JP2009202054A - High-activity catalyst for cleaning car exhaust gas in reducing atmosphere - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst having catalytic cleaning activity prevented from being lowered when the combustion state of an engine is changed to an A/F of a rich atmosphere. <P>SOLUTION: The catalyst contains, in a catalyst coated layer, an OSC material capable of emitting 0.4%-or-higher O<SB>2</SB>, detected as CO<SB>2</SB>, continuously for 30 s or longer in the OSC capability evaluation test. The OCS capability evaluation test comprises laying 3.0 g of an OSC material in an evaluation instrument through which a model gas flows, raising the temperature to 600°C at a programming rate of 20°C/min, pretreating the material by exposing to the atmosphere of 1%-O<SB>2</SB>/N<SB>2</SB>, 10 L/min, for 30 min, measuring the concentration of CO<SB>2</SB>in the outlet gas at 600°C 30 min after switching to the atmosphere of 2%-CO/N<SB>2</SB>, 10 L/min, in order to evaluate the OSC capability of the catalyst. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification catalyst for purifying components in exhaust gas discharged from an internal combustion engine.

  The exhaust gas from an internal combustion engine such as an automobile engine contains nitrogen oxide (NOx), carbon monoxide (CO), hydrocarbon (HC), etc., and these substances are purified by an exhaust gas purification catalyst. From the atmosphere. A typical exhaust gas purification catalyst used here is a ternary catalyst in which a noble metal such as platinum (Pt), rhodium (Rh), palladium (Pd) is supported on a porous metal oxide carrier such as γ-alumina. Catalysts are known.

In order for the oxidation of CO and HC and the reduction of NO _x to proceed efficiently in such a three-way catalyst, the air-fuel ratio (A / F) of the internal combustion engine is appropriately controlled, and the exhaust gas composition is a specific narrow Must be in range. That is, the air-fuel ratio of the internal combustion engine needs to be in the vicinity of the stoichiometric air-fuel ratio (stoichiometric). However, in practice, the exhaust gas composition may fluctuate to some extent due to a control system time delay or the like, and may deviate from a specific narrow range. When the internal combustion engine is operated under a condition where there is a lot of air relative to the fuel (lean) or a condition where there is a lot of fuel relative to the air (rich), the three-way catalyst cannot sufficiently exhibit its purification ability.

  Therefore, in order to absorb the fluctuation of the oxygen concentration in the exhaust gas and enhance the exhaust gas purification ability of the three-way catalyst, oxygen is stored when the oxygen concentration in the exhaust gas is high, and oxygen is released when the oxygen concentration in the exhaust gas is low. A material having a so-called oxygen absorption / release capability (OSC capability) is used in an exhaust gas purifying catalyst.

Such OSC materials, for example, ceria (CeO ₂₎ and ceria - such as zirconia (CeO ₂ -ZrO ₂₎ composite oxide, ceria material based on (CeO ₂₎ is known, is widely put into practical use Yes. In addition, in order to achieve stable purification of exhaust gas, an OSC material having a higher oxygen absorption / release capability is required and studied. As prior art of such a cerium-zirconium composite oxide, there are JP-A-10-194742, JP-A-6-279027, and the like.

Japanese Patent Laid-Open No. 10-194742 JP-A-6-279027

  However, the OSC material as described above is intended to absorb a slight A / F fluctuation in the vicinity of stoichiometry and keep the A / F in a state with the highest purification efficiency. However, the A / F may be controlled to a rich atmosphere from stoichiometry, and the oxygen supply cannot catch up with the OSC material as described above, and the oxygen-free state at this time may significantly reduce the purification activity of the catalyst. .

  With respect to this problem, an object of the present invention is to provide a catalyst that prevents a decrease in catalyst purification activity when the A / F is greatly changed in a rich atmosphere in the combustion state of the engine.

The present invention provides the following (1) to (4).
(1) In the OSC ability evaluation test, the catalyst coat layer includes an OSC material in which 0.4% or more of O ₂ (detected as CO ₂ ) can be continuously released for 30 seconds or more,
Here, the OSC ability evaluation test means that the OSC material (3.0 g) is placed in an evaluation apparatus in which the following model gas flows, and the temperature is raised to 600 ° C. at a temperature rising rate of 20 ° C./min. After pre-treatment by exposure to an atmosphere of -O ₂ / N ₂ (10 L / min) for 30 minutes, after switching to an atmosphere of 2% -CO / N ₂ (10 L / min), the outlet gas at a temperature of 600 ° C. for 30 minutes A catalyst which is a test for measuring the concentration of CO ₂ in the catalyst and evaluating the OSC ability of the catalyst.
(2) The catalyst according to (1), wherein the OSC material is obtained by supporting a noble metal on praseodymium oxysulfate.
(3) The catalyst as described in (2) whose precious metal is 0.5 mass% Pt with respect to praseodymium oxysulfate.
(4) The catalyst according to any one of (1) to (3), wherein the catalyst coat layer further comprises a porous metal oxide support on which a noble metal is supported.

  Conventional OSC materials are intended to absorb slight A / F fluctuations in the vicinity of stoichiometry and keep the A / F in a state with the highest purification efficiency. That is, it is aimed to quickly absorb and release oxygen so as to be able to cope with relatively small A / F fluctuations. For this reason, although it can respond to the rich condition for a relatively short period, it may not be able to cope with the rich condition for a long period.

In the present invention, a purification activity is selected by selecting a material that stores more oxygen and disposing a material that supplies oxygen relatively slowly in the catalyst coat layer when the A / F is rich. It is possible to suppress HC poisoning that causes a decrease and prevent a decrease in purification activity. Here, “relatively slowly” specifically means that 0.4% or more of O ₂ (detected as CO ₂ ) can be continuously released for 30 seconds or more in the OSC ability evaluation test.

The catalyst of the present invention is a catalyst containing an OSC material capable of continuously releasing 0.4% or more of O ₂ (detected as CO ₂ ) for 30 seconds or longer in the OSC ability evaluation test in the catalyst coating layer.

Here, the OSC ability evaluation test evaluates the concentration and the level of sustained release of O ₂ stored in the OSC material when the lean condition is switched to the rich condition. It is. Specifically, the OSC material (3.0 g) is placed in an evaluation apparatus in which the following model gas is circulated, and the temperature is set to 600 ° C. at a temperature rising rate of 20 ° C./min. 1% −O ₂ / N ₂ was pretreated by exposure for 30 minutes to an atmosphere of (10L / min), CO ₂ concentration in the outlet gas at the temperature by switching to an atmosphere from the 30 minutes 600 ° C. for _{2% -CO / N 2 (10L} / min) Is a test for evaluating the OSC ability of the catalyst.

  As such an OSC material, for example, a material in which a noble metal is supported on praseodymium oxysulfate may be used.

Pr ₂ O ₂ SO ₄ , which is praseodymium oxysulfate, can be prepared by any method known to those skilled in the art. Pr ₂ O ₂ SO ₄ can be prepared, for example, by a method using a surfactant as a supply source of the sulfate group moiety. Specifically, Pr ₂ O ₂ SO ₄ is a compound containing praseodymium as a cation, for example, praseodymium nitrate, and a basic solvent for hydrating it, for example, ammonia water, and supply of a sulfate group. A surfactant as a source, for example, sodium dodecyl sulfate (SDS), is mixed at a predetermined concentration, and a precipitate obtained by mixing is washed, dried, pulverized, and calcined.

Surfactant sodium dodecyl sulfate or the like used as a source of sulfate moiety in the Pr ₂ O ₂ SO ₄ is introduced at a concentration sufficient to produce a Pr ₂ O ₂ SO ₄ with respect to the raw material of praseodymium nitrate be able to. In addition, in the method for preparing Pr ₂ O ₂ SO ₄ using such a surfactant, the surfactant acts not only as a sulfate group supply source but also as a carbon chain part as an organic template. Conceivable. Therefore, the precipitate obtained by mixing the raw materials can have a regular layer, and it can be fired to decompose and remove the organic template portion to obtain a high surface area Pr ₂ O ₂ SO _4. Can do.

The precipitate can be dried and calcined at a temperature and for a time sufficient to decompose and remove the organic template portion of the surfactant and to obtain a high surface area Pr ₂ O ₂ SO ₄ . For example, drying can be carried out at room temperature under reduced pressure or can be carried out at a temperature of 80 to 250 ° C. for 12 to 24 hours, and baking can be carried out at a temperature of 500 to 700 ° C. for 1 to 20 hours. .

As another method for preparing Pr ₂ O ₂ SO ₄ , Pr sulfate, that is, praseodymium sulfate octahydrate (Pr ₂ (SO ₄ ) ₃ .8H ₂ O) is used at a temperature of 800 ° C. or higher in air. It can also be prepared by heating and decomposing some of the sulfate groups. In this case, the temperature at which the Pr sulfate is heated may be 800 ° C. or higher, and may be set as appropriate within a range in which a part of the sulfate group is decomposed as described above.

  Examples of the noble metal supported on the OSC material include Pt, Rh, Pd, Ir, and Ru, but it is particularly preferable to support Pt. The amount of noble metal supported on the OSC material is generally 0.01 to 5% by mass, particularly 0.1 to 2% by mass, based on the OSC material.

Supporting these precious metals on Pr ₂ O ₂ SO ₄ can be performed by any method known to those skilled in the art.

For example, these noble metals are supported by using a compound containing the noble metal as a cation as a metal source and immersing Pr ₂ O ₂ SO ₄ in a solution having a predetermined concentration of the compound, followed by drying and firing, or the like. The above-mentioned noble metal complex is used as a noble metal source, Pr ₂ O ₂ SO ₄ is immersed in a solution of a predetermined concentration of this complex, and then dried, fired, and the like.

Drying and firing of Pr ₂ O ₂ SO ₄ immersed in a solution containing a compound or complex of these noble metals can be performed at a temperature and time sufficient to support the noble metal on Pr ₂ O ₂ SO _4. . For example, drying can be performed at a temperature of 80 to 250 ° C. for 12 to 24 hours, and baking can be performed at a temperature of 400 to 500 ° C. for 2 to 10 hours.

  The catalyst coat layer may further include a porous metal oxide support on which a noble metal is supported. Thereby, it is possible to improve the performance as a three-way catalyst.

  The porous metal oxide support may not have OSC ability, and may be a heat-resistant inorganic oxide such as cordierite, mullite, and alumina.

As the noble metal supported on the porous metal oxide support, the above-mentioned noble metal supported on the OSC material may be used, but it is particularly preferable to support Pt. The amount of the noble metal supported on the porous metal oxide carrier is generally 0.01 to 5% by mass, particularly 0.1 to 2% by mass, based on the porous metal oxide carrier. The noble metal can be supported on the porous metal oxide support by any method known to those skilled in the art. For example, the method of supporting the precious metal on Pr ₂ O ₂ SO ₄ as described above may be applied by changing from Pr ₂ O ₂ SO ₄ to a porous metal oxide.

  The catalyst of the present invention can also be used as a catalyst for purifying exhaust gas by coating a monolithic carrier such as a ceramic honeycomb.

  EXAMPLES Hereinafter, although an Example demonstrates this invention more concretely, this invention is not limited to these Examples.

  In this example, a catalyst containing an OSC material according to the present invention and a catalyst containing an existing OSC material (cerium-zirconium composite oxide) were prepared, and their OSC ability and NOx purification activity were examined.

[Sample preparation of Example]
The praseodymium oxysulfate Pr ₂ O ₂ SO ₄ used in the examples was prepared as follows. First, praseodymium nitrate hexahydrate (Pr (NO ₃ ) ₃ .6H ₂ O), sodium dodecyl sulfate (SDS) as a supply source of the sulfate group, ammonia, and water are respectively 1: 2: 30. Was introduced into the flask at a molar ratio of 60. Next, the flask was stirred for 1 hour at a rotational speed of 350 rpm with a stirrer in an oil bath at 40 ° C. in an open air system. Thereafter, the temperature was raised to 60 ° C. and stirred for 9 hours, and then cooled to room temperature (pH = about 11). The resulting precipitate was separated by centrifugation, washed several times with distilled water, and dried under reduced pressure at room temperature. The dried sample was pulverized into powder and calcined in a draft at a temperature of 500 ° C. or higher for 5 hours to obtain praseodymium oxysulfate Pr ₂ O ₂ SO ₄ .

In this example, platinum (Pt) was supported on the compound consisting of praseodymium oxysulfate Pr ₂ O ₂ SO ₄ prepared above to prepare Pt / Pr ₂ O ₂ SO ₄ . First, 1 mL of a platinum (Pt) nitrate solution was prepared at a concentration at which the supported amount of Pt was 0.5 wt% with respect to 3.0 g of Pr ₂ O ₂ SO ₄ . Next, about 3.0 g of the Pr ₂ O ₂ SO ₄ powder prepared above was placed in a crucible, and the crucible was placed on a hotting stirrer and set to a temperature at which the platinum (Pt) nitrate solution was dried. After the crucible was warmed, it was dried while adding a nitrate solution of platinum (Pt) little by little, and after the dropping was completed, it was left at room temperature for about 1 hour and dried overnight at 120 ° C. in a dryer. The dried sample was lightly loosened and then calcined in air at 400 ° C. for 2 hours to obtain a 0.5 wt% -Pt / Pr ₂ O ₂ SO ₄ OSC material.

Separately, a predetermined amount of Al ₂ O ₃ powder was impregnated with a predetermined amount of a rhodium nitrate aqueous solution having a predetermined concentration and baked at 250 ° C. for 1 hour to obtain 0.5 wt% -Rh / Al ₂ O ₃ .

The slurry prepared by mixing 0.5 wt% -Pt / Pr ₂ O ₂ SO ₄ prepared above and 0.5 wt% -Rh / Al ₂ O ₃ at a mass ratio of 150: 80 was prepared. A cordierite-made honeycomb substrate (φ30 mm, L50 mm, wall thickness 4 mil, 400 cells) manufactured by NGK (NGK) was used as a test piece by coating 10 g with a dry mass by a wash coat method.

[Sample preparation for comparative example]
The cerium-zirconium composite oxide used in the comparative example was prepared as follows. First, a solution prepared by dissolving 41.16 g of zirconium oxynitrate in 100 g of ion-exchanged water was added to 1193.33 g of ceria sol (15 mass% as CeO ₂ , manufactured by Taki Chemical Co., Ltd., Needle U-15) and stirred. A uniform suspension was created. This suspension was subjected to heating at 120 ° C. for 24 hours to evaporate water and then subjected to calcination at 700 ° C. for 5 hours, and the following composition (mass ratio): CeO ₂ / ZrO ₂ = 58/38 A cerium-zirconium composite metal oxide was obtained.

Next, 50 g of this composite oxide was dispersed in 300 g of ion-exchanged water to prepare a slurry. To this slurry, 11.36 g of an aqueous solution of dinitrodiammine platinum complex (Pt concentration 2.2 mass%) was added and stirred for 2 hours. did. Next, the slurry was subjected to heating at 120 ° C. for 24 hours to evaporate water, and then subjected to calcination at 500 ° C. for 2 hours to obtain an OSC of _0.5 wt% -Pt / (Ce _0.5 , Zr _0.5 ) O ₂ . Obtained material.

Similarly to the example, a predetermined amount of Al ₂ O ₃ powder was impregnated with a predetermined amount of a rhodium nitrate aqueous solution having a predetermined concentration and baked at 250 ° C. for 1 hour to obtain 0.5 wt% -Rh / Al ₂ O ₃ . It was.

The slurry prepared by mixing _0.5 wt% -Pt / (Ce _0.5 , Zr _0.5 ) O ₂ prepared above and 0.5 wt% -Rh / Al ₂ O ₃ at a mass ratio of 100: 80 was prepared. A cordierite honeycomb substrate (φ30 mm, L50 mm, wall thickness 4 mil, 400 cells) manufactured by NGK, Inc. (NGK) was used as a test piece.

[OSC ability evaluation test]
The OSC material of the example (0.5 wt% -Pt / Pr ₂ O ₂ SO ₄ ) and the OSC material of the comparative example 0.5 wt% -Pt / (Ce _0.5 , Zr _0.5 ) O ₂ ) of 3.0 g each were compressed. The OSC ability was evaluated using what was crushed and pelletized.

These OSC materials are placed in an evaluation apparatus in which the following model gas flows, and the temperature is set to 600 ° C. at a temperature rising rate of 20 ° C./min, and an atmosphere of 1% -O ₂ / N ₂ (10 L / min). after pretreated by exposure for 30 minutes, the CO ₂ concentration in the outlet gas at a temperature from 30 minutes 600 ° C. is switched to an atmosphere of _{2% -CO / N 2 (10L} / min) was measured, of the catalyst OSC Noh was evaluated.

The measurement results are shown in FIG. It has been shown that the catalyst according to the present invention can continuously release O _{2 of} 0.4% or more (measured CO ₂ concentration is converted to O ₂ concentration) for 30 seconds or more.

[Purification activity evaluation test]
Each honeycomb type exhaust gas purification catalyst (test piece) obtained was placed in an evaluation apparatus in which the following model gas circulates, and the temperature was raised to 600 ° C. at a rate of temperature increase of 20 ° C./min. After pretreatment by exposure for 30 minutes, the atmosphere was switched to a rich atmosphere, and the NOx concentration in the outlet gas at a temperature of 600 ° C. was measured. From the measurement result of the NOx concentration, the average NOx purification rate for 600 seconds from the atmosphere switching was determined by the following equation.
NOx purification rate = [(input gas concentration−output gas concentration) ÷ input gas concentration] × 100

  The composition of rich gas and lean gas used is shown in Table 1. Here, 35 ccTP (basic fuel injection amount) was adopted.

  The average NOx purification rate was 91% in the comparative example and 98% in the example.

  From the results of the OSC ability evaluation test (FIG. 1), it is clear that the catalyst (Example) according to the present invention can continuously supply oxygen from the material in a reducing atmosphere as compared with the comparative example. Accordingly, as the purification activity evaluation test result shows, the average NOx purification rate is clearly superior to the comparative example.

An OSC ability evaluation result is shown.

Claims (4)

In the OSC ability evaluation test, the catalyst coat layer includes an OSC material in which 0.4% or more of O ₂ (detected as CO ₂ ) can be continuously released for 30 seconds or more,
Here, the OSC ability evaluation test means that the OSC material (3.0 g) is placed in an evaluation apparatus in which the following model gas flows, and the temperature is raised to 600 ° C. at a temperature rising rate of 20 ° C./min. After pre-treatment by exposure to an atmosphere of -O ₂ / N ₂ (10 L / min) for 30 minutes, after switching to an atmosphere of 2% -CO / N ₂ (10 L / min), the outlet gas at a temperature of 600 ° C. for 30 minutes A catalyst which is a test for measuring the concentration of CO ₂ in the catalyst and evaluating the OSC ability of the catalyst.   The catalyst according to claim 1, wherein the OSC material is obtained by supporting noble metal on praseodymium oxysulfate.   The catalyst according to claim 2, wherein the noble metal is 0.5% by mass of Pt based on praseodymium oxysulfate.   The catalyst according to any one of claims 1 to 3, wherein the catalyst coat layer further comprises a porous metal oxide support on which a noble metal is supported.

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