Preparation method of micro-nano rhodium powder

Abstract

The invention relates to a preparation method of micro-nano rhodium powder, which comprises the following steps: preparing nano rhodium powder by adopting a chemical liquid phase reduction method, grinding and uniformly mixing the nano rhodium powder and a metal oxide with excellent thermal stability, then carrying out heat treatment on the mixed powder, and preparing the micro-nano rhodium powder by controlling the environment atmosphere and temperature in the heat treatment process. And the micro-nano rhodium powder with good dispersibility is prepared. According to the method, the nano rhodium powder is converted into the micro-nano rhodium powder, the particle structure of the powder is changed, the dispersity is improved, the particle size distribution of the rhodium powder is properly increased, the reasonable grading of the particle granularity is realized, the particle close packing slurry can be prepared, the cofiring matching property of the powder and the substrate is improved, the adhesive strength is enhanced, and the catalytic efficiency is further improved; the service life is prolonged.

Description

Technical Field

[0001] The invention belongs to the technical field of precious metal powder material preparation, and relates to a method for preparing micro-nano rhodium powder. Background Art

[0002] Precious metal rhodium powder has the characteristics of high catalytic activity, high catalytic selectivity, long catalytic life, and excellent thermal stability. It is widely used in the automotive, chemical, aerospace, electronics and other industries. Improving the performance and utilization efficiency of precious metal rhodium powder has extremely high economic value.

[0003] Rhodium powder is NO x Important materials for catalytic electrodes of sensors, in NO x During the catalytic reaction, its morphology, particle size distribution, tap density and other properties directly affect the microstructure and catalytic activity of the electrode. The slurry prepared with micro-nano rhodium powder with high sphericity, wide particle size distribution, appropriate particle size gradation and dense packing has a low shrinkage rate and dense structure after sintering, which can improve the electrode response rate, sensitivity and catalytic efficiency.

[0004] At present, chemical liquid phase reduction is the main method for preparing rhodium powder. The prepared powder has a small particle size of nanometer size. Electrodes are prepared with this nano rhodium powder, and the nano rhodium powder is mixed with a binder and an organic dispersant to form an electronic paste, which is then applied to NO using processes such as screen printing. x Sensor catalytic electrode. Due to the small particle size, large surface area, high surface energy, easy agglomeration between particles, difficulty in dispersion, and loose porosity of nano-rhodium powder, higher technical requirements are put forward for the preparation of slurry with densely packed powder particles; and the surface of nano-rhodium powder adsorbs a large amount of gas and organic dispersant in the preparation process, and the adsorbed gas will volatilize and the adsorbed dispersant will decompose during the sintering process, so that the nano-particles will partially agglomerate after sintering, resulting in coarsening of particles, enlargement of particle size, volume shrinkage, and discontinuous state of catalytic phase rhodium, which reduces the co-firing matching with the substrate, not only affecting its catalytic efficiency but also shortening the service life of the electrode.

[0005] Therefore, how to prepare the powder particle size gradation, achieve close packing of powder and reduce porosity is the key and difficult problem to improve the co-firing matching of powder and substrate, enhance the adhesion strength, make the structure dense, and thus improve the catalytic efficiency. Summary of the invention

[0006] The purpose of the present invention is to provide a method for preparing micro-nano rhodium powder. The method comprises the following steps: uniformly mixing nano rhodium powder with a metal oxide having excellent thermal stability and then subjecting the nano rhodium powder to heat treatment to convert the nano rhodium powder into micro-nano rhodium powder, thereby changing the particle structure of the powder, improving the dispersibility, increasing the particle size of the rhodium powder and its distribution. The slurry prepared from the micro-nano rhodium powder can achieve reasonable particle size grading, close packing between particles, reduce porosity, improve the co-firing matching of the powder and a substrate, improve the adhesion strength, achieve a dense structure, and improve the catalytic efficiency.

[0007] The method of the invention adopts a chemical liquid phase reduction method to prepare nano rhodium powder, grinds and uniformly mixes the nano rhodium powder and a metal oxide with excellent thermal stability, and then heat treats the mixed powder. By controlling the ambient atmosphere and temperature during the heat treatment process, micro-nano rhodium powder with good dispersibility and wide particle size distribution is prepared.

[0008] The specific technical solution of the present invention is: The preparation method of micro-nano rhodium powder comprises the following steps: S1. Take chlororhodium acid as a precursor, configure a rhodium content of 0.1g / L -20g / L aqueous solution, add a reducing agent solution to mix, the reducing agent aqueous solution mass concentration of 0.5%, stir until the mixed solution system is completely clarified, separate the precipitate, and obtain rhodium powder; S2. The rhodium powder obtained in step S1 is dried at 80-150 ° C for >20 h to prepare nano rhodium powder; S3. The nano-rhodium powder and the metal oxide having excellent thermal stability and the dispersant are added to the ball mill, ball milled, ground and mixed uniformly; drying to obtain a mixture of nano-rhodium powder and the metal oxide; S4. The mixture of step S3 is heat treated under an oxidizing or reducing atmosphere; the heat treatment temperature is 500-1500°C and the time is 1h-5h; S5. Add the mixture after the heat treatment in step S4 into dilute acid to convert the metal oxide into a water-soluble salt, dissolve and wash with deionized water to remove the soluble salt, and obtain micro-nano rhodium powder after drying.

[0009] The reducing agent in step S1 is any one of sodium borohydride, hydrazine hydrate, sodium citrate and glucose.

[0010] The metal oxide with excellent thermal stability in step S3 is any one of CaO, MgO, and Al2O3.

[0011] The dispersant in step S3 is an organic solvent with a low boiling point and high volatility.

[0012] The organic solvent is ethanol or acetone.

[0013] In step S3, the mass ratio of the nano-rhodium powder, the metal oxide and the dispersant is 1:1-10:2-15.

[0014] The ball milling dispersion in step S3 has a rotation speed of 100 rpm-400 rpm and a ball milling dispersion time of 1 h-6 h.

[0015] The oxidizing atmosphere in step S4 is air atmosphere; the reducing atmosphere is hydrogen, the flow rate of hydrogen is 0.1 L / min-1 L / min, hydrogen is passed for 5-10 minutes before heat treatment, and hydrogen is continuously passed during heat treatment.

[0016] The dilute acid in step S5 is hydrochloric acid or nitric acid with a mass concentration of 3%-5%.

[0017] The drying temperature in step S5 is 80° C.-150° C., and the drying time is 12 h-24 h.

[0018] The ratio of the nano rhodium powder, the metal oxide with excellent thermal stability and the dispersant, as well as the rotation speed and time of grinding and dispersion of the present invention can ensure that the nano rhodium powder and the metal oxide are evenly mixed, the metal oxide has stable performance during the heat treatment process, and a local micro space is formed, so that the nano rhodium powder distributed therein can be melted, agglomerated and grown, and the structure is dense, and the widening of the rhodium powder particle size distribution is effectively controlled, and the dispersibility is improved.

[0019] The metal oxide is converted into a water-soluble salt by reacting with dilute acid, and then completely removed by dissolving and washing with deionized water to ensure the purity of the prepared micro-nano rhodium powder.

[0020] The beneficial effects of the present invention are as follows: nano rhodium powder is prepared by a chemical liquid phase reduction method, a metal oxide with excellent thermal stability is ground and mixed with the nano rhodium powder, and after heat treatment, micro-nano rhodium powder is prepared from the nano rhodium powder by changing the heat treatment atmosphere and temperature, thereby improving the dispersibility of the powder and widening the particle size distribution.

[0021] The applicant's experiments show that the atmosphere and heat treatment temperature during the heat treatment process have a significant effect on the morphology and particle size of the rhodium powder. Under the action of air atmosphere heat treatment, the nano-rhodium powder is transformed into rod-shaped rhodium oxide; under the condition of hydrogen reducing atmosphere at a low temperature (800°C), the morphology of the rhodium powder does not change significantly. Under the condition of 1100°C hydrogen reducing atmosphere, the particle size of the rhodium powder becomes larger and the particle size distribution becomes wider, transforming from nano-rhodium powder to micro-nano-rhodium powder, and the dispersibility is improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Microscopic morphology of nano-rhodium powder (magnification 20,000 times); Figure 2 Microscopic morphology of rhodium powder heat treated in air atmosphere at 1100℃ (magnification 20000 times); Figure 3 Microscopic morphology of rhodium powder heat treated at 800℃ in hydrogen atmosphere (magnification 20,000 times); Figure 4 Microstructure of rhodium powder heat treated at 1100℃ in hydrogen atmosphere (magnification 20,000 times). DETAILED DESCRIPTION

[0023] The preparation method of the micro-nano rhodium powder of the present invention comprises the following steps: using chlororhodic acid as a precursor, preparing the nano-rhodium powder by a chemical liquid phase reduction method, mixing the nano-rhodium powder with a metal oxide having excellent thermal stability, and preparing the highly dispersed micro-nano rhodium powder by controlling the atmosphere and temperature during the heat treatment process. Example 1

[0024] S1. Chloro-rhodic acid is used as a precursor to prepare an aqueous solution with a rhodium content of 10 g / L, and sodium borohydride is used as a reducing agent, and the mass concentration of the aqueous solution is 0.5%; S2. The precursor in step S1 is added to the aqueous solution of rhodium chloride acid at room temperature and the reducing agent sodium borohydride solution is stirred and mixed evenly; S3. After the aqueous solution of chlororhodium acid and the aqueous solution of the reducing agent are mixed in step S2, stirring, the mixed solution system is completely clarified, and the chlororhodium acid is completely reduced to rhodium powder and deposited at the bottom of the reactor; S4. The rhodium powder prepared by reduction in step S3 was repeatedly washed with deionized water, precipitated, soluble salts were removed, and dried at 100 ° C for 24h to obtain a serious agglomeration of the nano rhodium powder, the particle size of which was 50-100nm (such as attached Figure 1 as shown). Example 2

[0025] S1-S4 are the same as in Example 1.

[0026] S5. The nano-rhodium powder obtained in step S4, the metal oxide MgO and the dispersant acetone are added to a ball mill for dispersion, grinding and mixing; the mass ratio of the nano-rhodium powder, MgO and the dispersant acetone is 1:9:10, the speed of the ball mill is 300rpm, and the grinding and dispersion time is 4h; the mixed mixture is ground and dried at 70°C for 24h, and the dispersant acetone is removed by drying to obtain a uniform mixture of the nano-rhodium powder and MgO; S6. The uniform mixture of nano-rhodium powder and MgO in step S5 is placed in an atmosphere furnace and heat treated in an air atmosphere at a heat treatment temperature of 1100°C for 2h; S7. The mixture after heat treatment in step S6 is added to a 3% concentration of dilute hydrochloric acid to remove the barrier agent MgO by reaction, and then repeatedly washed with deionized water until there is no Cl in the solution. -, dried at 105 °C for 20 h, and obtained rod-shaped rhodium oxide powder (rod diameter of about 90 nm, rod length of 1.1 μm). Figure 2 shown. Example 3

[0027] Steps S1-S4 are the same as in Example 1, and step S5 is the same as in Example 2.

[0028] S6. The uniform mixture of nano-rhodium powder and MgO in step S5 is placed in an atmosphere furnace and subjected to barrier-heat treatment in a hydrogen atmosphere at a hydrogen flow rate of 0.2 L / min. After the air in the furnace cavity is removed by hydrogen for 10 min, the heat treatment temperature is 800 ° C. for 2 h, and hydrogen is continuously passed during the heat treatment process; S7. Add the mixture after heat treatment in step S6 to 3% dilute hydrochloric acid to remove MgO by reaction, and then wash repeatedly with deionized water until there is no Cl in the solution. - , dried at 105℃ for 20h, and obtained nano rhodium powder. Compared with the powder prepared by chemical liquid phase reduction, the particle size is slightly larger (50-140nm), and the agglomeration between powder particles is still significant and the dispersion is poor. Figure 3 shown. Example 4

[0029] Steps S1-S4 are the same as in Example 1, and step S5 is the same as in Example 2.

[0030] S6. The uniform mixture of nano-rhodium powder and MgO in step S5 is placed in an atmosphere furnace and heat treated in a hydrogen atmosphere at a hydrogen flow rate of 0.3 L / min. After the air in the furnace cavity is removed by hydrogen for 10 min, the heat treatment temperature is 1100 ° C. The treatment time is 2 h, and hydrogen is continuously passed during the heat treatment process; S7. The mixture after heat treatment in step S6 is added to a 3% dilute hydrochloric acid to remove the barrier agent MgO by reaction, and then repeatedly washed with deionized water until there is no Cl in the solution. - , and dried at 105°C for 20 h to obtain micro-nano rhodium powder. Figure 4 The micro-nano rhodium powder shown is spherical or quasi-spherical, has no obvious agglomeration phenomenon, has good dispersibility, and has a larger particle size and a wider particle size distribution (particle size is 0.07-0.7μm).

[0031] Conclusion: Using chlororhodic acid as a precursor, nanopowders with a particle size of 50-100nm were prepared by chemical liquid phase reduction, and then mixed evenly with metal oxide MgO with excellent thermal stability. The nanorhodium powder was melted, agglomerated, and grown to spherical or quasi-spherical micro-nano rhodium powder in a hydrogen reducing atmosphere at a temperature above 1100°C, which changed the particle structure of the powder, improved the dispersibility, and increased the particle size and distribution of the rhodium powder. The large particle (primary particle) had a particle size of 0.62μm, and the secondary particle size was 0. The maximum particle size is 0.20 μm (about 0.414 times the primary particle size), and the minimum particle size is 0.076 μm (about 0.116 times the primary particle size), which basically conforms to the Horsfield distribution model of the powder. The slurry prepared from the micro-nano rhodium powder has a reasonable particle gradation, which can achieve close stacking of rhodium powder particles and reduce porosity. The volume shrinkage of the sintered rhodium electrode is reduced, the structure is dense, the matching with the substrate is improved, and the adhesion strength is enhanced, thereby improving the catalytic efficiency of the rhodium electrode and extending its service life.

Claims

1. A method for preparing micro-nano rhodium powder, characterized in that, The following steps are involved: S1. Take chlororhodium acid as a precursor, configure a rhodium content of 0.1g / L -20g / L aqueous solution, add a reducing agent solution to mix, the reducing agent aqueous solution mass concentration of 0.5%, stir until the mixed solution system is completely clarified, separate the precipitate, and obtain rhodium powder; S2. The rhodium powder obtained in step S1 is dried at 80-150 ° C for >20 h to prepare nano rhodium powder; S3. The nano-rhodium powder and the metal oxide having excellent thermal stability and the dispersant are added to the ball mill, ball milled, ground and mixed uniformly; drying to obtain a mixture of nano-rhodium powder and the metal oxide; S4. The mixture of step S3 is heat treated under an oxidizing or reducing atmosphere; the heat treatment temperature is 500-1500°C and the time is 1h-5h; S5. Add the mixture after the heat treatment in step S4 into dilute acid to convert the metal oxide into a water-soluble salt, dissolve and wash with deionized water to remove the soluble salt, and obtain micro-nano rhodium powder after drying.

2. The preparation method according to claim 1, characterized in that: The reducing agent in step S1 is any one of sodium borohydride, hydrazine hydrate, sodium citrate and glucose.

3. The preparation method according to claim 1, characterized in that: The metal oxide with excellent thermal stability in step S3 is any one of CaO, MgO, and Al2O3.

4. The preparation method according to claim 1, characterized in that: The dispersant in step S3 is an organic solvent with a low boiling point and high volatility.

5. The preparation method according to claim 4, characterized in that: The organic solvent is ethanol or acetone.

6. The preparation method according to claim 1, characterized in that: In step S3, the mass ratio of the nano-rhodium powder, the metal oxide and the dispersant is 1:1-10:2-15.

7. The preparation method according to claim 1, characterized in that: The ball milling dispersion in step S3 has a rotation speed of 100 rpm-400 rpm and a ball milling dispersion time of 1 h-6 h.

8. The preparation method according to claim 1, characterized in that: The oxidizing atmosphere in step S4 is air atmosphere; the reducing atmosphere is hydrogen, the flow rate of hydrogen is 0.1 L / min-1 L / min, hydrogen is passed for 5-10 minutes before heat treatment, and hydrogen is continuously passed during heat treatment.

9. The preparation method according to claim 1, characterized in that: The dilute acid in step S5 is hydrochloric acid or nitric acid with a mass concentration of 3%-5%.

10. The preparation method according to claim 1, characterized in that: The drying temperature in step S5 is 80° C.-150° C., and the drying time is 12 h-24 h.

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