JP2010227804A - Exhaust gas purifying catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust gas purifying catalyst which controls migration of rhodium(Rh) to a lower layer and sintering and exerts high NO<SB>x</SB>purifying performance over a wide temperature range including high-temperature ranges. <P>SOLUTION: The catalyst includes the first catalyst layer with platinum and/or palladium supported and the second catalyst layer in which rhodium is supported in a support containing alumina solid-dissolved with lanthanum oxide and ceria zirconia sold-dissolved with lanthanum oxide in a ratio of the lanthanum oxide to the total mass of the support of 1-7 mass%, in this order from the side of the supporting substrate, on a supporting substrate. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

The present invention relates to an exhaust gas purification catalyst suitable for purification of gas such as NO _x contained in exhaust gas.

  In recent years, it is extremely important to purify harmful exhaust substances discharged from an internal combustion engine or the like due to a demand for environmental protection.

Conventionally, a three-way catalyst is generally put into practical use as a purification catalyst for purifying harmful substances in exhaust gas discharged in an automobile equipped with an internal combustion engine. In this three-way catalyst, metals such as platinum (Pt), rhodium (Rh), zirconium (Zr), cerium (Ce) are used, and these metal components are present in a single layer. Is widely known. However, if Rh, Pt, or the like, which is a catalyst metal, coexists in the same layer, for example, Rh can easily interact with Pt to form a solid solution, and when the solution is formed, the catalytic activity inherent to each metal decreases. , purification performance for NO _x is reduced. Generally Rh is excellent in _the NO _x purification, _the NO _x purification performance when the catalyst activity is reduced of Rh is significantly reduced.

Therefore, in recent years, as a method for efficiently purifying NO _x , the three-way catalyst has a layer structure of two or more layers, or has a zone coat structure, and Rh is an outer layer having a high gas contact property (upper layer separated from the base material). Placement is widely considered.

For example, as the above three-way catalyst, a catalyst using a complex oxide mainly composed of ZrO ₂ in the upper layer as a carrier for rhodium and a ceria zirconia complex oxide as a carrier in the lower layer is known.

  In this connection, it consists of two layers: a cerium oxide-containing alumina layer supporting platinum or palladium as an inner coating layer and a lanthanum oxide-containing alumina layer supporting rhodium as an outer coating layer. The lanthanum content is converted to lanthanum oxide. In other words, an exhaust gas purification catalyst with 0.5 to 10% by weight is disclosed (see, for example, Patent Document 1).

JP-A-62-197148

However, as described above, a composite oxide mainly composed of ZrO ₂ is used for the rhodium support in the upper layer, and a ceria zirconia-based composite oxide having a high oxygen absorption / release capability is supported in the lower layer on active noble metal species such as platinum and palladium. when used on a carrier use, the catalyst is exposed to high-temperature oxidizing atmosphere of above 950 ° C., the upper layer of the rhodium likely to move to the lower layer of ceria zirconia composite oxide, NO _x purification performance of the catalyst is remarkably lowered Tend.

  The same applies to the above exhaust gas purification catalyst in which the cerium oxide-containing alumina layer supporting platinum or palladium was used as the inner coating layer, and the lanthanum oxide-containing alumina layer supporting rhodium was used as the outer coating layer. There is a problem that the rhodium supported on the upper layer moves or the sintering performance is lowered due to sintering.

The present invention has been made in view of the above, and provides an exhaust gas purification catalyst that has high NO _x purification performance over a wide temperature range including a high temperature range, in which rhodium (Rh) migration and sintering are suppressed. It aims at providing and makes it a subject to achieve this objective.

The present invention improves the purification performance of rhodium (Rh) from the surface of the carrier on which rhodium (Rh) is supported. Specifically, in particular, strongly basic lanthanum oxide is dissolved in the upper layer of Rh carrier. , when the particular concentration of lanthanum oxide in the upper layer of the carrier, easily maintained rhodium suppresses the movement of rhodium at a high temperature to the metal state, the reduction factor in the behavior in the low temperature and high temperature are different _the NO _x purification performance It has been achieved based on the knowledge that it can be reduced.

The exhaust gas purifying catalyst of the present invention includes a supporting base, a first catalyst layer provided on the supporting base and carrying platinum (Pt) and / or palladium (Pd), and the first catalyst layer. provided in the upper, comprising a lanthanum oxide _(La 2 _{O 3)} alumina solid solution _(Al 2 _{O 3),} and a lanthanum oxide ceria zirconia _(La 2 _{O 3)} is solid-solved _{(CeO 2} -ZrO 2) The support is provided with a second catalyst layer on which rhodium (Rh) is supported, and the ratio of lanthanum oxide in the second catalyst layer to the total mass of the support is 1 to 7% by mass.

  In the exhaust gas purification catalyst of the present invention, when the catalyst structure has two or more catalyst layers supporting Pt and / or Pd on the lower layer side close to the supporting base material and supporting Rh on the upper layer side, By dissolving the highly basic lanthanum oxide in the second catalyst layer carrier, the movement of Rh is suppressed and the formation of a solid solution with Pt in the lower first catalyst layer is prevented. By using a composite oxide carrier in which alumina and ceria zirconia are mixed, a carrier in which lanthanum oxide is dissolved has high heat resistance, and heat shrinkage and grain growth of ceria zirconia particles in the carrier are suppressed, and Rh Its own sintering is suppressed. This sintering suppression effect of Rh is particularly large in a system using La, and the catalytic activity of Rh can be kept higher. In addition, when lanthanum oxide is present in the lower layer, Rh in the upper layer easily takes an oxide state. However, the presence of lanthanum oxide in the upper layer supporting Rh keeps Rh in a metal state, thereby improving the catalytic activity of Rh. Enhanced.

Thus, high to maintain the catalytic activity of Rh, it is possible to remarkably improve the _the NO _x purification performance of the exhaust gas purifying catalyst. Here, when lanthanum oxide (La ₂ O ₃ ) is present in the carrier, the NO _x purification performance is improved as the proportion of La ₂ O _{3 in} the carrier increases at a high temperature range, while the NO _x emission amount is suppressed. Conversely, in the low temperature range, the NO _x purification performance decreases as the proportion of La ₂ O ₃ increases, and the NO _x emission amount tends to increase. Therefore, in the present invention, by setting the ratio of lanthanum oxide to the total mass of the carrier in the range of 1 to 7% by mass, stable and high NO _x purification performance is ensured in a wide temperature range from the low temperature range to the high temperature range. It is possible.

In the exhaust gas purification catalyst of the present invention, it is desirable that the alumina particles and ceria zirconia particles in the second catalyst layer, which is the upper layer, are at least partially mixed at the primary particle level. The Rh carrier carrying Rh in the upper layer is a composite oxide containing Al, Zr, Ce, and La, and this powder structure is composed of La ₂ O ₃ -added Al ₂ O ₃ particles and La ₂ O ₃ -added CeO ₂ -ZrO _2. By including particles at the primary particle level, the effect of suppressing thermal shrinkage and grain growth of CeO ₂ —ZrO ₂ particles is high, and the effect of suppressing Rh sintering is increased.

In the present invention, when the exhaust gas atmosphere is a rich atmosphere with excessive reducing components such as stoichiometric to hydrogen, it is excellent in exhaust gas purification performance by increasing the catalytic activity of Rh, that is, the reduction activity of NO _x and reducing it with high efficiency. Purification system can be constructed.

According to the present invention, is suppressed movement and sintering to the underlying rhodium (Rh) is, it is possible to provide an exhaust gas purifying catalyst having a high _the NO _x purification performance over a wide temperature range including high temperature range.

The La ₂ O ₃ is a graph showing the difference in the change in accompanied specific surface area in the heat treatment temperature in the case of physical mixing when solid solution in _Al 2 _{O 3} and _La 2 _{O 3} and _Al 2 _{O 3.} It is a graph showing the relationship between the existing ratio of La ₂ O ₃ the upper and lower layers of the concentration and Rh in Rh supporting carrier. Rh La ₂ O ₃ concentration in the particles for supports in the carrier is a graph showing the change of the NO _x emissions on the high temperature region and low temperature region. It is a graph showing the relationship between the _La 2 _{O 3} concentration and COP purification rate in the Rh supporting carrier.

Hereinafter, the exhaust gas purification catalyst of the present invention will be described in detail.
The exhaust gas purifying catalyst of the present invention includes a first catalyst layer in which Pt (platinum) or Pd (palladium) or Pt and Pd are supported on a support base, and Al ₂ in which La ₂ O ₃ is dissolved. Rh (rhodium) is supported on a composite oxide support including CeO ₂ —ZrO _{2 in} which O ₃ and La ₂ O ₃ are dissolved, and the ratio of La ₂ O _{3 to} the total mass of the composite oxide support is 1 to 7 The second catalyst layer having a mass% is provided in order from the support base material side.

The first catalyst layer in the exhaust gas purifying catalyst of the present invention contains at least Pt or Pd, or Pt and Pd (preferably supported on a carrier), and is provided between a second catalyst layer described later and the supporting substrate. Are provided as a lower layer of the second catalyst layer. In addition to Pt and Pd, the first catalyst layer can further contain a NO _x storage material such as an alkali metal such as Li, K, Na, Mg, Ca, St, or Ba, or an alkaline earth metal.

Pt is excellent in NO oxidation activity and oxidizes NO in a lean atmosphere to generate NO _x . The concentration of Pt in the first catalyst layer is preferably in the range of 0.1 to 1.5 g / L from the viewpoint of NO oxidation efficiency.

Pd has high NO _x reduction activity, but has NO oxidation activity. Pd has a high reducing activity of the NO _x, reduction efficiency of the NO _x occluded in the lean atmosphere is enhanced. In addition, since Pd is highly stable in a lean atmosphere, the presence of Pd together with Pt can suppress the growth of Pt grains and keep the NO oxidation activity of Pt high.

Pt and Pd can be supported on a desired carrier and contained in the layer. As the carrier here, particles of oxide such as zirconium dioxide (ZrO ₂ ), aluminum oxide (Al ₂ O ₃ ), silica, silica-alumina, ceria (CeO ₂ ), zeolite, and mixed particles thereof are used. be able to.

When Pt and Pd are contained in the layer, Pt-supported oxide particles supporting Pt and Pd-supported oxide particles supporting Pd may be mixed, or both Pt and Pd are supported. Oxide particles may be used. The Pt-supported oxide is, for example, a mixture of an oxide powder or granular material such as ZrO ₂ powder and a diammineplatinum platinum solution, a platinum chloride solution, an ammineplatinum solution, and the like, and is dried and fired (for example, 400 to For example, about 800 ° C.). The oxide particles supporting both Pt and Pd are, for example, oxide powder such as ZrO ₂ powder and granular materials, diammine platinum nitrate solution, platinum chloride solution, ammine platinum solution, and palladium nitrate. It can be obtained by mixing a solution, a palladium chloride solution and the like, drying, firing (for example, about 400 to 800 ° C.), and the like.

The first catalyst layer is, for example, a powdery or granular material of Pt-supported oxide or Pd-supported oxide or oxide particles supporting Pt and Pd in a medium such as water. (And other components as necessary) can be added and stirred to form a slurry, and the resulting slurry can be coated on a desired support substrate and fired to form.
What is necessary is just to select suitably about a coating method, baking conditions, etc. by a composition, a scale, etc.

The second catalyst layer in the exhaust gas purifying catalyst of the present invention, the composite oxide support and the carrier to _{which La} 2 _{O 3 Al} 2 _{O 3} in a solid solution and _La 2 _{O 3} contains _CeO 2 -ZrO ₂ solid-solved And is provided as an upper layer of the first catalyst layer. In addition to Rh, the second catalyst layer can further contain palladium (Pd) and a NO _x storage material. For the NO _x storage material, alkali metals and alkaline earth metals that can be used in the first catalyst layer are suitable.

Rh is excellent in reducing activity of NO _x, with separately the presence of Rh and a first catalyst layer containing Pt, solid solution of La ₂ O ₃ to a carrier for Rh supported in the second catalyst layer By making it, the movement to the lower layer of Rh is suppressed and solid solution formation with Pt is suppressed. By including La ₂ O ₃ in solid solution with Al ₂ O ₃ particles or CeO ₂ —ZrO ₂ particles, as shown in FIG. 1, the case of solid solution is simply Al ₂ O ₃ particles and La. Compared with the case where ₂ O ₃ particles are physically mixed, the specific surface area (SSA) of the carrier can be improved, and as a result, the effect of suppressing sintering of the noble metal can be enhanced.
Thus, favorably reducing NO _x under a rich atmosphere, to purify NO _x efficiently.

The content of La ₂ O ₃ in the carrier of the second catalyst layer is in the range of 1% by mass to 7% by mass with respect to the total mass of the carrier in the second catalyst layer. When the content of La ₂ O ₃ is less than 1% by mass, the movement of Rh to the lower layer (first catalyst layer) cannot be sufficiently suppressed (see FIG. 2), and particularly in a high temperature range (for example, 550 ° C. As described above, the NO _x purification ability decreases, and the NO _x emission amount increases (see FIG. 3). Further, when the content of La ₂ O ₃ exceeds 7% by mass, NO _x emission is suppressed in a high temperature range (for example, 550 ° C. or higher), but in a relatively low temperature range (for example, 400 ° C. or lower), In addition, as the La ₂ O ₃ concentration increases, the NO _x purification capacity decreases and the NO _x emission increases (see FIG. 3).
Among the above, from the viewpoint of effectively suppressing the movement of Rh and increasing the purification rate at the point (crossover point) where the HC purification rate curve and the NO _x purification rate curve intersect, it is based on the total mass of La ₂ O ₃ carrier. The ratio is preferably in the range of 1 to 4% by mass (more preferably 1 to 3% by mass). In addition to this, from the viewpoint of further enhancing the effect of suppressing the movement of Rh, a range of 2 to 3% by mass is preferable.

Rh is carried on the composite oxide support, the composite oxide support, and _Al 2 _{O 3 particles La} 2 _{O 3} in a solid solution state, and a _CeO 2 -ZrO _{2 particles La} 2 _{O 3} in a solid solution state At least. Al The ₂ particle size of O ₃ and _CeO 2 -ZrO _2, a volume mean diameter 5~10nm of _Al 2 _{O 3 particles,} if the volume average diameter of the CeO 2 -ZrO ₂ particles are 5~10nm preferable. The volume average diameter in the present invention is a value measured by X-ray diffraction method and TEM observation.
The composite oxide carrier carrying Rh can be obtained by mixing the composite oxide carrier with a rhodium nitrate solution, a rhodium chloride solution, etc., drying, firing (for example, about 400 to 800 ° C.), and the like.

The concentration of Rh in the second catalyst layer is preferably in the range of 0.15 g / L to 0.4 g / L, and more preferably in the range of 0.15 g / L to 0.30 g / L. When the concentration of Rh is within the above range, the NO _x purification performance is good.

The second catalyst layer, from the viewpoint of enhancing the reduction efficiency of the oxidation efficiency and NO _x NO, the form in which Pd is contained together with Rh is preferred. When Pd is contained, the concentration of Pd is preferably in the range of 100 to 1000 g / L with respect to the amount of Rh supported of 100 g / L from the same viewpoint as described above.

The second catalyst layer can be formed as follows, for example.
First, a nitric acid system in which an aluminum nitrate solution, a lanthanum nitrate solution, zirconium nitrate oxide (zirconyl nitrate dihydrate), and a cerium nitrate solution are mixed so that the ratio of Al, La, Zr, and Ce is a desired molar ratio. A precursor solution is prepared and added dropwise to an alkaline solution such as aqueous ammonia, sodium carbonate, or sodium bicarbonate. After the dropwise addition, the mixture was fired the precipitate formed to obtain a complex oxide containing _CeO 2 -ZrO _{2 particles La} 2 _{Al O 3} in a solid solution 2 _{O 3} particles and _La 2 _{O 3} in a solid solution state. The Al ₂ O ₃ particles and CeO ₂ —ZrO ₂ particles (and other components as necessary) in which La ₂ O ₃ is dissolved are added to a medium such as water and stirred to obtain a solution such as a slurry. The obtained slurry or the like can be coated on the first catalyst layer and baked.
What is necessary is just to select suitably about a coating method, baking conditions, etc. by a composition, a scale, etc.

As the content level of the NO _x storage material in the whole catalyst layer containing the first and second catalyst layer is preferably 0.01 to 5 mols per liter of the supporting substrate, 0.1 to 0.5 A molar range is more preferred. When the loading amount of the NO _x storage material is 0.01 mol / L or more, the NO _x purification ability becomes better, and when it is 5 mol / L or less, the reduction activity of Rh can be maintained without loss.

-Support substrate-
The supporting base material supports the first catalyst layer and the second catalyst layer, and a known material made of carbon or metal can be selected depending on the purpose or the case. Specific examples include a porous carbon support, a monolith support (honeycomb support), a metal support, and the like.

The first catalyst layer and the second catalyst layer are each provided with only one layer, and also provided with two or more first catalyst layers and / or second catalyst layers. It may be configured as follows. In the case where two or more second catalyst layers are provided, a carrier containing Al ₂ O _{3 in} which La ₂ O ₃ is dissolved and CeO ₂ —ZrO ₂ in which La ₂ O ₃ is dissolved in at least one layer is provided. It is only necessary that rhodium is supported and the ratio of La ₂ O _{3 to} the total mass of the carrier satisfies the above range.

  In addition, the exhaust gas purification catalyst of the present invention may be composed of three or more layers by providing another layer together with the first catalyst layer and the second catalyst layer.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples unless it exceeds the gist thereof. Unless otherwise specified, “part” is based on mass.

(Preparation of lower layer composite oxide)
0.3 mol / L zirconium nitrate, 0.3 mol / L cerium nitrate solution, 0.3 mol / L lanthanum nitrate solution, and 0.3 mol / L yttrium nitrate solution are oxidized A nitric acid-based precursor solution mixed so as to have a quantitative ratio of ZrO ₂ / CeO ₂ / La ₂ O ₃ / Y ₂ O ₃ = 60/30/5/5 was prepared, and this was dropped into aqueous ammonia. After completion of the dropwise addition, the resulting precipitate was baked at 850 ° C. to obtain a Zr, Ce, La, Y composite oxide.

(Preparation of upper layer composite oxide)
0.3 mol / L aluminum nitrate solution, 0.3 mol / L lanthanum nitrate solution, 0.3 mol / L zirconium nitrate oxide (zirconyl nitrate dihydrate), 0.3 mol / L A nitric acid-based precursor solution mixed with a cerium nitrate solution was prepared and added dropwise to aqueous ammonia. After completion of the dropping, the generated precipitate was baked at 900 ° C. to obtain a composite oxide containing Al ₂ O ₃ particles and CeO ₂ —ZrO ₂ particles. At this time, the _Al 2 _{O 3} particles dissolved is _La 2 _{O 3,} also CeO 2 -ZrO ₂ particles _La 2 _{O 3} in solid solution.
In the preparation of the nitric acid precursor solution, the ratio of La ₂ O ₃ to the total mass of the support (total mass of Al ₂ O ₃ , La ₂ O ₃ , CeO ₂ , and ZrO ₂ ) is 0% by mass and 1% by mass. , 1.5% by mass, 2% by mass, 2.5% by mass, 3% by mass, 5% by mass, 7% by mass, and 10% by mass, and the ratio of Al / Zr / Ce is 2 / 1/5 was performed.

It was confirmed by X-ray diffractometry and TEM observation that the obtained composite oxide was mixed with Al ₂ O ₃ particles and CeO ₂ —ZrO ₂ particles at the primary particle level.

(Formation of lower catalyst coat layer)
The Zr, Ce, La, Y composite oxide obtained above was mixed with 0.5% diammineplatinum dinitrate solution, dried and fired at 500 ° C., and Pt 0.75 g / L was supported. Pt-supported oxide particle powder A was produced.

120 g / L of this Pt-supported oxide particle powder A, 40 g / L of commercially available γ-Al ₂ O ₃ powder, and 160 g / L of ion-exchanged water were mixed to form a slurry, and the resulting slurry was 875 ml of cordier. A light monolith substrate was wash coated. Thereafter, drying and firing were performed to form a lower catalyst coat layer (first catalyst layer). The amount of Pt supported was 0.75 g / L.

(Formation of upper catalyst coat layer)
Next, 0.1% rhodium nitrate solution is mixed with the composite oxide containing Al ₂ O ₃ and CeO ₂ —ZrO _{2 in} which La ₂ O ₃ is dissolved as described above, dried, and dried at 500 ° C. Firing was performed to prepare Rh-supported oxide particle powder B on which Rh 0.15 g / L was supported.

The Rh-supported oxide particle powder B was mixed with 60 g / L, commercially available γ-Al ₂ O ₃ powder 25 g / L, and ion-exchanged water 85 g / L to obtain a slurry. Wash coating was performed on the catalyst coat layer, followed by drying and firing to form an upper catalyst coat layer (second catalyst layer). The amount of Rh supported was 0.15 g / L.

Thereafter, the monolith substrate on which the lower catalyst coat layer and the upper catalyst coat layer were formed was immersed in a mixed solution containing barium acetate and potassium acetate, and the excess solution was blown off, followed by drying at 120 ° C. . Then, it baked at 500 degreeC and produced the exhaust gas purification catalyst.
At this time, in the exhaust gas purification catalyst, the ratio of La ₂ O ₃ in the upper catalyst coat layer is 0 mass%, 1 mass%, 1.5 mass%, 2 mass%, 2.5 mass%, 3 mass%, 5 Nine types of mass%, 7 mass%, and 10 mass% were produced.

(Evaluation)
The exhaust gas purification catalyst obtained above was evaluated as follows.
-1. Interlayer movement of Rh
Each of the obtained exhaust gas purification catalysts was mounted on an engine bench, and an endurance test was conducted in which exhaust gas in a lean and rich atmosphere was flown at a catalyst bed temperature of 1000 ° C. for 50 hours. About each purification catalyst after an endurance test, the distribution state of Rh in the lower catalyst coat layer and the upper catalyst coat layer is obtained with an X-ray microanalyzer (EPMA; Electron Probe Micro Analyzer), and the graph shown in FIG. 2 is created. Rh interlayer movement was evaluated.

As shown in FIG. 2, La ₂ O ₃ was contained in the support in the upper catalyst coat layer, and the interlayer movement of Rh to the lower layer was suppressed as the content concentration was increased.

-2. Unreduced NO _x amount-
Each of the obtained exhaust gas purification catalysts was mounted on an engine bench, and repeated operation was performed with a steady operation under a lean atmosphere for 120 seconds and a rich treatment under a rich atmosphere for 120 seconds. The amount of NO _x was measured. The measurement results are shown in FIG.

As shown in FIG. 3, in the example in which the La ₂ O ₃ concentration in the carrier for supporting Rh is 1 to 7% by mass, the temperature environment at both 550 ° C. and 400 ° C. is compared with the comparative example outside this range. Below, the amount of unreduced NO _x emissions was reduced. That is, as shown in FIG. 3, the behavior of the NO _x purification ability when the La ₂ O ₃ concentration is increased at high and low temperatures is reversed, but the La ₂ O ₃ concentration is 1 to 7% by mass. It was confirmed that the NO _x purification performance can be kept high in both the high temperature region and the low temperature region within the range.

-3. COP purification rate
Each of the obtained exhaust gas purification catalysts was mounted on an engine bench, evaluated at 400 ° C. constantly while repeating the operation in a lean and rich atmosphere at 1 Hz, and continuously measured the HC purification rate and NO _x purification rate during that time. did. Then, to determine the purification rate of the point of intersection and the HC purification rate curve and _the NO _x purification rate curve (crossover point). The result is shown in FIG. 4 as the COP purification rate.

As shown in FIG. 4, a high value was obtained in the range of La ₂ O ₃ concentration (1 to 7% by mass) where high NO _x purification performance was obtained, and the maximum was obtained particularly in the range of 1 to 3% by mass. .

Claims (2)

On the support substrate, in order from the support substrate side,
A first catalyst layer on which at least one of platinum and palladium is supported;
A second catalyst layer in which rhodium is supported on a support containing alumina in which lanthanum oxide is dissolved and ceria zirconia in which lanthanum oxide is dissolved, and the ratio of lanthanum oxide to the total mass of the support is 1 to 7% by mass;
An exhaust gas purifying catalyst.   The exhaust gas purifying catalyst according to claim 1, wherein at least a part of the alumina and the ceria zirconia are primary particles.

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