palladium powder prepared in Example 6 of the present invention; Figure 8 This is a SEM image of the palladium powder prepared in Comparative Example 1 of this invention. Figure 9 This is a SEM image of the palladium powder prepared in Comparative Example 2 of this invention; Figure 10 This is a SEM image of the palladium powder prepared in Comparative Example 3 of this invention. Figure 11 This is a SEM image of the palladium powder prepared in Comparative Example 4 of this invention; Figure 12 This is a SEM image of the palladium powder prepared in Comparative Example 5 of the present invention. Detailed Implementation

[0019] This invention provides a method for preparing palladium powder with a low specific surface area and a bimodal particle size distribution, comprising the following steps: The pH of the palladium source solution is adjusted to 7-9 using a composite alkaline solution to obtain a palladium control solution. The alkaline in the composite alkaline solution includes complexed alkaline and non-complexed alkaline. The molar ratio of the complexed alkaline to the non-complexed alkaline is 1:(0.2-1). The complexed alkaline refers to an alkaline substance that can form a stable complex with palladium ions, and the opposite is a non-complexed alkaline. An aqueous solution of the dispersant is mixed with the palladium regulating solution to obtain a pre-reaction solution; A reducing agent aqueous solution is added dropwise to the pre-reaction solution to carry out a reduction reaction. After standing and aging, the solid and liquid are separated to obtain the low specific surface area palladium powder with bimodal particle size distribution.

[0020] Unless otherwise specified, all raw materials used in this invention are commercially available products well known in the art.

[0021] This invention uses a composite alkaline solution to adjust the pH value of the palladium source solution to 7-9 to obtain a palladium control solution.

[0022] In this invention, the palladium source solution is preferably a chloropalladium acid solution or a palladium nitrate solution, more preferably a chloropalladium acid solution; the palladium content in the palladium source solution is preferably 19-21 wt%, and in a specific embodiment it can be 20 wt%. In a specific embodiment, the pH value of the palladium source solution can be adjusted to 7.5, 8, 8.5 or 9.

[0023] In this invention, the alkali in the composite alkaline solution includes complexed alkali and non-complexed alkali; the complexed alkali refers to an alkaline substance capable of forming a stable complex with palladium ions, and the opposite is true for non-complexed alkali. In this invention, the complexed alkali preferably includes ammonia; the non-complexed alkali preferably includes one or two of sodium hydroxide, sodium carbonate, and sodium bicarbonate. In this invention, the molar ratio of the complexed alkali to the non-complexed alkali is 1:(0.2~1), and in specific embodiments it can be 1:0.2, 1:0.3, 1:0.4, 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9, or 1:1. In this invention, the complexed alkali can form a stable complex with palladium ions, delaying the reduction process of palladium, while the non-complexed alkali only provides an alkaline environment and has no complexing effect. The combination of the two can precisely control the nucleation-growth kinetics of the system, thereby obtaining palladium powder with a bimodal particle size distribution. It is worth noting that when the amount of complexing base is too small or too large, palladium powder with a bimodal particle size distribution cannot be obtained. In this invention, the molar ratio of complexing base to non-complexing base and the pH value are controlled within the above range, so that some palladium ions are complexed while others are not, resulting in different reduction rates.

[0024] In this invention, the total concentration of complexed and non-complexed alkalis in the composite alkali solution is preferably 1~10 mol / L, and in specific embodiments it can be 2, 4, 6, 8 or 10 mol / L.

[0025] After obtaining the palladium regulating solution, the present invention mixes the aqueous solution of the dispersant with the palladium regulating solution to obtain a pre-reaction solution.

[0026] In this invention, the dispersant is preferably one or more of polyvinylpyrrolidone, gum arabic, gelatin, and polyvinyl alcohol; the polyvinylpyrrolidone is preferably polyvinylpyrrolidone K30 and / or polyvinylpyrrolidone K90; the concentration of the aqueous solution of the dispersant is preferably 0.5~8 g / L, and in specific embodiments it can be 0.5, 1, 2, 3, 4, 5, 6, 7 or 8 g / L; the mass ratio of the dispersant to palladium in the palladium regulating solution is preferably (0.02~0.3):1, and in specific embodiments it can be 0.05:1, 0.1:1, 0.15:1, 0.2:1, 0.25:1 or 0.3:1. In this invention, the concentration of palladium in the pre-reaction solution is preferably 0.05~0.25 mol / L, and in specific embodiments it can be 0.05, 0.10, 0.15, 0.20 or 0.25 mol / L.

[0027] After obtaining the pre-reaction solution, the present invention adds an aqueous solution of reducing agent to the pre-reaction solution to carry out a reduction reaction. After standing and aging, the solid and liquid are separated to obtain the palladium powder with a bimodal particle size distribution and low specific surface area.

[0028] In this invention, the reducing agent preferably comprises sodium borohydride; the concentration of the aqueous solution of the reducing agent is preferably 0.5~1.0 mol / L, and in specific embodiments it can be 0.5, 0.6, 0.7, 0.8, 0.9 or 1.0 mol / L; the molar ratio of the reducing agent to palladium in the pre-reaction solution is preferably (0.5~3):1, and in specific embodiments it can be 0.5:1, 1:1, 1.5:1, 2:1, 2.5:1 or 3:1. In this invention, the dropping rate of the aqueous solution of the reducing agent is preferably 1~3 mL / s, and in specific embodiments it can be 1, 1.5, 2, 2.5 or 3 mL / s. In this invention, the dropping of the aqueous solution of the reducing agent and the reduction reaction are preferably carried out under stirring conditions, and the stirring rate is preferably 100~300 rpm, and in specific embodiments it can be 100, 150, 200, 250 or 300 rpm.

[0029] In this invention, the temperature of the reduction reaction is preferably 20~35℃, and in specific embodiments it can be 20, 25, 30 or 35℃; the time of the reduction reaction is preferably 30~60 min, and in specific embodiments it can be 30, 40, 50 or 60 min. During the reduction reaction, uncomplexed palladium rapidly and synchronously undergoes reduction and nucleation, resulting in fine-sized palladium powder; while complexed palladium slows down the reduction process, resulting in coarser-sized palladium powder.

[0030] In this invention, the temperature of the static aging is preferably the same as the temperature of the reduction reaction; the static aging time is preferably 20-30 min.

[0031] The present invention does not have any special requirements for the solid-liquid separation method; any solid-liquid separation method well known in the art can be used, such as filtration.

[0032] After solid-liquid separation, the present invention preferably further includes washing and drying the obtained solid to obtain the low specific surface area palladium powder with a bimodal particle size distribution. In the present invention, the washing is preferably performed sequentially with water and then with ethanol. The number of water and ethanol washes is preferably multiple, in a specific embodiment three times. The drying temperature is preferably 40-60°C, in a specific embodiment 50°C.

[0033] This invention provides palladium powder with a bimodal particle size distribution and a low specific surface area, prepared by the method described above, with a specific surface area of ​​2.0 m². 2 / g or less; the low specific surface area palladium powder with bimodal particle size distribution is composed of coarse particles and fine particles, wherein the particle size of the coarse particles is 300~500 nm and the particle size of the fine particles is 50~200 nm.

[0034] In this invention, the specific surface area of ​​the low specific surface area palladium powder with bimodal particle size distribution is preferably 1.5~2.0 m². 2 / g.

[0035] The palladium powder prepared by this invention has a dual-scale structure, which retains the main fine particles while introducing an appropriate amount of coarse particles to reduce the specific surface area, thus breaking through the technical bottleneck of the difficulty in achieving both small particle size and low specific surface area.

[0036] The following detailed description, in conjunction with embodiments, illustrates the low specific surface area palladium powder with bimodal particle size distribution and its preparation method provided by the present invention. However, these descriptions should not be construed as limiting the scope of protection of the present invention.

[0037] Example 1 S1. Take 60 mL of chloropalladium acid solution with a palladium content of 20 wt%, add a certain amount of composite alkaline solution with a molar ratio of ammonia and sodium hydroxide of 1:0.5, the total alkaline concentration in the composite alkaline solution is 6 mol / L, adjust the pH value of the chloropalladium acid solution to 8, and obtain palladium control solution.

[0038] S2. Dissolve 1.2 g of gelatin in 400 mL of deionized water to obtain a dispersant solution. Mix the dispersant solution with the above palladium control solution thoroughly to obtain a pre-reaction solution. The molar concentration of palladium ions in the pre-reaction solution is 0.11 mol / L, and the mass ratio of gelatin to palladium ions is 0.1:1.

[0039] S3. Dissolve 4 g of sodium borohydride in 150 mL of deionized water to obtain a reducing agent solution with a concentration of 0.7 mol / L.

[0040] S4. Under stirring conditions of 30℃ and 250 rpm, the reducing solution prepared in step S3 is added to the pre-reaction solution in step S2 at a dropping rate of 2 mL / s to carry out the reduction reaction, and a suspension containing palladium powder is prepared. The molar ratio of sodium borohydride to palladium ions in the pre-reaction solution is 1:1.

[0041] S5. The palladium powder suspension from step S4 was allowed to stand for 30 min, then solid-liquid separation was performed by filtration. The suspension was washed three times with deionized water and ethanol, respectively, and then dried in a forced-air drying oven at 50°C to obtain palladium powder with a low specific surface area and a bimodal particle size distribution.

[0042] Example 2 The difference from Example 1 is that the alkaline solution used to adjust the pH of the chloropalladic acid solution is a composite solution of ammonia and sodium carbonate in a molar ratio of 1:0.5.

[0043] Example 3 The difference from Example 1 is that the alkaline solution used to adjust the pH of the chloropalladic acid solution is a composite solution of ammonia and sodium bicarbonate in a molar ratio of 1:0.5.

[0044] Example 4 The difference from Example 1 is that the alkaline solution used to adjust the pH of the chloropalladium acid solution is a composite solution of ammonia and sodium carbonate (sodium bicarbonate) in a molar ratio of 1:0.5, wherein the molar ratio of sodium carbonate to sodium bicarbonate is 1:1.

[0045] Example 5 The only difference from Example 1 is that the molar concentration of palladium ions in the pre-reaction solution is 0.15 mol / L, the amount of dispersant remains unchanged, and the mass ratio of dispersant to palladium ions in the pre-reaction solution is 0.075:1.

[0046] Example 6 The only difference from Example 1 is that the concentration of sodium borohydride in the reducing agent solution is 1.0 mol / L, and the molar ratio of reducing agent to palladium ions remains the same as in Example 1.

[0047] Comparative Example 1 The difference from Example 1 is that the alkaline solution used to adjust the pH of the palladium solution is a composite solution of sodium hydroxide and sodium carbonate in a 1:1 molar ratio.

[0048] Comparative Example 2 The difference from Example 1 is that the alkaline solution used to adjust the pH of the palladium solution is a composite solution of sodium carbonate and sodium bicarbonate in a 1:1 molar ratio.

[0049] Comparative Example 3 The difference from Example 1 is that the alkaline solution used to adjust the pH of the palladium solution is a composite alkaline solution of ammonia and sodium hydroxide in a molar ratio of 1:10.

[0050] Comparative Example 4 The difference from Example 1 is that the alkaline solution used to adjust the pH of the palladium solution is an ammonia solution.

[0051] Comparative Example 5 The difference from Example 1 is that the alkaline solution used to adjust the pH of the palladium solution is a sodium hydroxide solution.

[0052] Characterization tests: Morphology analysis was performed on the palladium powder samples obtained in Examples 1-6 and Comparative Examples 1-5. Figure 1 Here is a SEM image of the palladium powder obtained in Example 1. Figure 1 It can be seen that the palladium powder prepared by the present invention has no irregular irregular structure such as flakes or dendrites, and the particle morphology is regular and uniform. Figure 2 This is a particle size distribution diagram of the palladium powder prepared in Example 1 of the present invention. Figure 2 It is evident that the powder exhibits a dual-scale particle size distribution, specifically consisting of fine particles of 98.40 nm (90.4%) and coarse particles of 321.7 nm (9.6%). This is attributed to the synergistic regulation of the sodium hydroxide and ammonia composite alkaline solution: ammonia forms a stable complex with palladium ions, slowing down the palladium reduction process, while sodium hydroxide only provides an alkaline environment without complexation. The combination of these two solutions allows for precise control of the nucleation-growth kinetics of the system, optimizing the palladium ion reduction rate. This dual-scale structure utilizes both the high activity imparted by small-diameter particles and the surface energy and packing state of the system adjusted by coarse particles, thus achieving a balance between microstructure and macroscopic properties.

[0053] Figures 3-7 The images are SEM images of the palladium powders obtained in Examples 2-6, respectively. Figures 3-7 It can be seen that the palladium powder prepared in Examples 2-6 of this invention has similar morphology, particle size, and dispersibility to the palladium powder prepared in Example 1, and the powder exhibits a dual-scale particle size distribution (e.g., ...). Figures 3-7 In the diagram, the area within the circle represents fine particles, and the area within the square represents coarse particles. (This is an illustrative example and does not represent all fine and coarse particles.) This indicates that the process parameter window of the present invention is wide and has good tolerance to minor fluctuations in reaction conditions. Within the range defined by the present invention, process parameters such as palladium ion concentration, reducing agent concentration, and composite alkali solution have little impact on the morphology, particle size, and dispersibility of palladium powder.

[0054] Figure 8 The image shows a SEM image of the palladium powder obtained in Comparative Example 1. Figure 9 This is a SEM image of the palladium powder obtained in Comparative Example 2. Figure 8 and Figure 9It can be seen that the palladium powder prepared by the inorganic composite alkali system without ammonia in Comparative Example 1 and Comparative Example 2 has a small overall particle size and only exhibits a single particle size characteristic. This is because the pure inorganic alkali system can only provide an alkaline reaction environment and does not have the ability to complex palladium ions or regulate reduction kinetics, so it cannot achieve a dual-scale particle size distribution.

[0055] Figure 10 The image shows a SEM image of the palladium powder obtained in Comparative Example 3. Figure 10 It can be seen that Comparative Example 3 uses a composite alkali system with a 1:10 molar ratio of ammonia and sodium hydroxide to prepare palladium powder. In this low ammonia ratio system, only a small amount of palladium ions can undergo a complexation and slow-release reaction with ammonia. The vast majority of palladium ions undergo rapid and synchronous reduction and nucleation growth in an alkaline environment, resulting in palladium powder with a fine overall particle size. These fine palladium particles have high surface energy and are prone to agglomeration and adhesion, exhibiting a flocculent appearance and poor powder packing performance.

[0056] Figure 11 This is a SEM image of the palladium powder obtained in Comparative Example 4. Figure 12 This is a SEM image of the palladium powder obtained in Comparative Example 5. Figure 11 and Figure 12 It can be seen that the palladium powder prepared by Comparative Example 4 using a single alkali system (ammonia water) has a larger overall particle size, while the palladium powder prepared by Comparative Example 5 using a single alkali system (sodium hydroxide) has a smaller overall particle size and only exhibits a single particle size characteristic.

[0057] The above results indicate that, compared with the sodium hydroxide-ammonia water composite alkali system with optimized ratio of the present invention, the ammonia-free or low ammonia water ratio cannot fully exert the synergistic regulatory effect of complexation and alkalinity, and it is difficult to take into account the high activity of palladium powder with small particle size and the stacking performance of low specific surface area, resulting in defects in the comprehensive application performance of the powder.

[0058] The specific surface area of ​​palladium powder samples prepared in Examples 1-5 and Comparative Examples 1-3 was tested using a fully automated specific surface area analyzer. The results are shown in Table 1.

[0059] Table 1. Specific surface area of ​​palladium powders obtained in Examples 1-5 and Comparative Examples 1-3

[0060] As shown in Table 1, the specific surface area of ​​the palladium powders prepared in Examples 1-5 is all between 1.5 and 2.0 m². 2 Within the range of / g. Combined with the aforementioned SEM characterization results, it can be seen that the palladium powder prepared in this invention possesses both excellent characteristics of low specific surface area and small particle size.

[0061] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing palladium powder with a low specific surface area and a bimodal particle size distribution, characterized in that, Includes the following steps: The pH of the palladium source solution is adjusted to 7-9 using a composite alkaline solution to obtain a palladium control solution. The alkaline in the composite alkaline solution includes complexed alkaline and non-complexed alkaline. The molar ratio of the complexed alkaline to the non-complexed alkaline is 1:(0.2-1). The complexed alkaline refers to an alkaline substance that can form a stable complex with palladium ions, and the opposite is a non-complexed alkaline. An aqueous solution of the dispersant is mixed with the palladium regulating solution to obtain a pre-reaction solution; A reducing agent aqueous solution is added dropwise to the pre-reaction solution to carry out a reduction reaction. After standing and aging, the solid and liquid are separated to obtain the low specific surface area palladium powder with bimodal particle size distribution.

2. The preparation method according to claim 1, characterized in that, The complexing base includes ammonia.

3. The preparation method according to claim 1 or 2, characterized in that, The non-complexing base includes one or more of sodium hydroxide, sodium carbonate, and sodium bicarbonate.

4. The preparation method according to claim 1, characterized in that, The total concentration of complexed and non-complexed alkalis in the composite alkaline solution is 1~10 mol / L.

5. The preparation method according to claim 1, characterized in that, The reduction reaction is carried out at a temperature of 20-35°C for a time of 30-60 minutes.

6. The preparation method according to claim 1, characterized in that, The dispersant is one or two of polyvinylpyrrolidone, gum arabic, gelatin and polyvinyl alcohol; the mass ratio of the dispersant to palladium in the palladium control solution is (0.02~0.3):

1.

7. The preparation method according to claim 1, characterized in that, The reducing agent includes sodium borohydride; the concentration of the aqueous solution of the reducing agent is 0.5~1.0 mol / L; the molar ratio of the reducing agent to palladium in the pre-reaction solution is (0.5~3):

1.

8. The preparation method according to claim 1, characterized in that, The concentration of palladium in the pre-reaction solution is 0.05~0.25 mol / L.

9. The preparation method according to claim 1, characterized in that, The palladium source solution is a chloropalladium acid solution or a palladium nitrate solution.

10. The low specific surface area palladium powder with a bimodal particle size distribution prepared by the preparation method according to any one of claims 1 to 9, characterized in that, Specific surface area is 2.0 m² 2 / g or less; the low specific surface area palladium powder with bimodal particle size distribution is composed of coarse particles and fine particles, wherein the particle size of the coarse particles is 300~500 nm and the particle size of the fine particles is 50~200 nm.

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