CA2485893C - Catalyst for purifying exhaust gases - Google Patents
Catalyst for purifying exhaust gases Download PDFInfo
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Abstract
A catalyst for purifying exhaust gases includes a substrate including an exhaust-gas passage, a coating layer formed on the exhaust-gas passage, and a catalytic ingredient loaded on the coating layer. A loading density of the catalytic ingredient on an upstream area discriminated from a downstream area by a predetermined length from an upstream end of the substrate is greater than a loading density of the catalytic ingredient on the downstream area. The catalytic ingredient is loaded more on the downstream area of the exhaust-gas passage than on the upstream area thereof. Thus, it is possible to suppress the catalytic ingredient being poisoned by poisoning substances.
Description
CATALYST FOR PURIFYING EXHAUST GASES
BACKGROUND OF THE INVENTION
Field of the Invention X0001] The present invention relates to a catalyst for purifying exhaust gases, mainly for automotive applications. More particularly, it relates to a catalyst for purifying exhaust gases which can suppress the degradation by poisoning.
Description of the Related Art (0002 Automotive exhaust systems have been equipped with a variety of catalysts for purifying exhaust gases, such as oxidizing catalysts, three-way catalysts and NOX sorbing-and-reducing catalysts, in order to remove HC, CO and NOX in exhaust gases by oxidation and/or reduction. For example, three-way catalysts have been produced using a honeycomb-shaped substrate formed of cordierite or metallic foils in the following manner. A coating layer is formed on a surface of the cellular passages of the substrate using alumina and/or ceria. Then, a catalytic ingredient, such as Pt and Rh, is loaded on the coating layer. The resulting three-way catalysts have been used in an exhaust atmosphere produced by burning an air-fuel mixture whose air-fuel (A/F) ratio is controlled around 14.6, the stoichiometric ratio. Thus, the three-way catalysts purify HC and CO by oxidizing them, and simultaneously purify NOX
by reducing them.
~0003~ When fuels containing additives such as Pb and Mn are used, exhaust gases contain Pb and Mn components therein. Accordingly, there arises a drawback that the catalytic-ingredient active points in catalysts have been covered with the Pb and Mn components to degrade the activity. Moreover, it has been known that P, Zn and Ca contained in engine oils cause similar drawbacks.
0004 ~ Japanese Unexamined Patent Publication (KOKAI) No.
2002-172,329 discloses a catalytic structure comprising a trapping layerfor collecting catalyst-poisoning componentsin exhaustgases.
The trapping layer is disposed in proximity to the upstream-end surface of an NOX sorbing-and-reducing catalyst. The catalytic structure can suppress the NOX sorbing member of the NOX
sorbing-and-reducing catalyst being poisoned bysulfur, because the trapping layer collects sulfur components.
(0005 The trapping layer disclosed in the patent publication contains a trapping component which can react with sulfur components to trap sulfur components, thereby suppressing the sulfur poisoning of the NOX sorbing member only. However, the patent publication neither sets forth nor suggests suppressing the poisoning of catalytic ingredients such as Pt . Moreover, the patent publication is silent on another poisoning substances, such as Pb and Mn, other than sulfur.
0006) The present invention has been developed in view of such circumstances . It is therefore an obj ect of the present invention to suppress the poisoning degradation of catalytic ingredients by poisoning substances, such as Pb and Mn, thereby improving the durability of catalysts for purifying exhaust gases.
~0007~ A catalyst for purifying exhaust gases according to the present invention comprises:
a substrate comprising an exhaust-gas passage;
a coating layer formed on the exhaust-gas passage; and a catalytic ingredient loaded on the coating layer, wherein a loading density of the catalytic ingredient on a downstream area discriminated from an upstream area by a predetermined length from an upstream end of the substrate is greater than a loading density of the catalytic ingredient on the ~zpstream area.
(0008) It is desirable that the upstream area of the exhaust-gas passage can be free from the catalytic ingredient loaded.
(0009) Moreover, it is preferable that the predetermined length from an upstream end can fall in a range of less than 30~ of an overall length of the substrate. In addition, it is desirable that the predetermined length from an upstream end can fall in a range of from 5 to 20~ of an overall length of the substrate.
(0010) The present catalyst for purifying exhaust gases can suppress the poisoning substances such as Pb and Mn poisoning the catalytic ingredient. Therefore, the present catalyst exhibits high activities even after it is subjected to a durability test, because the degradation of the activities of the catalytic ingredient is suppressed.
(OOliJ In particular, when the length of .the upstream area of the exhaust-gas passage which is free from the loaded catalytic ingredient falls in a range of less than 30 0 of the overall length of the substrate, the present catalyst exhibits activities equal to or higher than those of conventional catalysts in which catalytic ingredients are loaded over the entire length.
(0012) A more complete appreciation of the present invention and many of its advantages will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings and detailed specification, all of which forms a part of the disclosure.
(0013) Fig. 1 is a cross-sectional view for roughly illustrating an arrangement of a catalyst for purifying exhaust gases according to Example No. 1 of the present invention.
(0014) Fig. 2 is an explanatory diagram for illustrating how to find a cross conversion.
(0015) Fig. 3 is a graph for illustrating relationships between CO-NOX cross conversions and proportions of a length of an upstream area of an exhaust-gas passage with respect to an overall length of a substrate.
DETAILED DESCRIpTT_ON OF THE PREFERRED EMBODT_MENTS
(0016) Having generally described the present invention, a further understanding can be obtained by reference to the specific preferred embodiments which are provided herein for the purpose of illustration only and not intended to limit the scope of the appended claims.
(0017) According to studies carried out by the present inventors, it has been apparent that the poisoning degradation of catalytic ingredients by poisoning substances such as Pb and Mn occur intensively on the upstream part of catalysts when catalysts are brought into contact with exhaust gases containing Pb and Mn.
Accordingly, in the present catalyst for purifying exhaust gases, the loading density of the catalytic ingredient on the downstream area is greater than that on the upstream area discriminated by a predetermined length from the upstream end of the substrate.
Consequently, since the catalytic ingredient is loaded less on the upstream area in which the catalytic ingredient is likely to be degraded due to poisoning by poisoning substances, and is loaded more on the downstream area of the exhaust-gas passage excepting the upstream area, it is suppressed that the catalytic ingredient is poisoned by poisoning substances. Therefore, not only the present catalyst can utilize the catalytic ingredient effectively, but also it can suppress the activities degrading after being subjected to a durability test.
(0018 Poisoning substances adsorb or deposit mainly onto the upstream area of the exhaust-gas passage. Hence, when the loading density of the catalytic ingredient on the upstream area is made less than the loading density of the catalytic ingredient on the downstream area of the exhaust-gas passage excepting the upstream area, it is possible to suppress the poisoning degradation of the catalytic ingredient. For example, the loading density of the catalytic ingredient on the upstream area can be made less than the loading density of the catalytic ingredient on the downstream area by a certain extent. It is preferable, however, to reduce the loading density of the catalytic ingredient on the upstream area from large to small stepwise or gradually in the direction approaching the upstream end of the substrate, because the upstream area is more likely to be poisoned than the downstream area is.
Moreover, it is preferable as well to reduce the loading density of the catalytic ingredient at parts of the upstream area at which the flow rate of exhaust gases is fast, because the degradation of the catalytic ingredient is more likely to occur deeply at parts of catalysts at which the flow rate of exhaust gases is fast.
~0019~ It is often difficult, however, to provide the loading density of the catalytic ingredient with a distribution in view of production processes. Hence, it is preferable to arrange the present catalyst for purifying exhaust gases so that the upstream area of the exhaust-gas passage can be free from the loaded catalytic ingredient . Such a catalyst can be produced with extreme readiness .
Thus, with such an arrangement, it is possible to completely prevent the upstream area of the exhaust-gas passage from being degraded by poisoning. Moreover, it is possible to additionally load the catalytic ingredient, which is to be loaded on the upstream area, on the downstream area of the exhaust-gas passage so that the present catalyst can fully show the activities.
~0020~ Note that poisoning substances deposit on the upstream area of the exhaust-gas passage gradually. However, the deposition of poisoning substances does not pose any problem, because the deposited poisoning substances are emitted to the outside together with exhaust gases when the deposition reaches a certain amount and the flow rate of exhaust gases enlarges.
~0021~ The length of the upstream area of the exhaust-gas passage can preferably fall in a range of less than 30 0 of an overall length of the substrate. According to experiments conducted by the present inventors, it has been found out that, when the length of area on which no catalytic ingredient is loaded exceeds 30 0 of the overall length of the substrate, the activities of the resulting catalysts degrade after being subjected to a durability test, compared with those of conventional catalysts in which catalytic ingredients are loaded over the entire length of the substrate. The reason has not been clear yet. It is believed, however, as follows. Let the loading amount of the catalytic ingredient, the absolute value, be identical in the present catalyst and the conventional catalysts, the loading density of the catalytic ingredient increases too much in the downstream area of the present catalyst. As a result, the granular growth of the catalytic ingredient occurs after the present catalyst is subjected to a durability test so that the number of the active points decreases.
(0022 Moreover, it is especially preferable that the length of the upstream area of the exhaust-gas passage can fall in a range of from 5 to 200 of the overall length of the substrate. When the length of the upstream area of the exhaust-gas passage falls in the range, the activities of the present catalyst for purifying exhaust gases improve more, compared with those of conventional catalysts in which catalytic ingredients are loaded over the entire length of the substrate.
(0023 The substrate is formed as a honeycomb shape or a foamed shape. It is possible to use the following for making the substrate:
honeycomb structures or foamed structures formed of heat-resistant ceramicssuch ascordierite; honeycombstructuresformed of metallic foils; and foamed structures formed of metallic fibers. Note that the cellular density and porosity of the substrate can be equal to those of substrates used conventionally.
(0024 The coating layer is formed of a single member selected from the group consisting of alumina, titania, zirconia, ceria and silica, or a mixture of the oxides. Alternatively, the coating layer can be formed of a composite oxide composed of a plurality of the oxides .
It is possible to use conventional raw materials for making the coating layer. Depending on the types of catalyst, it is preferable to select an optimum raw material. For example, when the present catalyst for purifying exhaust gases is turned into a three-way catalyst, it is preferable to mix alumina with ceria or a ceria-zirconia composite oxide having an oxygen sorbing-and-releasing ability and use the resulting mixture for making the coating layer.
(0025) Note that, even when no coating layer is formed on the upstream area of the exhaust-gas passage, the resulting present catalyst for purifying exhaust gases likewise operates and effects advantages . Accordingly, when the present catalyst is arranged so that the upstream area of the exhaust-gas passage is free from the loaded catalytic ingredient, it is preferable not to form the coating layer on the upstream area. With such an arrangement, the ventilation resistance exerted to exhaust gases lowers in the upstream area so that the pressure loss can be reduced. Moreover, when the cellular density in the upstream area is increased to make the pressure loss equal to that of conventional catalysts, the cellular superficial area contacting with exhaust gases enlarges.
Consequently, the adsorption or deposition of poisoning substances enlarges in the upstream area so that it is possible to further suppress the poisoning degradation of the catalytic ingredient in the downstream area.
(0026) The coating layer can be formed by utilizing a wash coating method using a slurry and followed by drying and calcining the slurry, as it has been done conventionally. In accordance with the wash coating method, it is possible not to form the coating layer on the upstream area of the exhaust-gas passage with extreme readiness, because the wash coating method makes it possible not to deposit the slurry on the upstream area. Note that the forming amount of the coating layer can be as usual, for example, from 100 to 300 g with respect to 1 L of the substrate.
(0027) It is possible to appropriately select and use a noble metal, such as Pt, Rh, Pd, Ir and Ru, for the catalytic ingredient, depending on the types and applications of the present catalyst for purifying exhaust gases. Moreover, in certain applications, it is possible to use a transition metal, such as Fe, Ni, Co, Cu and W, for the catalytic ingredient. The loading amount of the catalytic ingredient can be from 0. 1 to 10 g with respect to 1 L of the substrate, but can be changed appropriately, depending on the types of and applications of the present catalyst.
(0028) The catalytic ingredient can be loaded on the coating layer by a water absorption method or an adsorption method in the same manner as having been done conventionally. In the water absorption method, the coating layer is impregnated with a solution in which a compound of the catalytic ingredient is solved, and is dried and calcined subsequently. In the adsorption method, the substrate provided with the coating layer is immersed into and taken up from a solution in which a compound of the catalytic ingredient is solved, and is dried and calcined subsequently. Alternatively, the catalytic ingredient can be loaded on an oxide powder making the coating layer by the water absorption method or the adsorption method in advance, and the coating layer can be formed using the resulting catalytic powder.
(0029) Hereinafter, the present invention will be described in detail with reference to examples and comparative examples.
(Example No. 1) (0030) Fig. 1 roughly illustrates a cross-sectional view of a catalyst for purifying exhaust gases according to Example No. 1 of the present invention. The catalyst is a three-way catalyst, and comprises a honeycomb-shaped substrate 1, a coating layer 2 formed on an downstream area of the substrate 1 alone and loaded with catalytic ingredients, and Pt and Rh loaded on the coating layer 2 as the catalytic ingredients. The substrate 1 has an overall length of 105 mm. The substrate 1 comprises an upstream area 10 extending from the upstream-end surface to the downstream-end surface by a length of 5.25 mm (i.e., 5o of the overall length of the substrate 1). Note that the coating layer 2 is not formed on the upstream area 10, and that Pt and Rh are not loaded on the upstream area 10.
(0031) Hereinafter, the production process of the exhaust gas-purifying catalyst according to Example No. 1 will be described instead of describing the arrangement of the exhaust gas-purifying catalyst.
X0032) 100 parts by weight of an A1203 powder, 80 parts by weight of a Ce02-ZrOz composite oxide powder, 40 parts by weight an alumina sol, and 130 parts by weight of water were mixed. Note that the solid content of the alumina sol was 10 o by weight . The resulting mixture was milled to prepare a slurry.
0033) Subsequently, a honeycomb-shaped substrate 1 was prepared.
The substrate 1 was made of cordierite, and had a diameter of 103 mm, an overa:Ll length of 105 mm and a cellular density of 600 cells/inch2. The substrate 1 was immersed into the slurry over a range by 95% of the overall length from the downstream-end surface.
The substrate 1 was taken up from the slurry, and the excessive slurry was blown off from the substrate 1. Thereafter, the substrate 1 was dried at 250 °C for 2 hours, and was further calcined at 500 °C
for 2 hours, thereby forming the coating layer 2. Thus, the coating layer 2 was foamed only on the downstream area excepting the upstream area 10 which extended from the upstream-end surface over a range by 5% of the overall length of the substrate 1. The forming amount of the coating layer 2 was 200 g with respect to 1 L of the substrate 1.
(0034 Then, a platinum dinitrodiammine aqueous solution having a prescribed concentration was absorbed into the substrate 1 provided with the coating layer 2 in a predetermined amount . After drying, the substrate 1 was calcined at 500 °C for 1 hour, thereby loading Pt thereon. Moreover, a rhodium nitrate aqueous solution having a prescribed concentration was absorbed into the substrate 1 in a predetermined amount. After drying, the substrate 1 was calcined at 500 °C for 1 hour, thereby loading Rh thereon. Note that Pt and Rh were loaded in an amount of 1. 3 g and 0. 35 g, respectively, with respect to one piece of the resulting catalyst. In addition, no Pt and Rh were loaded on the upstream area 10 on which no coating layer 2 was formed.
(Example No. 2) (0035) Except that the length of the upstream area 10 was set at l00 of the overall length of the substrate 1 (i.e., 10.5 mm), a catalyst for purifying exhaust gases according to Example No. 2 of the present invention was produced in the same manner as Example No. 1. Note that Pt and Rh were loaded likewise in an amount of 1.3 g and 0.35 g, respectively, with respect to one piece of the resulting catalyst.
(Example No. 3) X0036) Except that the length of the upstream area 10 was set at 20% of the overall length of the substrate 1 (i.e., 21.0 mm), a catalyst for purifying exhaust gases according to Example No. 3 of the present invention was produced in the same manner as Example No. 1. Note that Pt and Rh were loaded likewise in an amount of 1.3 g and 0.35 g, respectively, with respect to one piece of the resulting catalyst.
(Example No. 4) ~0037~ Except that the length of the upstream area 10 was set at 300 of the overall length of the substrate 1 (i.e., 31.5 mm), a catalyst for purifying exhaust gases according to Example No. 4 of the present invention was produced in the same manner as Example No. 1. Note that Pt and Rh were loaded likewise in an amount of 1.3 g and 0.35 g, respectively, with respect to one piece of the resulting catalyst.
(Example No. 5) ~0038~ Except that the length of the upstream area 10 was set at 500 of the overall length of the substrate 1 (i.e., 52.5 mm), a catalyst for purifying exhaust gases according to Example No. 5 of the present invention was produced in the same manner as Example No. 1. Note that Pt and Rh were loaded likewise in an amount of 1.3 g and 0.35 g, respectively, with respect to one piece of the resulting catalyst.
(Comparative Example No. 1) ~0039~ Except that the coating layer 2 with Pt and Rh loaded was formed over the entire length of the substrate 1 (i.e., 105 mm), a catalyst for purifying exhaust gases according to Comparative Example No. 1 was produced in the same manner as Example No. 1. Note that Pt and Rh were loaded likewise in an amount of 1.3 g and 0.35 g, respectively, with respect to one piece of the resulting catalyst .
(Test and Evaluation) (0040) The catalysts according to Example Nos. 1 through 5 and Comparative Example No. 1 were disposed in an exhaust system of an engine testing bench equipped with a gasoline engine whose displacement was 1.8 L, respectively. Then, the gasoline engine was driven using gasoline which contained Mn in an amount of 35 mg/L.
The gasoline engine was driven while changing the air-fuel ratio A/F. Under the conditions that the exhaust gases exhibited a catalyst inlet temperature of 400 °C and a space velocity of 100, 000 hr-1, the respective catalysts were examined for the HC, CO and NOX
conversions. The thus measured conversions were labeled the initial conversions.
(0041 Subsequently, the catalysts according to Example Nos. 1 through 5 and Comparative Example No. 1 were subjected to a durability test on the same engine testing bench, respectively. In the durability test, the gasoline engine was driven using gasoline which contained Mn in an amount of 35 mg/L similarly, but the catalysts were held in the exhaust gases whose catalyst inlet temperature was controlled at 900 °C for 50 hours . Then, in the same manner as the examination for the initial HC, CO and NOX conversions, the catalysts, which had been subjected to the durability test, were examined for the HC, CO and NOX conversions after the durability test.
(0042 The thus obtained results were summarized as a graph in which the A/F ratios were plotted against the horizontal axis and the conversions were plotted against the vertical axis. Moreover, as illustrated in Fig. 2, an HC-NOX cross conversion and an CO-NOX cross conversion, the intersecting points of the A/F ratio-HC conversion curve and A/F ratio-NOX conversion curve and the A/F ratio-CO
conversion curve and A/F ratio-NOx conversion curve, were determined for each of the catalysts according to Example Nos. 1 through 5 and Comparative Example No. 1 as shown in Fig. 2. All of the catalysts according to Example Nos . 1 through 5 and Comparative Example No .
I exhibited a great correlation between the HC-NOX cross conversion and the CO-NOX cross conversion. Accordingly, the CO-NOX cross conversions were selected as a representative characteristic, and were summarized as a graph in which the CO-NOX cross conversions were plotted against the vertical axis and the proportions of the upstream area 10 with respect to the overall length of the substrate 1 were plotted against the horizontal axis. Fig. 3 illustrates the results.
(0043 It is understood from Fig. 3 that the catalysts according to Example Nos. 1 through 4 exhibited exhaust-gas purifying performance equal to or higher than that of the catalyst according to Comparative Example No. 1 after the durability test, though the catalyst according to Comparative Example No. 1 exhibited the highest exhaust-gas purifying performance initially. It is apparent that this resulted from the fact that no Pt and Rh were loaded on the upstream area 10 of the catalysts according to Example Nos. I through 4. It is believed that the catalysts according to Example Nos. 1 through 4 effected the advantage because it is suppressed that Pt and Rh are being poisoned by Mn.
(0044 Moreover, the catalysts according to Example Nos . 1 through 3 showed upgraded exhaust-gas purifying performance after the durability test, compared with that of the catalyst according to Comparative Example No. 1. Therefore, it is evident that the length of the upstream area 10 can particularly desirably fall in a range of from 5 to 20% of the overall length of the substrate 1.
~0045~ The present invention can be applied to oxidizing catalysts, three-way catalysts, NOX-selective-reducing catalysts and NOX
sorbing-and-reducing catalysts. Moreover, it can be applied to filter catalysts as well. For example, filter catalysts comprise a diesel particulate-material filter (i.e., DPF) whose cellular passages are clogged alternately at the opposite ends and in which a coating layer with a catalytic ingredient loaded is formed in the pores of the cellular walls in the DPF.
~0046~ Having now fully described the present invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the present invention as set forth herein including the appended claims.
BACKGROUND OF THE INVENTION
Field of the Invention X0001] The present invention relates to a catalyst for purifying exhaust gases, mainly for automotive applications. More particularly, it relates to a catalyst for purifying exhaust gases which can suppress the degradation by poisoning.
Description of the Related Art (0002 Automotive exhaust systems have been equipped with a variety of catalysts for purifying exhaust gases, such as oxidizing catalysts, three-way catalysts and NOX sorbing-and-reducing catalysts, in order to remove HC, CO and NOX in exhaust gases by oxidation and/or reduction. For example, three-way catalysts have been produced using a honeycomb-shaped substrate formed of cordierite or metallic foils in the following manner. A coating layer is formed on a surface of the cellular passages of the substrate using alumina and/or ceria. Then, a catalytic ingredient, such as Pt and Rh, is loaded on the coating layer. The resulting three-way catalysts have been used in an exhaust atmosphere produced by burning an air-fuel mixture whose air-fuel (A/F) ratio is controlled around 14.6, the stoichiometric ratio. Thus, the three-way catalysts purify HC and CO by oxidizing them, and simultaneously purify NOX
by reducing them.
~0003~ When fuels containing additives such as Pb and Mn are used, exhaust gases contain Pb and Mn components therein. Accordingly, there arises a drawback that the catalytic-ingredient active points in catalysts have been covered with the Pb and Mn components to degrade the activity. Moreover, it has been known that P, Zn and Ca contained in engine oils cause similar drawbacks.
0004 ~ Japanese Unexamined Patent Publication (KOKAI) No.
2002-172,329 discloses a catalytic structure comprising a trapping layerfor collecting catalyst-poisoning componentsin exhaustgases.
The trapping layer is disposed in proximity to the upstream-end surface of an NOX sorbing-and-reducing catalyst. The catalytic structure can suppress the NOX sorbing member of the NOX
sorbing-and-reducing catalyst being poisoned bysulfur, because the trapping layer collects sulfur components.
(0005 The trapping layer disclosed in the patent publication contains a trapping component which can react with sulfur components to trap sulfur components, thereby suppressing the sulfur poisoning of the NOX sorbing member only. However, the patent publication neither sets forth nor suggests suppressing the poisoning of catalytic ingredients such as Pt . Moreover, the patent publication is silent on another poisoning substances, such as Pb and Mn, other than sulfur.
0006) The present invention has been developed in view of such circumstances . It is therefore an obj ect of the present invention to suppress the poisoning degradation of catalytic ingredients by poisoning substances, such as Pb and Mn, thereby improving the durability of catalysts for purifying exhaust gases.
~0007~ A catalyst for purifying exhaust gases according to the present invention comprises:
a substrate comprising an exhaust-gas passage;
a coating layer formed on the exhaust-gas passage; and a catalytic ingredient loaded on the coating layer, wherein a loading density of the catalytic ingredient on a downstream area discriminated from an upstream area by a predetermined length from an upstream end of the substrate is greater than a loading density of the catalytic ingredient on the ~zpstream area.
(0008) It is desirable that the upstream area of the exhaust-gas passage can be free from the catalytic ingredient loaded.
(0009) Moreover, it is preferable that the predetermined length from an upstream end can fall in a range of less than 30~ of an overall length of the substrate. In addition, it is desirable that the predetermined length from an upstream end can fall in a range of from 5 to 20~ of an overall length of the substrate.
(0010) The present catalyst for purifying exhaust gases can suppress the poisoning substances such as Pb and Mn poisoning the catalytic ingredient. Therefore, the present catalyst exhibits high activities even after it is subjected to a durability test, because the degradation of the activities of the catalytic ingredient is suppressed.
(OOliJ In particular, when the length of .the upstream area of the exhaust-gas passage which is free from the loaded catalytic ingredient falls in a range of less than 30 0 of the overall length of the substrate, the present catalyst exhibits activities equal to or higher than those of conventional catalysts in which catalytic ingredients are loaded over the entire length.
(0012) A more complete appreciation of the present invention and many of its advantages will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings and detailed specification, all of which forms a part of the disclosure.
(0013) Fig. 1 is a cross-sectional view for roughly illustrating an arrangement of a catalyst for purifying exhaust gases according to Example No. 1 of the present invention.
(0014) Fig. 2 is an explanatory diagram for illustrating how to find a cross conversion.
(0015) Fig. 3 is a graph for illustrating relationships between CO-NOX cross conversions and proportions of a length of an upstream area of an exhaust-gas passage with respect to an overall length of a substrate.
DETAILED DESCRIpTT_ON OF THE PREFERRED EMBODT_MENTS
(0016) Having generally described the present invention, a further understanding can be obtained by reference to the specific preferred embodiments which are provided herein for the purpose of illustration only and not intended to limit the scope of the appended claims.
(0017) According to studies carried out by the present inventors, it has been apparent that the poisoning degradation of catalytic ingredients by poisoning substances such as Pb and Mn occur intensively on the upstream part of catalysts when catalysts are brought into contact with exhaust gases containing Pb and Mn.
Accordingly, in the present catalyst for purifying exhaust gases, the loading density of the catalytic ingredient on the downstream area is greater than that on the upstream area discriminated by a predetermined length from the upstream end of the substrate.
Consequently, since the catalytic ingredient is loaded less on the upstream area in which the catalytic ingredient is likely to be degraded due to poisoning by poisoning substances, and is loaded more on the downstream area of the exhaust-gas passage excepting the upstream area, it is suppressed that the catalytic ingredient is poisoned by poisoning substances. Therefore, not only the present catalyst can utilize the catalytic ingredient effectively, but also it can suppress the activities degrading after being subjected to a durability test.
(0018 Poisoning substances adsorb or deposit mainly onto the upstream area of the exhaust-gas passage. Hence, when the loading density of the catalytic ingredient on the upstream area is made less than the loading density of the catalytic ingredient on the downstream area of the exhaust-gas passage excepting the upstream area, it is possible to suppress the poisoning degradation of the catalytic ingredient. For example, the loading density of the catalytic ingredient on the upstream area can be made less than the loading density of the catalytic ingredient on the downstream area by a certain extent. It is preferable, however, to reduce the loading density of the catalytic ingredient on the upstream area from large to small stepwise or gradually in the direction approaching the upstream end of the substrate, because the upstream area is more likely to be poisoned than the downstream area is.
Moreover, it is preferable as well to reduce the loading density of the catalytic ingredient at parts of the upstream area at which the flow rate of exhaust gases is fast, because the degradation of the catalytic ingredient is more likely to occur deeply at parts of catalysts at which the flow rate of exhaust gases is fast.
~0019~ It is often difficult, however, to provide the loading density of the catalytic ingredient with a distribution in view of production processes. Hence, it is preferable to arrange the present catalyst for purifying exhaust gases so that the upstream area of the exhaust-gas passage can be free from the loaded catalytic ingredient . Such a catalyst can be produced with extreme readiness .
Thus, with such an arrangement, it is possible to completely prevent the upstream area of the exhaust-gas passage from being degraded by poisoning. Moreover, it is possible to additionally load the catalytic ingredient, which is to be loaded on the upstream area, on the downstream area of the exhaust-gas passage so that the present catalyst can fully show the activities.
~0020~ Note that poisoning substances deposit on the upstream area of the exhaust-gas passage gradually. However, the deposition of poisoning substances does not pose any problem, because the deposited poisoning substances are emitted to the outside together with exhaust gases when the deposition reaches a certain amount and the flow rate of exhaust gases enlarges.
~0021~ The length of the upstream area of the exhaust-gas passage can preferably fall in a range of less than 30 0 of an overall length of the substrate. According to experiments conducted by the present inventors, it has been found out that, when the length of area on which no catalytic ingredient is loaded exceeds 30 0 of the overall length of the substrate, the activities of the resulting catalysts degrade after being subjected to a durability test, compared with those of conventional catalysts in which catalytic ingredients are loaded over the entire length of the substrate. The reason has not been clear yet. It is believed, however, as follows. Let the loading amount of the catalytic ingredient, the absolute value, be identical in the present catalyst and the conventional catalysts, the loading density of the catalytic ingredient increases too much in the downstream area of the present catalyst. As a result, the granular growth of the catalytic ingredient occurs after the present catalyst is subjected to a durability test so that the number of the active points decreases.
(0022 Moreover, it is especially preferable that the length of the upstream area of the exhaust-gas passage can fall in a range of from 5 to 200 of the overall length of the substrate. When the length of the upstream area of the exhaust-gas passage falls in the range, the activities of the present catalyst for purifying exhaust gases improve more, compared with those of conventional catalysts in which catalytic ingredients are loaded over the entire length of the substrate.
(0023 The substrate is formed as a honeycomb shape or a foamed shape. It is possible to use the following for making the substrate:
honeycomb structures or foamed structures formed of heat-resistant ceramicssuch ascordierite; honeycombstructuresformed of metallic foils; and foamed structures formed of metallic fibers. Note that the cellular density and porosity of the substrate can be equal to those of substrates used conventionally.
(0024 The coating layer is formed of a single member selected from the group consisting of alumina, titania, zirconia, ceria and silica, or a mixture of the oxides. Alternatively, the coating layer can be formed of a composite oxide composed of a plurality of the oxides .
It is possible to use conventional raw materials for making the coating layer. Depending on the types of catalyst, it is preferable to select an optimum raw material. For example, when the present catalyst for purifying exhaust gases is turned into a three-way catalyst, it is preferable to mix alumina with ceria or a ceria-zirconia composite oxide having an oxygen sorbing-and-releasing ability and use the resulting mixture for making the coating layer.
(0025) Note that, even when no coating layer is formed on the upstream area of the exhaust-gas passage, the resulting present catalyst for purifying exhaust gases likewise operates and effects advantages . Accordingly, when the present catalyst is arranged so that the upstream area of the exhaust-gas passage is free from the loaded catalytic ingredient, it is preferable not to form the coating layer on the upstream area. With such an arrangement, the ventilation resistance exerted to exhaust gases lowers in the upstream area so that the pressure loss can be reduced. Moreover, when the cellular density in the upstream area is increased to make the pressure loss equal to that of conventional catalysts, the cellular superficial area contacting with exhaust gases enlarges.
Consequently, the adsorption or deposition of poisoning substances enlarges in the upstream area so that it is possible to further suppress the poisoning degradation of the catalytic ingredient in the downstream area.
(0026) The coating layer can be formed by utilizing a wash coating method using a slurry and followed by drying and calcining the slurry, as it has been done conventionally. In accordance with the wash coating method, it is possible not to form the coating layer on the upstream area of the exhaust-gas passage with extreme readiness, because the wash coating method makes it possible not to deposit the slurry on the upstream area. Note that the forming amount of the coating layer can be as usual, for example, from 100 to 300 g with respect to 1 L of the substrate.
(0027) It is possible to appropriately select and use a noble metal, such as Pt, Rh, Pd, Ir and Ru, for the catalytic ingredient, depending on the types and applications of the present catalyst for purifying exhaust gases. Moreover, in certain applications, it is possible to use a transition metal, such as Fe, Ni, Co, Cu and W, for the catalytic ingredient. The loading amount of the catalytic ingredient can be from 0. 1 to 10 g with respect to 1 L of the substrate, but can be changed appropriately, depending on the types of and applications of the present catalyst.
(0028) The catalytic ingredient can be loaded on the coating layer by a water absorption method or an adsorption method in the same manner as having been done conventionally. In the water absorption method, the coating layer is impregnated with a solution in which a compound of the catalytic ingredient is solved, and is dried and calcined subsequently. In the adsorption method, the substrate provided with the coating layer is immersed into and taken up from a solution in which a compound of the catalytic ingredient is solved, and is dried and calcined subsequently. Alternatively, the catalytic ingredient can be loaded on an oxide powder making the coating layer by the water absorption method or the adsorption method in advance, and the coating layer can be formed using the resulting catalytic powder.
(0029) Hereinafter, the present invention will be described in detail with reference to examples and comparative examples.
(Example No. 1) (0030) Fig. 1 roughly illustrates a cross-sectional view of a catalyst for purifying exhaust gases according to Example No. 1 of the present invention. The catalyst is a three-way catalyst, and comprises a honeycomb-shaped substrate 1, a coating layer 2 formed on an downstream area of the substrate 1 alone and loaded with catalytic ingredients, and Pt and Rh loaded on the coating layer 2 as the catalytic ingredients. The substrate 1 has an overall length of 105 mm. The substrate 1 comprises an upstream area 10 extending from the upstream-end surface to the downstream-end surface by a length of 5.25 mm (i.e., 5o of the overall length of the substrate 1). Note that the coating layer 2 is not formed on the upstream area 10, and that Pt and Rh are not loaded on the upstream area 10.
(0031) Hereinafter, the production process of the exhaust gas-purifying catalyst according to Example No. 1 will be described instead of describing the arrangement of the exhaust gas-purifying catalyst.
X0032) 100 parts by weight of an A1203 powder, 80 parts by weight of a Ce02-ZrOz composite oxide powder, 40 parts by weight an alumina sol, and 130 parts by weight of water were mixed. Note that the solid content of the alumina sol was 10 o by weight . The resulting mixture was milled to prepare a slurry.
0033) Subsequently, a honeycomb-shaped substrate 1 was prepared.
The substrate 1 was made of cordierite, and had a diameter of 103 mm, an overa:Ll length of 105 mm and a cellular density of 600 cells/inch2. The substrate 1 was immersed into the slurry over a range by 95% of the overall length from the downstream-end surface.
The substrate 1 was taken up from the slurry, and the excessive slurry was blown off from the substrate 1. Thereafter, the substrate 1 was dried at 250 °C for 2 hours, and was further calcined at 500 °C
for 2 hours, thereby forming the coating layer 2. Thus, the coating layer 2 was foamed only on the downstream area excepting the upstream area 10 which extended from the upstream-end surface over a range by 5% of the overall length of the substrate 1. The forming amount of the coating layer 2 was 200 g with respect to 1 L of the substrate 1.
(0034 Then, a platinum dinitrodiammine aqueous solution having a prescribed concentration was absorbed into the substrate 1 provided with the coating layer 2 in a predetermined amount . After drying, the substrate 1 was calcined at 500 °C for 1 hour, thereby loading Pt thereon. Moreover, a rhodium nitrate aqueous solution having a prescribed concentration was absorbed into the substrate 1 in a predetermined amount. After drying, the substrate 1 was calcined at 500 °C for 1 hour, thereby loading Rh thereon. Note that Pt and Rh were loaded in an amount of 1. 3 g and 0. 35 g, respectively, with respect to one piece of the resulting catalyst. In addition, no Pt and Rh were loaded on the upstream area 10 on which no coating layer 2 was formed.
(Example No. 2) (0035) Except that the length of the upstream area 10 was set at l00 of the overall length of the substrate 1 (i.e., 10.5 mm), a catalyst for purifying exhaust gases according to Example No. 2 of the present invention was produced in the same manner as Example No. 1. Note that Pt and Rh were loaded likewise in an amount of 1.3 g and 0.35 g, respectively, with respect to one piece of the resulting catalyst.
(Example No. 3) X0036) Except that the length of the upstream area 10 was set at 20% of the overall length of the substrate 1 (i.e., 21.0 mm), a catalyst for purifying exhaust gases according to Example No. 3 of the present invention was produced in the same manner as Example No. 1. Note that Pt and Rh were loaded likewise in an amount of 1.3 g and 0.35 g, respectively, with respect to one piece of the resulting catalyst.
(Example No. 4) ~0037~ Except that the length of the upstream area 10 was set at 300 of the overall length of the substrate 1 (i.e., 31.5 mm), a catalyst for purifying exhaust gases according to Example No. 4 of the present invention was produced in the same manner as Example No. 1. Note that Pt and Rh were loaded likewise in an amount of 1.3 g and 0.35 g, respectively, with respect to one piece of the resulting catalyst.
(Example No. 5) ~0038~ Except that the length of the upstream area 10 was set at 500 of the overall length of the substrate 1 (i.e., 52.5 mm), a catalyst for purifying exhaust gases according to Example No. 5 of the present invention was produced in the same manner as Example No. 1. Note that Pt and Rh were loaded likewise in an amount of 1.3 g and 0.35 g, respectively, with respect to one piece of the resulting catalyst.
(Comparative Example No. 1) ~0039~ Except that the coating layer 2 with Pt and Rh loaded was formed over the entire length of the substrate 1 (i.e., 105 mm), a catalyst for purifying exhaust gases according to Comparative Example No. 1 was produced in the same manner as Example No. 1. Note that Pt and Rh were loaded likewise in an amount of 1.3 g and 0.35 g, respectively, with respect to one piece of the resulting catalyst .
(Test and Evaluation) (0040) The catalysts according to Example Nos. 1 through 5 and Comparative Example No. 1 were disposed in an exhaust system of an engine testing bench equipped with a gasoline engine whose displacement was 1.8 L, respectively. Then, the gasoline engine was driven using gasoline which contained Mn in an amount of 35 mg/L.
The gasoline engine was driven while changing the air-fuel ratio A/F. Under the conditions that the exhaust gases exhibited a catalyst inlet temperature of 400 °C and a space velocity of 100, 000 hr-1, the respective catalysts were examined for the HC, CO and NOX
conversions. The thus measured conversions were labeled the initial conversions.
(0041 Subsequently, the catalysts according to Example Nos. 1 through 5 and Comparative Example No. 1 were subjected to a durability test on the same engine testing bench, respectively. In the durability test, the gasoline engine was driven using gasoline which contained Mn in an amount of 35 mg/L similarly, but the catalysts were held in the exhaust gases whose catalyst inlet temperature was controlled at 900 °C for 50 hours . Then, in the same manner as the examination for the initial HC, CO and NOX conversions, the catalysts, which had been subjected to the durability test, were examined for the HC, CO and NOX conversions after the durability test.
(0042 The thus obtained results were summarized as a graph in which the A/F ratios were plotted against the horizontal axis and the conversions were plotted against the vertical axis. Moreover, as illustrated in Fig. 2, an HC-NOX cross conversion and an CO-NOX cross conversion, the intersecting points of the A/F ratio-HC conversion curve and A/F ratio-NOX conversion curve and the A/F ratio-CO
conversion curve and A/F ratio-NOx conversion curve, were determined for each of the catalysts according to Example Nos. 1 through 5 and Comparative Example No. 1 as shown in Fig. 2. All of the catalysts according to Example Nos . 1 through 5 and Comparative Example No .
I exhibited a great correlation between the HC-NOX cross conversion and the CO-NOX cross conversion. Accordingly, the CO-NOX cross conversions were selected as a representative characteristic, and were summarized as a graph in which the CO-NOX cross conversions were plotted against the vertical axis and the proportions of the upstream area 10 with respect to the overall length of the substrate 1 were plotted against the horizontal axis. Fig. 3 illustrates the results.
(0043 It is understood from Fig. 3 that the catalysts according to Example Nos. 1 through 4 exhibited exhaust-gas purifying performance equal to or higher than that of the catalyst according to Comparative Example No. 1 after the durability test, though the catalyst according to Comparative Example No. 1 exhibited the highest exhaust-gas purifying performance initially. It is apparent that this resulted from the fact that no Pt and Rh were loaded on the upstream area 10 of the catalysts according to Example Nos. I through 4. It is believed that the catalysts according to Example Nos. 1 through 4 effected the advantage because it is suppressed that Pt and Rh are being poisoned by Mn.
(0044 Moreover, the catalysts according to Example Nos . 1 through 3 showed upgraded exhaust-gas purifying performance after the durability test, compared with that of the catalyst according to Comparative Example No. 1. Therefore, it is evident that the length of the upstream area 10 can particularly desirably fall in a range of from 5 to 20% of the overall length of the substrate 1.
~0045~ The present invention can be applied to oxidizing catalysts, three-way catalysts, NOX-selective-reducing catalysts and NOX
sorbing-and-reducing catalysts. Moreover, it can be applied to filter catalysts as well. For example, filter catalysts comprise a diesel particulate-material filter (i.e., DPF) whose cellular passages are clogged alternately at the opposite ends and in which a coating layer with a catalytic ingredient loaded is formed in the pores of the cellular walls in the DPF.
~0046~ Having now fully described the present invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the present invention as set forth herein including the appended claims.
Claims (5)
1. A catalyst for purifying exhaust gases from combustion of fuels containing at least one of lead and manganese, comprising:
a substrate having a length comprising an exhaust-gas passage, said exhaust gas passage having an upstream area and a downstream area;
a coating layer formed on the exhaust-gas passage;
and a noble metal loaded on the coating layer only in the downstream area of the exhaust-gas passage, wherein a loading density of the noble metal on the downstream area is discriminated from the upstream area free from noble metal loading by a predetermined length of the upstream area of at least 5% to 30% of the overall length of the substrate from an upstream end of the substrate, whereby poisoning degradation of catalytic ingredients including platinum by lead and manganese is suppressed.
a substrate having a length comprising an exhaust-gas passage, said exhaust gas passage having an upstream area and a downstream area;
a coating layer formed on the exhaust-gas passage;
and a noble metal loaded on the coating layer only in the downstream area of the exhaust-gas passage, wherein a loading density of the noble metal on the downstream area is discriminated from the upstream area free from noble metal loading by a predetermined length of the upstream area of at least 5% to 30% of the overall length of the substrate from an upstream end of the substrate, whereby poisoning degradation of catalytic ingredients including platinum by lead and manganese is suppressed.
2. The catalyst set forth in claim 1 or 2, wherein the upstream area is free from the coating layer.
3. The catalyst set forth in claim 1 or 2, wherein the predetermined length of the upstream area from the upstream end falls in a range of from 5 to 20% of an overall length of the substrate.
4. The catalyst set forth in claim 1 or 2, wherein the predetermined length from an upstream end falls in a range of less than 31.5 mm.
5. The catalyst set forth in claim 4, wherein the predetermined length from an upstream end falls in a range of from 5 to 21 mm.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2003370983A JP2005131551A (en) | 2003-10-30 | 2003-10-30 | Catalyst for purifying exhaust gas |
| JP2003-370983 | 2003-10-30 |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2485893A1 CA2485893A1 (en) | 2005-04-30 |
| CA2485893C true CA2485893C (en) | 2010-07-27 |
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ID=34510405
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2485893 Expired - Fee Related CA2485893C (en) | 2003-10-30 | 2004-10-25 | Catalyst for purifying exhaust gases |
Country Status (3)
| Country | Link |
|---|---|
| JP (1) | JP2005131551A (en) |
| CN (1) | CN1311893C (en) |
| CA (1) | CA2485893C (en) |
Families Citing this family (8)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP4736724B2 (en) * | 2005-11-09 | 2011-07-27 | トヨタ自動車株式会社 | Exhaust gas purification device for internal combustion engine |
| WO2008126330A1 (en) * | 2007-03-30 | 2008-10-23 | Ibiden Co., Ltd. | Honeycomb structure |
| EP2763785A4 (en) * | 2011-10-06 | 2015-06-24 | Mack Trucks | Diesel oxidation catalyst and method of treating engine exhaust gas |
| JP5720949B2 (en) * | 2011-12-08 | 2015-05-20 | トヨタ自動車株式会社 | Exhaust gas purification catalyst |
| JP5780247B2 (en) * | 2013-01-23 | 2015-09-16 | トヨタ自動車株式会社 | Catalytic converter |
| JP5757297B2 (en) * | 2013-01-23 | 2015-07-29 | トヨタ自動車株式会社 | Catalytic converter |
| JP5821887B2 (en) * | 2013-04-03 | 2015-11-24 | トヨタ自動車株式会社 | Catalytic converter |
| JP7035780B2 (en) * | 2018-05-08 | 2022-03-15 | トヨタ自動車株式会社 | Catalyst structure |
Family Cites Families (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5846502A (en) * | 1996-01-16 | 1998-12-08 | Ford Global Technologies, Inc. | Mini-cascade catalyst system |
| KR100584795B1 (en) * | 1998-04-28 | 2006-06-02 | 엥겔하드 코포레이션 | Monolithic catalysts and related process for manufacture |
| TW475027B (en) * | 2000-06-02 | 2002-02-01 | Emitec Emissionstechnologie | Catalyst carrier body with protective zone |
| LU90900B1 (en) * | 2002-03-07 | 2003-10-27 | Delphi Tech Inc | Multizone catalytic converter |
-
2003
- 2003-10-30 JP JP2003370983A patent/JP2005131551A/en active Pending
-
2004
- 2004-10-25 CA CA 2485893 patent/CA2485893C/en not_active Expired - Fee Related
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Also Published As
| Publication number | Publication date |
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| CN1623635A (en) | 2005-06-08 |
| CN1311893C (en) | 2007-04-25 |
| CA2485893A1 (en) | 2005-04-30 |
| JP2005131551A (en) | 2005-05-26 |
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