Catalytic converter

Abstract

A catalytic converter includes a substrate (1) and a catalyst layer (3). The catalyst layer includes a bottom catalyst layer (4), a first top catalyst layer (6) and a second top catalyst layer (7). The second top catalyst layer is provided on a downstream side relative to the first top catalyst layer. The first top catalyst layer is made of a ceria-free zirconia composite oxide support and rhodium. The second top catalyst layer is made of a ceria-containing zirconia composite oxide support and rhodium. The first top catalyst layer has a length that is X % of the entire length of the substrate. The second top catalyst layer has a length that is 100?X % of the entire length of the substrate. A ratio of a density of rhodium in the first top catalyst layer to a density of rhodium in the second top catalyst layer is at least 1 and at most 3.

Claims

1. A catalytic converter comprising: a substrate including a cell structure configured to allow exhaust gas to flow through the cell structure; and a catalyst layer formed on a surface of a cell wall of the substrate, wherein the catalyst layer includes a bottom catalyst layer, a first top catalyst layer and a second top catalyst layer, the bottom catalyst layer is provided on a surface of the substrate over an entire length of the substrate, the first top catalyst layer is provided on a surface of the bottom catalyst layer on an upstream side of the substrate in an exhaust gas flow direction, the second top catalyst layer is provided on a surface of the bottom catalyst layer on a downstream side of the substrate in the exhaust gas flow direction, the bottom catalyst layer is made of a support and palladium supported on the support, the first top catalyst layer is made of a ceria-free zirconia composite oxide support and rhodium supported on the ceria-free zirconia composite oxide support, and is a ceria-free catalyst layer, the second top catalyst layer is made of a ceria-containing zirconia composite oxide support and rhodium supported on the ceria-containing zirconia composite oxide support, the first top catalyst layer starts from an upstream side end of the substrate and has a length that is X % of the entire length of the substrate in the exhaust gas flow direction, X being from 30% to 70%, the second top catalyst layer starts from a downstream side end of the substrate and has a length that is 100-X % of the entire length of the substrate in the exhaust gas flow direction, and a ratio of a support density of rhodium supported in the first top catalyst layer to a support density of rhodium supported in the second top catalyst layer is at least 1 and at most 3, wherein the support density is the amount of rhodium per unit volume of the support.

2. The catalytic converter of claim 1, wherein the ratio of the support density of rhodium supported in the first top catalyst layer to the support density of rhodium supported in the second top catalyst layer is at least 5/3 and at most 3.

3. The catalytic converter of claim 1, wherein the ceria-free zirconia composite oxide is Al.sub.2O.sub.3ZrO.sub.2 composite oxide.

4. The catalytic converter of claim 1, wherein the ceria-containing zirconia composite oxide is CeO.sub.3ZrO.sub.2 composite oxide or Al.sub.2O.sub.3CeO.sub.2ZrO.sub.2 composite oxide.

5. The catalytic converter of claim 3, wherein the Al.sub.2O.sub.3ZrO.sub.2 composite oxide includes LaO.sub.3, Y.sub.2O.sub.3 and Nd.sub.2O.sub.3.

6. The catalytic converter of claim 4, wherein the CeO.sub.3ZrO.sub.2 composite oxide or Al.sub.2O.sub.3CeO.sub.2ZrO.sub.2 composite oxide includes LaO.sub.3, Y.sub.2O.sub.3 and Nd.sub.2O.sub.3.

7. The catalytic converter of claim 1, wherein the first top catalyst layer consists of a ceria-free zirconia composite oxide support and rhodium supported on the ceria-free zirconia composite oxide support, and the second top catalyst layer consists of a ceria-containing zirconia composite oxide support and rhodium supported on the ceria-containing zirconia composite oxide support.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Features, advantages, and the technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

(2) FIG. 1A is a schematic view of a catalytic converter according to an embodiment of the invention;

(3) FIG. 1B is an enlarged view of some of cells in the catalytic converter according to the embodiment of the invention;

(4) FIGS. 2A and B are longitudinal sectional views showing catalyst layers in the embodiment of the invention;

(5) FIG. 3 is a graph showing experimental results which determine a relationship between a ratio SD1/SD2 and NOx conversion efficiency under a high load, the ratio SD1/SD2 corresponding to a ratio of the density of supported Rh SD1 in the first top catalyst layer to the density of supported Rh SD2 in the second top catalyst layer; and

(6) FIG. 4 is a graph showing experimental results which determine a relation between the ratio SD1/SD2 and the HC 50% conversion time.

DETAILED DESCRIPTION OF EMBODIMENTS

(7) Embodiments of the catalytic converter of the invention are described below in conjunction with the diagrams.

(8) (Exhaust System)

(9) First, a system for emitting exhaust gases having located therein a catalytic converter according to an embodiment of the invention is described. In the exhaust system where the catalytic converter according to an embodiment of the invention is employed, an engine, a catalytic converter, a three-way catalytic converter, a submuffler and a main muffler are arranged and connected to each other by system pipes. Exhaust gases produced by the engine flow through each part and are emitted by way of the system pipes. An embodiment of the catalytic converter is described below.

(10) (Embodiment of Catalytic Converter)

(11) FIG. 1A is a schematic view of a catalytic converter according to an embodiment of the invention, and FIG. 1B is an enlarged view of some of the cells in the catalytic converter. FIGS. 2A and 2B are longitudinal sectional views showing embodiments of the catalyst layer in the catalytic converter.

(12) The catalytic converter 10 shown in FIGS. 1A and 1B is formed of a tubular substrate 1 having numerous cells and a catalyst layer 3 formed on the surfaces of cell walls 2 making up the cells. The cell structure may be regarded as being constituted by the numerous cells.

(13) Here, the material making up the substrate 1 is exemplified by ceramic materials such as cordierite (made of a composite oxide of magnesium oxide, aluminum oxide and silicon dioxide) and silicon carbide, and materials other than ceramic materials, such as metallic materials.

(14) The substrate 1 has a honeycomb structure constituted by numerous lattices, e.g., tetragonal, hexagonal or octagonal cells. Exhaust gases that have flowed into the cells on, in the direction of exhaust gas flow, the upstream side (Fr side) end of the substrate 1 pass through the interior of the substrate 1 and are purified in the course of such passage. The purified exhaust gases flow out (X direction) from the substrate 1 on, in the direction of exhaust gas flow, the downstream side (Rr side) end thereof.

(15) Next, an embodiment of the catalyst layer is described while referring to FIGS. 2A and 2B.

(16) The catalyst layer 3 shown in FIG. 2A is constituted by a bottom catalyst layer 4 formed on the surface of the substrate 1 and a top catalyst layer 5 formed on the surface of the bottom catalyst layer 4. The top catalyst layer 5 is additionally constituted by a first top catalyst layer 6 on the upstream side of the substrate 1 in the direction of exhaust gas flow and a second top catalyst layer 7 on the downstream side of the substrate 1 in the direction of exhaust gas flow. The top catalyst layer 5 is a zone-coated catalyst layer.

(17) The bottom catalyst layer 4 has a length that extends the entire length of the substrate 1, and is formed by supporting the noble metal catalyst Pd on an oxide support.

(18) In this embodiment, oxides composed of any one from among ceria (CeO.sub.2), zirconia (ZrO.sub.2) and alumina (Al.sub.2O.sub.3), composite oxides composed of two or more of these, and so on may be used as the oxide support forming the bottom catalyst layer 4. The composite oxides are, for example, CeO.sub.2ZrO.sub.2 compounds (available as CZ materials) and Al.sub.2O.sub.3CeO.sub.2ZrO.sub.2 tertiary composite oxides (available as ACZ materials). Incidentally, Al.sub.2O.sub.3 is introduced into Al.sub.2O.sub.3CeO.sub.2ZrO.sub.2 tertiary composite oxides as a diffusion barrier.

(19) The first top catalyst layer 6 in the top catalyst layer 5 of the embodiment shown in FIG. 2A has a length that is 30% of the entire length of the substrate 1, and is formed by supporting the noble metal catalyst Rh on an oxide support. The second top catalyst layer 7 has a length that is 70%, of the entire length of the substrate 1, and is formed by supporting the noble metal catalyst Rh on the oxide support.

(20) In this embodiment, CeO.sub.2ZrO.sub.2 compounds (available as CZ materials) which are zirconia complex oxides that include at least ceria (CeO.sub.2), Al.sub.2O.sub.3CeO.sub.2ZrO.sub.2 tertiary composite oxides (available as ACZ materials), and so on may be used as the oxide support forming the second top catalyst layer 7. Incidentally, Al.sub.2O.sub.3 is introduced into the Al.sub.2O.sub.3CeO.sub.2ZrO.sub.2 tertiary composite oxides as a diffusion barrier.

(21) On the other hand, Al.sub.2O.sub.3ZrO.sub.2 binary complex oxides (AZ materials), for example, may be used as the oxide support forming the first top catalyst layer 6. As described above, the Al.sub.2O.sub.3ZrO.sub.2 binary complex oxides are zirconia composite oxides that do not contain ceria.

(22) There exists a tradeoff between the fact that a high OSC is achieved by supporting Rh on a ceria-containing support and the fact that increasing the amount of ceria in the support lowers the NOx conversion performance that is characteristic of Rh. This dilemma can be addressed by employing, as shown in the illustrated embodiment, a zone-coated configuration having a ceria-containing catalyst layer and a ceria-free catalyst layer, and thereby forming catalyst layers in which the OSC and the NOx conversion performance are both good.

(23) In addition, the ratio SD1/SD2 of the density of supported Rh SD1 in the first top catalyst layer 6 to the density of supported Rh SD2 in the second top catalyst layer 7 is adjusted to at least 1 and at most 3.

(24) As is apparent also from the subsequently described experimental results, the inventors have empirically found that a catalyst having both a good NOx conversion performance under high engine load and a good catalyst warm-up performance can be obtained by adjusting the ratio SD1/SD2 to at least 1 and at most 3.

(25) Meanwhile, in the top catalyst layer 5A of the catalyst layer 3A shown in FIG. 2B, the first top catalyst layer 6A and the second top catalyst layer 7A have lengths which are respectively 70% and 30% of the entire length of the substrate 1, and these lengths are the reverse of the lengths of the layers in the embodiment shown in FIG. 2A.

(26) As subsequently described, the inventors have empirically found that when the length of the first top catalyst layer is in the range of 30 to 70% (and conversely, when the length of the second top catalyst layer is in the range of 70 to 30%) of the entire length of the substrate 1, a catalyst layer having both a good OSC and a good NOx conversion performance can be obtained.

(27) Experiments and the results of those experiments are described as below. One of the experiments is an experiment to determine the relationship between the ratio SD1/SD2 of the density of supported Rh SD1 in the first top catalyst layer to the density of supported Rh SD2 in the second top catalyst layer and the NOx conversion efficiency under high loading. The other of the experiments is an experiment to determine the relationship between the ratio SD1/SD2 and the HC 50% conversion time. The inventors produced catalyst slurries and catalytic converters by the following method, conducted durability tests and engine bench tests, and carried out performance evaluations of the catalytic converters. The relationship between the ratio SD1/SD2 of the density of supported Rh SD 1 in the first top catalyst layer to the density of supported Rh SD2 in the second top catalyst layer and the NOx conversion efficiency under high loading was then determined, and the relationship between the ratio SD1/SD2 and the HC 50% conversion time was also determined. The optimal range in SD1/SD2 was identified from these results. The 12 types of catalyst layers in Examples 1 to 5 and Comparative Examples 1 to 7 shown in Table 1 below were produced, catalytic converters equipped with these respective catalyst layers were produced, and durability tests were carried out.

(28) The methods used to prepare the catalyst slurries were as follows. A slurry to form the bottom catalyst layer (Pd catalyst layer) was prepared by first impregnating a support 65 g/L of Al.sub.2O.sub.3 composite oxide with a Pd nitrate solution so as to produce a 1.0 wt % supporting powder. Then, a Pd catalyst slurry was prepared by adding 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 (wt %)) in an amount corresponding to 85 g/L, Ba acetate in an amount corresponding to 10 g/L, water, Al.sub.2O.sub.3 binder, acetic acid, a thickener and the like to the supporting powder in given amounts and mixing the mixture.

(29) In a separate procedure, a slurry to form the top catalyst layer (Rh catalyst layer) was produced by compounding 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 (wt %)) or 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 (wt %)) to a concentration of 65 g/L, then supporting Rh in the amounts shown in Table 1 below onto the respective supports (the amount of Rh in the catalysts was made uniform). In addition, La-doped Al.sub.2O.sub.3 in an amount equivalent to 25 g/L, Ba acetate in an amount equivalent to 10 g/L, water, Al.sub.2O.sub.3 binder, acetic acid, a thickener and the like were added thereto in given amounts and mixed, thereby giving the Rh catalyst slurries. As for the catalytic layer in Comparative Example 3, a CeO.sub.2ZrO.sub.2 composite oxide and a ZrO.sub.2 composite oxide were blended together in a 1:1 ratio, and the total amount was made uniform.

(30) A monolithic substrate (875 cc) was furnished, and was coated by a suction process with the slurries prepared as described above (with the Pd catalyst layer being applied over 100% of the substrate length, and the respective Rh catalyst layers being applied as indicated in Table 1 below). In Table 1 below, AZLNY stands for Al.sub.2O.sub.3/ZrO.sub.2/La.sub.2O.sub.3/Y.sub.2O.sub.3/Nd.sub.2O.sub.3, and AZLCNY stands for 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.

(31) TABLE-US-00001 TABLE 1 Density of Density of Length Amount supported supported of of Rh Rh top First Second CeO.sub.2 in SD1 in SD2 in catalyst top top top first top second top layer catalyst catalyst catalyst catalyst catalyst (Fr/Rr) layer layer layer layer layer (%) support support (g) (g/L) (g/L) Comp. 100/0 AZLCNY 11.4 0.20 Ex. 1 Comp. 100/0 AZLNY 0 0.20 Ex. 2 Comp. 100/0 AZLNY + 5.7 0.20 Ex. 3 AZLCNY Exam- 50/50 AZLNY AZLCNY 5.7 0.20 0.20 ple 1 Comp. 50/50 AZLNY AZLCNY 5.7 0 0.40 Ex. 4 Comp. 50/50 AZLNY AZLCNY 5.7 0.10 0.30 Ex. 5 Comp. 50/50 AZLNY AZLCNY 5.7 0.15 0.25 Ex. 6 Exam- 50/50 AZLNY AZLCNY 5.7 0.25 0.15 ple 2 Exam- 50/50 AZLNY AZLCNY 5.7 0.30 0.10 ple 3 Comp. 50/50 AZLNY AZLCNY 5.7 0.40 0 Ex. 7 Exam- 30/70 AZLNY AZLCNY 5.7 0.20 0.20 ple 4 Exam- 70/30 AZLNY AZLCNY 5.7 0.20 0.20 ple 5

(32) The catalytic converters produced were set directly below a working engine, and a 50-hour durability test was conducted at a bed temperature of 1,000? C. under a complex pattern where the air-fuel ratio is cyclically changed.

(33) Engine bench testing is described below. The catalytic converter subjected to a durability test was then set in another working engine, and the time from stoichiometric engine startup until the concentration of HCs falls to 50% or below was measured to determine the catalyst warm-up performance. Also, the NOx conversion efficiency when running the engine under operating conditions corresponding to a vehicle speed of 160 km/h was measured to determine the NOx conversion performance under a high engine load. The test results are shown in Table 2 below and in FIGS. 3 and 4.

(34) TABLE-US-00002 TABLE 2 NOx conversion Time to 50% efficiency under conversion of high engine load HCs (%) (seconds) Comparative Example 1 90.5 16.3 Comparative Example 2 92.8 15.5 Comparative Example 3 94.3 15.8 Example 1 95.5 15.4 Comparative Example 4 83.0 18.8 Comparative Example 5 85.6 17.5 Comparative Example 6 90.8 16.2 Example 2 98.2 15.2 Example 3 98.5 14.4 Comparative Example 7 88.5 13.2 Example 4 93.8 15.5 Example 5 95.2 15.4

(35) As shown in Table 1, in Examples 1 to 5, the support used on the upstream side and the support used on the downstream side were of different types. Moreover, in Examples 2 and 3, the density of supported Rh was made higher in the top catalyst layer on the upstream side than in the top catalyst layer on the downstream side. It is apparent from Table 2 and FIG. 3 that, compared with Comparative Examples 1 to 7, the catalytic converters in Examples 1 to 5 resulted in higher NOx conversion efficiencies under high engine loading. It is also apparent that, in the absence of Rh in the top catalyst layer on the downstream side (Comparative Example 7), the conversion performance abruptly decreases. It was also demonstrated from this experiment that it is desirable: to form the upstream-side top catalyst layer that starts from the upstream side end of the substrate and has a length that is 30 to 70% (X %) of the entire length of the substrate; to form the downstream-side top catalyst layer that starts from the downstream side end of the substrate and has a length that is 100?X % of the entire length of the substrate; and to set the ratio SD1/SD2 of the density of supported Rh SD1 on the upstream side top catalyst layer to the density of supported Rh SD2 on the downstream side top catalyst layer in the range of at least 1 and at most 3. It was additionally found that, with regard to the NOx conversion efficiency, an SD1/SD2 ratio of at least 5/3 and at most 3 is more preferred.

(36) From Table 2 and FIG. 4, it was found that, compared with Comparative Examples 1 to 7, the catalytic converters in Examples 1 to 5 resulted in an improved catalyst warm-up performance. Moreover, an SD1/SD2 ratio of at least 5/3 and at most 3 is also preferred with regard to the catalyst warm-up performance. From the results in each of the above examples, it was demonstrated that the SD1/SD2 ratio is preferably 1 or more. This is consistent with the results for NOx conversion performance under high engine load.

(37) From the above two sets of experimental results, it was found that by setting the SD1/SD2 ratio in the range of at least 1 and at most 3, a catalytic converter having an excellent OSC, an excellent NOx conversion performance under high engine load, and an excellent catalyst warm-up performance can be provided.

(38) The embodiment of the invention has been described above in detail with reference made to the drawings. However, the specific structure of the invention is not limited to the embodiment, and design modifications not departing from the gist of the invention are also included in the scope of the invention.