Catalytic converter

Abstract

Provided is a catalytic converter capable of obtaining superior NOx purification performance while reducing the amount of a noble metal catalyst. A catalytic converter 10 includes: a substrate 1 having a cell structure in which exhaust gas flows; and catalyst layers 3 that are formed on cell wall surfaces 2 of the substrate 1. The catalyst layers 3 include a first catalyst layer 4 disposed on an upstream side of the substrate 1 in an exhaust gas flow direction and a second catalyst layer 5 disposed on a downstream side of the substrate in the exhaust gas flow direction. The first catalyst layer 4 is formed of a support and rhodium which is a noble metal catalyst supported on the support. The second catalyst layer 5 is formed of a support and palladium or platinum which is a noble metal catalyst supported on the support. The first catalyst layer 4 is formed in a range of 80% to 100% of a total length of the substrate 1 starting from an end of the substrate on the upstream side, and the second catalyst layer 15 5 is formed in a range of 20% to 50% of the total length of the substrate 1 starting from an end of the substrate on the downstream side.

Claims

1. A catalytic converter comprising: a substrate having a cell structure in which exhaust gas flows; and catalyst layers that are formed on cell wall surfaces of the substrate, wherein the catalyst layers include a first catalyst layer disposed on an upstream side of the substrate in an exhaust gas flow direction and a second catalyst layer disposed on a downstream side of the substrate in the exhaust gas flow direction, the first catalyst layer is formed of a support and rhodium which is a noble metal catalyst supported on the support, the second catalyst layer is formed of a support and a noble metal catalyst selected from a group consisting of palladium and platinum, the noble metal catalyst being supported on the support, the first catalyst layer is formed in a first range of 80% to 100% of a total length of the substrate, the first range starting from an end of the substrate on the upstream side, and the second catalyst layer is formed in a second range of 20% to 50% of the total length of the substrate, the second range starting from an end of the substrate on the downstream side, wherein the support of the first catalyst layer contains no cerium, the first catalyst layer is formed on the second catalyst layer in a portion where the first catalyst layer and the second catalyst layer are overlapped with each other, the support of the first catalyst layer comprises a first composite oxide, the support of the second catalyst layer comprises a second composite oxide, the first composite oxide is different from the second composite oxide, and the first catalyst layer and the second catalyst layer are arranged so as to form a layer structure as follows: (i) a single layer structure of the first catalyst layer, (ii) a two-layer structure in which the first catalyst layer is stacked on top of the second catalyst layer, and (iii) a single layer structure of the second catalyst layer are arranged in this order from the upstream side of the substrate in the exhaust gas flow direction to the downstream side of the substrate in the exhaust gas flow direction.

2. The catalytic converter according to claim 1, wherein the first catalyst layer directly contacts the substrate on the upstream side of the substrate, and the second catalyst layer directly contacts the substrate on the downstream side of the substrate.

3. The catalytic converter according to claim 1, wherein the second catalyst layer directly contacts the substrate along the total length of the second catalyst layer.

4. The catalytic converter according to claim 1, wherein the first catalyst layer directly contacts the substrate along 50% to 80% of the total length of the substrate.

5. The catalytic converter according to claim 2, wherein the first catalyst layer directly contacts the substrate along 50% to 80% of the total length of the substrate.

6. The catalytic converter according to claim 3, wherein the first catalyst layer directly contacts the substrate along 50% to 80% of the total length of the substrate.

7. The catalytic converter according to claim 1, wherein the second catalyst layer is formed in a second range of from 20% to less than 50% of the total length of the substrate.

8. The catalytic converter according to claim 1, wherein the second catalyst layer is formed in a second range of 20% to 30% of the total length of the substrate.

9. The catalytic converter according to claim 1, wherein the second catalyst layer is formed in a second range of 30% to 50% of the total length of the substrate.

10. An exhaust system comprising: an engine; the catalytic converter according to claim 1; and a muffler, wherein the engine, the catalytic converter and the muffler are configured for fluid communication such that exhaust gas from the engine can flow from the engine on one end of the exhaust system to the muffler on another end of the exhaust system; the catalytic converter is disposed on a downstream side of the engine in the exhaust gas flow direction, and the muffler is disposed on a downstream side of the catalytic converter in the exhaust gas flow direction.

11. The exhaust system according to claim 10, wherein the first catalyst layer directly contacts the substrate on the upstream side of the substrate, and the second catalyst layer directly contacts the substrate on the downstream side of the substrate.

12. The exhaust system according to claim 10, wherein the second catalyst layer directly contacts the substrate along the total length of the second catalyst layer.

13. The exhaust system according to claim 10, wherein the first catalyst layer directly contacts the substrate along 50% to 80% of the total length of the substrate.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1(a) is a schematic diagram showing a catalytic converter according to the present invention, and FIG. 1(b) is an enlarged view showing a part of cells.

(2) FIG. 2(a) is a vertical sectional view showing Embodiment 1 of catalyst layers, and FIG. 2(b) is a vertical sectional view showing Embodiment 2 of catalyst layers.

(3) FIG. 3(a) is a vertical sectional view showing Embodiment 3 of catalyst layers, and FIG. 3(b) is a vertical sectional view showing Embodiment 4 of catalyst layers.

(4) FIG. 4(a) is a vertical sectional view showing Embodiment 5 of catalyst layers, and FIG. 4(b) is a vertical sectional view showing Embodiment 6 of catalyst layers.

(5) FIG. 5 is a graph showing the experiment results of measuring the emission amount of NOx when the length of the first catalyst layer is fixed to 80% with respect to the length of the substrate, and when the length of the second catalyst layer is changed.

(6) FIG. 6 is a graph showing the experiment results of measuring the emission amount of NOx regarding Comparative Examples and Examples.

MODES FOR CARRYING OUT THE INVENTION

(7) Hereinafter, an embodiment of a catalytic converter according to the present invention will be described with reference to the drawings.

(8) (Exhaust System for Exhaust Gas)

(9) First, an exhaust system for exhaust gas in which the catalytic converter according to the present invention is provided will be briefly described. In the exhaust system for exhaust gas to which the catalytic converter according to the present invention is applied, an engine, a catalytic converter, a three-way catalytic converter, a sub muffler, and a main muffler are disposed and connected to each other through a system pipe, and exhaust gas produced from the engine flows to each unit through the system pipe and is exhausted. Next, hereinafter, the embodiment of the catalytic converter will be described.

(10) (Embodiment of Catalytic Converter)

(11) FIG. 1(a) is a schematic diagram showing a catalytic converter according to the present invention, and FIG. 1(b) is an enlarged view showing a part of cells. In addition, FIGS. 2(a), 2(b), 3(a), 3(b), 4(a), and 4(b) are vertical sectional views showing Embodiments 1 to 6 of catalyst layers.

(12) Briefly, a catalytic converter 10 shown in FIG. 1 includes: a cylindrical substrate 1 having plural cells; and catalyst layers 3 that are formed on surfaces of cell walls 2 constituting the cells.

(13) Here, examples of a material of the substrate 1 include a ceramic material such as cordierite or silicon carbide which is formed of a composite oxide of magnesium oxide, aluminum oxide, and silicon dioxide; and a material other than a ceramic material such as a metal material. In addition, examples of a support constituting the catalyst layers 3 that are formed on the surfaces of the cell walls 2 of the substrate 1 include oxides containing at least one porous oxide of CeO.sub.2, ZrO.sub.2, and Al.sub.2O.sub.3 as a major component; one oxide among ceria (CeO.sub.2), zirconia (ZrO.sub.2), and alumina (Al.sub.2O.sub.3); and a composite oxide formed of two or more oxides among ceria (CeO.sub.2), zirconia (ZrO.sub.2), and alumina (Al.sub.2O.sub.3) (for example, a CeO.sub.2ZrO.sub.2 compound which is a CZ material, or an Al.sub.2O.sub.3CeO.sub.2ZrO.sub.2 ternary composite oxide (ACZ material) into which Al.sub.2O.sub.3 is introduced as a diffusion barrier).

(14) The substrate 1 has a honeycomb structure including cells having plural lattice contours which have, for example, rectangular, hexagonal, and octagonal shapes. Exhaust gas, which flows to the inside of cells at an end of the substrate 1 on an upstream side (Fr side) in an exhaust gas flow direction, flows to the inside of the substrate 1. During the flow process, the exhaust gas is purified, and the purified exhaust gas flows out from an end of the substrate 1 on a downstream side (Rr side) in the exhaust gas flow direction (X direction).

(15) Next, the catalyst layers formed on the surfaces of the cell walls 2 will be described with reference to FIGS. 2 to 4. In each drawing, upper and lower cell walls forming one cell are shown.

(16) FIG. 2(a) shows zone-coated catalyst layers 3 according to Embodiment 1.

(17) The catalyst layers 3 shown in the same drawing includes a first catalyst layer 4 and a second catalyst layer 5, in which the first catalyst layer 4 has a length of 80% with respect to the length (100%) of the substrate 1 starting from the end of the substrate 1 on the upstream side (Fr side) in the exhaust gas flow direction, the second catalyst layer 5 has a length of 20% with respect to the length (100%) of the substrate 1 starting from the end of the substrate 1 on the downstream side (Rr side) in the exhaust gas flow direction, and both the catalyst layers are not overlapped.

(18) In the first catalyst layer 4, rhodium is used as a noble metal catalyst supported on a support. In the second catalyst layer 5, palladium or platinum is used as a noble metal catalyst supported on a support.

(19) As the support for supporting rhodium in the first catalyst layer 4, a material not containing cerium is preferably used. Examples of the support include an oxide formed of one of zirconia (ZrO.sub.2) and alumina (Al.sub.2O.sub.3); and an Al.sub.2O.sub.3ZrO.sub.2 binary composite oxide (AZ material).

(20) According to the catalyst layers 3 shown in the drawing, rhodium is not used over the entire length thereof. Therefore, the amount of the rhodium used, which is the most expensive among the noble metal catalysts, can be reduced. Further, the first catalyst layer has a length of 80% on the upstream side with respect to the length of the substrate 1. The second catalyst layer 5 in which palladium or the like is used as the noble metal catalyst has a length of 20% on the downstream side with respect to the length of the substrate 1. As a result, the catalyst layer 3 having superior NOx purification performance is formed.

(21) On the other hand, FIG. 2(b) shows zone-coated catalyst layers 3A according to Embodiment 2. In the configuration of the catalyst layers 3A shown in the same drawing, a second catalyst layer 5A has a length of 50% with respect to the length of the substrate 1, a first catalyst layer 4A has a length of 80% with respect to the length of the substrate 1, and both the catalyst layers are overlapped in a range of 30%. Due to the catalyst layers 3A shown in the drawing, the amount of rhodium used is reduced, and superior NOx purification performance can be expected.

(22) On the other hand, FIG. 3(a) shows zone-coated catalyst layers 3B according to Embodiment 3. In the configuration of the catalyst layers 3B shown in the same drawing, the second catalyst layer 5 has a length of 20% with respect to the length of the substrate 1, a first catalyst layer 4B has a length of 90% with respect to the length of the substrate 1, and both the catalyst layers are overlapped in a range of 10%. Due to the catalyst layers 3B shown in the drawing, the amount of rhodium used is reduced, and superior NOx purification performance can be expected.

(23) On the other hand, FIG. 3(b) shows zone-coated catalyst layers 3C according to Embodiment 4. In the configuration of the catalyst layers 3C shown in the same drawing, the second catalyst layer 5A has a length of 50% with respect to the length of the substrate 1, the first catalyst layer 4B has a length of 90% with respect to the length of the substrate 1, and both the catalyst layers are overlapped in a range of 40%. Due to the catalyst layers 3C shown in the drawing, the amount of rhodium used is reduced, and superior NOx purification performance can be expected.

(24) On the other hand, FIG. 4(a) shows zone-coated catalyst layers 3D according to Embodiment 5. In the configuration of the catalyst layers 3D shown in the same drawing, the second catalyst layer 5 has a length of 20% with respect to the length of the substrate 1, a first catalyst layer 4C has a length of 100% with respect to the length of the substrate 1, and both the catalyst layers are overlapped in a range of 20%. Due to the catalyst layers 3D shown in the drawing, superior NOx purification performance can be expected.

(25) Further, FIG. 4(b) shows zone-coated catalyst layers 3E according to Embodiment 6. In the configuration of the catalyst layers 3E shown in the same drawing, the second catalyst layer 5A has a length of 50% with respect to the length of the substrate 1, the first catalyst layer 4C has a length of 100% with respect to the length of the substrate 1, and both the catalyst layers are overlapped in a range of 50%. Due to the catalyst layers 3E shown in the drawing, superior NOx purification performance can be expected.

(26) In addition to the examples shown in the drawings, there are various combination embodiments which satisfy the following configurations: the first catalyst layer is formed in a range of 80% to 100% of the total length of the substrate 1 starting from the end of the substrate 1 on the upstream side; and the second catalyst layer is formed in a range of 20% to 50% of the total length of the substrate starting from the end of the substrate 1 on the downstream side.

(27) [Experiment (Part 1) for Determining Optimum Range of Second Catalyst Layer, and Results Thereof]

(28) The present inventors defined the length of the first catalyst layer to be 80% with respect to the length of the substrate, and changed the length of the second catalyst layer to be 0%, 10%, 30%, 50%, 80%, and 100% with respect to the length of the substrate. A catalytic converter including catalyst layers of each case was prepared, a durability test was performed, and an experiment of measuring the amount of NOx in a normal rich state was performed.

(29) (Regarding Method of Preparing Catalyst Slurry)

(30) Regarding the preparation of a slurry for forming the second catalyst layer (Pd was used as the noble metal catalyst), 65 g/L of an Al.sub.2O.sub.3 composite oxide as a support was impregnated with a palladium nitrate solution. As a result, 1.0 mass % of support powder was prepared. Next, 85 g/L of a CeO.sub.2ZrO.sub.2 composite oxide (CeO.sub.2/ZrO.sub.2/La.sub.2O.sub.3/Y.sub.2O.sub.3=30/60/5/5 (mass %)), 10 g/L of barium acetate, water, an Al.sub.2O.sub.3 binder, acetic acid, a thickener, and the like were mixed with each other in predetermined amounts. As a result, a Pd catalyst slurry was obtained.

(31) On the other hand, regarding the preparation of a slurry for forming the first catalyst layer (Rh was used as the noble metal catalyst), 65 g/L of a CeO.sub.2ZrO.sub.2 composite oxide (Al.sub.2O.sub.3/CeO.sub.2/ZrO.sub.2/La.sub.2O.sub.3/Y.sub.2O.sub.3/Nd.sub.2O.sub.330/20/44/2/2/2 (mass %)) was prepared, and 0.3 mass % of Rh was supported on each support. Further, 25 g/L of La-added Al.sub.2O.sub.3, 10 g/L of barium acetate, water, an Al.sub.2O.sub.3 binder, acetic acid, a thickener, and the like were mixed with each other in predetermined amounts. As a result, a Rh catalyst slurry was obtained.

(32) 875 cc of a monolith substrate was prepared and was coated with the above-described slurries using a suction method.

(33) The second catalyst layers (Pd-supported catalyst layers), which had lengths of 0%, 10%, 30%, 50%, 80%, and 100% with respect to the length of the substrate starting from the end of the substrate on the Rr side, were formed by coating using the same amount of slurry.

(34) On the other hand, the first catalyst layer (Rh-supported catalyst layer), which had a length of 80% with respect to the length of the substrate starting from the end of the substrate on the Fr side, was formed by coating using the slurry.

(35) (Regarding Durability Test)

(36) The prepared catalytic converter was set immediately below an actual engine, and a durability test was performed thereon at a bed temperature of 1000 C. for 50 hours under a composite pattern where an A/F ratio cyclically changed.

(37) (Regarding Engine Bench Evaluation)

(38) After the durability test, the catalytic converter was set in another actual engine, and the purification performance was calculated as the average emission amount of NOx when an A/F ratio was changed in a rectangular shape from a rich state to a lean state and was maintained in the rich state for 120 seconds. The test results are shown in FIG. 5.

(39) In the same drawing, the emission amount of NOx had an inflection point when the length of the second catalyst layer was 50% with respect to the length of the substrate. When the length of the second catalyst layer was longer than 50%, the emission amount of NOx was increased to be in an unfavorable range as the purification performance and approached 500 ppm. On the other hand, when the length of the second catalyst layer was 50% or shorter, the emission amount of NOx was significantly decreased and saturated at 200 ppm or less.

(40) Based on the experiment results, the upper limit of a ratio of the length of the second catalyst layer to the length of the substrate can be defined as 50%.

(41) Next, hereinafter, in another experiment, the lower limit of the ratio of the length of the second catalyst layer will be defined.

(42) [Experiment (Part 2) for Determining Optimum Range of Second Catalyst Layer, and Results Thereof]

(43) The present inventors prepared a catalytic converter including the catalyst layers according to each of Examples and Comparative Examples, a durability test was performed, and an experiment of measuring the amount of NOx in a normal rich state was performed.

(44) (Regarding Method of Preparing Catalyst Slurry)

(45) Regarding the preparation of a slurry for forming the second catalyst layer (Pd was used as the noble metal catalyst), 65 g/L of an Al.sub.2O.sub.3 composite oxide as a support was impregnated with a palladium nitrate solution. As a result, 1.0 mass % of support powder was prepared. Next, 85 g/L of a CeO.sub.2ZrO.sub.2 composite oxide (CeO.sub.2/ZrO.sub.2/La.sub.2O.sub.3/Y.sub.2O.sub.3=30/60/5/5 (mass %)), 10 g/L of barium acetate, water, an Al.sub.2O.sub.3 binder, acetic acid, a thickener, and the like were mixed with each other in predetermined amounts. As a result, a Pd catalyst slurry was obtained.

(46) On the other hand, regarding the preparation of a slurry for forming the first catalyst layer (Rh was used as the noble metal catalyst), 65 g/L of a CeO.sub.2ZrO.sub.2 composite oxide (Al.sub.2O.sub.3/CeO.sub.2/ZrO.sub.2/La.sub.2O.sub.3/Y.sub.2O.sub.3/Nd.sub.2O.sub.3=30/20/44/2/2/2 (mass %)) was prepared. Here, regarding Example 2, the same amount of a ZrO.sub.2 composite oxide (Al.sub.2O.sub.3/ZrO.sub.2/La.sub.2O.sub.3/Nd.sub.2O.sub.3=50/46/2/2 (mass %)) was used. 0.3 mass % of Rh was supported on each support. Further, 25 g/L of La-added Al.sub.2O.sub.3, 10 g/L of barium acetate, water, an Al.sub.2O.sub.3 binder, acetic acid, a thickener, and the like were mixed with each other in predetermined amounts. As a result, a Rh catalyst slurry was obtained.

(47) 875 cc of a monolith substrate was prepared and was coated with the above-described slurries using a suction method.

(48) In Comparative Example 1, catalyst layers having a two-layer structure, in which a Pd-supported catalyst layer and a Rh-supported catalyst layer were laminated using the above-described slurries, were formed over the entire length of the substrate.

(49) In Comparative Example 2, catalyst layers having a three-layer structure, in which a Pd-supported catalyst layer and a Rh-supported catalyst layer were laminated using the above-described slurries, were formed over the entire length of the substrate.

(50) On the other hand, in Example 1, as shown in FIG. 2(a), the first catalyst layer having a length of 80% was formed on the upstream side, and the second catalyst layer having a length of 20% was formed on the downstream side (both the layers were not overlapped).

(51) Further, the configuration of the catalyst layers of Example 2 was the same as that of Example 1; however, in the slurry for forming the first catalyst layer, a ZrO.sub.2 composite oxide not containing cerium was used.

(52) (Regarding Durability Test)

(53) The prepared catalytic converter was set immediately below an actual engine, and a durability test was performed thereon at a bed temperature of 1000 C. for 50 hours under a composite pattern where an A/F ratio cyclically changed.

(54) (Regarding Engine Bench Evaluation)

(55) After the durability test, the catalytic converter was set in another actual engine, and the purification performance was calculated as the average emission amount of NOx when an A/F ratio was changed in a rectangular shape from a rich state to a lean state and was maintained in the rich state for 120 seconds. The test results are shown in FIG. 6.

(56) It was verified from the same drawing that the emission amount of NOx of Example 1 can be reduced by about 60% to 70% as compared to that of Comparative Examples 1 and 2.

(57) Further, when Example 1 was compared to Example 2, it was verified that the emission amount of NOx of Example 2 can be reduced to about 20% of that of Example 1.

(58) It was verified from the experiment results that the ratio of the length of the second catalyst layer to the length of the substrate is preferably secured to be 20% or higher. This value of 20% can be defined as the lower limit of the ratio of the length of the second catalyst layer. In consideration of the above results and the results of Experiment part 1, the ratio of the length of the second catalyst layer to the length of the substrate can be defined to be in a range of 20% to 50%.

(59) In the above-described experiments, the ratio of the length of the first catalyst layer to the length of the substrate was fixed to 80%. It is needless to say that, as the length of the first catalyst layer containing rhodium increases, a catalytic converter having superior NOx purification performance can be obtained. Accordingly, the above-described value of 80% can be defined as the lower limit of the ratio of the length of the first catalyst layer, and the value of 100% where the length of the first catalyst layer is the same as the length of the substrate can be defined as the upper limit of the ratio of the length of the first catalyst layer. From the viewpoint of reducing the amount of rhodium used, it is preferable that the length of the first catalyst layer is set to be about 80%. Therefore, the length of the first catalyst layer may be appropriately adjusted in a length ratio range of 80% to 100%.

(60) In addition, it was verified that, when the support constituting the first catalyst layer does not contain cerium, the NOx purification performance can be further improved.

(61) Hereinabove, the embodiments of the present invention have been described with reference to the drawings. However, a specific configuration is not limited to the embodiments, and design changes and the like which are made within a range not departing from the scope of the present invention are included in the invention.

REFERENCE SIGNS LIST

(62) 1 . . . SUBSTRATE, 2 . . . CELL WALL, 3, 3A, 3B, 3C, 3D, 3E . . . CATALYST LAYER, 4, 4A, 4B, 4C . . . FIRST CATALYST LAYER, 5, 5A . . . SECOND CATALYST LAYER, 10 . . . CATALYTIC CONVERTER, Fr . . . UPSTREAM SIDE IN EXHAUST GAS FLOW DIRECTION, Rr . . . DOWNSTREAM SIDE IN EXHAUST GAS FLOW DIRECTION