JP2007330879A - Catalyst for cleaning exhaust gas - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst for cleaning exhaust gas, in which nickel is not used but which can reduce the amount of hydrogen sulfide to be discharged while keeping its exhaust gas cleaning performance. <P>SOLUTION: In a catalyst-coated layer formed on a base material, the density of a noble metal to be deposited per 1 liter base material in the upstream side part of exhaust gas is made higher than that in the downstream side part of exhaust gas, and the amount of an oxygen storage/release material to be incorporated per 1 liter base material in the downstream side part of exhaust gas is made larger than that in the upstream side part of exhaust gas. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an exhaust gas purification catalyst, and more particularly to an exhaust gas purification catalyst that can reduce the emission of hydrogen sulfide.

Automobile fuels such as gasoline contain a sulfur component S, and this sulfur component becomes sulfur dioxide SO ₂ with combustion. This sulfur dioxide SO ₂ may react with hydrogen H ₂ by a catalytic reaction to generate hydrogen sulfide H ₂ S when the exhaust gas purifying catalyst is in a reducing atmosphere. Since this hydrogen sulfide may cause off-flavors, it is desired to suppress the generation of hydrogen sulfide.

  As a solution for suppressing the generation of hydrogen sulfide proposed so far, a means of adding nickel to the exhaust gas purifying catalyst has been generally used (Japanese Patent Laid-Open No. 63-310637, Japanese Patent Laid-Open No. 242149 and JP-A-8-290063 and Catalysis Today, 9, (1991) 105-112).

  However, in recent years, several countries including Europe have designated nickel and nickel compounds as environmentally hazardous substances, and these cannot be used as catalysts.

JP-A-63-161037 JP-A-1-242149 JP-A-8-290063 Catalysis Today, 9, (1991) 105-112

  Accordingly, an object of the present invention is to provide an exhaust gas purifying catalyst that reduces the amount of hydrogen sulfide emission while maintaining the exhaust gas purifying performance without using nickel.

In the mechanism in which the exhaust gas purifying catalyst generates hydrogen sulfide in a reducing atmosphere, platinum (catalyst component) and ceria (oxygen storage and release material) in the exhaust gas purifying catalyst are converted from SO ₂ to H ₂ S by the mechanism shown in FIG. May be generated. As a result of the study of the above mechanism by the present inventor, nickel was used by separating the noble metal as a catalyst component contributing to the adsorption / desorption reaction of the sulfur component and the oxygen storage / release material as much as possible on the substrate. It has been found that hydrogen sulfide emissions can be reduced while maintaining exhaust gas purification performance. Furthermore, it has also been clarified that the amount of hydrogen sulfide discharged is further reduced by reducing the specific surface area of the oxygen storage / release material in the exhaust gas purification catalyst and suppressing the adsorption of sulfur components to the oxygen storage / release material.

  Thus, according to the present invention, in the catalyst coat layer on the base material, the loading density of the precious metal contained in the upstream portion of the exhaust gas per 1 L of the base material is larger than the downstream portion of the exhaust gas, and the downstream portion is the upstream portion. An exhaust gas purifying catalyst containing a larger amount of oxygen storage / release material per liter of base material.

  According to the exhaust gas purifying catalyst of the present invention, the noble metal and the oxygen storage / release material, which serve as a window for adsorption / desorption of sulfur components in the exhaust gas purifying catalyst, are separated as much as possible to arrange the conventional exhaust gas purifying catalyst. As a result, the amount of hydrogen sulfide discharged can be reduced while maintaining the same exhaust gas purification performance. Among precious metals, platinum is particularly effective in reducing hydrogen sulfide emissions by separation from the oxygen storage / release material. Such an effect of reducing the amount of hydrogen sulfide emission is further improved by suppressing the adsorption of sulfur to the oxygen storage / release material by reducing the specific surface area of the oxygen storage / release material. The present invention is desirable in comparison with conventional exhaust gas purification catalysts in that the use of nickel having a high environmental load can be avoided while maintaining the exhaust gas purification performance.

  The “substrate” used in the exhaust gas purifying catalyst of the present invention carries a slurry-like coating solution containing a catalyst component and an oxygen storage / release material, and is generally used in the manufacture of exhaust gas purifying catalysts. Means a carrier. As the base material, a porous material such as a honeycomb formed of a material such as cordierite is preferable from the viewpoint of dispersing and supporting the catalyst component.

  In the present invention, a catalyst coat layer containing a catalyst component and an oxygen storage / release material is formed on the substrate. Here, the catalyst coat layer in the present invention is divided into an exhaust gas upstream portion and an exhaust gas downstream portion in order to separate and arrange the noble metal and the oxygen storage / release material as much as possible. Moreover, as an aspect of a catalyst coat layer, what does not contain nickel is preferable. In the present specification, the “exhaust gas upstream portion” refers to the inlet gas side of the catalyst coat layer on the substrate, while the “exhaust gas downstream portion” refers to the outlet gas side of the catalyst coat layer on the substrate. . Although the length of the exhaust gas upstream portion is not limited, it is preferably adjusted so that the ratio is about 10 to 75% with respect to the total length of the base material. Most preferably, the upstream length is about 50% of the total length of the substrate from the viewpoint of improving the purification performance.

The catalyst coat layer contains one or more kinds of noble metals as a catalyst component. Here, in order to separate the noble metal as much as possible from the oxygen storage / release material disposed in the downstream portion, the precious metal is disposed in the upstream portion in comparison with the downstream portion. The noble metal used in the upstream portion is preferably platinum or palladium, and platinum is particularly preferable among these. Note that rhodium is not preferable if it is arranged upstream due to its high hydrogenation action, because it may reduce sulfur adhering to the entire catalyst and generate a large amount of hydrogen sulfide. The noble metal used in the downstream portion is preferably rhodium, palladium and / or platinum. However, among these noble metals, platinum has a greater effect of reducing hydrogen sulfide emissions by separation from the oxygen storage / release material than other noble metals, so the noble metal contained in the downstream portion together with the oxygen storage / release material is rhodium or More preferably, it is palladium. However, when platinum is included in the catalyst coat layer in the downstream portion, the same H ₂ S production suppressing effect as palladium or rhodium can be achieved if the supporting density per liter of the substrate is low. In fact, the platinum supported on the downstream portion can suppress the generation of H ₂ S more than the conventional exhaust gas purifying catalyst by suppressing the supporting density per 1 L of the base material to 0.3 g or less. In addition, rhodium exhibits excellent exhaust gas purification performance in combination with platinum, and therefore is particularly preferable as a noble metal contained in the downstream portion when platinum is disposed in the upstream portion. As described above, rhodium may generate a large amount of hydrogen sulfide when arranged in the upstream portion, but the possibility can be minimized by arranging it in the downstream. The upstream and downstream catalyst components are not limited to those described above, and may include other noble metal elements. However, nickel is excluded.

  The noble metal supporting density (per 1 L of the base material) as a catalyst component contained in the upstream catalyst coat layer is not particularly limited as long as it is higher than that of the downstream portion. For example, when rhodium, palladium and / or platinum is used as the catalyst component in the downstream portion, rhodium in the downstream portion is preferably 0.1 to 1 g, more preferably 0. 2 to 0.6 g, most preferably 0.4 g, palladium is preferably 0.1 to 10 g, more preferably 0.1 to 2.0 g, most preferably 1.0 g, platinum as the support density per liter of the base material. Is contained in an amount of not more than 0.3 g, more preferably not more than 0.1 g per 1 L of the base material, whereas the noble metal loading density in the upstream portion is appropriately adjusted so as not to fall below the noble metal loading density in the downstream portion. . In the case of using platinum in the upstream portion and rhodium in the downstream portion, the ratio of the platinum loading density in the upstream portion to the rhodium loading density in the downstream portion is preferably 1: 1 to 10: 1, and more than 5: 1. It is particularly preferred. In addition, the noble metal carrying density in the said downstream part should not be interpreted limitedly. In addition, since the catalytic activity differs depending on the type of the noble metal, depending on the noble metal used, the catalyst is such that the upstream portion has a higher catalytic activity than the downstream portion in consideration of not only the loading density but also the overall catalytic activity of each layer. It may be necessary to consider the placement of the components.

  The oxygen storage / release material contained in the catalyst coat layer, when used in the present specification, has the ability to occlude oxygen under an oxidizing atmosphere and release the oxygen occluded under a reducing atmosphere (oxygen storage / release capacity (OSC)). ) And serves as a carrier for supporting the catalyst component. However, the carrier used for supporting the catalyst component in the present invention is not limited to the oxygen storage / release material, and for example, alumina, zirconia, and composite oxides thereof may be used together.

Examples of oxygen storage / release materials include ceria (also referred to herein as cerium oxide), rare earth metal oxides such as Pr ₆ O ₁₁ , transition metal oxides such as Fe ₂ O ₃ , CuO, and Mn ₂ O ₅ , Ce -Zr composite oxide can be mentioned. From the viewpoint of oxygen storage / release capability, the oxygen storage / release material in the present invention is preferably ceria or Ce-Zr composite oxide.

By reducing the specific surface area of the oxygen storage / release material, it is possible to suppress the sulfur component from adsorbing on the surface of the oxygen storage / release material, and thus to the hydrogen sulfide under a reducing atmosphere. The reaction can be suppressed. The specific surface area can be reduced, for example, by baking an oxygen storage / release material having a predetermined specific surface area at a high temperature. In the present invention, it is preferable to use an oxygen storage / release material having a specific surface area of 30 m ² / g or less from the viewpoint of improving hydrogen sulfide purification performance. More preferably, the specific surface area is 10 m ² / g or less. Here, the “specific surface area” used in the present specification represents the BET specific surface area of the catalyst per 1 g of the catalyst, and is measured by a specific surface area measuring device (Micro Data 4232 manufactured by Kawachu Corporation).

In order to separate as much as possible the noble metal mainly contained in the upstream part in the catalyst coat layer, the oxygen storage / release material is arranged more in the downstream part than in the upstream part. For example, when ceria is used as the oxygen storage / release material and the amount of cerium in the downstream portion is 0.6 mol, the amount of cerium in the upstream portion per 1 L of the substrate may be 0.1 mol or less. However, if importance is placed on the suppression of H ₂ S generation, the upstream catalyst coat layer preferably does not contain an oxygen storage / release material.

The catalyst coat layer is not limited to the catalyst component and the oxygen storage / release material, and includes any material that is considered to be supported on the base material in constituting the catalyst. For example, the catalyst coat layer may include an oxide such as neodymium, iron, praseodymium, strontium, and barium that is an effective material for suppressing H ₂ S emission. These substances have a property of occluding sulfur, and as a result, H ₂ S production can be suppressed. By including the substance in the catalyst coat layer, the H ₂ S purification performance of the present invention is further improved. Although the results are not shown, iron oxide is particularly preferable among these substances because of its excellent effect of reducing H ₂ S emission. Note that these H ₂ S emissions material is coated on a substrate by including in the slurry-like coating solution together with the catalyst components and the like. A preferable amount of the H ₂ S emission suppressing substance is 0.2 to 0.7 mol as a loading density per 1 L of the substrate, but is not limited thereto. The H ₂ S emission suppressing substance may be disposed in the upstream portion, the downstream portion, or both of the catalyst coat layer.

  In the catalyst of the present invention, for example, the substrate is immersed in a slurry-like coating solution containing a catalyst component, an oxygen storage / release material, a carrier and the like, the coating solution is adsorbed on the substrate surface, dried, and fired repeatedly. Can be manufactured. However, it is not limited to these methods. For example, after the catalyst components are previously supported on the support, a slurry containing them may be coated on the substrate.

  The invention of the present invention will be described more specifically with reference to the following examples. The present invention is not limited to these examples.

Example 1
A slurry 1 was prepared by mixing 45 g of alumina and 50 g of alumina sol (10 wt%). Separately, a solution obtained by dissolving rhodium in nitric acid (hereinafter referred to as a rhodium nitrate solution: equivalent to 0.2 g of rhodium), 45 g of alumina, 52 g of ceria having a specific surface area of about 100 m ² / g, 50 g of alumina sol (10 wt%), pure 50 g of water was mixed to prepare slurry 2.

  The slurry 1 was coated from the upstream of a monolith honeycomb carrier having a volume of 1 L (diameter 114 mm × length 98 mm) to a length of 49 mm, dried at 150 ° C. for 1 hour, and then fired at 500 ° C. for 1 hour. Subsequently, the coated part of the carrier (carrier length 1/2) is a solution in which platinum is dissolved in nitric acid (hereinafter referred to as platinum nitrate solution: 1.0 g of platinum per liquid amount that the carrier can absorb water). In this case, 1.0 g of platinum was supported per 0.5 L of the carrier.

Next, the slurry 2 was coated to 1/2 length from the downstream side of the carrier, dried at 150 ° C. for 1 hour, and then calcined at 500 ° C. for 1 hour to prepare the catalyst of the present invention (Example 1). . The final composition of the catalyst of Example 1 is as follows:
Upstream part: platinum 2.0 g / L, alumina 100 g / L
Downstream part: rhodium 0.4 g / L, alumina 100 g / L, ceria 104 g / L

(Example 2)
Nitric acid-based platinum solution (including platinum equivalent of 1.0 g), alumina 35 g, cerium oxide stabilized zirconia 50 g (cerium: zirconium (molar ratio) = 10: 90) having a specific surface area of about 25 m ² / g, alumina sol (10 wt% ) 50 g and 20 g of pure water were mixed to prepare slurry 3. Separately, a rhodium nitrate solution (including rhodium 0.2 g equivalent), 35 g of alumina, 60 g of cerium oxide-stabilized zirconia having a specific surface area of about 25 m ² / g (cerium: zirconium (molar ratio) = 70: 30), alumina sol (10 wt. %) 50 g and 40 g of pure water were mixed to prepare slurry 4.

The slurry 3 was coated from the upstream side to 1/2 the length of a monolith honeycomb carrier having a volume of 1 L, dried at 150 ° C. for 1 hour, and then fired at 500 ° C. for 1 hour. Next, the slurry 4 was coated to a length ½ from the downstream side of the carrier, dried at 150 ° C. for 1 hour, and then calcined at 500 ° C. for 1 hour to prepare the catalyst of the present invention (Example 2). . The final composition of the catalyst of Example 2 is as follows:
Upstream part: platinum 2.0 g / L, alumina 80 g / L, ceria 14 g / L, zirconia 86 g / L
Downstream part: rhodium 0.4 g / L, alumina 80 g / L, ceria 92 g / L, zirconia 28 g / L

(Example 3)
A slurry 5 was prepared by mixing a nitrate-based platinum solution (including platinum equivalent to 1.0 g), 35 g of alumina, 50 g of zirconia, 50 g of alumina sol (10 wt%), and 20 g of pure water. Separately, rhodium nitrate solution (including rhodium 0.2 g equivalent), alumina 35 g, cerium oxide stabilized zirconia 67.5 g (cerium: zirconium (molar ratio) = 70: 30), alumina sol (10 wt%) 50 g, pure water 50 g Were mixed to prepare slurry 6.

The slurry 5 was coated from the upstream side to 1/2 the length of a monolith honeycomb carrier having a volume of 1 L, dried at 150 ° C. for 1 hour, and then fired at 500 ° C. for 1 hour. Next, the slurry 6 was coated to a length ½ from the downstream side of the carrier, dried at 150 ° C. for 1 hour, and then calcined at 500 ° C. for 1 hour to prepare a catalyst of the present invention (Example 3). . The final composition of the catalyst of Example 3 is as follows:
Upstream part: platinum 2.0 g / L, alumina 80 g / L, zirconia 100 g / L
Downstream part: rhodium 0.4 g / L, alumina 80 g / L, ceria 103 g / L, zirconia 32 g / L

Example 4
In the same manner as in Example 1, the slurry 1 was used for coating, and then the upstream part was prepared by dipping in a platinum nitrate solution. Rhodium nitrate solution (containing corresponding rhodium 0.2 g), alumina 45 g, about 10 m ² / g of specific surface area of ceria 52 g, iron oxide (Fe ₂ O ₃₎ 24g, alumina sol (10 wt%) 50 g, mixture of pure water 60g Then, slurry 7 was prepared, and coated with slurry 7 to a length ½ from the downstream side of the carrier, dried at 150 ° C. for 1 hour, and then calcined at 500 ° C. for 1 hour to prepare the catalyst of the present invention. (Example 4). The final composition of the catalyst of Example 4 is as follows:
Upstream part: platinum 2.0 g / L, alumina 100 g / L
Downstream part: rhodium 0.4 g / L, alumina 100 g / L, ceria 104 g / L, iron oxide 48 g / L

(Comparative Example 1)
Nitric acid-based platinum solution (including platinum equivalent to 1.0 g), rhodium nitrate solution (including rhodium equivalent to 0.2 g), alumina 90 g, ceria 52 g with a specific surface area of about 100 m ² / g, alumina sol (10 wt%) 100 g, pure 30 g of water was mixed to prepare slurry 8. A monolith honeycomb carrier having a volume of 1 L is coated with the slurry, dried at 150 ° C. for 1 hour, and then fired at 500 ° C. for 1 hour. From the catalyst coating layer that does not contain nickel and is not divided into an upstream portion and a downstream portion A catalyst was prepared (Comparative Example 1). The final composition of the catalyst of Comparative Example 1 is as follows:
Platinum 1.0g / L, Rhodium 0.2g / L, Alumina 100g / L, Ceria 52g / L

(Comparative Example 2)
Nitric acid-based platinum solution (including platinum equivalent to 1.0 g), rhodium nitrate solution (including rhodium equivalent to 0.2 g), alumina 90 g, ceria 52 g having a specific surface area of about 100 m ² / g, nickel oxide 4.5 g, alumina sol (10 wt%) 100 g and pure water 30 g were mixed to prepare slurry 9. The slurry is coated with a monolithic honeycomb carrier having a volume of 1 L, dried at 150 ° C. for 1 hour, and then fired at 500 ° C. for 1 hour to comprise a catalyst coating layer containing nickel and not separated into an upstream portion and a downstream portion. A catalyst was prepared (Comparative Example 2). The final composition of the catalyst of Comparative Example 2 is as follows:
Platinum 1.0g / L, Rhodium 0.2g / L, Alumina 100g / L, Ceria 52g / L, Nickel oxide (NiO) 4.5g / L

1. H ₂ S emission measurement Using an inline 4-cylinder 1.5L engine vehicle, running at 40km / h, adsorbing sulfur to the catalyst, and accelerating in the state of WOT (wide open throttle), the speed reaches 100km / h The amount of H ₂ S emitted was measured. The results are shown in FIG.

As shown in FIG. 2, the catalysts of Examples 1 to 4 are not divided into an upstream portion and a downstream portion, and H ₂ S emissions are compared with a catalyst composed of a catalyst coat layer that does not contain nickel (Comparative Example 1). The amount was much lower. Furthermore, the catalysts of Examples 1 to 3 showed almost the same H ₂ S purification performance as the catalyst (Comparative Example 2) which was not divided into the upstream part and the downstream part and was composed of a catalyst coating layer containing nickel. On the other hand, the catalyst of Example 4 in which iron oxide was added as a co-catalyst in the downstream portion had a lower amount of about 40% H ₂ S emission than the catalyst of Comparative Example 2.

2. Purification performance measurement The catalyst was endured for 5 hours at a catalyst-containing gas temperature of 800 ° C. with an engine with a displacement of 4 L, and then mounted on a real vehicle having an engine with a displacement of 2.2 L. The vehicle was driven with the operation mode set to the LA # 4 mode, and the NOx, HC, and CO emission effects of the catalyst were measured. The results for NOx emissions are shown in Table 1 and FIG.

  As shown in Table 1 and FIG. 3, the catalysts of Examples 1 to 4 are not divided into an upstream portion and a downstream portion, and are composed of a catalyst coating layer containing no nickel (Comparative Example 1), and an upstream portion and a downstream portion. Compared with the catalyst (Comparative Example 2) which is not divided into parts and is composed of a catalyst coat layer containing nickel, the NOx emission was reduced to the same level or lower. Among them, the catalysts of Example 2 and Example 3 had the lowest NOx emission. Moreover, although a result is not shown, also about the HC and CO emission effect, the catalyst of Examples 1-4 did not show deterioration of purification performance compared with the thing of a comparative example.

3. Examination of the amount of oxygen storage / release material that can be added to the upstream In order to determine the upper limit of the amount of oxygen storage / release material that can be added upstream, 0.6 mol (103 g) The catalyst containing various amounts of ceria in the upstream portion of the catalyst of Example 1 containing ceria was prepared, and their H ₂ S purification performance was measured as described above. As a result of comparison using the catalyst of Comparative Example 2 (ceria loading density of about 0.3 mol / L) as a control (100%), the lower the loading density per 1 L of the upstream ceria substrate, the lower the H ₂ S emission. , if it does not contain ceria (0 mol / L), compared with the catalyst of Comparative example 2 containing conventional nickel not divided into upstream and downstream showed H ₂ S emissions comparable (See Table 2 and FIG. 4).

4). Examination of specific surface area of oxygen storage / release material Subsequently, the relationship between the specific surface area of the oxygen storage / release material used in the catalyst coating layer and the H ₂ S emission amount was examined. First, ceria having a specific surface area of 100 m ² / g was reduced to a specific surface area of 75, 45, 25 m ² / g by firing in an electric furnace at various temperatures for 5 hours. Next, a catalyst separated into an upstream portion and a downstream portion having the same catalyst component composition as in Example 1 was prepared using ceria having these specific surface areas. As a result of measuring the H ₂ S purification performance of these catalysts prepared using ceria having different specific surface areas as described above, the amount of H ₂ S emission decreased as the specific surface area of ceria decreased. In addition, the present invention has a specific surface area of 45 m ² / g when compared to the catalyst of Comparative Example 2, ie, a prior art nickel-containing catalyst that has a specific surface area of 100 m ² / g and is not divided upstream and downstream. the catalyst was reduced H ₂ S emissions to the same extent, and the catalyst having a specific surface area of 25 m ² / g showed a lower H ₂ S emissions. The results are shown in Table 3 and FIG.

These results, the exhaust gas purifying catalyst of the present invention, without the use of nickel which is an environmental load substance, showing a conventional high H ₂ S purification performance of the catalyst, and also other exhaust gas purification performance and the like NOx It became clear that it could be maintained.

FIG. 1 shows a schematic diagram of the mechanism of H ₂ S formation by ceria and platinum. FIG. 2 shows a graph of H ₂ S emission (%) of the catalysts of Examples 1 to 4 and Comparative Examples 1 and ₂ . Here, the H ₂ S emission amount (%) in the graph is shown with the value of Comparative Example 1 as 100%. FIG. 3 shows the NOx emissions (g / mile) of the catalysts of Examples 1-4 and Comparative Examples 1-2. FIG. 4 is a graph showing changes in H ₂ S emission (%) when the amount of ceria (mol / L) of the catalyst used in Example 1 is changed. Here, the H ₂ S emission amount (%) in the graph indicates that calculated with the value of Comparative Example 2 as 100%. FIG. 5 shows a graph of H ₂ S emission (%) against the specific surface area (m ² / g) of ceria used in the catalyst of Example 1. Here, the H ₂ S emission amount (%) in the graph indicates that calculated with the value of Comparative Example 2 as 100%.

Claims (7)

  In the catalyst coat layer on the base material, the loading density of the precious metal contained in the upstream portion of the exhaust gas per 1 L of the base material is higher than that of the downstream portion of the exhaust gas, and the downstream portion is based on the oxygen storage / release material than the upstream portion. Exhaust gas purification catalyst that contains a large amount per liter of material.   The exhaust gas purifying catalyst according to claim 1, wherein the noble metal contained in the upstream portion of the exhaust gas is platinum and the noble metal contained in the downstream portion of the exhaust gas is rhodium.   2. The exhaust gas purifying catalyst according to claim 1, wherein a ratio between a platinum carrying density contained in the exhaust gas upstream portion and a rhodium carrying density contained in the exhaust gas downstream portion is 1: 1 to 10: 1. The exhaust gas-purifying catalyst according to any one of claims 1 to 3, wherein the oxygen storage / release material has a specific surface area of 30 m ² / g or less.   The catalyst for exhaust gas purification according to any one of claims 1 to 4, wherein a catalyst coat layer in an upstream portion of the exhaust gas does not contain the oxygen storage / release material.   The exhaust gas purifying catalyst according to any one of claims 1 to 5, wherein the oxygen storage / release material contains ceria.   The catalyst for exhaust gas purification according to any one of claims 1 to 6, wherein the catalyst coat layer in the exhaust gas upstream portion and / or the exhaust gas downstream portion contains iron oxide.

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