RU2569535C1 - Production of superdispersed powders of various oxides with narrow separation of particles by sizes - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: mesoporous SiO₂ is used as a porous medium to be impregnated with the reaction solution containing the metal nitrides and water-soluble organic compounds, for example, glycine. Synthesis temperature is controlled by application of ammonium nitrate. After every step of impregnation, air- of vacuum-drying is performed. Then, impregnated SiO₂ is compacted to get the compacted blank and to initiate the chemical reaction therein with the help of tungsten coiled filament. Now, said blank is cooled down and ground to required fineness and leached in concentrated solution of sodium hydroxide at 80°C for 12 hours. Obtained oxide consisting of superfine particles sized to about 3 nm is extracted by filtering and washing.

EFFECT: nanoparticles with narrow size distribution and high specific surface.

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Description

The invention relates to the field of powder metallurgy, in particular to methods for producing ultrafine powders of metal oxides with particle sizes less than 10 nm. It is known that nanosized powders of metal oxides have non-traditional mechanical, physical and chemical properties that are different from micron-sized powders, which allows their use in catalysis, electronics, medicine, as well as in various other fields of science and technology. So, for example, in the case of catalysts, the use of ultrafine powders of metal oxides can increase the surface area of the catalyst.

Several methods are known for producing finely dispersed oxides and materials based on them: plasma-chemical decomposition of salts; coprecipitation and subsequent calcination of hydroxides or drying of an oxide deposit, mechanical grinding, oxidation of metal powders, electric explosion of metal wires in an oxidizing environment, spray thermolysis of organic or aqueous-organic solutions of metal-containing compounds.

For example, a method for the synthesis of complex oxide compounds of rare-earth metals is known, in which a mixture of the corresponding hydrated or dehydrated metal nitrates, hydrated or dehydrated lanthanum nitrates (La (NO ₃ ) ₃ or (La (NO ₃ ) ₃ · 9H ₂ O) and introduced into the mixture is further completely burned off energy additive - tetraformil triazine (C ₄ H ₁₆ N ₆ O ₂₎ This additive during the synthesis decomposes with the release of energy and a large volume of gases (CO, NO, etc.) and pre-synthesized as a result of the shape of the reaction. degida with hydrazine hydrate at T = 0 ° C with constant stirring. The mixture of lanthanum nitrate, nitrate of appropriate metal and energy supplements in stoichiometric proportions dissolved in a vessel of water of 300 cm ^3. The dissolved mixture was placed in a muffle furnace where heated at 500 ° C Hydrated mixtures are completely dehydrated and, after heating, self-ignite, turning into a foamy mass in the volume of the vessel. The flame temperature during combustion of mixtures reaches T = 1100 ° C ± 100 ° C. The whole process takes 5 minutes. In some cases, after the combustion process proceeds, an X-ray amorphous product is formed in the system, which, upon subsequent heating to 800 ° C and holding for 1 h, crystallizes in the single-phase compound LnMO ₃ (S. Sundar Manoharan, KC Patil, Combustion route to fine particle perovskite oxides, Journal of Solid State Chemistry, 1993, v. 102, pp. 267-276).

The disadvantage of this method is the multi-stage process, the need for high-temperature heating of the mixtures to their ignition temperature and, in some cases, the implementation of additional long-term high-temperature processing of combustion products.

The prototype of the proposed invention is a method for producing fine powders of metal oxides (RU 2318723 C2, publ. 10.03.2008), comprising preparing a mixture containing an aqueous solution, a melt or an aqueous suspension of a salt of at least one metal exhibiting oxidizing properties, and an organic reducing agent. The resulting mixture is homogenized and heated by continuously feeding it to the hot surface of a rotating heater, and evaporation and ignition occur. The evolving gaseous substances and the resulting solid finished product are continuously removed from the surface of the heater. The invention allows to obtain fine powders of metal oxides with an average particle size of 100 nm and a specific surface area of 50 m ² / g

The disadvantage of this method is the wide distribution of particle sizes (0.028-0.175 μm), as well as obtaining the final product with a relatively low specific surface area.

In the proposed invention, the following technical result is achieved:

- the formation of crystalline nanomaterials occurs without additional calcination;

- the formation of nanoparticles with a narrow size distribution;

- the final product has a high specific surface of the order of 132 m ² / g;

- removal of organic impurities without the formation of coke.

The technical result is achieved as follows.

A method of obtaining ultrafine powders of transition metal oxides (Ni, Cu, Fe), which consists in the fact that mesoporous SiO _{2 is} impregnated with a reaction solution containing metal nitrates and water-soluble organic compounds, after which the impregnated mesoporous SiO ₂ is dried at room temperature, then impregnated mesoporous SiO ₂ is compressed to compact the sample and initiate a chemical reaction in a sample using the tungsten coil, and the resulting powder was cooled, pulverized and then subjected vyschel Chivanov molar solution in concentrated sodium hydroxide at a temperature of 60-80 ° C for 8-12 hours, then the resulting metal oxide powder is filtered and washed. As metal nitrates, iron nitrate and / or nickel nitrate and / or copper nitrate are used. As water soluble organic compounds, glycine and / or hydrazine and / or urea are used. The impregnated mesoporous SiO _{2 is} dried in air or in vacuum.

The invention is illustrated in the drawing, where in FIG. 1 shows the microstructure of the obtained iron oxide, and FIG. 2 shows the particle size distribution. From the above images it is seen that approximately 95% of the particles of iron oxide are less than 5 nm with an average size of 3.5 nm.

The method is as follows.

The so-called combustion method of reactionary porous media was developed for the production of transition metal nanooxides with a narrow particle size distribution. In this method, mesoporous SiO ₂ (SBA-15) is used as a porous medium (average pore diameter of 7 nm), which can be synthesized according to published data (D. Zhao, et al. Triblock Copolymer Syntheses of Mesoporous Silica with Periodic 50 to 300 Angstrom Pores, SCIENCE 279 (1998) 548-552). A typical synthesis process consists in impregnating SBA-15 with a reaction solution containing metal nitrates and water-soluble organic compounds, for example glycine, hydrazine, urea (fuel). In some cases, ammonium nitrate is used to adjust the synthesis temperature. Depending on the specific material, various methods of impregnation are used, for example, impregnation by moisture capacity.

To achieve uniform distribution of the reaction solution within the porous structure, stepwise impregnation of SBA-15 is required. After each step of the SBA-15 impregnation, it is dried in air or in vacuum. Then, the impregnated SBA-15 is pressed into a compact sample with dimensions: d = 25 mm and h = 3 mm and is briefly initiated using a tungsten spiral. After initiation, a chemical reaction, in the form of a combustion wave front, quickly propagates through the sample. After cooling, the product is ground to fine particles and then leached in a concentrated unimolar solution of sodium hydroxide at a temperature of 60-80 ° C for 8-12 hours. The resulting oxide is then recovered by filtration and washing.

This method allows the production of various ultrafine powders of transition metal oxides with an average particle size of ~ 3 nm. Nanosizes of combustion products and their high specific surface area are due to several reasons:

- the mixing of the reagents in the solution occurs at the molecular level, which provides a very small scale of heterogeneity of the initial reaction mixture;

- the maximum reaction temperature is about 1000 ° C. Such a high temperature promotes the formation of crystalline materials without additional calcination;

- a large number of gas-phase products, the intensive release of which prevents the agglomeration of solid-phase products;

- the formation of a solid-phase product in the form of thin layers or suspensions, which leads to a high cooling rate of the product, i.e. short duration of the high-temperature zone behind the combustion wave, which, in turn, promotes the formation of nanoparticles with a narrow size distribution;

- the synthesis process effectively removes organic impurities without the formation of coke.

Example 1

The synthesis of ultrafine powder Fe ₂ O ₃ consists of the following stages:

Impregnation of a reaction solution containing: 1.1 g Fe (NO ₃ ) ₃ · 6H ₂ O; 0.35 g of C ₂ H ₅ NO ₂ and 3.5 g of NH ₄ NO ₃ in 0.5 g of SBA-15. SBA-15 is impregnated gradually over 6 times. For each step, 0.5 ml of the reaction solution is added. After the next impregnation step, the SBA-15 is vacuum dried at room temperature. After the impregnation is completed, the resulting powder is pressed into a compact cylindrical sample with dimensions: d = 25 mm and h = 3 mm. The sample is initiated in air using a tungsten helix. After initiation, a chemical reaction, in the form of a combustion wave front, quickly propagates through the sample. After cooling, the product is ground to fine particles and then leached in a concentrated unimolar solution of sodium hydroxide at a temperature of 80 ° C for 12 hours, then the obtained iron oxide is recovered by filtration and washing at room temperature. The finished product has a high specific surface area of 132 m ² / g, an average particle size of 3.5 nm.

Example 2

To obtain an ultrafine NiO powder, a reaction mixture containing 3.89 g of nickel nitrate, 1.4 g of glycine and 1.4 g of ammonium nitrate is dissolved in 6 ml of distilled water and thoroughly mixed. The aqueous solution obtained after mixing is impregnated in 2 g of SBA-15 in three steps. In the first step, 6 g of the solution is added to SBA-15, then the resulting mixture is stirred and dried at 70 ° C for 2 hours. This process is repeated two more times, but with the addition of 1.5 g of the solution. The dried material is pressed into a cylindrical sample d = 25 mm and is initiated by local external heating with a tungsten wire. After cooling the product, grinding, leaching and filtration should be carried out as described in Example 1. The specific surface of the material is 155 m ² / g.

Example 3

The procedure for the synthesis of ultrafine CuO powder is as follows. A solution containing 2.36 g of copper nitrate and 2.25 g of urea is added to 2 g of SBA-15. Impregnation of SBA-15 is carried out gradually over 6 times by the method of impregnation by moisture capacity. The further procedure corresponds to example 1. The resulting product has a specific surface area of about 140 m ² / g.

Claims (4)

1. The method of producing ultrafine powders of metal oxides, namely, that the mesoporous SiO _{2 is} impregnated with a reaction solution containing metal nitrates and water-soluble organic compounds, then the impregnated mesoporous SiO ₂ is dried at room temperature, then the impregnated mesoporous SiO ₂ is pressed to compact sample and initiate a chemical reaction in the sample using a tungsten spiral, after which the resulting powder is cooled, ground and then leached to a concentration of trined unimolar solution of sodium hydroxide at a temperature of 60-80 ° C for 8-12 hours, then the obtained metal oxide powder is filtered and washed. 2. The method according to p. 1, characterized in that iron nitrate and / or nickel nitrate and / or copper nitrate are used as metal nitrates. 3. The method according to p. 1, characterized in that glycine and / or hydrazine and / or urea are used as water-soluble organic compounds. 4. The method according to p. 1, characterized in that the drying of the impregnated mesoporous SiO _{2 is} carried out in air or in vacuum.

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