Diesel engine exhaust emission control system

RELATED APPLICATION DATA

The Japanese priority application No. 2003-433302 upon which the present application is based is hereby incorporated in its entirety herein by reference.

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

The present invention relates to a diesel engine exhaust emission control system.

THE RELATED ART

Conventionally, as an exhaust emission control system for cleaning up NOx contained in the exhaust gas of a diesel engine, a system employing an NOx-selective reduction type catalyst is known. This catalyst is a mixture of Pt particles and a zeolite that can adsorb HC which serves as a reducing agent, but has a problem that the NOx clean-up temperature range is extremely small. This is due to a correlation between the average particle size of the Pt particles and the maximum temperature at which cleaning up of NOx is conducted, where the smaller the average particle size of the Pt particles, the lower the maximum temperature at which cleaning up of NOx is conducted, and the larger the average particle size of the Pt particles, the higher the maximum temperature at which cleaning up of NOx is exhibited.

In order to expand the NOx clean-up temperature range, an exhaust emission control system in which the average particle size of Pt particles disposed on the upstream side of an exhaust pipe is set to be smaller than the average particle size of Pt particles disposed on the downstream side has been disclosed (see Japanese Patent Application Laid-open No. 9-103651).

However, the ability of cleaning up NOx in this system is poor in a low exhaust gas temperature region, leading to a problem that NOx cannot be sufficiently cleaned throughout the entire exhaust gas temperature region.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an exhaust emission control system that can sufficiently clean up NOx throughout the entire exhaust gas temperature region.

In order to attain this object, in accordance with the present invention, there is provided a diesel engine exhaust emission control system that includes, in an exhaust pipe of a diesel engine, a first cleaner and a second cleaner installed sequentially from the diesel engine side, the first cleaner including Pt particles having an average particle size d₁ of ≤ 10 nm, CeO₂, and a zeolite, and the second cleaner including Pt particles having an average particle size d₂ of ≥ 15 nm, and a zeolite.

With this arrangement, in a low exhaust gas temperature region, the zeolite of the first cleaner adsorbs HC which serves as a reducing agent, and the Pt particles having a small average particle size function as a catalyst, thus carrying out reductive cleaning up of NOx. Furthermore, since CeO₂ exhibits the function of adsorbing NOx, the amount of NOx emitted into the atmosphere is reduced. On the other hand, in a high exhaust gas temperature region, since mainly the zeolite of the second cleaner adsorbs HC and the Pt particles having a large average particle size function as a catalyst, reductive cleaning up of NOx is carried out. The NOx adsorbed on the CeO₂ is released at about 200°C or higher, and the thus-released NOx is subjected to reductive cleaning up mainly in the second cleaner, in the same manner as stated above.

As described above, in accordance with the present invention, it is possible to provide an exhaust emission control system for diesel engine that can sufficiently clean up NOx throughout the entire exhaust gas temperature region.

The above-mentioned object, other objects, characteristics, and advantages of the present invention will become apparent from an explanation of a preferred embodiment that will be described in detail below by reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

<ul id="ul0001" list-style="none" compact="compact"><li>FIG. 1 is a diagram for explaining an NOx clean-up system.</li><li>FIG. 2 is a diagram for explaining an experimental NOx clean-up system.</li><li>FIG. 3 is a sectional view of an essential part of Example (1) of a first cleaner.</li><li>FIG. 4 is a sectional view of an essential part of Example (1) of a second cleaner.</li><li>FIG. 5 is a sectional view of an essential part of Example (2) of the first cleaner.</li><li>FIG. 6 is a sectional view of an essential part of Example (3) of the first cleaner.</li></ul>

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An exhaust emission control system 1 shown in FIG. 1 includes, in an exhaust pipe 3 of a diesel engine 2, two first cleaners 4 and a second cleaner 5 installed sequentially from the diesel engine 2 side. One of the first cleaners 4 is disposed immediately below the diesel engine 2, and the other first cleaner 4 and the second cleaner 5 are disposed beneath a vehicle floor. Either of the two first cleaners 4 may be omitted.

Each first cleaner 4 includes Pt particles having an average particle size d₁ of ≤ 10 nm, CeO₂, and a zeolite (Zeo). It is not desirable for the average particle size d₁ of the Pt particles to be greater than 10 nm, since the maximum temperature at which cleaning up of NOx is conducted shifts toward the exhaust gas high temperature region side, whereas the lower limit value for the average particle size d₁ of the Pt particles is preferably 3 nm. If d₁ is &lt; 3 nm, the NOx clean-up temperature range tends to become narrow.

The Pt particles are supported on an SiO₂-Al₂O₃ support. Examples of the zeolite used include β zeolite, ZSM-5 zeolite, Y zeolite, and mordenite. As the other components, TiO₂ is used for the purpose of improving the durability against sulfate, if necessary. These components are attached in the form of layers, together with a binder, to an inner face of each cell of a ceramic honeycomb support, thus forming a washcoat. Examples of the binder include alumina and silica.

The content of each component of the washcoat is set as follows.

Amount of Pt added <u>a</u>: 2.5 g/L ≤ a ≤ 4 g/L; it is undesirable if a &lt; 2.5 g/L since the number of active sites decreases and sufficient NOx clean-up performance cannot be achieved, whereas it is undesirable if a &gt; 4 g/L since the NOx clean-up performance saturates.

SiO₂-Al₂O₃ content <u>b</u>: 50 g/L ≤ b ≤ 90 g/L; if b &lt; 50 g/L, the area of contact with exhaust gas becomes small, whereas if b &gt; 90 g/L, since the Pt concentration is diluted, the NOx clean-up rate decreases.

CeO₂ content <u>c</u>: 15 g/L ≤ c ≤ 25 g/L; if c &lt; 15 g/L, since the amount of NOx adsorbed decreases, cleaning up of NOx in the exhaust gas low temperature region is insufficient, whereas if c &gt; 25 g/L, the NOx clean-up rate tends to decrease.

Zeolite content <u>d</u>: 35 g/L ≤ d ≤ 65 g/L; it is undesirable if d &lt; 35 g/L since adsorption of HC which is necessary for cleaning up NOx, cannot be carried out efficiently, whereas it is undesirable if d &gt; 65 g/L since the Pt concentration is diluted. When a plurality of zeolites are used, the content of each zeolite is set in the above range.

TiO₂ content <u>e</u>: 3.5 g/L ≤ e ≤ 6.5 g/L; it is undesirable if e &lt; 3.5 g/L since an effect of suppressing sulfur cannot be obtained and thus the NOx clean-up performance is degraded, whereas it is undesirable if e &gt; 6.5 g/L since the effect of suppressing sulfur saturates.

The second cleaner 5 includes Pt particles having an average particle size d₂ of ≥ 15 nm and a zeolite (Zeo). It is undesirable if the average particle size d₂ of the Pt particles is &lt; 15 nm, since the maximum temperature at which cleaning up of NOx is conducted shifts toward the exhaust gas low temperature region side. It is preferable for the upper limit value of the average particle size d₂ of the Pt particles to be 30 nm. If d₂ &gt; 30 nm, the NOx clean-up rate tends to decrease accompanying a decrease in the specific surface area of the Pt particles.

The Pt particles are supported on an SiO₂-Al₂O₃ support. Examples of the zeolite used include β zeolite, ZSM-5 zeolite, Y zeolite, and mordenite. As the other components, TiO₂ is used for the purpose of improving the durability against sulfate, if necessary. These components are attached in the form of layers, together with a binder, to an inner face of each cell of a ceramic honeycomb support, thus forming a washcoat. Examples of the binder include alumina and silica.

The content of each component of the washcoat is set as follows.

Amount of Pt added <u>a</u>: 1.3 g/L ≤ a ≤ 2.5 g/L; it is undesirable if a &lt; 1.3 g/L since the number of active sites decreases and sufficient NOx clean-up performance cannot be achieved, whereas it is undesirable if a &gt; 2.5 g/L since the NOx clean-up performance saturates.

SiO₂-Al₂O₃ content <u>b</u>: 40 g/L ≤ b ≤ 65 g/L; if b &lt; 40 g/L, the area of contact with exhaust gas becomes small, whereas if b &gt; 65 g/L, since the Pt concentration is diluted, the NOx clean-up rate decreases.

Zeolite content <u>d</u>: 55 g/L ≤ d ≤ 85 g/L; it is undesirable if d &lt; 55 g/L since adsorption of HC which is necessary for cleaning up NOx, cannot be carried out efficiently, whereas it is undesirable if d &gt; 85 g/L since the Pt concentration is diluted.

TiO₂ content <u>e</u>: 3.5 g/L ≤ e ≤ 6.5 g/L; it is undesirable if e &lt; 3.5 g/L since the effect of suppressing sulfur cannot be obtained and the NOx clean-up performance is degraded, whereas it is undesirable if e &gt; 6.5 g/L since the effect of suppressing sulfur saturates.

Specific examples are explained below.

As shown in FIG. 2, an experimental NOx clean-up system 1 is equipped with a model gas sapply source 6, which corresponds to the diesel engine 2. A first cleaner 4 or a first comparison cleaner is installed on the upstream side of an exhaust pipe 3 extending from the model gas supply source 6. A second cleaner 5 or a second comparison cleaner is installed on the downstream side. An NOx gauge 7 is connected to the exit of the exhaust pipe 3.

Assuming the case of exhaust gas of a diesel engine, the following model gas was prepared. Model gas: NO 160 ppm; HC 420 ppm C; CO 1100 ppm; CO₂ 3.6%; O₂ 15%; H₂O 35%; balance N₂.

(Example-I)

As shown in FIG. 3, in Example (1) of the first cleaner 4, the inner face of each cell 9 of a honeycomb support 8 had a washcoat 10 with a single layer structure. The average particle size d₁ of Pt particles was 10 nm, and each component of the washcoat 10 and the content thereof were as follows. Amount of Pt added <u>a</u>: 3.2 g/L; SiO₂-Al₂O₃ content <u>b</u>: 71.8 g/L; CeO₂ content <u>c</u>: 20 g/L; β zeolite content <u>d</u>: 50 g/L; ZSM-5 zeolite content <u>d</u>: 50 g/L; TiO₂ content <u>e</u>: 5 g/L.

For comparison, a first comparison cleaner was prepared that had the same arrangement as that of Example (1) of the first cleaner 4, except that it contained no CeO₂.

As shown in FIG. 4, in Example (1) of the second cleaner 5, the inner face of each cell 12 of a honeycomb support 11 had a washcoat 13 with a single layer structure. The average particle size d2 of Pt particles was 15 nm, and each component of the washcoat 13 and the content thereof were as follows. Amount of Pt added <u>a</u>: 1.9 g/L; SiO₂-Al₂O₃ content <u>b</u>: 53.1 g/L; β zeolite content <u>d</u>: 70 g/L; TiO₂ content <u>e</u>: 5 g/L.

For comparison, a second comparison cleaner was prepared that had the same arrangement as that of Example (1) of the second cleaner 5, except that the average particle size d₂ of Pt particles was set at 10 nm.

As shown in Table 1, Example-A of the experimental NOx control system 1 was a combination of Example (1) of the first cleaner 4 and Example (1) of the second cleaner 5; Example-B was a combination of the first and second comparison cleaners; and Example-C was a combination of Example (1) of the first cleaner 4 and the second comparison cleaner. In Example-A to Example-C, while increasing the temperature of the model gas in the model gas supply source 6 at a temperature increase rate of 20°C/min from room temperature to 300°C, the model gas was passed through the exhaust pipe 3 at a space velocity SV of 50,000/h, and the average NOx clean-up rate of the model gas at temperatures of 50°C to 300°C was measured by the NOx gauge 7. <tables id="tabl0001" num="0001"><table frame="all"><title>Table 1</title><tgroup cols="4" colsep="1" rowsep="1"><colspec colnum="1" colname="col1" colwidth="39.37mm" /><colspec colnum="2" colname="col2" colwidth="39.37mm" /><colspec colnum="3" colname="col3" colwidth="39.37mm" /><colspec colnum="4" colname="col4" colwidth="39.37mm" /><thead valign="top"><row><entry namest="col1" nameend="col3" align="center">Experimental NOx control system</entry><entry namest="col4" nameend="col4" align="center">Average NOx clean-up rate</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="center">Example-A</entry><entry namest="col2" nameend="col2" align="center">Example (1) of first cleaner Pt (d₁): 10 nm; with CeO₂</entry><entry namest="col3" nameend="col3" align="center">Example (1) of second cleaner Pt (d₂): 15 nm</entry><entry namest="col4" nameend="col4" align="center">24%</entry></row><row><entry namest="col1" nameend="col1" align="center">Example-B</entry><entry namest="col2" nameend="col2" align="center">First comparison cleaner Pt (d₁): 10 nm; without CeO₂</entry><entry namest="col3" nameend="col3" align="center">Second comparison cleaner Pt (d₂): 10 nm</entry><entry namest="col4" nameend="col4" align="center">15%</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="center">Example-C</entry><entry namest="col2" nameend="col2" align="center">Example (1) of first cleaner Pt (d₁): 10 nm; with CeO₂</entry><entry namest="col3" nameend="col3" align="center">Second comparison cleaner Pt (d₂): 10 nm</entry><entry namest="col4" nameend="col4" align="center">15%</entry></row></tbody></tgroup></table></tables>

As is clear from Table 1, in Example-A, in the low exhaust gas temperature region the Pt particles of Example (1) of the first cleaner 4 exhibited catalytic activity so as to clean up the NOx, and the CeO₂ adsorbed NOx, whereas in the high exhaust gas temperature region the Pt particles of Example (1) of the second cleaner 5 exhibited catalytic activity so as to clean up the NOx in the exhaust gas and the NOx released from the CeO₂, thus giving a high average NOx clean-up rate.

In Example-B, since NOx could not be adsorbed at all in the low exhaust gas temperature region, and cleaning up of NOx was insufficient in the high exhaust gas temperature region, the average NOx clean-up rate was lower than that of Example-A. In Example-C, although cleaning up of the NOx was good in the low exhaust gas temperature region, cleaning up of the NOx was insufficient in the high exhaust gas temperature region, and thus the average NOx clean-up rate was lower than that of Example-A.

(Example-II)

Example (2) of the first cleaner 4 shown in FIG. 5 had a washcoat 10 having a two layer structure on the inner face of each cell 9 of the honeycomb support 8. The average particle size d₁ of the Pt particles was 10 nm, and each component and the content thereof in a first layer 14 which was on the inside in the washcoat 10 were as follows. Amount of Pt added <u>a</u>: 3.2 g/L; SiO₂-Al₂O₃ content <u>b</u>: 71.8 g/L; β zeolite content <u>d</u>: 50 g/L; TiO₂ content <u>e</u>: 5 g/L. Each component and the content thereof in a second layer 15 which was on the outside in the washcoat 10 were as follows. CeO₂ content <u>c</u>: 20 g/L; ZSM-5 zeolite content <u>d</u>: 50 g/L.

Example (3) of the first cleaner 4 shown in FIG. 6 had an arrangement in which the first layer 14 of Example (2) was placed on the outside and the second layer 15 was placed on the inside.

That is, in Example (1) of the first cleaner 4 shown in FIG. 3 the CeO₂ was in a mixed state, in Example (2) of the first cleaner 4 shown in FIG. 5 the CeO₂ was on the outside, and in Example (3) of the first cleaner 4 shown in FIG. 6 the CeO₂ was on the inside.

As shown in Table 2, Example-D of the experimental NOx control system 1 was a combination of Example (2) of the first cleaner 4 and Example (1) of the second cleaner 5; and Example-E was a combination of Example (3) of the first cleaner 4 and Example (1) of the second cleaner 5. In Example-D and Example-E, while increasing the temperature of the model gas in the model gas supply source 6 at a temperature increase rate of 20°C/min from room temperature to 300°C, the model gas was passed through the exhaust pipe 3 at a space velocity SV of 50,000/h, and the average NOx clean-up rate of the model gas at temperatures from 50°C to 300°C was measured by the NOx gauge 7. Table 2 includes data related to Example-A for comparison purposes. <tables id="tabl0002" num="0002"><table frame="all"><title>Table 2</title><tgroup cols="4" colsep="1" rowsep="1"><colspec colnum="1" colname="col1" colwidth="39.37mm" /><colspec colnum="2" colname="col2" colwidth="39.37mm" /><colspec colnum="3" colname="col3" colwidth="39.37mm" /><colspec colnum="4" colname="col4" colwidth="39.37mm" /><thead valign="top"><row><entry namest="col1" nameend="col3" align="center">Experimental NOx control system</entry><entry namest="col4" nameend="col4" align="center">Average NOx clean-up rate</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="center">Example-A</entry><entry namest="col2" nameend="col2" align="center">Example (1) of first cleaner CeO₂: in mixed state</entry><entry namest="col3" nameend="col3" morerows="2" align="left">Example (1) of second cleaner</entry><entry namest="col4" nameend="col4" align="center">24%</entry></row><row><entry namest="col1" nameend="col1" align="center">Example-D</entry><entry namest="col2" nameend="col2" align="center">Example (2) of first cleaner CeO₂: on outside</entry><entry namest="col4" nameend="col4" align="center">21 %</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="center">Example-E</entry><entry namest="col2" nameend="col2" align="center">Example (3) of first cleaner CeO₂: on inside</entry><entry namest="col4" nameend="col4" align="center">29%</entry></row></tbody></tgroup></table></tables>

From Table 2, it was found that, when CeO₂ was placed on the inside as shown in FIG. 6, the average NOx clean-up rate was the highest. It is surmised that NOx released from the CeO₂ is cleaned by the Pt particles disposed on the outside and the β zeolite in cooperation.

(Example-III)

Example (4) of the first cleaner 4 was prepared in the same manner as in the case of Example (3) of the first cleaner 4, except that the average particle size d₁ of the Pt particles was changed to 5 nm in Example (3) of the first cleaner 4 shown in FIG. 6, that is, one in which CeO₂ was placed on the inside. Similarly, Example (5) of the first cleaner 4 was prepared in the same manner as in the case of Example (3) of the first cleaner 4, except that the average particle size d₁ of the Pt particles was changed to 3 nm.

Example (2) of the second cleaner 5 was prepared in the same manner as in the case of Example (1) of the second cleaner 5, except that the average particle size d₂ of the Pt particles was changed to 20 nm in Example (1) of the second cleaner 5 shown in FIG. 4. Similarly, Example (3) of the second cleaner 5 was prepared in the same manner as in the case of Example (1) of the second cleaner 5, except that the average particle size d₂ of the Pt particles was changed to 30 nm. Similarly, Example (4) of the second cleaner 5 was prepared in the same manner as in the case of Example (1) of the second cleaner 5, except that the average particle size d₂ of the Pt particles was changed to 50 nm.

As shown in Table 3, Example-F of the experimental NOx control system 1 was a combination of Example (4) of the first cleaner 4 and Example (1) of the second cleaner 5; Example-G was a combination of Example (5) of the first cleaner 4 and Example (1) of the second cleaner 5; Example-H was a combination of Example (3) of the first cleaner 4 and Example (2) of the second cleaner 5; Example-I was a combination of Example (3) of the first cleaner 4 and Example (3) of the second cleaner 5; and Example-J was a combination of Example (3) of the first cleaner 4 and Example (4) of the second cleaner 5.

In Example-F to Example-J, while increasing the temperature of the model gas in the model gas supply source 6 at a temperature increase rate of 20°C/min from room temperature to 300°C, the model gas was passed through the exhaust pipe 3 at a space velocity SV of 50,000/h, and the average NOx clean-up rate of the model gas at temperatures from 50°C to 300°C was measured by the NOx gauge 7. Table 3 includes the data related to Example-E for comparison purposes. <tables id="tabl0003" num="0003"><table frame="all"><title>Table 3</title><tgroup cols="4" colsep="1" rowsep="1"><colspec colnum="1" colname="col1" colwidth="39.37mm" /><colspec colnum="2" colname="col2" colwidth="39.37mm" /><colspec colnum="3" colname="col3" colwidth="39.37mm" /><colspec colnum="4" colname="col4" colwidth="39.37mm" /><thead valign="top"><row><entry namest="col1" nameend="col3" align="center">Experimental NOx control system</entry><entry namest="col4" nameend="col4" align="center">Average NOx clean-up rate</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="center">Example-E</entry><entry namest="col2" nameend="col2" align="center">Example (3) of first cleaner Pt (d₁): 10 nm</entry><entry namest="col3" nameend="col3" morerows="2" align="center">Example (1) of second cleaner Pt (d₂): 15 nm</entry><entry namest="col4" nameend="col4" align="center">29%</entry></row><row><entry namest="col1" nameend="col1" align="center">Example-F</entry><entry namest="col2" nameend="col2" align="center">Example (4) of first cleaner Pt (d₁): 5 nm</entry><entry namest="col4" nameend="col4" align="center">32%</entry></row><row><entry namest="col1" nameend="col1" align="center">Example-G</entry><entry namest="col2" nameend="col2" align="center">Example (5) of first cleaner Pt (d₁): 3 nm</entry><entry namest="col4" nameend="col4" align="center">31 %</entry></row><row><entry namest="col1" nameend="col1" align="center">Example-H</entry><entry namest="col2" nameend="col2" morerows="2" align="center">Example (3) of first cleaner Pt(d₁): 10 nm</entry><entry namest="col3" nameend="col3" align="center">Example (2) of second cleaner Pt(d₂): 20 nm</entry><entry namest="col4" nameend="col4" align="center">33%</entry></row><row><entry namest="col1" nameend="col1" align="center">Example-I</entry><entry namest="col3" nameend="col3" align="center">Example (3) of second cleaner Pt (d₂): 30 nm</entry><entry namest="col4" nameend="col4" align="center">29%</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="center">Example-J</entry><entry namest="col3" nameend="col3" align="center">Example (4) of second cleaner Pt (d₂): 50 nm</entry><entry namest="col4" nameend="col4" align="center">26%</entry></row></tbody></tgroup></table></tables>

It can be seen from Table 3 that, in order to improve the average NOx clean-up rate, the average particle size d₁ of the Pt particles in the first cleaner 4 is preferably 3 nm ≤ d₁ ≤ 10 nm and the average particle size d₂ of the Pt particles in the second cleaner 5 is preferably 15 nm ≤ d₂ ≤ 30 nm. In the first cleaner 4, if the average particle size d₁ of the Pt particles is too small, the NOx clean-up temperature range becomes small, whereas in the second cleaner 5 if the average particle size d₂ of the Pt particles is too large, the average NOx clean-up rate decreases accompanying a decrease in the specific surface area.

(Example-IV)

Example (6) of the first cleaner 4 was prepared in the same manner as in the case of Example (4) of the first cleaner 4, except that the CeO₂ content <u>c</u> was changed to 10 g/L from Example (4) of the first cleaner 4 shown in FIG. 6, that is, one in which CeO₂ was disposed on the inside and the CeO₂ content <u>c</u> was set at 20 g/L. Similarly, Example (7) of the first cleaner 4 was prepared in the same manner as in the case of Example (4) of the first cleaner 4, except that the CeO₂ content <u>c</u> was changed to 15 g/L. Similarly, Example (8) of the first cleaner 4 was prepared in the same manner as in the case of Example (4) of the first cleaner 4, except that the CeO₂ content <u>c</u> was changed to 25 g/L. Similarly, Example (9) of the first cleaner 4 was prepared in the same manner as in the case of Example (4) of the first cleaner 4, except that the CeO₂ content <u>c</u> was changed to 30 g/L.

As shown in Table 4, Example-K of the experimental NOx control system 1 was a combination of Example (6) of the first cleaner 4 and Example (1) of the second cleaner 5; Example-L was a combination of Example (7) of the first cleaner 4 and Example (1) of the second cleaner 5; Example-M was a combination of Example (8) of the first cleaner 4 and Example (1) of the second cleaner 5; and Example-N was a combination of Example (9) of the first cleaner 4 and Example (1) of the second cleaner 5.

In Example-K to Example-N, while increasing the temperature of the model gas in the model gas supply source 6 at a temperature increase rate of 20°C/min from room temperature to 300°C, the model gas was passed through the exhaust pipe 3 at a space velocity SV of 50,000/h, and the average NOx clean-up rate of the model gas at temperatures from 50°C to 300°C was measured by the NOx gauge 7. Table 4 includes data related to Example-F for comparison purposes. <tables id="tabl0004" num="0004"><table frame="all"><title>Table 4</title><tgroup cols="4" colsep="1" rowsep="1"><colspec colnum="1" colname="col1" colwidth="39.37mm" /><colspec colnum="2" colname="col2" colwidth="39.37mm" /><colspec colnum="3" colname="col3" colwidth="39.37mm" /><colspec colnum="4" colname="col4" colwidth="39.37mm" /><thead valign="top"><row><entry namest="col1" nameend="col3" align="center">Experimental NOx control system</entry><entry namest="col4" nameend="col4" align="center">Average NOx clean-up rate</entry></row></thead><tbody valign="top"><row><entry namest="col1" nameend="col1" align="center">Example-K</entry><entry namest="col2" nameend="col2" align="center">Example (6) of first cleaner CeO₂ content <u>c</u>: 10 g/L</entry><entry namest="col3" nameend="col3" morerows="4" align="left">Example (1) of second cleaner</entry><entry namest="col4" nameend="col4" align="center">19%</entry></row><row><entry namest="col1" nameend="col1" align="center">Example-L</entry><entry namest="col2" nameend="col2" align="center">Example (7) of first cleaner CeO₂ content <u>c</u>: 15 g/L</entry><entry namest="col4" nameend="col4" align="center">25%</entry></row><row><entry namest="col1" nameend="col1" align="center">Example-F</entry><entry namest="col2" nameend="col2" align="center">Example (4) of first cleaner CeO₂ content <u>c</u>: 20 g/L</entry><entry namest="col4" nameend="col4" align="center">32%</entry></row><row><entry namest="col1" nameend="col1" align="center">Example-M</entry><entry namest="col2" nameend="col2" align="center">Example (8) of first cleaner CeO₂ content <u>c</u>: 25 g/L</entry><entry namest="col4" nameend="col4" align="center">27%</entry></row><row rowsep="1"><entry namest="col1" nameend="col1" align="center">Example-N</entry><entry namest="col2" nameend="col2" align="center">Example (9) of first cleaner CeO₂ content <u>c</u>: 30 g/L</entry><entry namest="col4" nameend="col4" align="center">20%</entry></row></tbody></tgroup></table></tables>

It can be seen from Table 4 that, in order to improve the average NOx clean-up rate, the CeO₂ content <u>c</u> is preferably 15 g/L ≤ c ≤ 25 g/L. When the CeO₂ content is too small, the amount of NOx adsorbed is small, and cleaning up of NOx is insufficient in the low exhaust gas temperature region; when it is too large, the average NOx clean-up rate tends to decrease.

A diesel engine exhaust emission control system includes, in an exhaust pipe 3 of a diesel engine 2, a first cleaner 4 and a second cleaner 5 installed sequentially from the diesel engine 2 side. The first cleaner 4 includes Pt particles having an average particle size d₁ of ≤ 10 nm, CeO₂, and a zeolite. The second cleaner 5 includes Pt particles having an average particle size d₂ of ≥ 15 nm, and a zeolite. Thus, it is possible to sufficiently clean up NOx throughout the entire exhaust gas temperature region.