RU2709862C1 - Method for preparing compositions based on oxides of zirconium and cerium - Google Patents

Abstract

FIELD: manufacturing technology.

SUBSTANCE: invention can be used in production of three-route catalysts for exhaust gases purification. Method of obtaining compositions based on zirconium and cerium oxides used in three-route catalysts involves preparing a solution containing nitrates of zirconium, cerium, lanthanum and another rare-earth element selected from yttrium and neodymium. Suspension is prepared by mixing said solution and basic compound. Concentration of metal oxides in solution containing nitrates of zirconium, cerium, lanthanum and other rare-earth element is at level from 20 to 30 g/dm³ in terms of final composition. Prior to precipitation, ammonium nitrate is added to solution to achieve concentration of 0.5 to 1 mol/dm^3. Suspension is prepared while stirring and maintaining the pH value at level of 8–10 due to controlled simultaneous dosing of the solution containing nitrates of zirconium, cerium, lanthanum and other rare-earth element, as well as ammonium nitrate and aqueous ammonia solution into the total reaction volume. Performing hydrothermal treatment with holding at temperature of 70–160 °C for 0.25–48 hours. Surfactants are added to the suspension, filtered, dried and calcined obtained residue at 500–1,000 °C.

EFFECT: invention increases specific surface of the product, reduces the amount of surfactant used, increases resistance of the composition to high temperatures.

1 cl, 1 tbl, 7 ex

Description

The invention relates to a technology for producing compositions based on cerium with high resistance to high temperatures intended for use in three-way catalysts.

Three-way catalysts are used to clean the exhaust gases of cars with gasoline engines, the function of which is the simultaneous conversion of pollutants, namely, the oxidation of hydrocarbons and carbon monoxide, as well as the reduction of nitrogen oxides. The performance of this function is possible only with a stoichiometric ratio of oxidizing agents and reducing agents in the gas phase. To maintain the stoichiometric ratio of the gas phase, materials capable of absorbing and releasing oxygen from their crystal lattice are used in the three-way catalysts, thereby maintaining the stability of the gas phase. The composition uses cerium oxide, which determines the ability of materials to reversibly release oxygen, as well as oxides of zirconium, yttrium and oxides of other rare earth elements necessary to increase resistance to high temperatures.

The current trend is to move the cleaning systems closer to the engine, this leads to a solution to the problem of cold start and increase the operating temperature of the catalysts. Long-term operation of automotive catalysts under conditions of elevated temperatures at the level of 1000 ° C leads to coarsening of particles of carrier materials, sintering of particles of noble metals, which leads to a decrease in the available surface area of noble metals and, accordingly, to a decrease in specific catalytic activity. Thus, the materials used in the three-way catalysts must be highly resistant to high temperatures. That is why it is urgent to develop new methods for producing compositions consisting of cerium and zirconium oxides, as well as at least one rare-earth oxide selected from yttrium, lanthanum and neodymium, which ensure the compositions are resistant to high temperatures, which is manifested in maintaining a high specific surface at a level not lower than 50 m ² / g after firing at a temperature of 1000 ° C for 4 hours.

Closest to this invention is a method for producing a composition based on cerium and zirconium with the addition of at least one rare earth element, which is described in the patent [RU 2648072, prior, from 05/06/2010, publ. 10/10/2014, IPC C01G 25/00, etc.]. According to the invention, the method includes preparing a solution containing zirconium, cerium, lanthanum and other rare earth elements, preparing a suspension by mixing the above solution and the basic compound, heating the resulting suspension, adding a surfactant to the suspension, filtering the suspension, drying and calcining the resulting precipitate.

The technical problem to which this invention is directed is the need to use a large amount of surfactant, washing the precipitate, as well as a low specific surface area of the final composition.

The technical result achieved by the implementation of the invention is to overcome the above-described disadvantages: increasing the specific surface area of the system with the same amount of surfactant used or significantly reducing the amount of surfactant irreversibly lost to achieve the same specific surface level by modifying the process of hydrolysis of metal salts to deposition stage.

The claimed method for producing compositions of cerium and zirconium oxides, as well as at least one rare earth element selected from yttrium, lanthanum and neodymium, providing high resistance to high temperatures, includes:

- preparation of a common solution containing soluble salts of zirconium, cerium, as well as one or more rare earth elements selected from yttrium, lanthanum and neodymium with a concentration of from 20 to 30 g / dm ³ in terms of the final composition;

- the introduction of ammonium nitrate in a common solution until the concentration of ammonium nitrate from 0.5 to 1 mol / DM ³ ;

- the introduction of distilled water into the reaction volume, the precipitation of hydrated oxides of cerium, zirconium, as well as at least one rare earth element selected from yttrium, lanthanum or neodymium, by dosing the above general solution into the reaction volume, in which a constant pH value is maintained at a level from 8 to 10 inclusive, preferably from 8.5 to 9.5, due to the controlled introduction of the precipitating reagent of the main nature in the reaction volume;

- carrying out hydrothermal treatment of the suspension at temperatures from 70 ° C to 160 ° C, preferably from 120 ° C to 130 ° C, with exposure at a given temperature for from 0.25 to 4 8 hours, preferably from 0.25 to 1 hour ;

- separation of the precipitate from the liquid part by any known method, preferably by suction filter filtration;

- additive to the precipitate surfactants from the group of anionic surfactants, nonionic surfactants, polyethylene glycols, saturated alcohols, carboxylic acids and their salts;

- drying and firing the precipitate obtained.

The ratio of cerium, zirconium and one or more rare-earth elements selected from yttrium, lanthanum and neodymium is expressed by the general formula: Ce _1-nm Zr _n Ln _m O _x , where Ln is the oxide or oxides of rare-earth elements selected from yttrium, lanthanum and neodymium , n is the mass fraction of zirconium oxide, which varies from 0.1 to 0.9, according to the preferred method of obtaining n = 0.5; m is the mass fraction of the oxide or oxides of rare earths selected from yttrium, lanthanum and neodymium, which varies from 0.1 to 0.2, according to the preferred method of obtaining the composition m = 0.1.

The authors found that the introduction of ammonium nitrate into a common solution containing soluble salts of zirconium, cerium, as well as one or more rare earth elements selected from yttrium, lanthanum and neodymium, allows you to modify the process of hydrolysis of metal salts at the deposition stage, which ultimately allows to obtain compositions resistant to high temperatures.

Without modifying the deposition process by introducing ammonium nitrate, the nuclei of hydrated metal oxides formed during hydrolysis as a result of coagulation processes are collected in dense agglomerates, which, during subsequent heat treatment, form a composition with lower resistance to high temperatures than the composition for which the deposition process was carried out in the presence of ammonium nitrate.

When ammonium nitrate is introduced into the general solution, it is possible to modify the process of hydrolysis of metal salts, while ammonium nitrate exerts a buffering effect and suppresses the coagulation of the nuclei of hydrated metal oxides by increasing the neutralization time of the droplet as it spreads in the reaction volume. In this case, the nuclei of hydrated metal oxides are collected in loose aggregates, which, upon subsequent heat treatment, form compositions with high resistance to high temperatures, characterized by developed fractal porosity. However, an excess of the concentration of ammonium nitrate in the general solution of critical value can lead to the intensification of coagulation processes, which ultimately leads to the formation of compositions with low resistance to high temperatures.

The invention is illustrated by figures, which depict:

- in FIG. 1 is a table of values of the specific surface of the samples obtained by various examples,

At the first stage of obtaining compositions based on zirconium, cerium with the addition of one or more rare earth elements selected from yttrium, lanthanum and neodymium, a general solution of the claimed elements is prepared in a liquid medium with a concentration of from 20 to 30 g / dm ³ in terms of the final composition. The use of a general solution with a concentration below 20 g / dm ³ in terms of the final composition requires large volume apparatuses, which is not expedient from a technological point of view, and the use of a solution with a concentration above 30 g / dm ³ in terms of the final composition does not allow reaching the declared technical result due to increased influence of salt background. Any liquid can be used as a liquid medium; in a preferred embodiment of the invention, the liquid medium is water. As compounds can be used salts of zirconium and REE, including nitrates, chlorides, sulfates, acetates. It is also possible to prepare zirconium and REE salts by dissolving carbonates or oxides in various mineral acids. According to the preferred method of implementing the invention, carbonates of the claimed metals and concentrated nitric acid are used to prepare a solution of zirconium and cerium salts, rare-earth oxides and concentrated nitric acid are used to obtain solutions of other rare earth elements selected from yttrium, lanthanum and neodymium.

In the second stage, ammonium nitrate is added to the general salt solution prepared in the first stage until the concentration in the solution is from 0.5 to 1 mol / dm ³ . When the concentration of ammonium nitrate in the total solution is below 0.5 mol / dm ^{3, the} effect of the buffer effect is reduced, while the time of drop neutralization is reduced and the activity of coagulation processes is increased, and at a concentration above 1 mol / dm ³ , coagulation processes are intensified due to the compression effect double electric layer, which leads to the formation of compositions with low resistance to high temperatures.

In the third stage, the initial reaction volume is prepared, which is distilled water. Next, the hydrated oxides of cerium, zirconium, and one or more rare earth elements selected from yttrium, lanthanum, or neodymium are precipitated. Precipitation is carried out by simultaneously dosing the total solution and the basic compound in the above initial reaction volume, while the pH of the reaction mixture is maintained at a level of from 8 to 10, preferably from 8.5 to 9.5, by controlling the feed rates of the general solution and the basic compound . As the main compound, solutions of ammonia, sodium or potassium hydroxide, tetramethylamine and other compounds can be used. An aqueous solution of ammonia is used in a preferred embodiment of the invention.

In the fourth stage, the precipitate obtained in the previous stage is heated in a liquid medium. The precipitate is heated to a temperature of at least 70 ° C, usually to a temperature of from 100 ° C to 160 ° C. The heating operation is preferably carried out in a closed vessel such as an autoclave. The duration of heating can vary within wide limits, for example, from 0.25 to 48 hours, preferably from 0.25 to 1 hour. The heating rate is not critical.

In the fifth stage, the precipitate is separated from the liquid part by any known method, it is preferable to use filtration using a suction filter.

At the sixth stage, surfactants from the group of anionic surfactants, nonionic surfactants, polyethylene glycols and carboxylic acids and their salts, as well as surfactants such as ethoxylates of fatty carboxymethylated alcohols and saturated alcohols are added to the precipitate obtained in the previous step.

The final stage is the drying and firing of the obtained precipitate. The mode of drying the precipitate is not critical. Usually, drying is carried out at a temperature of from room temperature to 200 ° C until the moisture is completely removed from the precipitate. Then carry out the firing obtained after drying the precipitate. The firing temperature can vary from 500 to 1000 ° C.

Example 1

This example relates to a composition of 50% by weight zirconia, 40% cerium dioxide 5% yttrium oxide and 5% lanthanum oxide.

40 cm ³ of zirconyl nitrate (158 g / dm ³ in terms of ZrO ₂ ), 31 cm ³ of cerium nitrate (163 g / dm ³ in terms of CeO ₂ ), 4.1 cm ³ of lanthanum are introduced into a beaker with stirring 152 g / dm ³ in terms of La ₂ O ₃ ) and 3.7 cm ³ of yttrium nitrate (166 g / dm ³ in terms of Y ₂ O ₃ ). Then add distilled water to obtain 500 cm ³ total nitric acid solution with a concentration of 25 g / DM ³ in terms of Zr ₀ . ₅ Ce _0.4 Y _0.05 La _0.05 O _x . 40 g of ammonium nitrate are added to the resulting solution to obtain a total solution with a concentration of ammonium nitrate in it of 1 mol / dm ³ . In parallel with this, a reaction volume is prepared by introducing 200 cm ^{3 of} distilled water into a reactor with a stirrer and a pH sensor. Next, the general solution and a 10% aqueous ammonia solution are simultaneously dosed into the reaction volume, while the pH in the reaction mixture is maintained at 9.

The resulting suspension is placed in an autoclave and heated to a temperature of 120 ° C and maintained at a given temperature for 15 minutes.

The suspension thus obtained is filtered on a suction vacuum filter. Next, the precipitate is placed in a beaker and treated with isopropanol with stirring until the concentration of isopropanol in the liquid part of the suspension reaches 90 ± 1% by mass.

Then the aqueous-alcoholic suspension is filtered on a vacuum suction filter, the resulting precipitate is dried at 120 ° C for 2 hours and calcined at a temperature of 500 ° C and 1000 ° C for 2 hours and 4 hours, respectively.

Example 2

This example relates to a composition of 50% by weight zirconia, 30% cerium dioxide, 10% lanthanum oxide and 10% neodymium oxide.

40 cm ³ of zirconyl nitrate (158 g / dm ³ in terms of ZrO ₂ ), 23 cm ³ of cerium nitrate (163 g / dm ³ in terms of CeO ₂ ), 8 cm ³ of lanthanum nitrate (152 g / dm ³ in terms of La ₂ O ₃ ) and 7 cm ³ of neodymium nitrate (167 g / dm ³ in terms of Nd ₂ O ₃ ). 40 g of ammonium nitrate are added to the resulting solution to obtain a total solution with a concentration of ammonium nitrate in it of 1 mol / dm ³ . Then distilled water is added to obtain 500 cm ^{3 of a} total nitric acid solution with a concentration of 25 g / dm ³ in terms of Zr _0.5 Ce _0.3 La _0.1 Nd _0.1 O _x .

Further operations are carried out as described in example 1.

Example 3

The composition is the same as in example 1.

An initial solution containing ions of cerium, zirconyl, yttrium and lanthanum is prepared as in Example 1. 20 g of ammonium nitrate are added to the resulting solution to obtain a total solution with a concentration of ammonium nitrate of 0.5 mol / dm ^{3 in it} .

Subsequent operations are carried out as in example 1.

Example 4

The composition is the same as in example 1.

31 cm ³ of zirconyl nitrate (158 g / dm ³ in terms of ZrO ₂ ), 24 ml of cerium nitrate (163 g / dm ³ in terms of CeO ₂ ), 3.2 cm ³ of lanthanum are introduced into a beaker with stirring, 15.2 g / dm ³ in terms of La ₂ O ₃ ) and 3 cm ³ of yttrium nitrate (166 g / dm ³ in terms of Y ₂ O ₃ ). Then add distilled water to obtain 500 cm ³ total nitric acid solution with a concentration of 20 g / DM ³ in terms of Zr ₀ . ₅ Ce _0.4 Y _0.05 La _0.05 O _x .

Subsequent operations are carried out as in example 1.

Example 5 (comparative). The composition is the same as in example 1

An initial solution containing ions of cerium, zirconyl, yttrium and lanthanum is prepared as in example 1, only ammonium nitrate is not added to the resulting solution.

Subsequent operations are carried out as in example 1.

Example 6 (comparative)

The composition is the same as in example 1

An initial solution containing ions of cerium, zirconyl, yttrium and lanthanum is prepared as in Example 1. 80 g of ammonium nitrate are added to the resulting solution to obtain a total solution with a concentration of ammonium nitrate in it of 2 mol / dm ³ .

Subsequent operations are carried out as in example 1.

Example 7 (comparative)

The composition is the same as in example 1

78 cm ³ of zirconyl nitrate (158 g / dm ³ in terms of ZrO ₂ ), 46 cm ³ of cerium nitrate (163 g / dm ³ in terms of CeO ₂ ), 16 cm ³ of lanthanum nitrate (152 g / dm ³ in terms of La ₂ O ₃ ) and 15 cm ³ of yttrium nitrate (166 g / dm ³ in terms of Y ₂ O ₃ ). Then, distilled water is added to obtain 500 cm ^{3 of a} total nitric acid solution with a concentration of 50 g / dm ³ in terms of Zr ₀ . ₅ Ce _0.4 Y _0.05 La _0.05 O _x

Subsequent operations are carried out as in example 1.

To determine the resistance of the compositions to high temperatures, all samples were subjected to heat treatment at a temperature of 1000 ° C for 4 hours. The results of determining the specific surface are presented in FIG. 1. The specific surface area of the compositions was determined using low-temperature nitrogen adsorption (-196 ° C) on a NOVA Quantachrorae 1200E instrument. It is shown that the process of deposition of hydrated oxides under conditions when the concentration of the initial total nitric acid solution is in the range from 20-30 g / dm ³ in terms of the final product, and the concentration of ammonium nitrate in the initial solution varies from 0.5 to 1 mol / dm ³ leads to an increase in the specific surface of the formed compositions after firing at 1000 ° C for 4 hours, which determines the benefit of using the proposed method.

Claims (1)

A method of obtaining compositions based on zirconium and cerium oxides intended for use in three-way catalysts, comprising preparing a solution containing zirconium, cerium, lanthanum and other rare-earth element nitrates, preparing a suspension by mixing the above solution and the main compound, carrying out hydrothermal treatment at a temperature 70-160 ° С with holding at this temperature for 0.25-48 hours, adding surfactants to the suspension, filtering the suspension, drying and calcining the resulting precipitate at 500- 1000 ° C, characterized in that the concentration of metal oxides in the prepared solution containing nitrates of zirconium, cerium, lanthanum and other rare earth elements is at a level of from 20 to 30 g / DM ³ in terms of the final composition, and before precipitation in the above 10 inclusive due to controlled simultaneous dispensing races ammonium nitrate solution is introduced until a concentration of the latter from 0.5 to 1 mol / dm ³ slurry preparation is carried out by stirring and maintaining a constant pH level of 8 thief containing zirconium nitrate, cerium, lanthanum and other rare earth elements, ammonium and nitrate and an aqueous ammonia solution in the total reaction volume.

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