JP2005288382A - Catalyst for purifying exhaust gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst for purifying exhaust gas, in which such a problem is solved that the activity of a noble metal of the catalyst having a NOx occulusion material is deteriorated. <P>SOLUTION: This catalyst for purifying exhaust gas is constituted so that basic carrier powder (1) having a NOx occulusion element is mixed with acid carrier powder (2) on which the noble metal (3) is deposited and which has the acidity higher than that of the basic carrier powder (1). In this catalyst for purifying exhaust gas, the satisfactory activity can be kept by the nobel metal (3) deposited on the acid carrier powder (2) while the ability of NOx occulusion is exhibited by the basic carrier powder (1). <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

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

The exhaust gas from an internal combustion engine such as an automobile engine contains nitrogen oxides (NO _x ), carbon monoxide (CO), hydrocarbons (HC), and the like. These substances can be released into the atmosphere after being removed by an exhaust gas purifying catalyst that oxidizes CO and HC and simultaneously reduces NO _x . As a typical exhaust gas purifying catalyst, a NO _x storage reduction catalyst in which a noble metal and a NO _x storage material are supported on a porous metal oxide carrier such as γ-alumina is known. Here, examples of the noble metal include platinum (Pt), rhodium (Rh), and palladium (Pd), and examples of the NO _x storage material include lithium, potassium, and barium.

In this use of the NO _x storage-reduction catalyst, normally by circulating oxygen excess exhaust gas (lean gas) to occlude NO _x in _the NO _x storage material, to oxidize CO and HC by the catalytic action of the noble metal. Then, the exhaust gas is intermittently controlled so that the fuel is excessive (by performing a rich spike), and the stored NO _x is reduced.

Regarding this NO _x storage reduction catalyst, in Patent Document 1, by using at least two kinds of NO _x storage materials and noble metals, the reaction between the NO _x storage material and the sulfur component in the exhaust gas generates sulfate, This solves the problem that the sulfate deteriorates (sulfur poisoning) the performance of the NO _x storage reduction catalyst. In this patent document 1, for example, an NO _x storage reduction catalyst using Ba, Li, La, Ce, and Zr and Pt or Pd is provided.

That is, as shown in FIG. 2, the conventional NO _x storage-reduction catalyst makes the entire support (1) basic with a NO _x storage material selected from the group consisting of alkali metals, alkaline earth metals, and rare earths. _x was occluded. Here, the precious metal (3) is all carried on the basic carrier (1). In this case, as shown in FIG. 3, the NO _x storage material (34) in the catalyst carrier (32) provides electrons to the noble metal (36), so that the noble metal (36) is oxidized (oxygen) particularly in a lean atmosphere. The catalyst activity may be reduced due to poisoning.

Such a decrease in the catalytic performance of the noble metal due to the NO _x storage material is shown in FIG. Here, at 250 ° C., 2,000 ppm C of decane, up to 10% oxygen and the balance of nitrogen are passed through the catalyst formed into pellets. As shown in FIG. 4, in a general NO _x storage reduction catalyst supporting NO _x storage and supporting 2 g of platinum / substrate-L, the oxidation rate of decane increases as the oxygen concentration increases. Decreases. This is because the exhaust gas-purifying catalyst that supports only 1 g of platinum / substrate-L and does not support the NO _x storage material achieves a decane oxidation rate of almost 100% in an oxygen concentration range up to 10%. Is symmetrical. Decreasing the decane oxidation rate in the NO _x storage-reduction catalyst indicates a decrease in HC oxidation activity due to oxidation of platinum, which is a catalyst, because the higher the oxygen concentration, the more the decane oxidation should be promoted. .

JP 7-51544 A

The present invention provides an exhaust gas purifying catalyst that solves the problem of reduced precious metal activity in a catalyst having a NO _x storage material.

The exhaust gas purifying catalyst of the present invention carries a basic carrier powder (1) containing a NO _x storage element and a noble metal (3), and an acidic carrier powder (2) that is more acidic than the basic carrier powder. And are mixed.

In the present description, "NO _x basic support powder comprising storage elements" or "basic support powder" has _the NO _x storage element, whereby sufficient base to occlude NO _x The carrier powder having the property is shown. The term “acidic carrier powder” indicates a carrier powder that is more acidic than the basic carrier powder.

According to the exhaust gas purifying catalyst of the present invention, since the noble metal is supported on the acidic carrier powder, the problem of a decrease in the catalytic activity of the noble metal due to the presence of the NO _x storage element does not occur. This suggests that sufficient catalytic activity can be obtained with a relatively small amount of noble metal supported. In the basic support powder, the presence of _the NO _x storage material, similarly to the conventional NO _x storage-reduction catalyst can occlude NO _x. That is, according to the exhaust gas purifying catalyst of the present invention, it is possible to maintain the catalytic activity of the noble metal while providing NO _x storage ability. This suggests that sufficient catalytic activity can be obtained with a relatively small amount of noble metal supported.

  In one aspect of the exhaust gas purifying catalyst of the present invention, both the basic carrier powder and the acidic carrier powder carry a noble metal.

According to this, occlusion of NO _x into the basic carrier powder and reduction of the occluded NO _x can be promoted by the noble metal supported on the basic carrier powder.

In another aspect of the exhaust gas purifying catalyst of the present invention, the basic carrier powder is a metal selected from the group consisting of alumina, titania, silica and ceria zirconia containing NO _x storage elements, and combinations thereof. Contains oxide powder.

In another embodiment of the exhaust gas purifying catalyst of the present invention, the basic support powder includes a metal oxide powder having the following chemical formula:
_{M a Ce b Zr (1-} ab) O x
(M: rare earth elements other than alkali metal, alkaline earth metal and / or cerium, a: 0.001 to 0.50, b: 0.001 to 0.20, and x: stoichiometric amount).

According to this, the PM (particulate matter) oxidation ability can be improved by this metal oxide powder, and therefore the exhaust gas purifying catalyst of the present invention is a 4-way (NO _x reduction, HC oxidation, CO oxidation and PM oxidation) catalyst. Can be used as

  In another aspect of the exhaust gas purifying catalyst of the present invention, the acidic carrier powder includes a metal oxide powder selected from the group consisting of alumina, titania, silica and ceria zirconia, and combinations thereof.

In another aspect of the exhaust gas purifying catalyst of the present invention, the acidic carrier powder does not contain an element selected from the group consisting of alkali metals and alkaline earth metals, or NO _x storage elements.

According to this, it is possible to prevent the noble metal supported on the acidic carrier powder from being inactivated or oxygen poisoned by an element selected from the group consisting of alkali metals and alkaline earth metals or NO _x storage elements. it can.

According to the exhaust gas purifying catalyst of the present invention, it is possible to maintain the catalytic activity of the noble metal while providing NO _x storage ability.

  The exhaust gas-purifying catalyst of the present invention can be used, for example, after being formed into a pellet form or coated on a substrate. Examples of such a substrate include cordierite honeycomb substrates and metal honeycomb substrates. The exhaust gas purifying catalyst of the present invention can be used with a binder such as a metal oxide sol.

  The exhaust gas-purifying catalyst of the present invention can be obtained by mixing a basic carrier powder and an acidic carrier powder and loading a noble metal on the mixture. The mixing ratio of these basic carrier powder and acidic carrier powder can be arbitrarily determined based on the required catalyst performance. For example, the weight ratio is 9: 1 to 1: 9, particularly 4: 6. -6: 4 can be mixed.

  In addition, the exhaust gas purifying catalyst of the present invention can carry a noble metal only on an acidic carrier powder and mix it with a basic carrier powder not carrying a noble metal.

  Furthermore, the exhaust gas purifying catalyst of the present invention can also be obtained by previously supporting a noble metal on both the basic carrier powder and the acidic carrier powder, and mixing these carriers.

The basic carrier powder that can be used in the exhaust gas purifying catalyst of the present invention comprises NO _x storage elements, that is, elements selected from the group consisting of alkali metals, alkaline earth metals, and rare earths, particularly alkali metals and alkaline earth metals. It may be any carrier that can occlude NO _x in the exhaust gas by having a mixture of an element selected from the group, more particularly an alkali metal and an alkaline earth metal such as a mixture of K, Ca and Ba.

This basic carrier powder can be obtained by any method. For example, a NO _x storage element such as lithium, calcium or barium and a carrier material such as alumina, silica, titanium, ceria zirconia and the like are co-precipitated from a solution. The obtained precipitate can be dried and calcined. It can also be obtained by impregnating a powder of a carrier material such as alumina or silica with a salt solution of an NO _x storage element, and drying and firing it. The amount of the NO _x storage element can be determined based on the NO _x storage capacity required in the obtained exhaust gas purification catalyst. For example, 0.01 mol to 1.0 mol per 100 g of the carrier to be supported. Can have.

The acidic carrier powder that can be used in the exhaust gas purifying catalyst of the present invention may be alumina, silica, titanium, ceria zirconia or the like that is generally used, and NO in a range in which the basicity does not become larger than the basic carrier powder. _x Occluded elements, especially rare earths such as cerium. Since cerium has an oxygen storage capacity, it may be preferable for the acidic carrier powder to have cerium.

  Examples of noble metals that can be used in the exhaust gas purifying catalyst of the present invention include platinum (Pt), rhodium (Rh), and palladium (Pd). These noble metals can be supported on the carrier by any method such as water absorption. The amount of the noble metal can be determined based on the catalytic activity required in the obtained exhaust gas purifying catalyst, but 0.1% to 3% by weight is supported on the basis of the weight of the support to be supported. be able to.

[Example 1 and Comparative Example 1]
Preparation of catalyst of Example 1 Alumina particles (acid carrier powder), titania particles (acid carrier powder), and lanthanum oxide-ceria-zirconia composite oxide particles (basic carrier powder) (La: 10 mol%, Ce: 10 mol%) And Zr: balance) were mixed in an equal weight ratio, and a platinum dinitrodiammine solution was used to support 1% by weight of platinum with respect to these mixtures to obtain the catalyst of Example 1.

Preparation of Catalyst of Comparative Example 1 Alumina particles, titania particles, and ceria-zirconia composite oxide particles were mixed at an equal weight ratio, and platinum 1% by weight was supported on these mixtures using a platinum dinitrodiammine solution. Further, using a nitrate solution, 0.5 mol of Li, 0.3 mol of K, and 0.3 mol of Ba were supported with respect to 100 g of the carrier to obtain a catalyst of Comparative Example 1.

Decane oxidation performance in a lean atmosphere Decan oxidation performance in a lean atmosphere was evaluated using the following conditions:
Catalyst temperature: 250 ° C
Space velocity: 50,000 / hour Lean gas composition: C ₁₀ H ₂₂ (2,000 ppmC), O ₂ (6%), CO ₂ (10%), H ₂ O (8%), NO (200 ppm), N ₂ (Remainder)

  The obtained results are shown in FIG. Here, the decane oxidation rate is obtained by dividing the difference between the decane concentration of the input gas and the decane concentration of the output gas by the decane concentration of the input gas. As understood from FIG. 5, the catalyst of Example 1 according to the present invention has a decane oxidation rate of about 90%, whereas the catalyst of Comparative Example 1 has a decane oxidation rate of about 20%. there were. This indicates that the catalyst of Example 1 according to the present invention has sufficient activity even in a lean gas atmosphere at 250 ° C.

Rich spike NO _x purification rate Lean gas composition: NO (50 ppm (150 ° C.), 200 ppm (400 ° C.)), O ₂ (6%), H ₂ O (8%), CO ₂ (8%), N ₂ (balance) )
Rich gas composition: C ₃ H ₆ (2,000 ppmC), H ₂ O (8%), CO ₂ (8%), N ₂ (balance)

The obtained result is shown in FIG. Here the rich spike _the NO _x purification rate _(RS-the NO _x purification ratio) is the ratio of the clarified NO _x for the introduced NO _x. As can be seen from FIG. 6, the catalyst of Example 1 according to the present invention has a rich spike NO _x purification rate at a relatively low temperature (150 ° C.) despite having no alkali metal and alkaline earth. Compared with Comparative Example 1, it is greatly improved. This indicates that the catalyst of Example 1 according to the present invention maintains the activity of the noble metal. At a relatively high temperature (400 ° C.), the rich spike NO _x purification rate of Example 1 is lower than that of Comparative Example 1, but the sufficient purification rate is still maintained. That is, the catalyst of Example 1 according to the present invention achieves a sufficient rich spike NO _x purification rate over a wide range from low temperature to high temperature.

[Example 2 and Comparative Example 2]
Preparation of catalyst of Example 2 Alumina particles (acid carrier powder), titania particles (acid carrier powder), and calcia-ceria-zirconia composite oxide particles (basic carrier powder) (Ca: 10 mol%, Ce: 10 mol%, And Zr: balance) were mixed in an equal weight ratio, and a platinum dinitrodiammine solution was used to carry 1% by weight of platinum with respect to these mixtures to obtain the catalyst of Example 2.

Preparation of Catalyst of Comparative Example 2 Alumina particles, titania particles and ceria-zirconia composite oxide particles were mixed at an equal weight ratio, and 1% by weight of platinum was supported on these mixtures using a platinum dinitrodiammine solution. Moreover, the catalyst of the comparative example 2 was obtained by carrying | supporting Li0.5mol, K0.3mol, and Ba0.3mol with respect to the support | carrier 100g using a nitrate solution.

Decane Oxidation Performance in Lean Atmosphere In the same manner as in Example 1 and Comparative Example 1, decane oxidation performance in a lean atmosphere was evaluated.

  The obtained results are shown in FIG. As can be seen from FIG. 7, the catalyst of Example 2 according to the present invention has a decane oxidation rate of about 90%, whereas the catalyst of Comparative Example 1 has an oxidation rate of about 20%. It was. This shows that the catalyst of Example 2 according to the present invention maintains the activity of the noble metal even in a 250 ° C. lean gas atmosphere.

Rich Spike NO _x Purification Rate In the same manner as in Example 1 and Comparative Example 1, the rich spike NO _x purification rate (%) was evaluated.

The obtained result is shown in FIG. As can be seen from FIG. 8, in the catalyst of Example 2 according to the present invention, the rich spike NO _x purification rate at a relatively low temperature (150 ° C.) is greatly improved as compared with Comparative Example 2. This indicates that the catalyst of Example 2 according to the present invention maintains the activity of the noble metal. At a relatively high temperature (400 ° C.), the rich spike NO _x purification rate of Example 2 is lower than that of Comparative Example 2, but a sufficient purification rate is still maintained. That is, the catalyst of Example 2 according to the present invention achieves a sufficient rich spike NO _x purification rate over a wide range from low temperature to high temperature.

PM oxidation performance Using a ceramic honeycomb in which PM was sufficiently deposited, PM oxidation rates were determined for the catalyst of Example 2 and the catalyst of Comparative Example 2 at 300 ° C. The obtained results are shown in FIG. As can be seen from FIG. 9, in the catalyst of Example 2 according to the present invention, the PM oxidation ability is improved by using complex oxide particles containing Ce.

It is a schematic diagram showing the exhaust gas-purifying catalyst of the present invention. It is a schematic view showing a conventional NO _{x storage-and-reduction} catalyst. It is a conceptual diagram showing a state in use of the conventional NO _x storage-reduction catalyst. It is a graph showing the decrease in decane oxidation ability of the conventional NO _x storage-reduction catalyst. It is a graph which compares the oxidation performance of the decane in the lean atmosphere of the catalyst of Example 1 and the catalyst of Comparative Example 1. 6 is a graph comparing rich spike NO _x purification rates of the catalyst of Example 1 and the catalyst of Comparative Example 1; It is a graph which compares the oxidation performance of the decane in the lean atmosphere of the catalyst of Example 2 and the catalyst of Comparative Example 2. 5 is a graph comparing rich spike NO _x purification rates of the catalyst of Example 2 and the catalyst of Comparative Example 2. FIG. 6 is a graph comparing PM oxidation performance of the catalyst of Example 2 and the catalyst of Comparative Example 2.

Explanation of symbols

1 ... basic support powder 2 ... acidic support powder 3 ... noble metal particles 32 ... catalyst carrier 34 ... NO _x storage material 36 ... noble

Claims (6)

A basic carrier powder comprising NO _x storage elements;
Carrying a noble metal, acidic carrier powder more acidic than the basic carrier powder;
A catalyst for purifying exhaust gas, in which is mixed.   The exhaust gas-purifying catalyst according to claim 1, wherein both the basic carrier powder and the acidic carrier powder carry a noble metal. The basic carrier powder comprises a metal oxide powder selected from the group consisting of alumina, titania, silica and ceria zirconia comprising NO _x storage elements, and combinations thereof. Exhaust gas purification catalyst. The exhaust gas-purifying catalyst according to any one of claims 1 to 3, wherein the basic carrier powder contains a metal oxide powder having the following chemical formula:
_{M a Ce b Zr (1-} ab) O x
(M: rare earth elements other than alkali metal, alkaline earth metal and / or cerium, a: 0.001 to 0.50, b: 0.001 to 0.20, and x: stoichiometric amount).   The exhaust gas-purifying catalyst according to any one of claims 1 to 4, wherein the acidic carrier powder includes a metal oxide powder selected from the group consisting of alumina, titania, silica and ceria zirconia, and combinations thereof.   The exhaust gas-purifying catalyst according to any one of claims 1 to 5, wherein the acidic carrier powder does not contain an element selected from the group consisting of alkali metals and alkaline earth metals.

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