Catalytic converter

Abstract

A catalytic converter with excellent OSC performance and NO.sub.x purification performance. The catalytic converter includes a substrate with a cell structure and catalyst layer. The catalyst layer includes lower and upper catalyst layers. The upper catalyst layer includes a zirconia compound support with rhodium carried thereon that contains zirconia, lanthanum oxide, and yttrium oxide; an alumina compound without rhodium carried thereon that contains alumina and lanthanum oxide; and a ceria-zirconia-based composite oxide containing ceria, zirconia, lanthanum oxide, and neodymium oxide. The lower catalyst layer includes an alumina compound support with platinum carried thereon that contains alumina and lanthanum oxide that are the same materials as those of the alumina compound of the upper catalyst layer; and a ceria-zirconia-based composite oxide without platinum carried thereon that contains ceria, zirconia, lanthanum oxide, and neodymium oxide that are the same materials as those of the ceria-zirconia-based composite oxide of the upper catalyst layer.

Claims

1. A catalytic converter comprising: a substrate with a cell structure through which exhaust gas flows; and a catalyst layer formed on a cell wall surface of the substrate, wherein the catalyst layer includes a lower catalyst layer and an upper catalyst layer, the lower catalyst layer being formed on a surface of the substrate, and the upper catalyst layer being formed on a surface of the lower catalyst layer, wherein, the upper catalyst layer includes a zirconia compound support with rhodium carried thereon, the zirconia compound support containing zirconia, lanthanum oxide, and yttrium oxide, an alumina compound without a noble metal catalyst carried thereon, the alumina compound containing alumina and lanthanum oxide, and a ceria-zirconia-based composite oxide containing ceria, zirconia, lanthanum oxide, and neodymium oxide, and the lower catalyst layer includes an alumina compound support with platinum carried thereon, the alumina compound support containing alumina and lanthanum oxide that are the same materials as those of the alumina compound of the upper catalyst layer, and a ceria-zirconia-based composite oxide without a noble metal catalyst carried thereon, the ceria-zirconia-based composite oxide containing ceria, zirconia, lanthanum oxide, and neodymium oxide that are the same materials as those of the ceria-zirconia-based composite oxide of the upper catalyst layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view of a catalytic converter of the present invention.

(2) FIG. 2 is a partially enlarged view of a cell.

(3) FIG. 3 is a longitudinal sectional view illustrating an embodiment of a catalyst layer.

(4) FIG. 4 is a graph showing the experimental results for verifying the NO.sub.x purification performance.

(5) FIG. 5 is a graph showing the experimental results for verifying the OSC performance.

DETAILED DESCRIPTION OF THE EMBODIMENT(S)

(6) Hereinafter, embodiments of a catalytic converter of the present invention will be described with reference to the drawings. The catalytic converter shown in the drawings has an upper catalyst layer that is formed in the range of 80% of the total length of a substrate from an end of the substrate on the downstream side of the exhaust gas flow direction, and also has a lower catalyst layer that is formed in the range of 80% of the total length of the substrate from an end of the substrate on the upstream side of the exhaust gas flow direction. It should be noted that the length over which each of the upper catalyst layer and the lower catalyst layer is formed is preferably in the range of 65 to 95% of the total length of the substrate.

(7) (Exhaust System for Exhaust Gas)

(8) First, an exhaust system for exhaust gas in which the catalytic converter of the present invention is provided will be briefly described. An exhaust system for exhaust gas to which the catalytic converter of the present invention is applied has a configuration in which an engine, a catalytic converter, a three-way catalytic converter, a sub-muffler, and a main muffler are arranged and are mutually connected with system pipes so that exhaust gas generated in the engine flows through each part via the system pipe and is then discharged. Next, an embodiment of the catalytic converter will be described.

(9) (Embodiment of Catalytic Converter)

(10) FIG. 1 is a schematic view of the catalytic converter of the present invention. FIG. 2 is a partially enlarged view of a cell. FIG. 3 is a longitudinal sectional view illustrating an embodiment of a catalyst layer.

(11) A catalytic converter 10 shown in FIG. 1 generally includes a cylindrical substrate 1 with a number of cells and a catalyst layer 3 formed on the surface of a cell wall 2 of each cell as shown in FIG. 2.

(12) Herein, examples of the substrate 1 include cordierite made of a composite oxide of magnesium oxide, aluminum oxide, and silicon dioxide, ceramic materials, such as silicon carbide, and materials other than ceramic materials, such as metal materials.

(13) The substrate 1 has a honeycomb structure with a number of cells whose lattice contour is a quadrangle, a hexagon, an octagon, or the like. Exhaust gas, which has entered a cell at an end of the substrate 1 on the upstream side (Fr side) of the exhaust gas flow direction, flows through the substrate 1, and is purified in this process, and then, the purified exhaust gas flows out from an end of the substrate 1 on the downstream side (Rr side) of the exhaust gas flow direction (x-direction).

(14) Next, an embodiment of the catalyst layer will be described with reference to FIGS. 2 and 3.

(15) The catalyst layer 3 shown in FIGS. 2 and 3 includes a lower catalyst layer 4 that is formed on the surface of a cell wall 2 and an upper catalyst layer 5 that is formed on the surface of the lower catalyst layer 4.

(16) The lower catalyst layer 4 is formed in the range of 80% of the total length of the substrate 1 from the end of the substrate 1 on the upstream side Fr of the exhaust gas flow direction, while the upper catalyst layer 5 is formed in the range of 80% of the total length of the substrate 1 from the end of the substrate 1 on the downstream side Rr of the exhaust gas flow direction.

(17) Herein, the lower catalyst layer 4 contains an alumina compound support (a compound of alumina (Al.sub.2O.sub.3) and lanthanum oxide (La.sub.2O.sub.3)) with platinum (Pt) carried thereon, and also contains a ceria-zirconia-based composite oxide (a compound of ceria (CeO.sub.2), zirconia (ZrO.sub.2) lanthanum oxide (La.sub.2O.sub.3), and neodymium oxide (Nd.sub.2O.sub.3)).

(18) Meanwhile, the upper catalyst layer 5 contains a zirconia compound support (a compound of zirconia (ZrO.sub.2), lanthanum oxide (La.sub.2O.sub.3), and yttrium oxide (Y.sub.2O.sub.3)) with rhodium (Rh) carried thereon, and also contains a ceria-zirconia-based composite oxide (a compound of ceria (CeO.sub.2), zirconia (ZrO.sub.2), lanthanum oxide (La.sub.2O.sub.3), and neodymium oxide (Nd.sub.2O.sub.3)) that are the same materials as those of the lower catalyst layer 4, and further contains an alumina compound (a compound of alumina (Al.sub.2O.sub.3) and lanthanum oxide (La.sub.2O.sub.3)) that are the same materials as those of the lower catalyst layer 4.

(19) In the upper catalyst layer 5, rhodium (Rh) is carried only on the zirconia compound support that does not contain ceria. Such a structure can improve the NO.sub.x purification rate.

(20) As each of the upper and lower catalyst layers 5 and 4 contains a promoter made of the same materials (a ceria-zirconia-based composite oxide and an alumina compound), the upper and lower catalyst layers 5 and 4 can have a good affinity at the interface and thus have high bond strength. Further, as the packed structure in each of the upper and lower catalyst layers 5 and 4 is optimized, the OSC performance is improved.

(21) Thus, by having the catalyst layer 3 having a two-layer structure with high bond strength, the catalytic converter 10 with the upper and lower catalyst layers 5, 4 shown in the drawing becomes a catalytic converter with excellent OSC performance and NO.sub.x purification performance.

(22) (Experiments for Verifying the NO.sub.x Purification Performance and OSC Performance and Results Thereof)

(23) The inventors conducted experiments for verifying the NO.sub.x purification performance and OSC performance of catalyst converters. Example 1 and Comparative Examples 1-4 were produced using methods described below.

Comparative Example 1

(24) In Comparative Example 1, the lower catalyst layer contains Pt as a catalyst (Pt(0.2)/Al.sub.2O.sub.3(25) CZ(30)), and the upper catalyst layer contains Rh as a catalyst (Rh(0.12)/CeO.sub.2ZrO.sub.2 composite oxide(40)+Al.sub.2O.sub.3(20)). Herein, the unit of the numerical values in the parentheses is g/L. First, using nitric acid Pt, Pt/Al.sub.2O.sub.3 (i.e., material 1) in which Pt is carried on Al.sub.2O.sub.3 was prepared. Impregnation was used as a method for causing Pt to be carried on Al.sub.2O.sub.3. Next, a slurry 1 was prepared by pouring the material 1, a CZ material, and a Al.sub.2O.sub.3-based binder into distilled water while agitating them. Further, the prepared slurry 1 was poured into a substrate, and unnecessary portions were wiped away with a blower, so that the wall surface of the substrate was coated with the slurry 1. At that time, the coating material for the Pt layer was prepared such that the content of Pt, the content of the material 1, and the content of the CZ material with respect to the volume of the substrate were 0.2 g/L, 25 g/L, and 30 respectively. Finally, moisture was dried with a dryer kept at 120 C. for two hours, and baking was performed with an electric furnace at 500 C. for 2 hours. Likewise, using nitric acid Rh, a Rh/CZ material (i.e., material 2) in which Rh is carried on a CZ material was prepared. Herein, the CeO.sub.2ZrO.sub.2 composite oxide contains 20-70 mass % ZrO.sub.2, 20-70 mass % CeO.sub.2, and 10-15 mass % La.sub.2O.sub.3, Y.sub.2O.sub.3, Pr.sub.6O.sub.11, and Nd.sub.2O.sub.3. Next, a slurry 2 was prepared by pouring the material 2, Al.sub.2O.sub.3, and an Al.sub.2O.sub.3-based binder into distilled water while agitating them such that the materials were suspended in the distilled water. The prepared slurry 2 was poured into the coated substrate, and unnecessary portions were wiped away with a blower, so that the wall surface of the substrate was coated with the slurry 2. At that time, the coating material for the Rh layer was prepared such that the content of Rh, the content of the material 2, and the content of Al.sub.2O.sub.3 with respect to the volume of the substrate were 0.12 g/L, 40 g/L, and 20 g/L, respectively. Finally, moisture was dried with a dryer kept at 120 C. for two hours, and baking was performed with an electric furnace at 500 C. for 2 hours.

Comparative Example 2

(25) In Comparative Example 2, the lower catalyst layer contains Pt as a catalyst (Pt(0.2)/Al.sub.2O.sub.3(25)+CZ(30)), and the upper catalyst layer contains Rh as a catalyst (Rh(0.12)/ZrO.sub.2(40)+Al.sub.2O.sub.3(20)). A slurry was prepared by changing the specifications of the Rh support (material 2) used for the slurry 2 in Comparative Example 1, and then, coating, drying, and baking were performed. Herein, ZrO.sub.2 contains 80-90 mass % ZrO.sub.2, and also contains 10-20 mass % La.sub.2O.sub.3, Y.sub.2O.sub.3, Pr.sub.6O.sub.11, and Nd.sub.2O.sub.3 as stabilizers.

Comparative Example 3

(26) In Comparative Example 3, the lower catalyst layer contains Pt as a catalyst (Pt(0.2)/Al.sub.2O.sub.3(25)+CZ(30)), and the upper catalyst layer contains Rh as a catalyst (Rh layer Rh(0.12)/CeO.sub.2(40)+Al.sub.2O.sub.3(20)). A slurry was prepared by changing the specifications of the Rh support (material 2) used for the slurry 2 in Comparative Example 1, and then, coating, drying, and baking were performed. With respect to the catalyst, the process was unchanged except that the composition of the material 2 in Comparative Example 1 was changed. Herein, as CeO.sub.2, greater than or equal to 99 mass % CeO.sub.2 was used.

Example 1

(27) In Example 1, the lower catalyst layer contains Pt as a catalyst (Pt(0.2)/Al.sub.2O.sub.3(25)+CZ(30)), and the upper catalyst layer contains Rh as a catalyst (Rh(0.12)/ZrO.sub.2(40)+CZ(15)+Al.sub.2O.sub.3(20)). A slurry 2 was prepared by changing the specifications of the Rh support (material 2) used for the slurry 2 in Comparative Example 1, and then, coating, drying, and baking were performed. With respect to the catalyst, the process was unchanged except that the composition of the material 2 in Comparative Example 1 was changed. Herein, ZrO.sub.2 contains 80-90 mass % ZrO.sub.2, and also contains 10-20 mass % La.sub.2O.sub.3, Y.sub.2O.sub.3, Pr.sub.6O.sub.11, and Nd.sub.2O.sub.3 as stabilizers. In addition, the CZ material contains 20-70 mass % ZrO.sub.2, 20-70 mass % CeO.sub.2, and 10-15 mass % La.sub.2O.sub.3, Y.sub.2O.sub.3, Pr.sub.6O.sub.11, and Nd.sub.2O.sub.3.

Comparative Example 4

(28) In Comparative Example 4, the lower catalyst layer contains Pt as a catalyst (Pt(0.2)/Al.sub.2O.sub.3(25)+CZ(30)), and the upper catalyst layer contains Rh as a catalyst (Rh(0.12)/CeO.sub.2(40)+CZ(15)+Al.sub.2O.sub.3(20)). A slurry 2 was prepared by changing the specifications of the Rh support (material 2) used for the slurry 2 in Comparative Example 1, and then, coating, drying, and baking were performed. With respect to the catalyst, the process was unchanged except that the composition of the material 2 in Comparative Example 1 was changed. Herein, as CeO.sub.2, greater than or equal to 99 mass % CeO.sub.2 was used. In addition, the CZ material contains 20-70 mass % ZrO.sub.2, 20-70 mass % CeO.sub.2, and 10-15 mass % La.sub.2O.sub.3, Y.sub.2O.sub.3, Pr.sub.6O.sub.11, and Nd.sub.2O.sub.3.

Evaluation Method

(29) A 4.3 L V8 cylinder gasoline engine was used, and the bed temperature of a catalyst on the downstream side was set to 950 C., so that a cycle that includes feedback, fuel cut, rich, and lean per minute as a condition was conducted for 50 hours.

(30) An aged catalytic converter was mounted, and the purification rate for when the entering gas atmosphere was periodically switched between the rich and lean sides of the A/F ratio was measured. In addition, an aged catalytic converter was mounted, and the purification rate for when the entering gas atmosphere was continuously maintained on the rich side of the A/F ratio was also measured.

(31) Table 1 below shows the materials used herein.

(32) TABLE-US-00001 TABLE 1 Name of Portion Material Producer Composition Upper Catalyst ZrO.sub.2 DAIICHI KIGENSO ZrO.sub.2 (84 mass %), La.sub.2O.sub.3 (6 mass %), Layer KAGAKU KOGYO Co., LTD. Y.sub.2O.sub.3 (10 mass %) (Rh Layer) CeO.sub.2ZrO.sub.2 DAIICHI KIGENSO CeO.sub.2 (21 mass %), ZrO.sub.2 (72 mass %), KAGAKU KOGYO Co., LTD. La.sub.2O.sub.3 (1.7 mass %), Nd.sub.2O.sub.3 (5.3 mass %) Al.sub.2O.sub.3 Sasol Al.sub.2O.sub.3 (99 mass %), La.sub.2O.sub.3 (1 mass %) Lower Catalyst CeO.sub.2ZrO.sub.2 DAIICHI KIGENSO CeO.sub.2 (21 mass %), ZrO.sub.2 (72 mass %), Layer KAGAKU KOGYO Co., LTD. La.sub.2O.sub.3 (1.7 mass %), Nd.sub.2O.sub.3 (5.3 mass %) (Pt Layer) Al.sub.2O.sub.3 Sasol Al.sub.2O.sub.3 (99 mass %), La.sub.2O.sub.3 (1 mass %)

Results of Experiment

(33) FIGS. 4 and 5 each show the experimental results. Herein, FIG. 4 is a graph showing the experimental results for verifying the NO.sub.x purification performance. FIG. 5 is a graph showing the experimental results for verifying the OSC performance. In each of FIGS. 4 and 5, the result of Comparative Example 1 is shown as a reference, and the results of other Comparative Examples and Example are shown as the proportions to the result of Comparative Example 1.

(34) FIG. 4 can confirm that Comparative Example 2 and Example 1 show excellent results of NO.sub.x purification rates, while the other samples show low NO purification rates.

(35) Meanwhile, FIG. 5 can confirm that all samples other than Comparative Example 2 have high OSC performance at substantially the same level.

(36) From the results of FIGS. 4 and 5, it can be confirmed that only Example 1 shows excellent results of both the NO purification performance and the OSC performance. This is considered to be due to the compositions of the upper catalyst layer and the lower catalyst layer of Example 1. This shows that the catalytic converter of the present invention has excellent NO purification performance and OSC performance.

(37) Although the embodiments of the present invention have been described in detail with reference to the drawings, specific structures are not limited thereto, and any design changes that may occur within the spirit and scope of the present invention are all included in the present invention.

DESCRIPTION OF SYMBOLS

(38) 1 Substrate 2 Cell Wall 3 Catalyst Layer 4 Lower Catalyst Layer 5 Upper Catalyst Layer 10 Catalytic converter Fr Upstream side of the exhaust gas flow direction Rr Downstream side of the exhaust gas flow direction