Catalytic converter

Abstract

A catalytic converter with excellent OSC performance and NO.sub.x purification performance. The converter includes a substrate with a cell structure through which exhaust gas flows, and a catalyst layer formed on a cell wall surface of the substrate. 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. The upper catalyst layer includes at least a zirconia support with rhodium carried thereon, and two types of ceria-zirconia-based composite oxides with different specific surface areas, each of the ceria-zirconia-based composite oxides having no rhodium carried thereon. The lower catalyst layer includes an alumina support with platinum carried thereon, and a ceria-zirconia-based composite oxide.

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, the upper catalyst layer includes at least a zirconia support with rhodium carried thereon, and two types of ceria-zirconia-based composite oxides with different specific surface areas, each of the ceria-zirconia-based composite oxides having no rhodium carried thereon, and the lower catalyst layer includes an alumina support with platinum carried thereon, and a ceria-zirconia-based composite oxide, of the two types of ceria-zirconia-based composite oxides with different specific surface areas, the ceria-zirconia-based composite oxide with a larger specific surface area has a specific surface area of greater than or equal to 40 m.sup.2/g, and the ceria-zirconia-based composite oxide with a smaller specific surface area has a specific surface area of less than or equal to 4 m.sup.2/g, wherein, of the two types of ceria-zirconia-based composite oxides with different specific surface areas, the ceria-zirconia-based composite oxide with a larger specific surface area is contained in the upper catalyst layer by an amount of greater than or equal to 12 g/L, and the ceria-zirconia-based composite oxide with a smaller specific surface area is contained in the upper catalyst layer by an amount of greater than or equal to 8 g/L.

2. The catalytic converter according to claim 1, wherein the zirconia support with rhodium carried thereon is contained in the upper catalyst layer by an amount of 40 g/L.

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 identifying the relationship between the Ce concentration in the Rh support and the NO.sub.x purification rate.

(5) FIG. 5 is a graph showing the experimental results for identifying the relationship between the added amounts of a high SSA composite oxide and a low SSA composite oxide in the upper catalyst layer and the NO.sub.x purification rate.

(6) FIG. 6 is a graph showing the experimental results for identifying the relationship between the amount of the Rh support and a pressure loss and the relationship between the amount of the Rh support and the purification temperature of low-temperature active NO.sub.x.

DETAILED DESCRIPTION OF THE EMBODIMENT(S)

(7) 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.

(8) (Exhaust System for Exhaust Gas)

(9) 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.

(10) (Embodiment of Catalytic Converter)

(11) 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.

(12) 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.

(13) 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.

(14) 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).

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

(16) 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.

(17) 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.

(18) Herein, the lower catalyst layer 4 contains an alumina support (Al.sub.2O.sub.3) with platinum (Pt) carried thereon, and a ceria-zirconia-based composite oxide (CeO.sub.2—ZrO.sub.2 composite oxide).

(19) Meanwhile, the upper catalyst layer 5 contains a zirconia support (ZrO.sub.2) with rhodium (Rh) carried thereon, and two types of ceria-zirconia-based composite oxides (CeO.sub.2—ZrO.sub.2 composite oxides) with different specific surface areas, and further contains alumina (Al.sub.2O.sub.3).

(20) Regarding the two types of ceria-zirconia-based composite oxides with different specific surface areas, the specific surface area of the ceria-zirconia-based composite oxide with a larger specific surface area is greater than or equal to 40 m.sup.2/g, while the specific surface area of the ceria-zirconia-based composite oxide with a smaller specific surface area is less than or equal to 4 m.sup.2/g.

(21) In the upper catalyst layer 5, rhodium (Rh) is carried only on zirconia (ZrO.sub.2) that does not contain ceria. Such a structure can improve the NO.sub.x purification rate. This has been verified through experiments described below.

(22) As the upper catalyst layer 5 contains a ceria-zirconia-based composite oxide with a large specific surface area and a ceria-zirconia-based composite oxide with a small specific surface area, more specifically, as the upper catalyst layer 5 contains a composite oxide with a large specific surface area by an amount of greater than or equal to 12 g/L and also contains a composite oxide with a small specific surface area by an amount of greater than or equal to 8 g/L, it is possible to suppress a pressure loss while exhibiting excellent OSC performance. This has been also verified through experiments described below.

(23) In addition, the zirconia support with rhodium carried thereon is contained in the upper catalyst layer 5 by an amount of 40 g/L.

(24) When the amount of the zirconia support with rhodium carried thereon is increased, the low-temperature activity, in particular, is improved. However, if the amount of the zirconia support is increased too much, a pressure loss is increased. Thus, from the perspective of achieving both excellent low-temperature activity and suppressed pressure loss, the amount of the zirconia support with rhodium carried thereon is defined as 40 g/L. This has been also verified through experiments described below.

(25) (Experiments for identifying the relationship between the Ce concentration in the Rh support and the NO.sub.x purification rate, experiments for identifying the relationship between the added amounts of a high SSA composite oxide and a low SSA composite oxide in the upper catalyst layer and the NO.sub.x purification rate, and experiments for identifying the relationship between the amount of the Rh support and a pressure loss and the relationship between the amount of the Rh support and the purification temperature of low-temperature active NO.sub.x)

(26) The inventors have conducted experiments for identifying the relationship between the Ce concentration in the Rh layer and the NO.sub.x purification rate, experiments for identifying the relationship between the added amounts of a high SSA composite oxide and a low SSA composite oxide in the upper catalyst layer and the NO.sub.x purification rate, and experiments for identifying the relationship between the amount of the Rh support and a pressure loss and the relationship between the amount of the Rh support and the purification temperature of low-temperature active NO.sub.x. Reference Examples 1-12, Example 1, and Comparative Examples 1-4 were produced using methods described below.

Reference Example 1

(27) In Reference 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)/Ce—Zr composite oxide A(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 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. Likewise, using nitric acid Rh, a Rh/Ce—Zr composite oxide A (i.e., material 2) in which Rh is carried on a Ce—Zr composite oxide A was prepared. Herein, the proportion of Ce in the Ce—Zr composite oxide A was 30 mass %. 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.

Reference Examples 2, 3, and 4

(28) In each of Reference Examples 2, 3, and 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)/Ce—Zr composite oxide B,C,D(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 Reference 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 shown in Reference Example 1 was changed. In Reference Example 2, a Ce—Zr composite oxide that contains a Ce concentration of 0 mass % (B) was used; in Reference Example 3, a Ce—Zr composite oxide that contains a Ce concentration of 20 mass % (C) was used; and in Reference Example 4, a Ce—Zr composite oxide that contains a Ce concentration of 60 mass % (D) was used.

Reference Examples 5, 6, 7, and 8

(29) In each of Reference Examples 5, 6, 7, and 8, 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)/Ce—Zr composite oxide B(40)+Al.sub.2O.sub.3(20)+low SSA Ce—Zr composite oxide(4,8,12,16)). A slurry was prepared by further pouring each amount of a low SSA Ce—Zr composite oxide into the slurry 2 in Reference Example 1, and then, coating, drying, and baking were performed. With respect to the catalyst, the process was unchanged except that a low SSA Ce—Zr composite oxide was added in the step of preparing the slurry 2 in Reference Example 1. In Reference Example 5, 4 parts by mass of a low SSA Ce—Zr composite oxide was added; in Reference Example 6, 8 parts by mass of a low SSA Ce—Zr composite oxide was added; in Reference Example 7, 12 parts by mass of a low SSA Ce—Zr composite oxide was added; and in Reference Example 8, 16 parts by mass of a low SSA Ce—Zr composite oxide was added. It should be noted that the “low SSA Ce—Zr composite oxide” herein means a material that initially has an SSA of less than or equal to 4 m.sup.2/g.

Reference Examples 9, 10, 11, and 12

(30) In each of Reference Examples 9, 10, 11, and 12, 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)/Ce—Zr composite oxide B(40)+Al.sub.2O.sub.3(20)+high SSA Ce—Zr composite oxide (4,8,12,16)). A slurry 2 was prepared by further pouring each amount of a high SSA Ce—Zr composite oxide into the slurry 2 in Reference Example 1, and then, coating, drying, and baking were performed. With respect to the catalyst, the process was unchanged except that a high SSA Ce—Zr composite oxide was added in the step of preparing the slurry 2 in Reference Example 1. In Reference Example 9, 4 parts by mass of a high SSA Ce—Zr composite oxide was added; in Reference Example 10, 8 parts by mass of a high SSA Ce—Zr composite oxide was added; in Reference Example 11, 12 parts by mass of a high SSA Ce—Zr composite oxide was added; and in Reference Example 12, 16 parts by mass of a high SSA Ce—Zr composite oxide was added. It should be noted that the “high SSA Ce—Zr composite oxide” herein means a material that initially has an SSA of greater than or equal to 40 m.sup.2/g.

Example 1 and Comparative Examples 1, 2, 3, and 4

(31) In each of Example 1 and Comparative Examples 1, 2, 3, and 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)/Ce—Zr composite oxide B(x)+Al.sub.2O.sub.3(20)+low SSA Ce—Zr composite oxide(8)+high SSA Ce—Zr composite oxide (12)). A slurry was prepared by changing the Rh support of the slurry 2 in Reference Example 1 to a Ce—Zr composite oxide B that contains 0 mass % C and pouring 8 parts by mass of a low SSA Ce—Zr composite oxide and 12 parts by mass of a high SSA Ce—Zr composite oxide into the slurry 2, and then, coating, drying, and baking were performed. With respect to the catalyst, the process was unchanged except that the material for the step of preparing the slurry 2 in Reference Example 1 was changed. In Comparative Example 1, 24 parts by mass of a Ce—Zr composite oxide B was added; in Comparative Example 2, 32 parts by mass of a Ce—Zr composite oxide B was added; in Example 1, 40 parts by mass of a Ce—Zr composite oxide B was added; in Comparative Example 3, 48 parts by mass of a Ce—Zr composite oxide B was added; and in Comparative Example 4, 56 parts by mass of a Ce—Zr composite oxide B was added.

(32) <Evaluation Method>

(33) 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.

(34) To measure the NO.sub.x purification rate under fluctuating conditions of the A/F ratio, a catalyst layer on the upstream side that 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, to measure the NO purification rate under the steady rich condition, 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 measured. Further, to measure a pressure loss, a pressure loss for when air was supplied to a catalyst at a constant flow rate (6 m.sup.3/minute) was measured. Furthermore, regarding NO.sub.x light-off performance NO.sub.x-T50, the temperature that had increased from a low temperature under the rich environment and at which the NO purification rate reached 50% was measured.

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

(36) TABLE-US-00001 TABLE 1 Portion Name of Material Producer Composition Upper Ce—Zr TOYOTA MOTOR CeO.sub.2 (x mass %), Catalyst Composite CORPORATION ZrO.sub.2 (100-x mass %) Oxide Layer High, TOYOTA MOTOR Al.sub.2O.sub.3 (30 mass %), (Rh Low SSA CORPORATION CeO.sub.2 (20 mass %), Layer) Ce—Zr ZrO.sub.2 (40 mass %), Composite Y.sub.2O.sub.3 (4 mass %), Oxide Nd.sub.2O.sub.3 (2 mass %), La.sub.2O.sub.3 (4 mass %) Alumina Sasol Al.sub.2O.sub.3 (99 mass %), La.sub.2O.sub.3 (1 mass %) Lower CZ Material Rhodia CeO.sub.2 (30 mass %), Catalyst ZrO.sub.2 (60 mass %), La.sub.2O.sub.3 (5 mass %), Y.sub.2O.sub.3 (5 mass %) Layer Alumina Sasol Al.sub.2O.sub.3 (99 mass %), (Pt La.sub.2O.sub.3 (1 mass %) Layer)
<Results of Experiment>

(37) FIGS. 4 to 6 each show the experimental results. Herein, FIG. 4 is a graph showing the experimental results for identifying the Ce concentration in the Rh support and the NO.sub.x purification rate. In addition, FIG. 5 is a graph showing the experimental results for identifying the relationship between the added amounts of a high SSA composite oxide and a low SSA composite oxide in the upper catalyst layer and the NO.sub.x purification rate. Further, FIG. 6 is a graph showing the experimental results for identifying the relationship between the amount of the Rh support and a pressure loss and the relationship between the amount of the Rh support and the NO.sub.x light-off performance.

(38) FIG. 4 can confirm that regarding the Ce concentration in the Rh support, the catalytic activity becomes the highest when the concentration is 0%. Accordingly, a Ce concentration of 0% in Reference Example 2 is adopted. It is considered that the activity of Rh is improved with a reduction in the amount of bases in the Rh support.

(39) In addition, FIG. 5 can confirm that the OSC performance of a low SSA-OSC material is saturated when it is added by an amount of greater than or equal to 8 g/L, and the OSC performance of a high SSA-OSC material is also saturated when it is added by an amount of greater than or equal to 12 g/L from the difference from the required OSC performance, and thus that the low SSA-OSC material is preferably added by an amount of greater than or equal to 8 g/L and the high SSA-OSC material is preferably added by an amount of greater than or equal to 12 g/L. It should be noted that the low SSA-OSC material is useful for reducing a pressure loss as it has a small volume relative to its weight. However, the low SSA-OSC material has a limitation in the oxygen release capacity of the OSC performance. Thus, compensating for the OSC performance that is insufficient with the low SSA-OSC by using a high-SSA OSC material can achieve both excellent OSC performance and a reduction in the pressure loss.

(40) In addition, FIG. 6 can confirm that when the amount of the Rh support is increased, the light-off performance is improved, but a pressure loss is increased.

(41) Increasing the amount of the Rh support to increase the dispersibility of Rh is useful, and the light-off performance will be improved with an increase in the amount of the Rh support. Meanwhile, as a pressure loss tends to increase, a Rh support with an amount of 40 g/L, which is shown by Example 1, can be defined as the optimal amount of the Rh support.

(42) It should be noted that the results shown in FIG. 6 indicate not a point that Comparative Examples 1-4 should be excluded from the scope of the present invention, but a point that Example 1 has an optimal amount of the Rh support that can achieve both excellent OSC performance and a reduction in the pressure loss.

(43) 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

(44) 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