EXHAUST PURIFICATION CATALYST

, BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates to an exhaust purification catalyst that purifies harmful components in exhaust gas from an automobile. More specifically, the present invention relates to an exhaust purification catalyst that exhibits increased oxidation activity at low temperatures. The exhaust purification catalyst according to the present invention is useful as an oxidation catalyst that is placed upstream of a particulate filter or a filter catalyst in the exhaust system of a diesel engine.

2. Description of the Related Art

[0002] Oxidation catalysts that includes a noble metal such as Pt supported on a porous carrier such as alumina are widely used as a catalyst for purifying exhaust gas from an automobile engine. Oxidation catalysts allow for efficient oxidation and purification of HC and CO.

[0003] ThuSj oxidation catalysts are commonly placed upstream of a particulate filter or a filter catalyst in the exhaust system of a diesel engine. As the temperature of exhaust gas increases due to the reaction heat in the oxidation catalyst, oxidation of particulates that are trapped the particulate filter or filter catalyst is promoted, thereby

making it possible to extend the length of time for which the oxidation catalyst can be used before a filter regeneration process is required.

[0004] However, oxidation catalyst, oxidation and purification of HC are difficult until the temperature of the noble metal reaches at least an activation temperature. This leads to the problem of low purification activity for HC at low temperatures, such as at start-up. In particular, the exhaust gas from a diesel engine contains a large amount of high-boiling HC, which further makes combustion at a low temperature difficult. Also, it is now being contemplated to add light oil into diesel exhaust gas for reduction and purification of NO _x . In this case, there is a problem in that the noble metal becomes deactivated as unburned light oil contained in the exhaust gas covers the noble metal.

[0005] Accordingly, as described in Japanese Patent Application Publication No. 06-205983 (JP-A-Oδ-205983), it is common to set the density of supported noble metal high on the upstream side of the catalyst. On the upstream side of the catalyst, exhaust gas that is yet to become a laminar flow collides against the cell walls of the catalyst, so the temperature of the catalyst increases quickly, and the noble metal reaches the activation temperature at a relatively early stage. Then, after reaching the activation temperature, the temperature further rises due to the reaction heat, and the temperature increase on the downstream side of the catalyst is promoted, thereby enhancing purification performance at low temperatures.

[0006] Also, Japanese Patent Application Publication No. 2002-210371 (JP-A-2002-210371) proposes making the heat capacity of a HC adsorbent layer small on the upstream side and large on the downstream side, in the case of an exhaust purification catalyst that has a two-layer structure formed by the HC adsorbent layer and a catalyst layer. According to this exhaust purification catalyst, because the temperature increases quickly on the upstream side, the catalyst layer activates at an early stage. On the other hand, because the temperature does not readily increase on the downstream side, HC released from the upstream side is re-adsorbed on the downstream side. Therefore, it is possible to reduce the amount of HC released until the noble metal in the catalyst layer on the downstream side activates.

[0007] However, it cannot be said that even the above-mentioned techniques according to the related art provide sufficient oxidation activity at low temperatures.

SUMMARY OF THE INVENTION

[0008] The present invention further enhances oxidation activity at low temperatures, in an exhaust purification catalyst, such as an oxidation catalyst.

[0009] An exhaust purification catalyst according to an aspect of the present invention includes: a carrier substrate; an upstream catalyst layer that is formed on an exhaust-gas upstream side of the carrier substrate, wherein the upstream catalyst layer has

on its upstream portion a high loading portion in which a loading density of noble metal is higher than in the other portion of the upstream catalyst layer; and a downstream catalyst layer that is formed on the carrier substrate on an exhaust-gas downstream side of the upstream catalyst layer, in which the downstream catalyst layer has a heat capacity higher than a heat capacity of the upstream catalyst layer.

[0010] The heat capacity of the upstream catalyst layer may be 40 to 75% of the heat capacity of the downstream catalyst layer, In addition, the loading density of noble metal on the high loading portion may be 3 to 7 g per liter of the carrier substrate. The high loading portion may be formed at a length that is 2/15 to 1/3 of a total length of the upstream catalyst layer and the downstream catalyst layer.

[0011] It should be noted that while the expression "per liter of the carrier substrate" means "per liter of the entire bulk volume including the net volume of the carrier substrate, plus the volume of the cell channel and the like", in this specification, this will be simply referred to as "per liter of the carrier substrate".

[0012] In an exhaust purification catalyst according to an aspect of the present invention, the heat capacity of the downstream catalyst layer is larger than the heat capacity of the upstream catalyst layer, and the upstream catalyst layer has on its upstream portion a high loading portion in which the loading density of noble metal is higher than in the other portion of the upstream catalyst layer. The noble metal loaded at a high density on the

high loading portion enhances oxidation activity at low temperatures. Moreover, since the high loading portion is formed in the upstream portion of the upstream catalyst layer with a low heat capacity, the downstream portion of the upstream catalyst layer is also warmed up at an early stage, thereby further enhancing oxidation activity.

[0013] Then, exhaust gas that has been raised in temperature due to the reaction heat in the upstream catalyst layer comes into contact with the downstream catalyst layer. At this time, since the exhaust gas temperature is sufficiently high, and the thermal energy per unit flow of exhaust gas is large, the downstream catalyst layer is also raised in temperature at an early stage, which contribute to oxidation reaction.

[0014] Therefore, with an exhaust purification catalyst according to an aspect of the present invention, exhaust gas can be sufficiently purified even from low temperatures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein:

FIG 1 is a perspective view showing an exhaust purification catalyst according to an embodiment of the present invention;

FIG 2 is a main-portion sectional view showing an exhaust purification catalyst according to an embodiment of the present invention;

FIG. 3 is a graph showing the CO purification rate after an endurance test;

FIG. 4 is a graph showing the relationship between the amount of formation of an upstream catalyst layer and the CO purification efficiency;

FIG. 5 is a graph showing the relationship between the loading density of Pt on a high loading portion and the CO purification efficiency; and

FIG. 6 is a graph showing the relationship between the length of the high loading portion and the CO purification efficiency.

DETAILED DESCRIPTION OF EMBODIMENTS

[0016] An exhaust purification catalyst according to an embodiment of the present invention includes a carrier substrate, an upstream catalyst layer, and a downstream catalyst layer. As the carrier substrate, one having a honeycomb shape, a foam shape, or a pellet shape may be used. The material of the carrier substrate is not particularly limited, and a known carrier substrate made of ceramic such as cordierite or SiC, or one made of metal can be used.

[0017] The upstream catalyst layer and the downstream catalyst layer are formed on the surface of the carrier substrate. The carrier that forms the base of each of

the upstream catalyst layer and the downstream catalyst layer may be selected from a simple substance of alumina, ceria, zirconia, titania, silica, zeolite, or the like or a mixture thereof, or a composite oxide made of a plurality of kinds selected from these. The upstream catalyst layer and the downstream catalyst layer may be formed on different carriers, or may be formed from the same carrier.

[0018] A noble metal is loaded on each of the upstream catalyst layer and the downstream catalyst layer. Examples of suitable noble metals include Pt, Pd, Rh, and Ag.

Depending on the case, a base metal such as Fe, Co, W, or Cu may be used in combination with the noble metal.

[0019] The loading density of the noble metal may be set in a range of 0.1 to 10 g per liter of the honeycomb substrate, with respect to each of the upstream catalyst layer and the downstream catalyst layer.

[0020] The heat capacity of the downstream catalyst layer is greater than the heat capacity of the upstream catalyst layer. This may be accomplished by using different carriers for the upstream catalyst layer and the downstream catalyst layer. Different heat capacities between the upstream and downstream catalyst layers may be imparted by adjusting the amount of carrier contained in each respective catalyst layer. Specifically, if carriers with substantially the same heat capacity are contained in the upstream catalyst layer and the downstream catalyst layer, the amount of carrier contained in the downstream

catalyst layer may be set greater than in the upstream catalyst layer. According to the method of adjusting the amount of carrier mentioned above, the upstream catalyst layer and the downstream catalyst layer may use the same kind of carrier , which proves advantageous in designing the performance of the catalyst.

[0021] It is desirable for the heat capacity of the upstream catalyst layer to be 40 to 75% of the heat capacity of the downstream catalyst layer, This may be accomplished by setting the amount of formation of the upstream catalyst layer be 40 to 75% of the amount of formation of the downstream catalyst layer. For example, if the amount of formation of the downstream catalyst layer is 200 g per liter of carrier substrate, the amount of formation of the upstream catalyst layer may be set to 80 g to 150 g per liter of carrier substrate.

[0022] It is desirable that each of the upstream catalyst layer and the downstream catalyst layer be formed in a range of 30 to 400 g per liter of carrier substrate. If the amount of formation falls below this range, grain growth may occur in the noble metal during use, resulting in degradation. If the amount of formation is beyond this range, exhaust backpressure increases.

[0023] Preferably, the proportional length of the upstream catalyst layer is within a range of 1/5 to 1/2 of the full length in the exhaust gas flow direction of the carrier substrate from the upstream end face of the carrier substrate, and the length of the

downstream catalyst layer extends over the remainder of the length of the carrier substrate. If the proportional length of the upstream catalyst layer is smaller than 1/5 of the length of the carrier substrate, low-temperature oxidation activity does not improve. In contrast, if the proportional length of the upstream catalyst layer is larger than 1/2 of the length of the carrier substrate, it is difficult to make the heat capacity of the downstream catalyst layer be larger than that of the upstream catalyst layer.

[0024] Further, the exhaust purification catalyst according to this embodiment has, at least in an upstream portion of the upstream catalyst layer, a high loading portion where the loading density of noble metal is higher than in the other portion of the upstream catalyst layer. The effect created by the provision of this high loading portion and the effect created by the low heat capacity work synergistically to enhance the low temperature oxidation activity of the upstream catalyst layer.

[0025] The loading density of noble metal on the high loading portion is desirably 3 to 7 g per liter of carrier substrate, If the loading density is less than 3 g/L, ignitability at low temperatures decreases, which degrades purification performance. If the loading density exceeds 7 g/L and the loading amount of noble metal is uniform over the entire catalyst, the loading density of noble metal becomes too low on the downstream side, causing a decrease in purification performance, To attain the same purification performance, it is necessary to increase the loading density of noble metal on the

downstream side, thereby increasing cost.

[0026] Preferably, the proportional length of the high loading portion is 2/15 to 1/3 of the total length of the upstream catalyst layer and downstream catalyst layer (full catalyst length). If the proportional length of the high loading portion is shorter than 2/15, ignitability at low temperature decreases,, which degrades purification performance. If the proportional length exceeds 1/3 and the loading amount of noble metal is uniform over the entire catalyst, the loading density of noble metal becomes too low on the downstream side, thereby reducing purification performance,

[0027] The noble metals Pt or Pd, both of which have excellent oxidation activity, are preferably loaded on the high loading portion.

[0028] Hereinafter, the present invention will be described specifically by way of Examples, Comparative Examples, and Test Examples.

[0029] (Example 1) FIGs. 1 and 2 schematically show an exhaust purification catalyst according to this embodiment. The exhaust purification catalyst includes a honeycomb substrate 1, an upstream catalyst layer 2 that is coated on the cell barrier surface within a range of 75 mm from the exhaust gas inflow-side end face of the honeycomb substrate 1, and a downstream catalyst layer 3 that is coated on the cell barrier surface within a range of 75 mm from the exhaust gas inflow-side end face of the honeycomb substrate 1. The upstream catalyst layer 2 has a high density portion 20, which is located

within a range of 30 mm from the exhaust gas inflow-side end, where the loading density of Pt is high, and a normal density portion 21 located downstream of the high density portion 20 where the loading density of Pt is equal to that in the downstream catalyst layer 3.

[0030] Hereinafter, a method of producing this exhaust purification catalyst will be described instead of giving a detailed description of its structure.

[0031] A powder of Y-Al ₂ Os stabilized with lanthanum (La) was impregnated with a dinitrodiamine platinum solution, dried and baked to prepare PtZAl ₂ Os catalyst powder. Then, 50 mass part of the Pt/Al ₂ θ ₃ catalyst powder, 50 mass part of zeolite (ZSM-5) powder, 10 mass part of alumina sol (AI2O3: 10 mass%) as a binder, and distilled water were mixed to prepare a slurry for the upstream catalyst layer.

10032] Next, a honeycomb substrate 1 made of cordieritε (with a diameter of 129 mm, a cell density of 400/in ² , and a length of 150 mm) is provided, and the portion of the honeycomb substrate 1 wthin 75 mm of the upstream end of the honeycomb substrate is immersed in the above slurry. After the honeycomb substrate 1 was taken out, and excess slurry was blown off, the honeycomb substrate 1 was dried and baked to form the upstream catalyst layer 2, The upstream catalyst layer 2 is formed in an amount of 101.5 g per liter of the honeycomb substrate 1, and 1.5 g of Pt is supported per liter of the honeycomb substrate 1.

[0033] Subsequently, a predetermined amount of dinitrodiamine platinum

solution was absorbed by the portion of the upstream catalyst layer 2 iocated within a range of 30 mm from the exhaust gas inflow-side end face of the honeycomb substrate 1, followed by drying and baking. Thus, an additional 2,5 g/L of Pt is supported on the high density portion 20 on the portion of the upstream catalyst layer 2 located within 30 mm from the upstream end.

[0034] That is, 4.0 g/L of Pt is supported at the high density portion 20 of the upstream catalyst layer 2, which has a length of 30 mm, and 1.5 g/L of Pt is supported at the normal density portion 21, which is located downstream of the high density portion 20, which has a length αf 45 mm. The upstream catalyst layer 2 is formed in 101.5 g/L as a whole, including the high density portion 20 and the normal density portion 21.

[0035] Subsequently, a powder of γ-Al ₂ U ₃ stabilized with La was impregnated with a dinitro diamine platinum solution, was dried and baked to prepare Pt/Al ₂ θ ₃ catalyst powder. Then, 50 mass part of this Pt/Al ₂ θ ₃ catalyst powder, 50 mass part of zeolite (ZSM-5) powder, 10 mass part of alumina sol (AI ₂ O ₃ : 10 mass%) as a binder, and distilled water were mixed to prepare a slurry for downstream catalyst layer.

[0036] The portion of the honeycomb substrate 1 on which the upstream catalyst layer 2 was formed is immersed in the above-mentioned slurry. After the honeycomb substrate 1 is removed, and excess slurry was blown off, the honeycomb substrate 1 is baked to form the downstream catalyst layer 3. The downstream catalyst layer 3 is formed

in an amount of 201.5 g per liter of the honeycomb substrate 1, and 1.5 g of Pt is supported per liter of the honeycomb substrate 1.

[0037] (Comparative Example 1) By using a slurry prepared in the same manner as in Example 1, a catalyst layer was formed on the entirety of the same honeycomb substrate 1 as that in Example 1. The catalyst layer is formed in an amount of 202 g per liter of the honeycomb substrate 1, and 2,0 g of Pt is supported per liter of the honeycomb substrate 1.

[0038] (Comparative Example 2) By using a slurry prepared in the same manner as in Example 1, a catalyst layer was formed on the entirety of the same honeycomb substrate 1 as that in Example 1. The catalyst layer is formed in an amount of 201.5 g per liter of the honeycomb substrate 1, and 1,5 g of Pt is supported per liter of the honeycomb substrate 1.

[0039] Next, a dinitrodiamine platinum solution was applied to and absorbed by the portion of the catalyst layer that is located within a range of 30 mm from the upstream end of the honeycomb substrate 1, followed by drying and baking. Thus, a high density portion where an additional 2.5 g/L of Pt is supported and hence 4.0 g/L of Pt is supported in total, is formed in the portion of the catalyst layer located within 30 mm on the upstream side.

[0040] (Comparative Example 3) By using a slurry prepared in the same

manner as in Example 1, the portion of the same honeycomb substrate 1 as that in Example 1 that is located within a range of 75 mm from the exhaust gas inflow-side end is immersed in the slurry. The honeycomb substrate 1 is then taken out, and excess slurry was blown off before being baked to form an upstream catalyst layer. The upstream catalyst layer is formed in an amount of 102 g per liter of the honeycomb substrate 1, and 2.0 g of Pt is supported per liter of the honeycomb substrate 1.

[0041] Subsequently, by using a slurry prepared in the same manner as in Example 1, the portion of the honeycomb substrate 1 with the upstream catalyst layer formed thereon which is located within a range of 75 mm from the downstream end was immersed in the slurry. After the honeycomb substrate 1 was taken out, and excess slurry was blown off, the honeycomb substrate 1 is baked to form a downstream catalyst layer. The downstream catalyst layer is formed in an amount of 202 g per liter of the honeycomb substrate 1, and 2.0 g of Pt is supported per liter of the honeycomb substrate 1.

[0042] <Test Example 1> Table 1 shows the configuration of catalysts according to Example 1 and Comparative Examples.

[Tablel]

Upstream catalyst layer Downstream catalyst layer

Pt density (g/L) Coating Pt density Coating

[0043] An endurance test was carried out with respect to the catalysts according to Example 1 and Comparative Examples by holding the respective catalysts in an electric furnace at 700 ⁰ C for five hours, The respective catalysts after the endurance test were placed on the underfloor (UF) of a vehicle equipped with a 2.2L diesel engine, and their respective CO purification efficiencies when the vehicle was running in an EC-Cold mode were measured. The results are shown in FIG 3.

[0044] It can be seen-from FIG. 3 that the catalyst according to Example 1 provides a higher CO purification efficiency than the catalysts according to Comparative Examples, and exhibits extremely high activity at low temperatures. Even the sum total of

the CO purification efficiencies of the catalysts according to Comparative Example 2 and Comparative Example 3 falls short of the CO purification efficiency of the catalyst according to Example 1. From this, it is apparent that in the case of ^' the catalyst according to Example 1, the effect created by making the heat capacity of the upstream catalyst layer smaller than that of the downstream catalyst layer, and the effect created by forming the high density portion 20 of Pt in an upstream portion of the upstream catalyst layer worked synergistically.

[0045] <Test Example 2> Eight catalysts having the upstream catalyst layer 2, the downstream catalyst layer 3, and the high density portion 20 were prepared in the same manner as in Example 1, except for varying the amount of formation of the upstream catalyst layer 2 between 31.5 g/Land 201.5 g/L. Therefore, a catalyst in which the amount of formation of the upstream catalyst layer is 201.5 g corresponds to the catalyst according to Comparative Example 2 mentioned above.

[0046] The same endurance test as that in Test Example 1 was carried out with respect to the respective catalysts, and the CO purification efficiencies of the respective catalysts after the endurance test were measured in the same manner as in Test Example 1. The results were arranged with respect to the amount of formation of the upstream catalyst layer 2, and are shown in FIG, 4.

[0047] It can be seen from FIG. 4 that the CO purification performance is

particularly high when the amount of formation of the upstream catalyst layer 2 is in a range of 81.5 g/L to 151.5 g/L, Therefore, it is apparent that the amount of formation of the upstream catalyst layer 2 is desirably 40 to 75% of the amount of formation (201.5 g/L) of the downstream catalyst layer 3. In addition, because the upstream catalyst layer 2 and the downstream catalyst layer 3 are formed from the same material, it is also apparent that the heat capacity of the upstream catalyst layer 2 is preferably 40 to 75% of the heat capacity of the downstream catalyst layer 3.

[0048] <Test Example 3> Catalysts were prepared in the same manner as in Example 1, except for varying the density of Pt supported on the high density portion 20 in nine levels within a range of 2.0 g/L to 9.0 g/L. The same endurance test as that in Test Example 1 was carried out for each catalyst, and the CO purification efficiencies of the respective catalysts after the endurance test were measured in the same manner as in Test Example 1. The results were arranged with respect to the density of Pt supported on the high density portion 20, and are shown in FIG. 5.

[0049] From FIG 5, it is apparent that the density of Ft supported on the high density portion 20 is desirably in a range of 3.0 g/L to 7.0 g/L, and particularly desirably in the vicinity of 4.0 g/L.

[0050] <Test Example 4> Catalysts were prepared in the same manner as in Example 1, except for varying the formation length of the high density portion 20 from the

exhaust gas inflow-side end face in eight levels from zero to 70 mm. The same endurance test as that in Test Example 1 was carried out for each catalyst, and the CO purification efficiencies of the respective catalysts after the endurance test were measured in the same manner as in Test Example 1. The results were arranged with respect to the length of the high density portion 20, and are shown in FIG. 6.

[0051] From FIG. 6, it is apparent that the length of the high density portion 20 is desirably in a range of 20 mm to 50 mm, and preferably in a range of 2/15 to 1/3 of the total length of the upstream catalyst layer 2 and downstream catalyst layer 3.