RU2738984C1 - Method for production of automotive three-way catalyst - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: disclosed is a method of producing automotive three-way catalyst. Method comprises steps of preparing an aqueous suspension containing a composition based on cerium and zirconium oxides, aluminium oxide, a modifier salt and a solution of a salt of precious metals; application of suspension onto substrate, drying and annealing. At the stage of preparing an aqueous suspension for grinding powders, compositions based on cerium and zirconium oxides and aluminium oxide are combined. Modifier salt used is barium nitrate or acetate. Addition of modifier to suspension is carried out before addition of precious metals. Addition of modifiers is carried out while stirring and maintaining the pH of the suspension in range of 4 to 7 units. PH is adjusted by adding an acetic acid solution if the pH is greater than 7 units and adding a solution of tetraethylammonium hydroxide in case the pH value is less than 4 units. Precious metals are added to the suspension after holding the suspension with modifier for at least 120 s.

EFFECT: invention discloses a method of producing automotive three-way catalyst.

4 cl, 1 dwg

Description

The invention relates to a method for the production of an automobile three-way catalyst with increased catalytic activity, intended for cleaning the exhaust gases of cars with gasoline engines.

The combustion of gasoline fuels produces three main groups of pollutants: unburned hydrocarbons, carbon monoxide and nitrogen oxides. A three-way catalyst (TWC) is used to ensure the simultaneous oxidation of carbon monoxide, hydrocarbons and the reduction of nitrogen oxides. This type of catalyst operates under very harsh conditions, characterized by high temperatures (> 1000 C) and constant fluctuations in the composition of the gas phase. Under these conditions, systems with catalytically active components, which are platinum metals such as Pt, Pd and Rh, have shown the greatest efficiency.

Supports of platinum metals, as well as various additives serving as promoters, play an essential role. As carriers of precious metals are used © -Al ₂ O ₃ stabilized ZrO ₂ , La ₂ O _3, and other oxide additives, as well as compositions based on cerium and zirconium oxides. The use of compositions based on cerium and zirconium oxides in TWC is associated with their ability to accumulate and release oxygen from the crystal lattice. This property makes it possible to compensate for fluctuations in the oxygen concentration in the exhaust gases associated with the peculiarities of the engine operation and to maintain the oxygen content on the catalyst surface close to stoichiometry. This achieves the simultaneous efficient flow of oxidative and reduction reactions.

When synthesizing catalysts based on platinum metals, additional modifiers are used. For example, in [TY Chou, CH Leu, CT Yeh. Effects of the addition of lanthana on the thermal stability of alumina-supported palladium // Catalysis Today 26 (1995) 53-58.] Found that the addition of lanthanum oxide to Pd / Al ₂ O ₃ to the catalyst not only improves the thermal stability of r- Al ₂ O ₃ , but also shifts the PdO - Pd transition temperature to the region above 800 ^o C, which is a barrier to the formation and sintering of metallic palladium at a high catalyst operating temperature.

In addition, it is believed that by using modifying additives, in particular alkaline earth and rare earth d - elements, it is possible to change the catalytic properties of Pd so that it reduces NO _x as well as oxidizes CO and hydrocarbons. [T. Kobayashi, T. Yamada, K. Kayano. Applied Catalysis B: Environmental 30 (2001) 287-292]. Using the XPS method, it was suggested that the addition of Ba increases the electron density around Pd, which probably begins to have an electronic configuration similar to Rh, and this gives palladium similar catalytic properties to rhodium.

Thus, the properties of TWC, namely the properties of the active components Pd and Rh, change after the introduction of modifiers. The most promising and currently used modifier is barium. However, the literature does not comprehensively consider the question of the influence of the method and order of introduction of barium and other modifiers on the properties of carriers and active components.

The known method [US20140241964A1, prior. from 26.02.2013, publ. 08/28/2014, IPC B01J23 / 44, B01D53 / 945, F01N3 / 101] for obtaining a catalyst for cleaning exhaust gases of internal combustion, which consists of a substrate and a layer with an active component on carrier particles, which has an uneven distribution of a noble metal or several noble metals. The creation of a concentration gradient of noble metals is carried out due to the fact that there is no additional stage of impregnation of oxide particles with one of the metals (possibly both metals), and their soluble salts are present in the suspension. A coating is formed from this suspension on a cordierite substrate, and during drying, different metals have different mobility, which can also be controlled by the introduction of salts with different anions (for example, nitrate ion and carboxylate ion). The advantage of this method is the ability to concentrate noble metals on the surface of the layer due to the different sorption capacity of the noble metal compounds. The disadvantage of this method is the lack of the possibility of controlled introduction of modifying additives, as a result of which, under reducing conditions of the gaseous environment, the formation of palladium metal particles prone to sintering at high temperatures on the surface of oxide carriers is possible, which leads to thermal deactivation of the catalyst.

Closest to this invention is the method [WO2013022958A1, prior. from 08/10/2011, publ. 02/14/2014, IPC B01D53 / 945, Y02T10 / 22, B01J37 / 0215] for obtaining a catalyst capable of simultaneously processing nitrogen oxides, carbon monoxide and hydrocarbons in exhaust emissions into less toxic compounds.

The result is achieved by the fact that the catalyst coating contains a composition based on cerium and zirconium oxides, a platinum subgroup metal, aluminum oxide stabilized with lanthanum oxide, and barium compounds. The method for the production of such catalysts consists of several stages: preparation of an aqueous suspension containing a composition based on cerium and zirconium oxides, aluminum oxide, a modifier salt and a solution of a salt of precious metals; application of the suspension to the substrate, drying and firing.

The stage of preparing an aqueous suspension contains the following technological operations: preparing an aqueous suspension of a powder of a composition based on cerium and zirconium oxides, preparing an aqueous suspension of aluminum oxide powder, mixing the resulting suspensions to obtain a total suspension. An aqueous suspension of a powder of a composition based on cerium and zirconium oxides is prepared by mixing water, a base (for example, tetraethylammonium hydroxide, tetramethylammonium hydroxide, tetrapropylammonium hydroxide, tetrabutylammonium hydroxide) and a powder of a composition based on cerium and zirconium oxides, grinding the resulting suspension (adding a transition metal salt for example Pd, Cu, Mn or Ni) to the slurry. An aqueous suspension of alumina powder is prepared by mixing water and alumina powder, grinding the resulting suspension in the presence of acetic acid, adding a salt of barium carbonate or calcium carbonate.

The advantage of this method is high catalytic activity in the reactions of nitrogen oxide reduction in the absence of rhodium in the catalyst. The disadvantage of this method is the complexity of the implementation of the invention, a large number of technological operations, as well as the need to use expensive organic compounds at the stage of preparing a suspension of a composition based on cerium and zirconium oxides.

The technical problem to be solved by the invention is the need to increase the catalytic activity, as well as the complexity of the three-way catalyst production process.

The technical result achieved during the implementation of the invention consists in increasing the thermal stability of the composition based on cerium oxides, which leads to an increase in catalytic activity in the reactions of conversion of carbon monoxide, hydrocarbons and nitrogen oxides. Moreover, a reduction in the number of steps in the manufacturing process of the automobile three-way catalyst is achieved.

The claimed method for the production of automobile three-way catalysts with increased catalytic activity contains the following stages: preparation of an aqueous suspension containing a composition based on oxides of cerium and zirconium, aluminum oxide, barium salt and a solution of a salt of precious metals; application of the suspension to the substrate, drying and firing. The claimed technical result is achieved by modifying the surface of oxide carriers with holding the suspension while maintaining a predetermined pH level before introducing precious metal salts.

It is known that the modification of aluminum oxide is the most widespread and effective way to increase its thermal stability. For example, the addition of barium as a modifier leads to an increase in the specific surface area of alumina due to inhibition of corundum formation when exposed to high temperatures. At the same time, the authors of the present invention have found that the interaction of barium and a composition based on cerium and zirconium oxides leads to a decrease in the specific surface area and porosity of the latter when exposed to high temperatures. This interaction can cause the formation of a separate phase of barium zirconate on the surface of the composition based on cerium and zirconium oxides, which in turn causes an abrupt increase in the diffusion coefficients of the components of the system and can lead to a significant decrease in the specific surface area and pore volume of the composition as a whole. In turn, a significant decrease in the specific surface area and pore volume of the composition, together with the formation of a barium zirconate phase on the surface of the composition, which hinders the absorption and evolution of oxygen, can cause a decrease in the oxygen capacity of the catalytic systems based on the above composition. The most obvious option for reducing the negative effect of barium on the properties of the composition based on cerium and zirconium oxides is the spatial separation of alumina and the composition based on cerium and zirconium oxides over different layers, while barium can be added only to the layer containing alumina. However, the use of this method leads to a significant complication of the technological process for the manufacture of catalysts.

The inventors have found that by adjusting the pH value of the suspension of alumina and the composition based on cerium and zirconium oxides in water and the order in which precious metals and barium are added to said suspension, preferential localization of barium on alumina can be ensured. This preferential localization of barium can be performed due to the high specific surface area of the alumina powders, which provide preferential sorption of barium from the liquid phase of the suspension.

The implementation of the present invention includes the introduction of barium into an aqueous suspension containing alumina and a composition based on cerium and zirconium oxides while maintaining the pH value in the range in which the sorption of barium occurs predominantly on the surface of the alumina. Thus, it becomes possible to significantly reduce the negative effect of barium on the properties of the composition based on cerium and zirconium oxides. In addition, the sorption of the modifier on the surface of oxide carriers prior to the introduction of precious metals leads to a decrease in the number of free sorption sites on the surface of oxide carriers intended for precious metals. Under conditions when a part of the sorption centers of the surface of the oxide carriers is occupied by the modifier ions, it becomes possible, on the one hand, to ensure an increase in the uniformity of the distribution of precious metal ions on the surface of the oxide carriers, and a significant increase in the interaction of the modifier phase with the precious metal phase, which together can reduce the migration of precious metals. metals and sintering under high temperatures and constant fluctuations in the composition of the gaseous medium during the operation of a three-way catalyst.

The essence of the invention is illustrated by a table of catalytic activity parameters for samples of catalytic blocks.

At the first stage of the production of an automobile three-way catalyst with increased catalytic activity, an aqueous suspension is prepared containing a composition based on cerium and zirconium oxides, aluminum oxide, a barium salt and a solution of a salt of precious metals. This stage contains the following technological steps.

Initially, water is introduced into the reaction volume, which is necessary for preparing the suspension, as well as for controlling the pH. The amount of water introduced into the reaction volume is not critical. However, it is preferable to add such an amount of water that the solids content in the formed suspension is from 15 to 45 wt.%, Since it is known from the prior art that such a solids content is more convenient when forming a coating on a substrate. Purified water can be used, including distilled or deionized water. It is preferable to use distilled water as a liquid medium.

Next, mixing of alumina powders and a composition based on cerium and zirconium oxides is carried out to obtain a mixture of powders. The composition may include oxides of cerium, zirconium, yttrium, lanthanum and praseodymium, it is preferable to use a composition of the composition Ce _0.33 Zr _0.57 Y _0.06 La _0.04 O ₂ , where subscripts indicate the molar fraction of the component in the composition. As the alumina, it is preferable to use alumina stabilized with zirconium oxide or lanthanum oxide.

After the resulting mixture of powders is introduced with stirring into a reaction volume filled with water to obtain a suspension.

Next, the suspension is milled as described in the prototype. Most preferably, grinding is carried out in a bead mill to achieve a d ₉₀ value of at most 9.5 μm. Hereinafter, the term "d ₉₀ " means a diameter with a percentage by weight of particles of a given size less than 90% of the total content. The specified particle size limit is generally accepted in the technology for the production of three-way catalysts, upon reaching which it is possible to achieve the required suspension viscosity and adhesion of the coating formed on the cordierite substrate. During grinding, it is possible to use acetic acid as described in the prototype.

Then the salt of the modifier is introduced into the milled suspension with constant stirring. Barium nitrate or barium acetate can be used as the modifier salt, preferably barium nitrate is used. The mass fraction of barium in terms of barium oxide in the catalytically active coating can be in the range from 1.5 to 5 wt. %, and even better from 2 to 3 wt. %. The pH value of the suspension with the addition of the modifier salt must be maintained at a level of 4 to 7 units. inclusive, preferably from 4 to 5 units. inclusive. Adjustment of the pH value can be performed by introducing an acid solution if the pH of the slurry has exceeded the upper acceptable range, and introducing an aqueous base solution if the pH of the suspension has exceeded the lower acceptable range. It is most preferable to use a solution of acetic acid, and a solution of tetraethylammonium hydroxide as the base. The suspension must be kept for at least 120 seconds, and even better 180 seconds to establish the equilibrium of the sorption process.

Then, precious metal salts are introduced into the suspension with constant stirring. Preferably, precious metal salts are added to the suspension in the form of solutions. As solutions of salts of precious metals, solutions of salts of palladium nitrate, rhodium nitrate or their mixture can be used, it is preferable to use a mixture of solutions of palladium nitrate and rhodium nitrate. The content of precious metals is described in the prototype, preferably it should be in the range from 5 to 100 g / ft ³ .

At the second stage of the production of a three-way catalyst, the suspension is applied to the substrate by the vacuum suction method to obtain a catalytic block.

In the third stage of the invention, the catalytically active coating is thermally cured by drying and firing.

The essence of the invention can be illustrated by the following examples.

Example 1

518 g of water is introduced into a beaker with stirring to prepare a reaction volume. Next, a mixture of powders of 195.3 g of aluminum oxide and 83.7 g of Ce _0.33 Zr _0.57 Y _0.06 La _0.04 O ₂ is prepared. The resulting mixture of powders with stirring is introduced into the reaction volume to obtain a suspension. The resulting suspension is milled using a bead mill to a d ₉₀ value of 7 μm. Next, 15.9 g of barium nitrate salt is introduced into the suspension, and the pH value in the reaction volume is maintained in the range from 4 to 7 units. The pH is adjusted by adding an acetic acid solution if the pH is more than 7 units. and adding a solution of tetraethylammonium hydroxide in case the pH value is less than 4 units. Then the suspension is kept for 120 seconds with stirring. Then 8.9 g of a palladium nitrate solution (mass concentration of palladium in the solution of 15.35%) and 3.0 g of a solution of rhodium nitrate (mass concentration of rhodium in the solution of 9.11%) are mixed to obtain a mixture of DM solutions. Next, a mixture of DM solutions is introduced into the suspension. The resulting suspension is used to form a coating on a cordierite substrate. The resulting catalyst unit was dried at 120 ^{° C} for 15 minutes and calcined at 550 ^{° C} for 40 minutes.

Example 2

518 g of water is introduced into a beaker with stirring to obtain a reaction volume. Next, a mixture of powders of 195.3 g of aluminum oxide and 83.7 g of Ce _0.33 Zr _0.57 Y _0.06 La _0.04 O ₂ is prepared. Then the resulting mixture of powders with stirring is introduced into the reaction volume to obtain a suspension. The resulting suspension is milled using a bead mill to a d ₉₀ value of 6 μm. Next, 7.74 g of barium acetate salt is introduced into the suspension, and the pH value in the reaction volume is maintained in the range from 4 to 7 units. The pH is adjusted by adding an acetic acid solution if the pH is more than 7 units. and adding a solution of tetraethylammonium hydroxide in case the pH value is less than 4 units. Then the suspension is kept for 180 seconds with stirring. Then 7.13 g of a solution of palladium nitrate (mass concentration of palladium in the solution of 15.35%) and 2.4 g of a solution of rhodium nitrate (mass concentration of rhodium in the solution of 9.11%) are mixed to obtain a mixture of DM solutions. Next, a mixture of DM solutions is introduced into the suspension. The resulting suspension is used to form a coating on a cordierite substrate. The resulting catalyst unit was dried at 120 ^{° C} for 15 minutes and calcined at 550 ^{° C} for 40 minutes.

Example 3 (comparative)

518 g of water is introduced into a beaker with stirring to prepare a reaction volume. Next, a mixture of powders of 195.3 g of aluminum oxide and 83.7 g of Ce _0.33 Zr _0.57 Y _0.06 La _0.04 O ₂ is prepared. The resulting mixture of powders with stirring is introduced into the reaction volume to obtain a suspension. The resulting suspension is milled using a bead mill to a d ₉₀ value of 10 μm. Then 8.9 g of a palladium nitrate solution (mass concentration of palladium in the solution of 15.35%) and 3.0 g of a solution of rhodium nitrate (mass concentration of rhodium in the solution of 9.11%) are mixed to obtain a mixture of DM solutions. Next, a mixture of DM solutions is introduced into the suspension. Then, 15.9 g of barium nitrate salt is introduced into the suspension. The resulting suspension is used to form a coating on a cordierite substrate. The resulting catalyst unit was dried at 120 ° C for 15 minutes and calcined at 550 ^{° C} for 40 minutes.

Example 4 (comparative)

518 g of water is introduced into a beaker with stirring to prepare a reaction volume. Next, a mixture of powders of 195.3 g of aluminum oxide and 83.7 g of Ce _0.33 Zr _0.57 Y _0.06 La _0.04 O ₂ is prepared. The resulting mixture of powders with stirring is introduced into the reaction volume to obtain a suspension. The resulting suspension is milled using a bead mill to a d ₉₀ value of 10 μm. Then 7.13 g of a solution of palladium nitrate (mass concentration of palladium in the solution of 15.35%) and 2.4 g of a solution of rhodium nitrate (mass concentration of rhodium in the solution of 9.11%) are mixed to obtain a mixture of DM solutions. Next, a mixture of DM solutions is introduced into the suspension. Next, 7.74 g of barium acetate salt is introduced into the suspension. The resulting suspension is used to form a coating on a cordierite substrate. The resulting catalyst unit was dried at 120 ° C for 15 minutes and calcined at 550 ^{° C} for 40 minutes.

The parameters of the catalytic activity of all samples were determined using a Horiba CTSJ.2003.12 gas analytical stand. The starting temperature of the catalyst was determined (temperature of 50% conversion or "ignition temperature" - "Light-off" test) and maximum conversion in a pulsed mode at a temperature of 400 ° C ("Perturbation" test). The dynamic oxygen capacity (OSC) was measured for all samples by the response method in the CO oxidation reaction.

Claims (4)

1. A method for the production of an automobile three-way catalyst, containing the stages of preparing an aqueous suspension containing a composition based on oxides of cerium and zirconium, aluminum oxide, a modifier salt and a solution of a salt of precious metals; application of the suspension to the substrate, drying and firing, characterized in that at the stage of preparing the aqueous suspension, the powders of the composition based on cerium and zirconium oxides and alumina are milled together, nitrate or barium acetate is used as the modifier salt, the modifier is added to the suspension before adding precious metals, the addition of modifiers is carried out with stirring and maintaining the pH of the suspension in the range from 4 to 7 units, pH adjustment is carried out by adding a solution of acetic acid in case the pH is more than 7 units, and adding a solution of tetraethylammonium hydroxide in case the pH value is less than 4 units, the addition of precious metals to the suspension is carried out after holding the suspension with the modifier for at least 120 s. 2. The method according to p. 1, characterized in that the mass fraction of barium in terms of barium oxide in the catalytically active coating is in the range from 1.5 to 5 wt. %, and even better from 2 to 3 wt. %. 3. The method according to PP. 1, 2, characterized in that the pH of the suspension is maintained in the range from 4 to 5 units. inclusive. 4. The method according to PP. 1-3, characterized in that the suspension is milled in a bead mill, ensuring the achievement of the diameter value with the percentage by weight of particles of a given size less than 90% of the total content (d ₉₀ ) less than 9.5 μm.

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