Catalyst

Abstract

catalyst (10) comprising: a substrate (1) with a cell structure in which exhaust gas flows; and catalyst layers (3, 3A, 3B, 3C, 3D, 3E) formed on the cell wall surfaces of the substrate (1), wherein the catalyst layers (3, 3A, 3B, 3C, 3D, 3E) comprise a first catalyst layer (4, 4A, 4B, 4C) arranged on an upstream side (Fr) of the substrate (1) in an exhaust gas flow direction and a second catalyst layer (5, 5A) arranged on an downstream side (Rr) of the substrate (1) in the exhaust gas flow direction, the first catalyst layer (4, 4A, 4B, 4C) is formed from a support and rhodium, which is a precious metal catalyst supported on the support, the second catalyst layer (5, 5A) is formed from a support and palladium or platinum, which is a precious metal catalyst supported on the support, the first catalyst layer (4, 4A, 4B, 4C) is formed in a range of 80% to 100% of the total length of the substrate (1) starting from one end of the substrate (1) on the upstream side (Fr), and the second catalyst layer (5, 5A) is formed in a range of 20% to 50% of the total length of the substrate (1) starting from one end of the substrate (1) on the downstream side (Rr); characterized in that the support of the first catalyst layer (4, 4A, 4B, 4C) does not contain cerium.

Description

TECHNICAL AREA

[0001] The present invention relates to a catalyst housed and attached in a pipeline representing an exhaust system for exhaust gas. STATE OF THE ART

[0002] Various industries have undertaken efforts to reduce environmental impacts on a global scale. Particularly in the automotive industry, progress has been made in developing technologies not only for the widespread adoption of gasoline-powered vehicles with superior fuel efficiency, but also for so-called ECO vehicles, such as hybrid or electric vehicles, and for further improving the performance of these vehicles. The development of such ECO vehicles has involved active research into exhaust gas purification catalysts, which clean the exhaust gases from the engine. These catalysts comprise an oxidation catalyst, a three-way catalyst, and a NOx storage reduction catalyst. Within these catalysts, a precious metal catalyst such as platinum (Pt), palladium (Pd), or rhodium (Rh) exhibits catalytic activity.In general, this precious metal catalyst is used in a state where it is supported on a substrate formed by a porous oxide such as aluminum oxide.

[0003] In an exhaust system connecting a vehicle engine and a muffler, a catalytic converter is generally used to clean the exhaust gases. The engine can emit environmentally harmful substances such as CO, NOx, or unburned HC or VOCs. To convert these harmful substances into environmentally friendly ones, the exhaust gas flows through a catalytic converter, so that CO is converted to CO2, NOx is converted to N2 and O2, and VOCs are combusted to produce CO2 and H2O. Within the catalytic converter, catalytic layers are formed on the cell wall surfaces of a substrate, using a precious metal catalyst such as Rh, Pd, or Pt.

[0004] Examples of the support material for precious metal catalysts include a CeO₂-ZrO₂ solid solution (also known as CZ material, cerium oxide-zirconium oxide mixed oxide, and the like). This support material is also called a CO catalyst and is an essential component of the three-way catalytic converter, which simultaneously removes harmful components from the exhaust gas such as CO, NOx, and HC. CeO₂ is an example of an essential component of the CO catalyst. The oxidation state of CeO₂ changes, for example, to Ce₂, depending on the partial pressure of oxygen in the exhaust gas to which the CeO₂ is exposed. 3+ or Ce 4+To compensate for the lack of charge, CeO2 exhibits an oxygen absorption and desorption function and an oxygen storage function (OSC: oxygen storage capacity). To maintain a clean window for the three-way catalytic converter, changes in the exhaust gas atmosphere can be absorbed and mitigated, allowing the clean window to be kept essentially at a theoretical air-fuel ratio.

[0005] Furthermore, from the perspective of reducing the material risk associated with rare metals and achieving cost competitiveness, the amount of precious metal catalyst used in the three-way catalyst is a crucial factor. However, if the amount of precious metal catalyst in the three-way catalyst is significantly reduced, the catalytic activity will also be significantly reduced. Consequently, the OSC (oxidation-free conversion) described above, the low-temperature activity, the NOx removal performance in a high-temperature environment, and similar properties will be significantly reduced. The reason is as follows: Along with a significant decrease in the amount of precious metal catalyst, the number of active sites is also significantly reduced, and the number of catalytic reaction sites is significantly reduced. As a result, the purification performance decreases significantly.

[0006] Of the precious metal catalysts, including Pt, Pd, and Rh, which are used particularly in the three-way catalyst, Rh exhibits the highest NOx removal efficiency but has the highest market price per unit weight. Furthermore, it is known that Rh, when supported on a cerium oxide (Ceria)-containing CO catalyst, exhibits high OSC. However, it is also known that a trade-off exists: increasing the amount of cerium oxide in the CO catalyst used to support Rh reduces its NOx removal efficiency. Therefore, when Rh is used as the precious metal catalyst in the three-way catalyst, the design criteria during its manufacture must be optimized to achieve both OSC and NOx removal efficiency simultaneously.

[0007] Regarding the production of the optimal 3-way catalyst, taking into account the fact that the performance of various catalytic precious metals and supports varies depending on their respective components, a zone-coated catalyst was actively investigated in which different components are arranged on an upstream side and a downstream side of a substrate, so that the properties of the respective components are efficiently exhibited.

[0008] With regard to this zone-coated catalyst, PTL 1 discloses an exhaust gas purification catalyst comprising: a substrate for forming a gas passage through which exhaust gas flows; and catalyst layers formed on the substrate. In particular, the catalyst layers used here comprise: a lower catalyst layer formed on a surface of the substrate; a front upper catalyst layer coated on a surface of the lower catalyst layer on an upstream side in a gas flow direction; and a rear upper catalyst layer coated on a surface of the lower catalyst layer on a downstream side of the front upper catalyst layer in the gas flow direction.Furthermore, at least one of the Pd and Pt elements is supported in the lower catalyst layer, Rh is supported in the rear upper catalyst layer, Pd is supported in the front upper catalyst layer, and a Y₂O₃-containing ZrO₂ mixed oxide serves as a support for Pd in ​​the front upper catalyst layer. According to this configuration, the purification properties of the catalytic noble metal can be sufficiently demonstrated, and the low-temperature purification performance of the catalyst can be improved. Moreover, by using the ZrO₂ mixed oxide containing Y₂O₃ as a support material for the front upper catalyst layer, which exhibits low specific heat and superior heat resistance, heat resistance can be ensured, while the catalyst temperature rise behavior is improved, and a long-life catalyst warm-up behavior can be achieved.

[0009] On the other hand, PTL 2 discloses an exhaust gas purification catalyst comprising: a substrate; a lower catalyst layer formed on the substrate and comprising at least one of Pd and Pt; and an upper catalyst layer formed on the lower catalyst layer and comprising Rh, wherein a region not comprising the upper catalyst layer is arranged on an exhaust gas upstream side of the exhaust gas purification catalyst, the lower catalyst layer comprises a front lower catalyst layer arranged on the exhaust gas upstream side, and a rear lower catalyst layer arranged on an exhaust gas downstream side, and the front lower catalyst layer comprises an oxygen storage material.With this configuration, the grain growth of the respective catalytic metals supported in the respective catalyst layers can be significantly suppressed, particularly in the rear lower catalyst layer and the upper catalyst layer on the exhaust gas downstream side. Furthermore, by providing the area on the exhaust gas upstream side that does not encompass the upper catalyst layer, the diffusion capacity of HC into the interior of the front lower catalyst layer can be improved, and the removal of HC from the front lower catalyst layer is accelerated, thus achieving sufficient catalyst warm-up performance.

[0010] Furthermore, PTL 3 discloses an exhaust gas purification catalyst in which catalyst layers comprise: a lower catalyst layer formed on the surface of a substrate; a front upper catalyst layer with which a surface of the lower catalyst layer is coated on an upstream side in the direction of gas flow; and a rear upper catalyst layer with which a surface of the lower catalyst layer is coated on a downstream side in the direction of gas flow of the front upper catalyst layer. In the exhaust gas purification catalyst, at least one of Pd and Pt is supported in the lower catalyst layer, Pd is supported in the front upper catalyst layer, Rh is supported in the rear upper catalyst layer, and the concentration of Pd supported in the front upper catalyst layer is 4.5 wt% to 12 wt%.With this configuration, the cleaning properties of the catalytic precious metal can be sufficiently demonstrated and the low-temperature cleaning performance of the catalyst can be improved.

[0011] Furthermore, PTL 4 reveals another state-of-the-art exhaust gas purification catalyst.

[0012] Thus, various techniques exist regarding the zone-coated catalyst. Under the above circumstances, the inventors have modified the configuration of the zone-coated catalyst and devised a catalyst capable of achieving superior NOx removal performance while reducing the amount of precious metal catalyst. LIST OF CITED DOCUMENTS Patent Literature PTL 1: JP 2012 - 040 547 A PTL 2: JP 2012 - 152 702 A PTL 3: JP 2012 - 020 276 A PTL 4: EP 1 825 912 A1 SUMMARY OF THE INVENTION

[0013] The present invention was made taking into account the problems described above, and one of its objectives is to provide a catalyst capable of achieving superior NOx removal performance while reducing the amount of a precious metal catalyst.

[0014] To achieve the above-described objective, according to one aspect of the invention, a catalyst is provided comprising: a substrate with a cell structure in which exhaust gas flows; and catalyst layers formed on the cell wall surfaces of the substrate, wherein the catalyst layers comprise a first catalyst layer arranged on an upstream side of the substrate in the direction of exhaust gas flow, and a second catalyst layer arranged on a downstream side of the substrate in the direction of exhaust gas flow, wherein the first catalyst layer is formed from a support and rhodium, which is a noble metal catalyst supported on the support, and wherein the second catalyst layer is formed from a support and palladium or platinum, which is a noble metal catalyst supported on the support.The first catalyst layer is formed in a region of 80% to 100% of the total length of the substrate, starting from an end of the substrate on the upstream side, and the second catalyst layer is formed in a region of 20% to 50% of the total length of the substrate, starting from an end of the substrate on the downstream side. The catalyst of the present invention is characterized in that the support of the first catalyst layer does not comprise cerium.

[0015] In the catalyst according to the present invention, the zone-coated catalyst is used as the catalyst layers formed on the cell walls of the substrate with a cell structure; the first catalyst layer is arranged on the upstream side (Fr side) of the substrate in the direction of exhaust gas flow; the second catalyst layer is arranged on the downstream side (Rr side) of the substrate in the direction of exhaust gas flow; rhodium is used as the noble metal catalyst of the first catalyst layer; palladium or platinum is used as the noble metal catalyst of the second catalyst layer; the length of the first catalyst layer is in the range of 80% to 100% with respect to the length of the substrate and the length of the second catalyst layer is in the range of 20% to 50% with respect to the length of the substrate.As a result, superior NOx removal performance can be achieved in the catalyst while reducing the amount of precious metal catalyst used, especially rhodium, as much as possible.

[0016] Here, the substrate with a cellular structure can be not only a ceramic material such as cordierite or silicon carbide, which is formed from a mixed oxide of magnesium oxide, aluminum oxide, and silicon dioxide, but also a material other than a ceramic, for example, a metal. Furthermore, a so-called honeycomb structure can be assumed in this configuration, comprising cells with multiple grid contours, such as rectangular, hexagonal, and octagonal shapes.

[0017] Furthermore, examples of the supports forming the first and second catalyst layers on the cell wall surfaces of the substrate include oxides comprising at least one porous oxide of CeO2, ZrO2, and Al2O2 as the main component; an oxide of cerium oxide (CeO2), zirconium oxide (ZrO2), and aluminum oxide (Al2O3); and a mixed oxide formed from two or more oxides of cerium oxide (CeO2), zirconium oxide (ZrO2), and aluminum oxide (Al2O3) (e.g., a CeO2-ZrO2 compound, which is a CZ material, or an Al2O3-CeO2-ZrO2 ternary mixed oxide (ACZ material), into which Al2O3 is incorporated as a diffusion barrier).

[0018] According to the inventors, it has been verified that the NOx removal performance is extremely superior under the following conditions: the length of the first catalyst layer, in which rhodium is used as the precious metal catalyst, is in the range of 80% to 100% with respect to the length of the substrate; and the length of the second catalyst layer, in which palladium or platinum is used as the precious metal catalyst, is in the range of 20% to 50% with respect to the length of the substrate.

[0019] For example, if the length of the first catalyst layer is 90% of the substrate length, and if the length of the second catalyst layer is 50% of the substrate length, the two catalyst layers overlap by 40% of the substrate length. In this case, for example, all the catalyst layers are formed such that the first catalyst layer directly overlaps the second catalyst layer.

[0020] It is likely that palladium forms an alloy with rhodium. Therefore, rhodium, where sintering is less likely due to its relatively high melting point, is used in the first catalyst layer, which forms on the upstream side of the substrate where exhaust gas flows at a relatively high temperature. Conversely, platinum, where sintering is likely due to its relatively low melting point, is used in the second catalyst layer on the downstream side of the substrate where exhaust gas flows at a relatively low temperature. As a result, NOx removal performance can be improved while alloying between the precious metal catalysts is suppressed.

[0021] By using platinum with a wider air-fuel ratio than that of palladium as the precious metal catalyst of the second catalyst layer, a catalyst with higher purification performance can be obtained.

[0022] In the catalyst of the invention, the support of the first catalyst layer does not include cerium.

[0023] Following confirmation of the invention, it was specified that the NOx removal performance can be further improved by not using cerium as the support that forms the first catalyst layer, in which rhodium is used as the precious metal catalyst.

[0024] The catalyst according to the present invention preferably comprises a cordierite honeycomb support with superior heat shock resistance, but can also be an electrically heated catalyst (EHC). In the electrically heated catalyst, for example, a pair of electrodes is attached to a honeycomb catalyst. The honeycomb catalyst is heated by ensuring that an electric current flows through the electrodes, thereby improving the activity of the catalyst and detoxifying the exhaust gas flowing through it. By applying this electrically heated catalyst to an exhaust system connecting a vehicle engine and a muffler, the exhaust gas can be cleaned not only at room temperature but also at cold temperatures by activating the catalyst via the electric heating element.

[0025] As can be seen from the above description, in the catalyst according to the present invention, the first catalyst layer is arranged on the upstream side of the substrate in the exhaust gas flow direction; the second catalyst layer is arranged on the downstream side of the substrate in the exhaust gas flow direction; rhodium is used as the precious metal catalyst of the first catalyst layer; palladium or platinum is used as the precious metal catalyst of the second catalyst layer; the length of the first catalyst layer is in the range of 80% to 100% of the length of the substrate; and the length of the second catalyst layer is in the range of 20% to 50% of the length of the substrate. As a result, superior NOx removal performance can be achieved in the catalyst while reducing the amount of precious metal catalyst used, in particular rhodium, as much as possible. BRIEF DESCRIPTION OF THE FIGURES Fig. 1(a) is a schematic diagram representing a catalyst according to the present invention and Fig. 1(b) is an enlarged view showing a portion of the cells. Fig. 2(a) is a vertical cross-sectional view showing embodiment 1 of catalyst layers and Fig. 2(b) is a vertical cross-sectional view showing embodiment 2 of catalyst layers. Fig. 3(a) is a vertical cross-sectional view showing embodiment 3 of catalyst layers and Fig. 3(b) is a vertical cross-sectional view showing embodiment 4 of catalyst layers. Fig. 4(a) is a vertical cross-sectional view showing embodiment 5 of catalyst layers and Fig. 4(b) is a vertical cross-sectional view showing embodiment 6 of catalyst layers. Fig. Figure 5 is a graph showing experimental results of measuring the emission fraction of NOx when the length of the first catalyst layer is set to 80% in relation to the length of the substrate and when the length of the second catalyst layer is changed. Fig. Figure 6 is a graph that, with regard to the comparison examples and examples, presents test results of measuring the emission fraction of NOx. WAYS TO EXECUTIVE THE INVENTION

[0026] An embodiment of a catalyst according to the present invention is described below with reference to the figures. (Exhaust system for exhaust gases)

[0027] First, an exhaust system for exhaust gases, in which the catalyst according to the present invention is provided, is briefly described. In the exhaust system for exhaust gases, in which the catalyst according to the present invention is used, an engine, a catalyst, a three-way catalyst, a sub-muffler, and a main muffler are arranged and connected to each other by a system pipe. Exhaust gas generated by the engine flows through the system pipe to each unit and is expelled. Next, the embodiment of the catalyst is described. (Form of the catalyst)

[0028] Fig. 1(a) is a schematic diagram representing a catalyst according to the present invention and Fig. 1(b) is an enlarged view showing a portion of the cells. Furthermore, Fig. 2(a), Fig. 2(b), Fig. 3(a), Fig. 3(b), Fig. 4(a) and Fig. 4(b) Vertical cross-sectional views showing embodiments 1 to 6 of catalyst layers.

[0029] In short, a Fig. 1 Catalyst 10 shown: a cylindrical substrate 1 with several cells; and catalyst layers 3 formed on surfaces of the cell walls 2 forming the cells.

[0030] Examples of substrate 1 material include a ceramic material such as cordierite or silicon carbide formed from a mixed oxide of magnesium oxide, aluminum oxide, and silicon dioxide; and a material other than a ceramic material, for example, a metallic material. Furthermore, examples of a support forming the catalyst layers 3 on the surfaces of the cell walls 2 of substrate 1 include oxides comprising at least a porous oxide of CeO2, ZrO2, and Al2O3 as a major component; and an oxide of cerium oxide (CeO2), zirconium oxide (ZrO2), and aluminum oxide (Al2O3). and a mixed oxide formed from two or more oxides of cerium oxide (CeO2), zirconium oxide (ZrO2) and aluminum oxide (Al2O3) (e.g. a CeO2-ZrO2 compound which is a CZ material, or an Al2O3-CeO2-ZrO2 ternary mixed oxide (ACZ material) into which Al2O3 is incorporated as a diffusion barrier).

[0031] Substrate 1 has a honeycomb structure comprising cells with a plurality of grid contours, including rectangular, hexagonal, and octagonal shapes. Exhaust gas, flowing into the interior of the cells at one end of substrate 1 on an upstream side (Fr side) in the direction of exhaust gas flow, enters the interior of substrate 1. During this flow process, the exhaust gas is cleaned, and the cleaned exhaust gas flows out of one end of the substrate on a downstream side (Rr side) in the direction of exhaust gas flow (X direction).

[0032] Next, the catalyst layers formed on the surfaces of the cell walls 2 will be described with reference to Fig. 2, Fig. 3 to Fig. 4 described. In each figure, the upper and lower cell walls forming a cell are shown.

[0033] Fig. 2(a) represents zone-coated catalyst layers 3 according to embodiment 1.

[0034] The catalyst layers 3 shown in the same figure comprise a first catalyst layer 4 and a second catalyst layer 5, wherein 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 direction of exhaust gas flow, 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 direction of exhaust gas flow, and the two catalyst layers do not overlap.

[0035] In the first catalyst layer 4, rhodium is used as a precious metal catalyst supported on a substrate. In the second catalyst layer 5, palladium or platinum is used as a precious metal catalyst supported on a substrate.

[0036] A cerium-free material is used as the support for rhodium in the first catalyst layer 4. Examples of the support include an oxide formed from a mixture of zirconium oxide (ZrO2) and aluminum oxide (Al2O3); and an Al2O3-ZrO2 binary mixed oxide (AZ material).

[0037] According to the catalyst layers 3 shown in the figure, rhodium is not used over their entire length. Therefore, the amount of rhodium used, which is the most expensive of the precious metal catalysts, can be reduced. Furthermore, the first catalyst layer has a length of 80% relative to the length of substrate 1 on the upstream side. The second catalyst layer 5, in which platinum or the like is used as the precious metal catalyst, has a length of 20% relative to the length of substrate 1 on the downstream side. As a result, catalyst layer 3 is formed with superior NOx removal performance.

[0038] Fig. 2(b) On the other hand, represents zone-coated catalyst layers 3A according to embodiment 2. In the configuration of the catalyst layers 3A shown in the same figure, a second catalyst layer 5A has a length of 50% relative to the length of the substrate 1, a first catalyst layer 4A has a length of 80% relative to the length of the substrate 1, and the two catalyst layers overlap by 30%. The catalyst layers 3A shown in the figure reduce the amount of rhodium used, and superior NOx removal performance can be expected.

[0039] Fig. 3(a) On the other hand, represents zone-coated catalyst layers 3B according to embodiment 3. In the configuration of the catalyst layers 3B shown in the same figure, the second catalyst layer 5 has a length of 20% relative to the length of the substrate 1, a first catalyst layer 4B has a length of 90% relative to the length of the substrate 1, and the two catalyst layers overlap by 10%. The catalyst layers 3B shown in the figure reduce the amount of rhodium used, and superior NOx removal performance can be expected.

[0040] Fig. 3(b) On the other hand, represents zone-coated catalyst layers 3C according to embodiment 4. In the configuration of the catalyst layers 3C shown in the same figure, the second catalyst layer 5A has a length of 50% relative to the length of the substrate 1, the first catalyst layer 4B has a length of 90% relative to the length of the substrate 1, and the two catalyst layers overlap by 40%. The catalyst layers 3C shown in the figure allow for a reduction in the amount of rhodium used and enable superior NOx removal performance.

[0041] Fig. 4(a) On the other hand, represents zone-coated catalyst layers 3D according to embodiment 5. In the configuration of the catalyst layers 3D shown in the same figure, the second catalyst layer has a length of 20% relative to the length of the substrate 1, a first catalyst layer 4C has a length of 100% relative to the length of the substrate 1, and the two catalyst layers overlap in a region of 20%. Superior NOx removal performance can be expected from the catalyst layers 3D shown in the figure.

[0042] Furthermore, it Fig. 4(b) Zone-coated catalyst layers 3E according to embodiment 6. In the configuration of the catalyst layers 3E shown in the same figure, the second catalyst layer 5A has a length of 50% relative to the length of the substrate 1, the first catalyst layer 4C has a length of 100% relative to the length of the substrate 1, and the two catalyst layers overlap in a region of 50%. Superior NOx removal performance can be expected from the catalyst layers 3E shown in the figure.

[0043] In addition to the examples shown in the figures, there are various combination configurations that fall under the following design criteria: the first catalyst layer is formed in a range of 80% to 100% of the total length of substrate 1, starting at the end of 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 substrate, starting from the end of substrate 1 on the downstream side. [Experiment (Part 1) to determine the optimal range of the second catalyst layer and its results]

[0044] The inventors defined the length of the first catalyst layer as 80% of the substrate length and varied the length of the second catalyst layer to 0%, 10%, 30%, 50%, 80%, and 100% of the substrate length. A catalyst comprising the catalyst layers of each of these cases was fabricated, a longevity test was performed, and an experiment measuring the NOx fraction under normal fuel conditions was conducted. (Regarding the process of producing a catalyst sludge)

[0045] 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 Al₂O₂ mixed oxide was impregnated with a palladium nitrate solution as a support. One result was 1.0 wt% support powder. Next, 85 g / L of a CeO₂-ZrO₂ mixed oxide (CeO₂ / ZrO₂ / La₂O₃ / Y₂O₃ = 30 / 60 / 5 / 5 (wt%)), 10 g / L barium acetate, water, an Al₂O₃ binder, acetic acid, a thickener, and the like were mixed together in predetermined amounts. One result was a Pd catalyst slurry.

[0046] 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 CeO₂-ZrO₂ mixed oxide (Al₂O₃ / CeO₂ / ZrO₂ / La₂O₃ / Y₂O₃ / Nd₂O₃ = 30 / 20 / 44 / 2 / 2 / 2 (wt%)) were prepared, and 0.3 wt% Rh was applied to each support. Furthermore, 25 g / L Al₂O₃ with added La, 10 g / L barium acetate, water, an Al₂O₃ binder, acetic acid, a thickening agent, and the like were mixed together in predetermined quantities. A Rh catalyst slurry was obtained as a result.

[0047] 875 cc of a monolithic substrate were produced and coated with the slurries described above using a suction process.

[0048] The second catalyst layers (Pd-supporting 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 at the Rr side, were formed by layering the slurry, using the same proportion of slurry.

[0049] The first catalyst layer, on the other hand (Rh-supporting catalyst layer), which had a length of 80% in relation to the length of the substrate starting from the end of the substrate on the Fr side, was formed by coating using the slurry. (Regarding the longevity test)

[0050] The manufactured catalyst was mounted directly under an actual engine and a longevity test was carried out on it at a bed temperature of 100°C for 50 hours under a mixture pattern in which an A / F ratio changed cyclically. (Regarding the engine bank evaluation)

[0051] After the longevity test, the catalyst was fitted to another actual engine, and the cleaning performance was calculated as the average NOx emission fraction when an air-fuel ratio in a rectangular shape was changed from a rich to a lean condition and held in the rich condition for 120 seconds. The test results are in Fig. 5 shown.

[0052] In the same figure, the NOx emission fraction exhibited an inflection point when the length of the second catalyst layer was 50% of the substrate length. If the length of the second catalyst layer was longer than 50%, the NOx emission fraction increased, resulting in an unfavorable purification performance approaching 500 ppm. Conversely, the NOx emission fraction was significantly reduced, approaching 200 ppm or less, when the length of the second catalyst layer was 50% or less.

[0053] Based on these experimental results, the upper limit of the ratio of the length of the second catalyst layer to the length of the substrate can be defined as 50%.

[0054] Next, in a different experiment, the lower limit of the ratio of the length of the second catalyst layer will be defined. [Experiment (Part 2) to determine the optimal range of the second catalyst layer and results thereof]

[0055] The inventors produced a catalyst that included the catalyst layers according to the respective examples and comparative examples, a longevity test was carried out, and an attempt was made to measure the proportion of NOx in a normal rich condition. (Regarding the process of producing the catalyst sludge)

[0056] 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 Al₂O₃ mixed oxide was impregnated with a palladium nitrate solution as a support. One result was 1.0 wt% support powder. Next, 85 g / L of a CeO₂-ZrO₂ mixed oxide (CeO₂ / ZrO₂ / La₂O₃ / Y₂O₃ = 30 / 60 / 5 / 5 (wt%)), 10 g / L barium acetate, water, an Al₂O₃ binder, acetic acid, a thickener, and the like were mixed together in predetermined amounts. One result was a Pd catalyst slurry.

[0057] Regarding the preparation of a slurry for forming the first catalyst layer (Rh was used as a noble metal catalyst), 65 g / L of a CeO₂-ZrO₂ mixed oxide (Al₂O₃ / CeO₂ / ZrO₂ / La₂O₃ / Y₂O₃ / Nd₂O₃ = 30 / 20 / 44 / 2 / 2 / 2 (wt%)) was prepared. In Example 2, the same proportion of ZrO₂ mixed oxide (Al₂O₃ / ZrO₂ / La₂O₃ / Nd₂O₃ = 50 / 46 / 2 / 2 (wt%)) was used. 0.3 wt% Rh was supported on each support. Furthermore, 25 g / L Al₂O₃ with added La, 10 g / L barium acetate, water, an Al₂O₃ binder, acetic acid, a thickening agent, and the like were mixed together in predetermined quantities. One result was a Rh catalyst sludge.

[0058] 875 cc of a monolithic substrate were produced and coated with the slurries described above using a suction process.

[0059] In comparative example 1, catalyst layers with a two-layer structure were formed over the entire length of the substrate, whereby a Pd-supporting catalyst layer and an Rh-supporting catalyst layer were laminated using the slurries described above.

[0060] In comparative example 2, catalyst layers with a three-layer structure were formed over the entire length of the substrate, whereby a Pd-supporting catalyst layer and an Rh-supporting catalyst layer were laminated using the slurries described above.

[0061] In Example 1 (not part of the invention), on the other hand, as in Fig. 2(a) shows the first catalyst layer with a length of 80% on the upstream side and the second catalyst layer with a length of 20% was formed on the downstream side (the two layers did not overlap).

[0062] Furthermore, the configuration of the catalyst layers of Example 2 was the same as that of Example 1 (not part of the invention); however, a ZrO2 mixed oxide not containing cerium was used in the slurry for forming the first catalyst layer. (Regarding the longevity test)

[0063] The manufactured catalyst was mounted directly under an actual engine and a longevity test was carried out on it at a bed temperature of 100°C for 50 hours under a mixture pattern in which an A / F ratio changed cyclically. (Regarding the engine bank evaluation)

[0064] After the longevity test, the catalyst was fitted to another actual engine, and the cleaning performance was calculated as the average NOx emission fraction when an air-fuel ratio in a rectangular shape was changed from a rich to a lean condition and held in the rich condition for 120 seconds. The test results are in Fig. 6 shown.

[0065] The same figure confirmed that the NOx emission share of example 1 can be reduced by approximately 60% to 70% compared to comparison examples 1 and 2.

[0066] Furthermore, when Example 1 (not part of the invention) was compared with Example 2, it was confirmed that the NOx emission rate of Example 2 can be reduced to approximately 20% of that of Example 1.

[0067] The experimental results confirmed that the ratio of the length of the second catalyst layer to the length of the substrate should preferably be 20% or higher. This value of 20% can be defined as the lower limit of the ratio of the second catalyst layer length. Considering 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 as lying in the range of 20% to 50%.

[0068] In the experiments described above, the ratio of the length of the first catalyst layer to the length of the substrate was set at 80%. It goes without saying that increasing the length of the first rhodium-containing catalyst layer will result in a catalyst with superior NOx removal performance. Accordingly, the 80% value described above can be defined as the lower limit of the first catalyst layer length ratio, and the value of 100%, where the first catalyst layer length equals the substrate length, can be defined as the upper limit. From the perspective of reducing the amount of rhodium used, it is preferable to set the length of the first catalyst layer to approximately 80%.Therefore, the length of the first catalyst layer can be appropriately adjusted within a length fraction range of 80% to 100%.

[0069] Furthermore, it was confirmed that the NOx removal performance can be further improved if the support forming the first catalyst layer does not contain cerium.

[0070] The embodiments of the present invention have been described above with reference to the figures. However, a specific configuration is not limited to these embodiments. REFERENCE MARK LIST 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 Catalyst Fr upstream side in the exhaust gas flow direction Rr downstream side in the exhaust gas flow direction

Claims

[1] Catalyst (10) comprising: a substrate (1) with a cell structure in which exhaust gas flows; and catalyst layers (3, 3A, 3B, 3C, 3D, 3E) formed on the cell wall surfaces of the substrate (1), wherein the catalyst layers (3, 3A, 3B, 3C, 3D, 3E) comprise a first catalyst layer (4, 4A, 4B, 4C) arranged on an upstream side (Fr) of the substrate (1) in an exhaust gas flow direction and a second catalyst layer (5, 5A) arranged on a downstream side (Rr) of the substrate (1) in the exhaust gas flow direction, the first catalyst layer (4, 4A, 4B, 4C) is formed from a support and rhodium, which is a precious metal catalyst supported on the support, the second catalyst layer (5, 5A) is formed from a support and palladium or platinum, which is a precious metal catalyst supported on the support, the first catalyst layer (4, 4A, 4B, 4C) is formed in a range of 80% to 100% of the total length of the substrate (1) starting from one end of the substrate (1) on the upstream side (Fr), and the second catalyst layer (5, 5A) is formed in a range of 20% to 50% of the total length of the substrate (1) starting from one end of the substrate (1) on the downstream side (Rr); characterized by , that the support of the first catalyst layer (4, 4A, 4B, 4C) does not contain cerium.

Images