JP5103052B2 - Oxidation catalyst of exhaust gas purification system for diesel engine - Google Patents

Description

  The present invention relates to an oxidation catalyst for an exhaust gas purification system for a diesel engine.

The exhaust gas (about 170 ° C to about 300 ° C) discharged from the diesel engine contains environmental pollutants such as HC (hydrocarbon) and NO _2. DE emission regulations are established. In order to comply with these DE exhaust gas regulations, it is necessary to sufficiently remove HC and NO ₂ from the exhaust gas. Therefore, in recent years, a forced regeneration system, a urea SCR system, or the like has been adopted depending on the engine characteristics of the diesel engine. In these systems, various oxidation catalysts are used, and these oxidation catalysts include HC oxidation performance that promotes HC oxidation reaction, or NO ₂ conversion characteristics that promote oxidation reaction that converts NO to NO _2. Either is requested.

By the way, DE exhaust gas regulations are expected to become even stricter in the future. Therefore, in recent years, an attempt has been made to develop an oxidation catalyst having both these HC oxidation performance and NO ₂ conversion characteristics. Since Pt / Pd-based catalyst components are excellent for HC oxidation performance and Pt-based catalyst components are known to be excellent for NO ₂ conversion characteristics, this characteristic has been used conventionally. Various oxidation catalysts have been proposed. As such conventional techniques, for example, an oxidation catalyst in which a mixture of these catalyst components in a slurry form is supported on a porous substrate as a catalyst carrier, or these catalyst components are supported on a catalyst carrier in a multilayer coat form. There is an oxidation catalyst (see, for example, FIG. 1B).

However, the prior art has the following problems. That is, as in the prior art, a mixture of Pt-based catalyst components and Pt / Pd-based catalyst components in the form of a slurry is supported on a catalyst carrier, or these catalyst components are supported on a catalyst carrier in a multilayer coating. Otherwise, the performance of each catalyst component will be canceled out. That is, in the prior art, the HC oxidation performance and the NO ₂ conversion characteristics are deteriorated, and it has not been possible to achieve a good balance between the HC oxidation performance and the NO ₂ conversion characteristics.

The present invention has been made in view of such problems, and an object thereof is to provide an oxidation catalyst of the HC oxidation performance and NO ₂ conversion characteristics can be successfully both diesel engine exhaust gas purification system.

  In order to solve the above-mentioned problems, the present inventors have intensively studied, and have focused on the following points (1) and (2) to complete the present invention.

That is, the present inventors (1) the surface of the porous substrate on the exhaust gas inflow side has a particularly high contact frequency between the catalyst component and the reactant due to the turbulent action of the exhaust gas, and (2) It was noted that the reaction rate for converting NO contained in NO to NO ₂ was slow, while the reaction rate for oxidizing HC contained in the exhaust gas was fast.

In view of these points, in the present invention, a catalyst component involved in the NO ₂ conversion reaction having a low reaction rate, that is, a Pt-based catalyst component excellent in NO ₂ conversion characteristics is applied to the entire surface of the porous substrate. Arranged. In addition, on the exhaust gas inflow side surface of the porous base material in which the contact frequency between the catalyst component and the reactant is particularly high, the catalyst component involved in the oxidation reaction of HC having a high reaction rate, that is, excellent HC oxidation performance. Pt / Pd catalyst components were concentrated and arranged.

  In short, the present invention is an oxidation catalyst for an exhaust gas purification system for a diesel engine for purifying exhaust gas discharged from a diesel engine. A catalyst component not containing (Pd) (hereinafter referred to as “Pt-based catalyst component”) is supported, and platinum (Pt) and platinum (Pt) and only on the exhaust gas inflow side surface of the porous substrate on which the catalyst component is supported A catalyst component containing both palladium (Pd) (hereinafter referred to as “Pt / Pd catalyst component”) is supported.

This makes it possible to further improve the HC oxidation performance while maintaining the NO ₂ conversion characteristics of the entire oxidation catalyst. That is, since the Pt / Pd-based catalyst component having excellent HC oxidation performance is supported on the surface area on the exhaust gas inflow side where the HC oxidation reaction is likely to occur, among the entire surface area of the porous base material, The performance will be fully demonstrated. Moreover, the Pt / Pd-based catalyst component is only locally supported only on the surface region on the exhaust gas inflow side of the porous substrate. Therefore, the Pt / Pd-based catalyst component does not significantly impair the characteristics of the catalyst component supported on the entire surface region of the porous substrate, that is, the NO ₂ conversion characteristic by the Pt-based catalyst component. , NO ₂ conversion characteristics will be sufficiently maintained. As described above, according to the present invention, these HC oxidation performance and NO ₂ conversion characteristics can be well balanced.

According to the oxidation catalyst of the exhaust gas purification system for a diesel engine of the present invention, it is possible to successfully achieve both HC oxidation performance and NO ₂ conversion characteristics.

  First, the configuration of the oxidation catalyst 100 of the present embodiment will be described with reference to FIG. FIG. 1 is a schematic view showing a state in which a catalyst component is supported on a porous substrate in each oxidation catalyst of the present embodiment and the prior art. FIG. 1A is an oxidation catalyst 100 of the present embodiment, ) Shows a prior art oxidation catalyst 200.

  As shown in FIG. 1 (a), the oxidation catalyst 100 of the present embodiment includes a catalyst component (hereinafter referred to as “Pt”) that contains platinum (Pt) but does not contain palladium (Pd) on the entire surface of the porous substrate. A catalyst component containing 10) and containing only platinum and palladium (Pt / Pd) only on the exhaust gas inflow side surface of the porous substrate on which the catalyst component 10 is supported. The component 20 (hereinafter referred to as “Pt / Pd catalyst component”) 20 is supported.

  On the other hand, as shown in FIG. 1 (b), the oxidation catalyst 200 of the prior art mixes the Pt-based catalyst component 10 and the Pt / Pd-based catalyst component 20 in a slurry state on the entire surface of the porous substrate. The catalyst component is supported on a catalyst carrier, or these catalyst components 10 and 20 are supported on a catalyst carrier in a multilayer coat form.

By the way, the surface of the porous substrate on the exhaust gas inflow side has a particularly high contact frequency between the catalyst component and the reactant due to the turbulent action of the exhaust gas. Also, the reaction rate for converting NO contained in the exhaust gas to NO ₂ is slow, while the reaction rate for oxidizing HC contained in the exhaust gas is fast. Therefore, in this embodiment, the catalyst component involved in the NO ₂ conversion reaction having a low reaction rate, that is, the Pt-based catalyst component 10 having excellent NO ₂ conversion characteristics is arranged on the entire surface of the porous substrate. . In addition, on the exhaust gas inflow side surface of the porous base material in which the contact frequency between the catalyst component and the reactant is particularly high, the catalyst component involved in the oxidation reaction of HC having a high reaction rate, that is, excellent HC oxidation performance. The Pt / Pd-based catalyst component 20 is arranged in a concentrated manner.

In short, in the oxidation catalyst 100 of the present embodiment, first, the Pt-based catalyst component 10 excellent in NO ₂ conversion characteristics is coated on the entire surface of the porous substrate, and then Pt excellent in HC oxidation performance is obtained. A / Pd-based catalyst component 20 is coated on the exhaust gas inflow side surface of the porous substrate. In the present embodiment, the region coated with the Pt / Pd-based catalyst component 20 is such that the region from the end on the exhaust gas inflow side of the porous substrate is 5 to 15% (volume) with respect to the entire porous substrate. Ratio) is preferable. If the region is less than 5%, the region where the Pt / Pd-based catalyst component 20 is supported is too narrow, and the HC oxidation performance due to the action of the Pt / Pd-based catalyst component 20 becomes insufficient. . On the other hand, when the region exceeds 15%, the region where the Pt / Pd catalyst component 20 is supported is too wide, and the Pt / Pd catalyst component 20 cancels the action of the Pt catalyst component 10. As a result, the NO ₂ conversion characteristics due to the action of the Pt-based catalyst component 10 become insufficient.

Next, the effect of the oxidation catalyst 100 of this embodiment is demonstrated, comparing with a prior art, referring FIG. FIG. 2 is a diagram showing a correlation between HC oxidation performance and NO ₂ conversion characteristics in each oxidation catalyst of the present embodiment and the prior art. Each performance shown in the figure shows the performance when the exhaust gas temperature is 170 to 300 ° C., and the HC oxidation performance on the vertical axis shows the temperature when HC is purified by 50% (“HC T50”). The NO ₂ conversion characteristic on the horizontal axis indicates the conversion rate from NO to NO ₂ (“NO ₂ C250”) when the exhaust gas temperature is 250 ° C.

As shown in FIG. 2, the oxidation catalyst 100 of the present embodiment is superior in terms of HC oxidation performance and also in terms of NO ₂ conversion characteristics as compared with the oxidation catalyst 200 of the prior art.

That is, in the case of the oxidation catalyst 200 of the prior art, NO ₂ conversion is performed as compared with a case where only the Pt / Pd catalyst component is coated on the entire surface of the porous substrate (hereinafter referred to as “Pt / Pd catalyst”). Although it shows excellent results in terms of characteristics, it shows inferior results in terms of HC oxidation performance. In addition, the oxidation catalyst 200 of the prior art is superior in terms of HC oxidation performance compared to a case where only the Pt-based catalyst component is coated on the entire surface of the porous substrate (hereinafter referred to as “Pt catalyst”). However, the results are inferior in terms of NO ₂ conversion characteristics.

In contrast, in the case of the oxidation catalyst 100 of the present embodiment, compared to the Pt / Pd catalyst, the oxidation catalyst 100 shows excellent results in terms of NO ₂ conversion characteristics, and also in terms of HC oxidation performance, The result will be almost equivalent. Further, in the case of the oxidation catalyst 100 of the present embodiment, the result is superior in terms of HC oxidation performance as compared with the Pt catalyst, and also in terms of NO ₂ conversion characteristics, the result is almost equivalent to that of this Pt catalyst. It will be. Then, the oxidation catalyst 100 of the present embodiment, in terms of both HC oxidation performance and NO ₂ conversion characteristics, and thus exert an excellent function and effect than the oxidation catalyst 200 of the prior art.

=== Example ===
Next, an embodiment of the present invention will be described with reference to FIGS. FIG. 3 is a graph showing the relationship between the HC oxidation performance (HC purification rate (%)) and the exhaust gas temperature in each oxidation catalyst of the present example and the conventional example, and FIG. 4 shows the present example and the conventional example. 6 is a graph showing the relationship between NO ₂ conversion characteristics (NO ₂ conversion (%)) and exhaust gas temperature in each of the oxidation catalysts.

  In this example and the conventional example, the oxidation catalyst shown in FIG. 1 was used. That is, in this example, as the oxidation catalyst, the Pt catalyst component is supported on the entire surface of the porous substrate, and the region from the end of the porous substrate on the exhaust gas inflow side is the entire region of the porous substrate. Only a region where the Pt / Pd catalyst component was supported was used in a region that was 10% (volume ratio) with respect to the region. On the other hand, in the conventional example, an oxidation catalyst in which Pt and Pt / Pd catalyst components were mixed in a nest slurry and supported on the entire surface of the porous substrate was used.

In this example and the conventional example, the HC oxidation performance and the NO ₂ conversion characteristics were evaluated as follows. That is, in this example and the conventional example, a cordierite honeycomb carrier having a diameter of 1 inch and a length of 2 inches was coated with the catalyst component described above as a sample. Then, the sample is passed through a mixed gas simulating an exhaust gas, the mixture gas + 30 ° C. / allowed to warm min, and was evaluated HC oxidation performance and NO ₂ conversion characteristics at the respective temperature zones. Table 1 shows the evaluation conditions at that time.

<HC oxidation performance>
As shown in FIG. 3, regarding the HC oxidation performance, in the region where the exhaust gas temperature is less than 170 ° C. and more than 300 ° C., the oxidation catalyst of this example showed almost the same result as the oxidation catalyst of the conventional example. It wasn't too much.

  More specifically, in the region where the exhaust gas temperature is less than 170 ° C., the oxidation catalyst of this example, the oxidation catalyst of the conventional example, the Pt catalyst, and the Pt / Pd catalyst are all HC oxidation performance indicators. The purification rate (%) was low, almost 0%. On the other hand, in the region where the exhaust gas temperature exceeds 300 ° C., all of these catalysts showed a high value of almost 100% in the HC purification rate (%).

  However, in the region where the exhaust gas temperature is 170 ° C. to 300 ° C., the oxidation catalyst of this example showed superior results in terms of HC oxidation performance as compared with the oxidation catalyst of the conventional example. That is, in the exhaust gas temperature range of 170 ° C. to 300 ° C., the oxidation catalyst of the conventional example showed a result superior to the Pt catalyst, but inferior to the Pt / Pd catalyst. In contrast, in the case of the oxidation catalyst of the present example, a result superior to that of the Pt catalyst was exhibited, and an excellent result substantially equivalent to that of the Pt / Pd catalyst was exhibited.

<NO ₂ conversion characteristics>
Further, as shown in FIG. 4, regarding the NO ₂ conversion characteristics, in the region where the exhaust gas temperature is less than 170 ° C. and more than 300 ° C., the oxidation catalyst of this example has almost the same result as the oxidation catalyst of the conventional example. It was only shown.

More specifically, in the region where the exhaust gas temperature is less than 170 ° C., the oxidation catalyst of this example, the oxidation catalyst of the conventional example, the Pt catalyst, and the Pt / Pd catalyst are all indexes of NO ₂ conversion characteristics. The NO ₂ conversion (%) showed a low value of almost 0%. On the other hand, in the region where the exhaust gas temperature exceeds 300 ° C., these catalysts all showed high values of NO ₂ conversion (%) of around 80%. However, in these catalysts, the conversion rate (%) tends to decrease when the exhaust gas temperature exceeds 350 ° C., and when the exhaust gas temperature is around 400 ° C., the conversion rate (%) reaches about 70%. Diminished.

  However, in the region where the exhaust gas temperature is 170 ° C. to 300 ° C., the oxidation catalyst of this example showed superior results in terms of HC oxidation performance as compared with the oxidation catalyst of the conventional example. That is, in the exhaust gas temperature range of 170 ° C. to 300 ° C., the oxidation catalyst of the conventional example showed a result superior to the Pt / Pd catalyst, but inferior to the Pt catalyst. On the other hand, in the case of the oxidation catalyst of the present example, a result superior to that of the Pt / Pd catalyst was shown, and an excellent result almost equivalent to that of the Pt catalyst was shown.

As described above, the oxidation catalyst of this example exhibits superior effects in both the HC oxidation performance and the NO ₂ conversion characteristics as compared with the conventional oxidation catalyst. Therefore, according to the oxidation catalyst of the present invention, the HC oxidation performance and the NO ₂ conversion characteristics can both be successfully achieved.

It is the schematic which shows the state which made the porous base material carry | support the catalyst component in each oxidation catalyst of this embodiment and a prior art. In this embodiment and each oxidation catalyst of the prior art such as a diagram showing the correlation between the HC oxidation performance ( "HC T50") and NO ₂ conversion characteristics ( _"NO 2 C250"). It is a graph which shows the relationship between HC oxidation performance and exhaust gas temperature in each oxidation catalyst of a present Example and a prior art example. In this example and the oxidation catalyst in the prior art such as is a graph showing the relationship between the NO ₂ conversion characteristics and the exhaust gas temperature.

Explanation of symbols

10 Pt-based catalyst component 20 Pt / Pd-based catalyst component 100 Oxidation catalyst 200 of the present embodiment Prior art oxidation catalyst

Claims (1)

An oxidation catalyst for an exhaust gas purification system for a diesel engine for purifying exhaust gas discharged from a diesel engine,
On the entire surface of the honeycomb carrier, a catalyst component containing platinum but not containing palladium is supported, and a region from the exhaust gas inflow side end portion of the honeycomb carrier on which the catalyst component is supported is the entire surface of the honeycomb carrier. Oxidation of an exhaust gas purification system for a diesel engine, characterized in that a catalyst component containing both platinum and palladium is supported only on the surface of a region that falls within a range of 5 to 15% by volume with respect to the region. catalyst.