Exhaust purification apparatus for internal combustion engine

An exhaust purification apparatus for an internal combustion engine is provided with an NOx storage and reduction type catalyst in an exhaust passage. The NOx storage and reduction type catalyst comprises a base member, an upstream side coat layer arranged on the base member, and a downstream side coat layer arranged at a downstream side in the direction of exhaust flow from the upstream side coat layer. The upstream side coat layer does not include a Ce-containing oxide but includes a precious metal catalyst. The downstream side coat layer contains a Ce-containing oxide and precious metal catalyst. A length of the upstream side coat layer is a length of 5 to 62.5% of the total length of the upstream side coat layer and the downstream side coat layer, while the remaining part of the coat layer aside from the upstream side coat layer is the downstream side coat layer.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority based on Japanese Patent Application No. 2016-202683 filed with the Japan Patent Office on Oct. 14, 2016, the entire contents of which are incorporated into the present specification by reference.

TECHNICAL FIELD

The present disclosure relates to an exhaust purification apparatus for an internal combustion engine.

BACKGROUND ART

Known in the art is an internal combustion engine provided with an NOxstorage and reduction type catalyst storing nitrogen oxides (NOx) in exhaust gas when an air-fuel ratio of an exhaust is lean and releasing and reducing the stored NOxwhen the air-fuel ratio of the exhaust is made rich.

WO2014/123232A discloses as such an NOxstorage and reduction type catalyst an NOxstorage and reduction type catalyst provided with a base member formed with a coat layer on its surface. The coat layer contains a precious metal carried on a support and ceria or a complex oxide of cerium (Ce) and strontium (St) or other Ce-containing oxide. In this NOxstorage and reduction type catalyst, when the air-fuel ratio of the exhaust is lean, the NOxin the exhaust is adsorbed at the Ce-containing oxide, while when the exhaust air-fuel ratio is made rich, the adsorbed NOxis discharged from the Ce-containing oxide and reduced.

SUMMARY OF THE DISCLOSURE

In this regard, in recent years, there has been a demand for reducing the size and lightening the weight of vehicles designed for improved fuel economy by using a single catalyst to remove the harmful substances contained in the exhaust of NOxand hydrocarbons (HC). Further, NOxand HC both have similar detrimental effects on the environment, so it is desirable to reduce the total amount of the amount of exhaust of NOxand the amount of exhaust of HC.

On the other hand, since the above-mentioned NOxstorage and reduction type catalyst contains a precious metal, the precious metal enables the HC to be oxidized and removed. However, Ce causes a drop in the reactivity of the precious metal, so if increasing the Ce-containing oxide, the amount of adsorption of NOxincreases, but the amount of removal of HC falls. Therefore, in the above-mentioned such NOxstorage and reduction type catalyst, there was the problem that it was difficult to use a single NOxstorage and reduction type catalyst to make the total amount of the amount of adsorption of NOxand the amount of removal of HC increase.

To solve the above problem, an exhaust purification apparatus for the internal combustion engine according to one aspect of the present disclosure is provided with an NOxstorage and reduction type catalyst in an exhaust passage. The NOxstorage and reduction type catalyst comprises a base member extending in a direction of exhaust flow and a coat layer formed on the base member. The coat layer comprises an upstream side coat layer arranged at an upstream side in the direction of exhaust flow and a downstream side coat layer arranged at a downstream side in the direction of exhaust flow from the upstream side coat layer. The upstream side coat layer does not include a Ce-containing oxide but includes a precious metal catalyst. The downstream side coat layer contains a Ce-containing oxide and precious metal catalyst. A length of the upstream side coat layer is a length of 5 to 62.5% of the total length of the upstream side coat layer and the downstream side coat layer and the remaining part of the coat layer aside from the upstream side coat layer is the downstream side coat layer.

According to the exhaust purification apparatus for the internal combustion engine according to such an aspect of the present disclosure, the drop in the HC oxidation action of the precious metal catalyst at the upstream side coat layer is suppressed by elimination of Ce from the upstream side coat layer. On the other hand, the length of the remaining part of the coat layer besides the upstream side coat layer, that is, the length of the downstream side coat layer, is a sufficient length of 37.5 to 95% with respect of the total length of the upstream side coat layer and the downstream side coat layer, so it is possible to sufficiently adsorb the NO2by the Ce in the downstream side coat layer. As a result, it is possible to use a single NOxstorage and reduction type catalyst to make the total amount of the NOxstorage amount and HC removal amount increase.

DESCRIPTION OF EMBODIMENTS

FIG. 1is a schematic system view showing an exhaust purification apparatus for an internal combustion engine100according to the present disclosure.

The internal combustion engine100is provided with a diesel engine1, an exhaust manifold2, an NOxstorage and reduction type catalyst (NSR)3connected to the exhaust manifold2, and a selective reduction type catalyst (SCR)4. Exhaust discharged from the diesel engine1is successively supplied through the exhaust manifold2to the NOxstorage and reduction type catalyst3and selective reduction type catalyst4. In this way, the exhaust purification apparatus for the internal combustion engine100according to the present embodiment is provided with the NOxstorage and reduction type catalyst3in an exhaust passage.

Next, while referring toFIGS. 2A and 2B, a structure of the NOxstorage and reduction type catalyst3will be explained.FIG. 2Ais a front view of the NOxstorage and reduction type catalyst3seen from the exhaust inflow end, whileFIG. 2Bis a side cross-sectional view of the NOxstorage and reduction type catalyst3cut along the direction of exhaust passage.

The NOxstorage and reduction type catalyst3forms a cylindrical shape having a uniform cross-section over its entire length and extending in a direction of exhaust flow (direction of arrow W inFIG. 2B). The inside of the NOxstorage and reduction type catalyst3is formed with a plurality of exhaust flow paths surrounded by partition walls. These exhaust flow paths are formed to be rectangular in cross-sections and to extend straight while maintaining certain widths. The base member5forming these exhaust flow paths is made of ceramic, for example, is formed from cordierite, mullite, or α-alumina. In some embodiments, the base member5is formed from cordierite. Furthermore, the surfaces of the partition walls are formed with a coat layer6.

FIG. 3is a view diagrammatically showing the surface of a catalyst support7contained in part of the coat layer6formed on the partition walls of the NOxstorage and reduction type catalyst3. As shown inFIG. 3, for example, a precious metal catalyst8and a Ce-containing oxide9for adsorption of NOxare carried on a catalyst support7comprised of alumina (Al2O3).

The precious metal catalyst8has an action of oxidizing NO to produce NO2and the action of reducing the NOx. This precious metal catalyst8is comprised of at least one precious metal of platinum (Pt), palladium (Pd), and rhodium (Rh).

As the Ce-containing oxide9, at least one of ceria and a complex oxide of Ce and strontium (St) is used. In some embodiments, ceria is the Ce-containing oxide9. This Ce-containing oxide9has an excellent NOxadsorption ability in an environment of a relatively low temperature (less than 300° C.)

Next, while referring toFIG. 3andFIG. 4, the action of removal of NOxat the NOxstorage and reduction type catalyst3will be explained in brief.FIG. 4is a graph showing a change in time of the air-flow ratio (A/F) of the exhaust flowing into the NOxstorage and reduction type catalyst3and an amount of NOxstored in the NOxstorage and reduction type catalyst3(NOxstorage amount).

As will be understood fromFIG. 4, the air-flow ratio of the exhaust flowing into the NOxstorage and reduction type catalyst3is usually maintained lean, so the exhaust flowing into the NOxstorage and reduction type catalyst3is usually in an state of oxygen excess. At this time, part of the NO contained in the exhaust is oxidized at the surface of the precious metal catalyst8and becomes NO2. Next, this NO2is believed to be adsorbed at the Ce-containing oxide9by the O forming the NO2and the Ce forming the Ce-containing oxide9chemically bonding.

In this regard, this NO2is adsorbed at the Ce-containing oxide9while the temperature of the exhaust flowing into the NOxstorage and reduction type catalyst3is a relatively low temperature (less than about 300° C.) On the other hand, if the temperature of the exhaust flowing into the NOxstorage and reduction type catalyst3becomes a relatively high temperature (about 300° C. or more), the thermal motion of NO2becomes larger and the NO2is released from the Ce-containing oxide9. Note that, NO, which has a smaller number of O than NO2, is not adsorbed at the Ce-containing oxide9much at all. That is, NO2is adsorbed at the Ce-containing oxide9when the air-flow ratio of the exhaust flowing into the NOxstorage and reduction type catalyst3is lean and the temperature of the NOxstorage and reduction type catalyst3is a relatively low temperature.

On the other hand, when the NO2is adsorbed at the Ce-containing oxide9, as shown inFIG. 4, if the air-flow ratio of the exhaust flowing into the NOxstorage and reduction type catalyst3is made rich, an oxygen concentration in the exhaust falls. At this time, the chemical bonds of Ce and O are cut and the NO2adsorbed at the Ce-containing oxide9is released. Next, the released NO2is reduced on the precious metal catalyst8by the HC and CO contained in the exhaust.

Now then, as shown inFIG. 3, the catalyst support7carries the Ce-containing oxide9and precious metal catalyst8. When the air-fuel ratio of the exhaust is lean, the HC contained in the exhaust reacts with oxygen on the surface of the precious metal catalyst8and is oxidized. On the other hand, as shown inFIG. 4, if the air-fuel ratio of the exhaust is made rich, the HC reacts with the NO2adsorbed on the surface of the precious metal catalyst8and the NO2is reduced by the same.

In this way, NO is oxidized to NO2by the precious metal catalyst8and the oxidized NO2is adsorbed at the Ce-containing oxide9. However, in this case, if the precious metal catalyst8and the Ce-containing oxide9are positioned close to each other, the HC oxidation ability of the precious metal catalyst8ends up being suppressed. The reason is presumed to be as follows:

That is, if the surface of the precious metal catalyst8is covered by oxygen atoms or oxygen molecules, the precious metal catalyst8falls in activity. This phenomenon is called “oxygen poisoning” and is widely known. Now, as shown inFIG. 5, if Ce and the precious metal catalyst8are positioned close to each other, the Ce and precious metal catalyst8will chemically bond through the oxygen. As a result, the precious metal catalyst8will enter a state of oxygen poisoning. Due to this, the precious metal catalyst8is reduced in oxidation ability. Therefore, if the Ce-containing oxide9and precious metal catalyst8are carried on the catalyst support7over the entire NOxstorage and reduction type catalyst3, the HC oxidation ability of the precious metal catalyst8is suppressed, so the HC removal amount is decreased and it becomes difficult to raise the total amount of the NOxadsorption amount and HC removal amount.

Therefore, in the examples of the present disclosure, as shown inFIG. 6AandFIG. 6B, the coat layer6is divided into an upstream side coat layer61arranged at the upstream side in the direction of exhaust flow and a downstream side coat layer62arranged at the downstream side in the direction of exhaust flow. Further, the upstream side coat layer61is configured so as to contain the precious metal catalyst8, but not contain the Ce-containing oxide9, while the downstream side coat layer62is configured to contain the precious metal catalyst8and the Ce-containing oxide9.

By doing this, at the upstream side coat layer61, the precious metal catalyst8and the Ce-containing oxide9are not copresent, so the activity of the precious metal catalyst8is never reduced by the Ce-containing oxide9and the activity of the precious metal catalyst8is maintained. As a result, the HC removal amount by the precious metal catalyst8increases.

Now then, if the activity of the precious metal catalyst8at the upstream side coat layer61is maintained in this way, NO is oxidized well to NO2at the upstream side coat layer61. On the other hand, at this time, the HC removal amount increases, so the amount of HC contacting the generated NO2and the NO2in the exhaust decreases. As a result, the amount of NO2reduced to NO by HC decreases, therefore, the exhaust flowing over the downstream side coat layer62is raised in NO2concentration. Furthermore, if the HC removal amount of the upstream side coat layer61increases, the amount of HC flowing over the downstream side coat layer62decreases, so the NO in the exhaust is oxidized to NO2on the precious metal catalyst8of the downstream side coat layer62as well and, therefore, the exhaust flowing over the downstream side coat layer62is sufficiently raised in NO2concentration. Due to the NO2concentration at the downstream side coat layer62being raised in this way, the opportunities for the Ce-containing oxide9and NO2to chemically bond increase and a greater amount of NO2is adsorbed at the Ce-containing oxide9.

Therefore, by forming the upstream side coat layer61and the downstream side coat layer62so that the upstream side coat layer61contains the precious metal catalyst8but does not contain the Ce-containing oxide9and the downstream side coat layer62contains the precious metal catalyst8and Ce-containing oxide9, it is possible to make the HC removal amount at the upstream side coat layer61increase and make the NOxadsorption amount at the downstream side coat layer62increase. As a result, it becomes possible to make the total value of the NOxadsorption amount and HC removal amount increase.

On the other hand, in order for the coat layer6to further absorb NOx, it may be made to further contain one or both of an alkali metal and alkali earth metal. In this case, as the alkali metal, potassium (K), rubidium (Rb), cesium (Cs), etc. may be mentioned. As the alkali earth metal, calcium (Ca), strontium (St), barium (Ba), etc. may be mentioned. These alkali metals and alkali earth metals have excellent NOxabsorbing abilities in an environment of a relatively high temperature (300° C. or more). Below, alkali metals and alkali earth metals absorbing NOxwill be referred to as “NOxabsorbing metals10”.

Next, the action of absorption of NOxwhen using Ba as such an NOxabsorbing metal10will be explained.FIG. 7is a view diagrammatically showing the NOxabsorbing action of an NOxabsorbing metal.

As shown inFIG. 7, when the air-flow ratio of the exhaust flowing into the NOxstorage and reduction type catalyst3is lean, NO2is further oxidized on the surface of the precious metal catalyst8and becomes nitric acid ions (NO3−) which then react with the NOxabsorbing metal10and are absorbed in the form of a nitrate. On the other hand, when the air-flow ratio of the exhaust flowing into the NOxstorage and reduction type catalyst3is made rich, the concentration of oxygen in the exhaust falls, so the reaction proceeds in the opposite direction and the NO3−which had been absorbed as nitrates is again discharged as NO2. Next, the discharged NO2is reduced by the HC and CO contained in the exhaust.

In this way, there are two actions on NOx: the action of adsorption of NOxby the Ce-containing oxide9and the action of absorption of NOxby the NOxabsorbing metal10. These two actions will be combined and called “storage”.

Note that, the action of absorption of NOxby Ba is stronger in force holding the NOxcompared with the adsorption action and has a harder time releasing NO2. For this reason, an NOxstorage and reduction type catalyst3containing both of an NOxabsorbing metal10and Ce-containing oxide9has an excellent NOxstoring ability with respect to a broad temperature region of the exhaust.

Note that, in the examples according to the present disclosure, as shown inFIG. 1, to reduce the size of the exhaust system and promote warmup of the NOxstorage and reduction type catalyst3, the NOxstorage and reduction type catalyst3is directly connected to the exhaust manifold2. An oxidation catalyst for oxidizing the HC is not arranged between the exhaust manifold2and the NOxstorage and reduction type catalyst3. However, in the examples according to the present disclosure, in this way, even if the oxidation catalyst is not arranged upstream of the NOxstorage and reduction type catalyst3, since the upstream side coat layer61of the NOxstorage and reduction type catalyst3has a powerful oxidation function, at the time of the engine warmup operation, a large amount of heat of oxidation reaction is generated at the upstream side coat layer61. As a result, the NOxstorage and reduction type catalyst3can be warmed up earlier.

Next, the method of production of Example 1 of the NOxstorage and reduction type catalyst3according to the present disclosure will be explained. First, to start, a slurry A for forming the upstream side coat layer61will be explained. First, a palladium nitrate solution, platinum dinitrodiammine solution, and rhodium nitrate solution were impregnated in alumina (Al2O3) to prepare a precious metal-carrying powder in which Pd: 0.6 wt %, Pt: 3.5 wt %, and Rh: 0.2 wt % are carried on alumina. This precious metal-carrying powder and water, an alumina binder, and a thickener were mixed to prepare the slurry A. This slurry A had an amount of precious metal-carrying powder per liter of 100 g/liter.

Next, a slurry B for forming the downstream side coat layer62will be explained. First, in the same way as the upstream side coat layer61, a precious metal-carrying powder was prepared. This precious metal-carrying powder and ceria (CeO2), water, an alumina binder, and a thickener were mixed to prepare the slurry B. This slurry B had an amount of precious metal-carrying powder per liter of 100 g/liter. The amount of ceria was 200 g/liter.

Next, the slurry A and slurry B were coated on a base member5. The upstream side part of this base member5in the direction of exhaust flow was immersed in the slurry A and the slurry A was sucked off from the downstream side end in the direction of exhaust flow to thereby coat the slurry A for forming the upstream side coat layer61on the partition walls of the base member5. Next, the downstream side part of the base member5in the direction of exhaust flow was immersed in the slurry B and the slurry B was sucked off from the upstream side end in the direction of exhaust flow to thereby coat the slurry B on the region in which the slurry A was not coated in the partition walls of the base member5.

In the examples according to the present disclosure, the slurry A and slurry B were coated so that the length of the total of the length Lf of the region in which the slurry A was coated and the length Lr of the region in which the slurry B was coated became the length L of the base member5in the direction of exhaust flow. In Example 1, the length Lf of the region in which the slurry A was coated was 5% with respect to the length L of the base member5in the direction of exhaust flow and the length Lr of the region in which the slurry B was coated was 95%. Note that, the volume of the base member5in this Example 1 was 2 liters, the length L in the direction of exhaust flow was 390 mm, and the diameter R of the cross-section of the base member5was 129 mm.

Next, the base member5was made to dry, then was impregnated with barium acetate. It was made to further dry, then was fired to make the partition walls sectioning the base member5carry barium Ba. In this example, the amount of barium Ba supported was 0.1 mol/liter. Due to the above process, the region in which the slurry A was coated became the upstream side coat layer61, the region in which the slurry B was coated became the downstream side coat layer62, and the NOxstorage and reduction type catalyst3of Example 1 was obtained.

Next, the methods of production of Examples 2 to 7 and Reference Examples 1 and 2 of the NOxstorage and reduction type catalyst according to present disclosure will be explained. Examples 2 to 7 and Reference Examples 1 and 2 and Example 1 differ in only the length of the region where the slurry A was coated, that is, the length Lf of the upstream side coat layer61, and the length of the region where the slurry B was coated, that is, the length Lr of the downstream side coat layer62. The methods of production were the same. Therefore, the explanation of the methods of production will be omitted.

The table shown inFIG. 8shows the ratios of the length Lf of the upstream side coat layer61and the length Lr of the downstream side coat layer62to the sum of the length Lf of the upstream side coat layer61and the length Lr of the downstream side coat layer62of Examples 1 to 7 and Reference Examples 1 and 2. In Examples 1 to 7 and Reference Examples 1 and 2, the concentrations of Ce in the slurry B for forming the downstream side coat layer62are made the same, so the length Lr of the downstream side coat layer62and the amount of Ce contained in the NOxstorage and reduction type catalyst are proportional.

As explained above, in Examples 1 to 7, the upstream side coat layer61and the downstream side coat layer62respectively contain Pt, Pd, and Rh as the precious metal catalyst8. Furthermore, the upstream side coat layer61and the downstream side coat layer62contain Ba, one of the alkali earth metals. Further, the downstream side coat layer62contains the Ce-containing oxide9.

Next, the method of production of Comparative Example 1 will be explained. Comparative Example 1 coats the slurry B with a composition the same as Example 1 on the partition walls over the entire region of the base member and therefore makes the Ce-containing oxide9be carried over the entire region of the base member. Other than this, the method is similar to Example 1, so the explanation will be omitted.

Finally, the methods of production of Comparative Examples 2 to 5 will be explained. Comparative Examples 2 to 4 and Comparative Example 1 differ only on the point of the difference in the concentration of the Ce-containing oxide9of the slurry B. On the rest of the points, Comparative Examples 2 to 4 are the same as Comparative Example 1.

That is, in each of Comparative Examples 2 to 4, the slurry B was prepared to give a content of Ce-containing oxide9as follows and the slurry B was coated on the partition walls over the entire region of the base member. Specifically, the slurry B was prepared so that, the content of ceria became 150 g/liter in Comparative Example 2, became 100 g/liter in Comparative Example 3, and became 50 g/liter in Comparative Example 4. In Comparative Example 5, the slurry A, that is, a slurry in which Ce is not contained, was coated on the partition walls over the entire region of the base member.

The table shown inFIG. 8shows the contents of Ce in Comparative Examples 1 to 5. Comparative Examples 1 to 5 had coat layers formed over the entire region of the catalyst, but as explained above, the contents of the Ce contained in the slurry forming the coat layer differ, so in the comparative examples, the Ce contents differ.

Next, the tests performed for Examples 1 to 7, Reference Examples 1 and 2, and Comparative Examples 1 to 5 will be explained. That is, first, to approach the performance of an actual vehicle, each NOxstorage and reduction type catalyst was arranged in the exhaust pipe of the diesel engine1. The exhaust generated by operating the diesel engine1was used to hold the NOxstorage and reduction type catalyst3as is at 750° C. for 50 hours to cause the NOxstorage and reduction type catalyst3to degrade. After that, the diesel engine1was operated so that the temperature of the NOxstorage and reduction type catalyst3became 150° C. The diesel engine1continued to be operated until the cumulative amount of NOxpassing through the NOxstorage and reduction type catalyst3became 500 mg. Note that, at this time, the cumulative amount of the HC passing through the NOxstorage and reduction type catalyst3was 390 mg.

Through this test, the NOxstorage rate and the HC removal rate were respectively evaluated. The NOxstorage rate is calculated by subtracting the amount of NOxdetected downstream of the NOxstorage and reduction type catalyst3from the amount of NOxdetected upstream of the NOxstorage and reduction type catalyst3to thereby obtain the NOxstorage amount of the NOxstorage and reduction type catalyst3, then dividing this NOxstorage amount by the amount of NOxdetected upstream of the NOxstorage and reduction type catalyst3.

Similarly, the HC removal rate is calculated by subtracting the amount of HC detected downstream of the NOxstorage and reduction type catalyst3from the amount of HC detected upstream of the NOxstorage and reduction type catalyst3to thereby obtain the HC removal amount of the NOxstorage and reduction type catalyst3, then dividing this HC removal amount by the amount of HC detected upstream of the NOxstorage and reduction type catalyst3.

FIG. 9shows the relationship between the Ce content and HC removal rate. The points e1to e7ofFIG. 9correspond to Examples 1 to 7, the points r1and r2correspond to Reference Examples 1 and 2, and the points c1to c5correspond to Comparative Examples 1 to 5.

First, Examples 1 to 7 shown by points e1to e7and Reference Examples 1 and 2 shown by points r1and r2will be explained. As an overall trend, it will be understood that the HC removal rate is improved along with a drop in the Ce content. In particular, in the region of a Ce content of 75% or more (Examples 1 to 4), the removal rate of HC rapidly rises along with the decrease of the Ce content, while in the region of a Ce content of less than 75% (Examples 5 to 7 and Reference Examples 1 and 2), the removal rate of HC slowly rises along with the decrease of the Ce content. Such a phenomenon is believed to occur as follows:

That is, the slurry A coated on the upstream side coat layer61does not contain Ce, so the precious metal catalyst8contained in the upstream side coat layer61suppresses the drop in activity due to Ce. On the other hand, the slurry B coated on the downstream side coat layer62contains Ce and the precious metal catalyst8mixed together, so Ce is sometimes positioned near the precious metal catalyst8. Therefore, part of the precious metal catalyst8falls in activity due to Ce.

Now then, the majority of the HC contained in the exhaust is oxidized within a range of 25% of the NOxstorage and reduction type catalyst3at the upstream side in the direction of exhaust flow. Therefore, the upstream side 25% range of precious metal catalyst8greatly contributes to the HC removal rate. Therefore, if there is no Ce in the upstream side 25% range, the HC removal rate is greatly improved. As opposed to this, the downstream side 75% range of precious metal catalyst does not greatly contribute to the HC purification, so even if there is no Ce in the downstream side 75% range, the HC removal rate does not rise that much. For this reason, the relationship between the Ce content and HC removal rate is believed to become the relationship such asFIG. 9.

Next, Comparative Examples 1 to 5 will be explained. In Comparative Examples 1 to 5, the HC removal rate rises along with a drop in the Ce content. In this case, in Comparative Examples 1 to 5, the correlation between the Ce content and the HC removal rate can be approximated by a line (see broken line ofFIG. 9). Comparing the approximation line formed by the comparative examples and Examples 1 to 7 and Reference Examples 1 and 2 corresponds to comparing the case of uniformly reducing the concentration of Ce of the NOxstorage and reduction type catalyst3(comparative examples) and the case of successively removing Ce from the upstream side (examples and reference examples). Examples 1 to 7 and Reference Examples 1 and 2 are positioned higher than the approximation line. Therefore, it will be understood that the HC removal performance is improved by removing Ce successively from the upstream side.

Next, the NOxstorage rate will be explained.FIG. 10shows the relationship between the Ce content and the NOxstorage rate. First, Examples 1 to 7 (points e1to e7) and Reference Examples 1 and 2 (points r1and r2) will be explained. Referring toFIG. 10, in the region where the Ce content is 87.5% or more (Examples 1 to 3), the NOxstorage rate greatly increases along with a decrease in the Ce content. This phenomenon can be understood as follows.

That is, due to the removal of Ce at the upstream side of the NOxstorage and reduction type catalyst3, the drop in activity of the precious metal catalyst8at the upstream side of the NOxstorage and reduction type catalyst3is suppressed. As a result, the oxidation of the HC by the precious metal catalyst8is promoted. Further, if an upstream side coat layer61with no Ce is formed, NO is oxidized well to NO2at the upstream side coat layer61. On the other hand, at this time, the HC removal amount at the upstream side coat layer61increases, so the amount of HC which contacts the generated NO2and the NO2in the exhaust decreases. As a result, the amount of NO2reduced to NO by the HC decreases, therefore, the exhaust flowing over the downstream side coat layer62is raised in NO2concentration. Furthermore, if the HC removal amount of the upstream side coat layer61increases, the amount of HC flowing into the downstream side coat layer62decreases, so the NO in the exhaust is oxidized to NO2on the precious metal catalyst8of the downstream side coat layer62as well and therefore the exhaust flowing over the downstream side coat layer62is sufficiently raised in NO2concentration.

In this way, due to the NO2concentration at the downstream side coat layer62being raised, the opportunities for the Ce-containing oxide9and NO2to chemically bond increase and a greater amount of NO2is adsorbed at the Ce-containing oxide9. Therefore, due to the elimination of Ce at the upstream side of the NOxstorage and reduction type catalyst3, Ce can adsorb more NO2. As a result, the NOxstorage rate greatly increases.

On the other hand, in the region where the Ce content is less than 87.5% (Examples 4 to 7 and Reference Examples 1 and 2), along with the reduction in the Ce content, the NOxstorage rate falls. That is, if the length of the downstream side coat layer62becomes shorter, the opportunities for Ce to adsorb NO2decrease. As a result, the amount of NO2adsorbed by Ce decreases. Therefore, as the Ce content decreases, that is, as the length of the downstream side coat layer62becomes shorter, the NOxstorage rate decreases.

Explained another way, in the region of a Ce content of less than 87.5%, the effect of the NOxstorage rate falling due to the amount of Ce able to adsorb NO2being reduced becomes greater than the effect of increasing the amount of NO2and improving the NOxstorage rate due to the drop in the activity of the precious metal catalyst being suppressed, so along with a reduction in the amount of Ce, the NOxstorage rate falls.

Next, Comparative Examples 1 to 5 (c1to c5) will be explained. In Comparative Examples 1 to 5, the NOxstorage rate falls along with the reduction in the Ce content. In this case, in Comparative Examples 1 to 5, the correlation between the Ce content and HC removal rate can be approximated by an approximation line (see broken line inFIG. 9). As shown inFIG. 10, at a Ce content of 50 to 95%, the NOxstorage rates of Examples 1 to 6 are higher values than the NOxstorage rates of the comparative examples obtained by the approximation line. Therefore, it will be understood that the NOxstorage rate is improved.

Note that, despite the Ce content being 0%, the NOxstorage rate is a value close to 40%, but this is believed to be an effect of the absorption of NOxby Ba.

FromFIG. 9andFIG. 10, it will be understood that if making the length Lf of the upstream side coat layer a length of 5 to 50% with respect to the length Lf+Lr of the total of the upstream side coat layer61and the downstream side coat layer62, both the removal rate of HC and the storage rate of NOxare improved.

Finally, the results of evaluation based on the NOxstorage amount and HC removal amount will be shown. First, to evaluate the effects of the Examples 1 to 7 and Reference Examples 1 and 2, comparative examples are compared with. For this reason, first, points on an approximation line of comparative examples having the same Ce contents as the working examples and reference examples are found. Next, the total amount of the NOxstorage amount and the HC removal amount at the point on the approximation line corresponding to each of the working examples and reference examples is subtracted from the total amount of the NOxstorage amount and HC removal amount of each of the working examples and reference examples to find the increase in the total amount of the NOxstorage amount and the HC removal amount.FIG. 11shows the relationship between the increase in the total amount of the NOxstorage amount and HC removal amount obtained in this way and the ratio (Lf/(Lf+Lr)) of the length Lf of the upstream side coat layer61with respect to the length Lf+Lr of the total of the length Lf of the upstream side coat layer61and the length Lr of the downstream side coat layer62.

InFIG. 11, the range where a positive value is taken is the range of increase of the total amount of the NOxstorage amount and HC removal amount with respect to the comparative examples. That is, it will be understood that the total amount of the NOxstorage amount and HC removal amount increases with respect to the comparative examples while the ratio Lf/(Lf+Lr) of the length Lf of the upstream side coat layer61with respect to the length Lf+Lr of the total of the length Lf of the upstream side coat layer61and the length Lr of the downstream side coat layer62is 5 to 62.5%. Therefore, in the examples according to the present disclosure, the length Lf of the upstream side coat layer61is made a length of 5 to 62.5% with respect to the length Lf+Lr of the total of the length Lf of the upstream side coat layer61and the length Lr of the downstream side coat layer62.

That is, the NOxstorage and reduction type catalyst3according to the present disclosure is provided with a base member5extending in the direction of exhaust flow and a coat layer6arranged on the base member5. The coat layer6is provided with an upstream side coat layer61arranged on the upstream side in the direction of exhaust flow and a downstream side coat layer62arranged at the downstream side in the direction of exhaust flow from the upstream side coat layer61. The upstream side coat layer61does not contain the Ce-containing oxide9but contains the precious metal catalyst8, while the downstream side coat layer62contains the Ce-containing oxide9and precious metal catalyst8. The length Lf of the upstream side coat layer61is a length of 5 to 62.5% with respect to the length Lf+Lr of the total of the upstream side coat layer61and the downstream side coat layer62. The remaining part of the coat layer besides the upstream side coat layer61becomes the downstream side coat layer62. That is, the length Lr of the downstream side coat layer62becomes a length of 37.5 to 95% with respect to the length Lf+Lr of the total of the upstream side coat layer61and the downstream side coat layer62.

If using such an NOxstorage and reduction type catalyst3provided with the upstream side coat layer61and downstream side coat layer62, it is possible to make the total amount of the NOxstorage rate and HC removal rate increase. This can be confirmed by the increase in the total amount of the NOxstorage amount and HC removal amount inFIG. 11being 0 or more. Note that, in this case, as will be understood fromFIG. 11, if making the length Lf of the upstream side coat layer61a length of 10 to 50% with respect to the length Lf+Lr of the total of the length Lf of the upstream side coat layer61and the length Lr of the downstream side coat layer62, the increase of the total amount of the NOxstorage amount and the HC removal amount becomes 20 mg or more and it is possible to greatly increase the total amount of the NOxstorage amount and the HC removal amount. In some embodiments, the length Lf of the upstream side coat layer61is made a length of 10 to 50% of the length Lf+Lr of the total of the length Lf of the upstream side coat layer61and the length Lr of the downstream side coat layer62.

In this regard, in legal regulations relating to exhaust of a vehicle provided with a diesel engine1, the total amount of the NOxexhaust amount and the HC exhaust amount is a parameter of the regulations. Therefore, by determining the length Lf of the upstream side coat layer61and the length Lr of the downstream side coat layer62so that the total amount of the NOxstorage amount and the HC removal amount becomes the maximum, it becomes possible to suitably deal with the legal regulations.

Note that, the relative relationship between the NOxamount and the HC amount discharged from the diesel engine1changes according to the operating state of the diesel engine1. However, for example, when the NOxexhaust amount is 500 mg, it is believed that the HC exhaust amount will only change by an extent of about 50 mg with respect to 390 mg.

In this case, even if the HC exhaust amount changes by this extent of amount with respect to the NOxexhaust amount, the total amount of the NOxadsorption amount and HC removal amount shown inFIG. 11does not change much at all. Therefore, it can be said to be practically sufficient to determine the length Lf of the upstream side coat layer61and the length Lr of the downstream side coat layer based on the total amount of the NOxadsorption amount and HC removal amount shown inFIG. 11.

Note that, as a modification, separate coat layers from the upstream side coat layer61and the downstream side coat layer62can be laminated above and below the upstream side coat layer61and the downstream side coat layer62. Even if using such a modification, the total amount of the NOxadsorption amount and HC removal amount have the relationship such as shown inFIG. 11.