Exhaust gas purification system and exhaust gas purification method

Abstract

The present disclosure provides an exhaust gas purification system capable of suppressing HC poisoning and coking. The exhaust gas purification system of the present disclosure has a first exhaust gas purification catalyst layer and a second exhaust gas purification catalyst layer. The first exhaust gas purification catalyst layer is used to purify the exhaust gas discharged from an internal combustion engine, and the second exhaust gas purification catalyst layer is used to further purify the exhaust gas purified by the first exhaust gas purification catalyst layer. The second exhaust gas purification catalyst layer contains an oxygen storage material, and the amount of basic sites per unit specific surface area (mmol-CO2 / m 2 ) of the oxygen storage material relative to the specific surface area (m 2 / g) is 4.50×10 ‑5 or less.

Description

Technical Field

[0001] The present disclosure relates to an exhaust gas purification system and an exhaust gas purification method. Background Art

[0002] Japanese Patent Application Laid-Open No. 2005-95761 discloses a catalyst for exhaust gas purification, which is characterized by comprising zeolite particles supporting a first noble metal and a porous oxide layer formed on the surface of the zeolite particles and supporting a second noble metal. Japanese Patent Application Laid-Open No. 2005-95761 describes that coking of the catalyst for exhaust gas purification is suppressed by such a structure.

[0003] Japanese Patent Application Laid-Open No. 2007-46494 describes that by controlling the air-fuel ratio according to predetermined conditions, it is possible to recover the ternary catalyst from HC poisoning before reaching coking by supplying oxygen.

[0004] Japanese Patent Application Laid-Open No. 2014-210229 discloses a catalyst composition for exhaust gas purification, which includes cerium-zirconium oxide particles. In the XRD pattern of the cerium-zirconium oxide particles, the peak attributable to the (111) plane is divided into two peak tops, and a specific transition metal is supported on the cerium-zirconium oxide particles. Summary of the Invention

[0005] In an exhaust gas purification system for purifying exhaust gas discharged from an internal combustion engine, HC poisoning and coking are likely to occur as the flow proceeds downstream of the exhaust gas purification catalyst layer. It is considered that this is because as the flow proceeds downstream of the exhaust gas purification catalyst layer, due to a decrease in the oxygen concentration in the exhaust gas or the like, the O2 / C ratio, H2O / C ratio, and temperature in the exhaust gas enter a specific region, thereby reaching the chemical equilibrium of carbon precipitation.

[0006] As a method for suppressing HC poisoning and coking, for example, it is conceivable to apply the technologies disclosed in Japanese Patent Application Laid-Open No. 2005-95761 and Japanese Patent Application Laid-Open No. 2007-46494 to the exhaust gas purification system.

[0007] However, there is a demand for a technology that suppresses HC poisoning and coking without applying these technologies.

[0008] The present disclosure provides an exhaust gas purification system and an exhaust gas purification method capable of suppressing HC poisoning and coking.

[0009] The present disclosure provides the above exhaust gas purification system and exhaust gas purification method obtained by the following means:

[0010] <<Mode 1>>

[0011] An exhaust gas purification system having a first exhaust gas purification catalyst layer and a second exhaust gas purification catalyst layer,

[0012] The first exhaust gas purification catalyst layer is used to purify the exhaust gas discharged from the internal combustion engine.

[0013] The second exhaust gas purification catalyst layer is used to further purify the exhaust gas purified by the first exhaust gas purification catalyst layer.

[0014] The second exhaust gas purification catalyst layer contains an oxygen storage material, and

[0015] The amount of base points per unit specific surface area (mmol-CO2 / m 2 ) of the oxygen storage material relative to the specific surface area (m 2 / g) is 4.50×10 -5 or less.

[0016] <<Mode 2>>

[0017] In the exhaust gas purification system according to Mode 1, the specific surface area of the oxygen storage material can be 40.0 m 2 / g to 110.0 m 2 / g.

[0018] <<Mode 3>>

[0019] In the exhaust gas purification system according to Mode 1 or 2, the oxygen storage material can be an oxide containing Ce, La, Pr, or a combination thereof.

[0020] <<Mode 4>>

[0021] In the exhaust gas purification system according to any one of Modes 1 to 3, the oxygen storage material can be CeO2, LaO2, PrO2, or a combination thereof.

[0022] <<Mode 5>>

[0023] In the exhaust gas purification system according to any one of Modes 1 to 4, the oxygen storage material can have a fluorite structure.

[0024] <<Mode 6>>

[0025] In the exhaust gas purification system according to any one of Modes 1 to 5, the second exhaust gas purification catalyst layer can further contain catalyst metal particles.

[0026] <<Mode 7>>

[0027] In the exhaust gas purification system according to Mode 6, the catalyst metal particles can be particles of Pt, Pd, or Rh.

[0028] <<Mode 8>>

[0029] The exhaust gas purification system according to any one of Modes 1 to 7, wherein the first exhaust gas purification catalyst layer may contain an amount of basic sites per unit specific surface area (mmol-CO2 / m 2 ) with respect to the specific surface area (m 2 / g) in a ratio greater than 4.50×10 -5 of an oxygen storage material.

[0030] "Mode 9"

[0031] In the exhaust gas purification system according to any one of Modes 1 to 8, the second exhaust gas purification catalyst layer may be located downstream of the first exhaust gas purification catalyst layer.

[0032] "Mode 10"

[0033] An exhaust gas purification method for purifying exhaust gas discharged from an internal combustion engine, comprising:

[0034] purifying the exhaust gas discharged from the internal combustion engine through a first exhaust gas purification catalyst layer; and

[0035] further purifying the exhaust gas purified by the first exhaust gas purification catalyst layer through a second exhaust gas purification catalyst layer, and

[0036] the second exhaust gas purification catalyst layer contains an oxygen storage material, and

[0037] the amount of basic sites per unit specific surface area (mmol-CO2 / m 2 ) of the oxygen storage material with respect to the specific surface area (m 2 / g) is 4.50×10 -5 or less.

[0038] "Mode 11"

[0039] In the exhaust gas purification method according to Mode 10, the first exhaust gas purification catalyst layer may contain an amount of basic sites per unit specific surface area (mmol-CO2 / m 2 ) with respect to the specific surface area (m 2 / g) in a ratio greater than 4.50×10 -5 of an oxygen storage material.

[0040] "Mode 12"

[0041] In the exhaust gas purification method according to Mode 10 or 11, the exhaust gas may flow through the exhaust gas purification system in the order of the first exhaust gas purification catalyst layer and the second exhaust gas purification catalyst layer.

[0042] According to the present disclosure, an exhaust gas purification system capable of suppressing HC poisoning and coking can be provided. Description of the Drawings

[0043] Hereinafter, with reference to the accompanying drawings, the features, advantages, technology and industrial significance of the exemplary embodiments of the present invention will be described, and the same reference numerals denote the same elements.

[0044] Figure 1 It is a schematic diagram of an exhaust gas purification system according to an embodiment of the present disclosure.

[0045] Figure 2 It is a coordinate diagram showing the relationship between the base point amount and the specific surface area of the oxygen storage material in each example.

[0046] Figure 3 It is a coordinate diagram comparing the amount of carbon deposition in the exhaust gas purification catalyst layer using the oxygen storage materials of Examples 2-4 (black circles) and Comparative Example 3 (white circles).

[0047] Figure 4 It is a coordinate diagram comparing the NMHC emissions in the exhaust gas purification catalyst layer using the oxygen storage materials of Examples 2-4 (black circles) and Comparative Example 3 (white circles).

[0048] Figure 5 It is a coordinate diagram comparing the reactivity of the carbon deposited in the exhaust gas purification catalyst layer using the oxygen storage materials of Examples 2-4 and Comparative Example 3. Detailed Embodiments

[0049] Hereinafter, the embodiments of the present disclosure will be described in detail. Furthermore, the present disclosure is not limited to the following embodiments, and various modifications can be made within the scope of the disclosed gist.

[0050] "Exhaust Gas Purification System"

[0051] The exhaust gas purification system of the present disclosure has a first exhaust gas purification catalyst layer and a second exhaust gas purification catalyst layer. The first exhaust gas purification catalyst layer is used to purify the exhaust gas discharged from the internal combustion engine, and the second exhaust gas purification catalyst layer is used to further purify the exhaust gas purified by the first exhaust gas purification catalyst layer. The second exhaust gas purification catalyst layer contains an oxygen storage material, and the amount of base points per unit specific surface area of the oxygen storage material (mmol-CO2 / m 2 ) relative to the specific surface area of the oxygen storage material (m 2 / g) is 4.50×10 -5 Hereinafter.

[0052] The internal combustion engine to which the present exhaust gas purification system can be applied can be, for example, an internal combustion engine mounted on a vehicle, more specifically a vehicle, and further specifically an internal combustion engine of an automobile.

[0053] Although not limited by the principle, in the present exhaust gas purification system of the present disclosure, the principle of suppressing HC poisoning and coking is as follows.

[0054] As described above, in an exhaust gas purification system that purifies exhaust gas discharged from an internal combustion engine, HC poisoning and coking are likely to occur as the flow proceeds downstream of the exhaust gas purification catalyst layer. This is considered to be caused by, for example, a decrease in the oxygen concentration in the exhaust gas as the flow proceeds downstream of the exhaust gas purification catalyst layer.

[0055] The exhaust gas purification system 2 of the first embodiment of the present disclosure is connected to the internal combustion engine 1, for example, Figure 1 as shown, through a flow path 3 through which the exhaust gas discharged from the internal combustion engine 1 flows. The exhaust gas purification system 2 includes a first exhaust gas purification catalyst layer 10 and a second exhaust gas purification catalyst layer 20. The first exhaust gas purification catalyst layer 10 is configured to purify the exhaust gas discharged from the internal combustion engine 1, and the second exhaust gas purification catalyst layer 20 is configured to further purify the exhaust gas purified by the first exhaust gas purification catalyst layer 10. More specifically, in the exhaust gas purification system 2, the first exhaust gas purification catalyst layer 10 and the second exhaust gas purification catalyst layer 20 are arranged in this order from the internal combustion engine 1 side. The black arrow indicates the flow direction 100 of the exhaust gas. The exhaust gas flows through the exhaust gas purification system 2 in the order of the first exhaust gas purification catalyst layer 10 and the second exhaust gas purification catalyst layer 20.

[0056] In this way, the second exhaust gas purification catalyst layer 20 is the exhaust gas purification catalyst layer located downstream in the exhaust gas purification system 2. Therefore, generally, the second exhaust gas purification catalyst layer 20 is likely to be poisoned by HC and coked.

[0057] Here, in the exhaust gas purification system 2 of the first embodiment of the present disclosure, the second exhaust gas purification catalyst layer 20 contains an oxygen storage material, and the amount of base points per unit specific surface area (mmol-CO2 / m 2 ) of the oxygen storage material relative to the specific surface area (m 2 / g) is 4.50×10 -5 or less.

[0058] Furthermore, the amount of basic sites of the oxygen storage material is the amount of basic sites determined as the amount of CO2 desorbed per 1 g of the oxygen storage material by temperature-programmed desorption of carbon dioxide. Specifically, in the temperature-programmed desorption of carbon dioxide, CO2 as a probe molecule is adsorbed on the solid sample at a temperature of 100°C (for example, by passing a gas containing 10% CO2 and the balance being N2 or He through the sample), and after purging the gas layer of CO2 with He gas or N2 gas, the concentration of the desorbed gas generated by continuously raising the temperature of the sample layer (for example, raising the temperature to 600°C at a rate of 10°C / minute) is measured. In the measurement of the gas concentration, commercially available catalyst evaluation devices (such as "CATA-5000" manufactured by Best Instruments Co., Ltd., "MEXA-4300FT" manufactured by Horiba, Ltd., etc.) can be used. In addition, before the measurement, the following operation can be performed: supply a gas composed of O2 and N2 (or a gas composed of O2 and He, etc.) to the sample at around 600°C, and then lower the temperature to around 100°C. Furthermore, since the CO2 adsorbed on the weak basic sites desorbs at a low temperature (l(low)-peak), and the CO2 adsorbed on the strong basic sites desorbs at a high temperature (h(high)-peak), the strength of the basic sites can be known. In addition, the specific surface area of the oxygen storage material can be measured by the B.E.T. method in accordance with JIS Z-8830.

[0059] In such an oxygen storage material, the amount of basic sites to which HC in the exhaust gas adheres is small.

[0060] Therefore, by disposing such an oxygen storage material in the second exhaust gas purification catalyst layer 20 where the oxygen concentration in the exhaust gas is low, HC poisoning and coking in the second exhaust gas purification catalyst layer 20 can be suppressed. Furthermore, since a lot of oxygen is disposed on the surface of such an oxygen storage material, it is easy to oxidize the HC adhering to the basic sites exposed on the surface and convert it into CO2. Thus, in the exhaust gas purification system 2 of the first embodiment of the present disclosure, HC poisoning and coking can be suppressed. On the contrary, when the metal is exposed without disposing oxygen on the surface, HC adsorbs on the metal, dehydrogenation occurs on the metal, and it becomes a state prone to coking.

[0061] Furthermore, Figure 1 is a schematic diagram of the exhaust gas purification system of the first embodiment of the present disclosure, that is, it is merely an example and does not limit the exhaust gas purification system of the present disclosure. Therefore, for example, in Figure 1 the first exhaust gas purification catalyst layer and the second exhaust gas purification catalyst layer are continuous, but they can also be independent of each other, for example, they are connected by a flow path through which the exhaust gas flows.

[0062] <The first exhaust gas purification catalyst layer>

[0063] The first exhaust gas purification catalyst layer purifies the exhaust gas discharged from the internal combustion engine. The first exhaust gas purification catalyst layer has an exhaust gas purification catalyst, such as a three-way catalyst.

[0064] The three-way catalyst may have catalyst metal particles such as Pt, Pd, or Rh, an oxygen storage material, and a carrier.

[0065] The oxygen storage material in the first exhaust gas purification catalyst layer may be the same as or different from the material of the second exhaust gas purification catalyst layer.

[0066] When the first exhaust gas purification catalyst layer contains an oxygen storage material, it is preferably different from the oxygen storage material of the second exhaust gas purification catalyst layer. More specifically, the amount of basic sites per unit specific surface area (mmol-CO2 / m 2 ) of the oxygen storage material contained in the first exhaust gas purification catalyst layer relative to the specific surface area (m 2 / g) is preferably greater than 4.50×10 -5 .

[0067] The amount of basic sites per unit specific surface area (mmol-CO2 / m 2 ) of the oxygen storage material of the first exhaust gas purification catalyst layer relative to the specific surface area (m 2 / g) may be greater than 4.50×10 -5 , be 5.00×10 -5 or more, 5.50×10 -5 or more, or 6.00×10 -5 or more, and may be 15.00×10 -5 or less, 10.00×10 -5 or less, 7.00×10 -5 or less, or 5.00×10 -5 or less.

[0068] The first exhaust gas purification catalyst layer purifies the exhaust gas before the second exhaust gas purification catalyst layer. Therefore, the O2 concentration in the exhaust gas flowing through the first exhaust gas purification catalyst layer is high. Therefore, HC poisoning and coking are less likely to occur than in the second exhaust gas purification catalyst layer. Therefore, in the first exhaust gas purification catalyst layer, by increasing the amount of basic sites, it is possible to more easily purify HC and the like.

[0069] That is, in the first exhaust gas purification catalyst layer, an oxygen storage material with a ratio of the amount of basic sites per unit specific surface area (mmol-CO2 / m 2 ) to the specific surface area (m 2 / g) greater than 4.50×10 -5 is used. At the same time, in the second exhaust gas purification catalyst layer, an oxygen storage material with a ratio of the amount of basic sites per unit specific surface area (mmol-CO2 / m 2 ) to the specific surface area (m2 The ratio of ( / g) is 4.50×10 -5 The following oxygen storage materials, thereby exhibiting high exhaust gas purification performance in the first exhaust gas purification catalyst layer and reducing the amount of HC flowing through the second exhaust gas purification catalyst layer, thereby enabling more reduction of HC poisoning and coking in the second exhaust gas purification catalyst layer.

[0070] Furthermore, the first exhaust gas purification catalyst layer can be formed on a substrate. The substrate has an exhaust gas flow upstream end and an exhaust gas flow downstream end. Here, the exhaust gas flow upstream end refers to the side where the exhaust gas discharged from the internal combustion engine flows into the end of the substrate when the exhaust gas purification device is in use. In addition, the exhaust gas flow downstream end refers to the side where the exhaust gas flows out at the end of the substrate.

[0071] As the substrate, in the exhaust gas purification device, any substrate for supporting the exhaust gas purification catalyst can be used. Such a substrate can be, for example, a ceramic or metal substrate. As a ceramic substrate, for example, substrates such as cordierite and SiC can be cited.

[0072] The substrate can have a flow path for the exhaust gas to pass through. The structure of the flow path can, for example, have a honeycomb structure, a foam structure, or a plate structure.

[0073] When the substrate is a honeycomb substrate, the catalyst can be disposed in the flow path of the honeycomb substrate.

[0074] The length from the exhaust gas flow upstream end to the exhaust gas flow downstream end of the substrate is not particularly limited and can be the same length as that generally used for the exhaust gas purification device.

[0075] The first exhaust gas purification catalyst layer can be formed by, for example, applying a slurry formed by dispersing an exhaust gas purification catalyst, an adhesive, etc. in a dispersion medium onto the substrate, optionally drying, and then firing.

[0076] <The second exhaust gas purification catalyst layer>

[0077] The second exhaust gas purification catalyst layer further purifies the exhaust gas purified by the first exhaust gas purification catalyst layer. Therefore, the second exhaust gas purification catalyst layer can be a part of the entire exhaust gas purification catalyst layer having a length of 50% or less, 40% or less, 30% or less, 20% or less, or 10% or less from the downstream side. Furthermore, the downstream side of the exhaust gas purification catalyst layer refers to the downstream side of the exhaust gas flow in the exhaust gas purification catalyst layer.

[0078] The second exhaust gas purification catalyst layer contains an oxygen storage material.

[0079] In addition to the oxygen storage material, the second exhaust gas purification catalyst layer may further include catalyst metal particles such as Pt, Pd, or Rh and a carrier. That is, the second exhaust gas purification catalyst layer contains a three-way catalyst, and as an element of the three-way catalyst, an oxygen storage material may be included.

[0080] The second exhaust gas purification catalyst layer may also be formed on the substrate in the same manner as the first exhaust gas purification catalyst layer. The substrate for forming the second exhaust gas purification catalyst layer may be the same as the substrate for the first exhaust gas purification catalyst layer.

[0081] The second exhaust gas purification catalyst layer may be formed on the same substrate as the first exhaust gas purification catalyst layer or on a separate substrate. When the second exhaust gas purification catalyst layer is formed on the same substrate as the first exhaust gas purification catalyst layer, the second exhaust gas purification catalyst layer may be a portion of the entire exhaust gas purification catalyst layer on the substrate with a length of 50% or less from the downstream side. In addition, the first exhaust gas purification catalyst layer and the second exhaust gas purification catalyst layer may partially overlap.

[0082] For example, the second exhaust gas purification catalyst layer may use an oxygen storage material having a ratio of the base point amount (mmol-CO2 / m 2 ) to the specific surface area (m 2 / g) of 4.50×10 -5 or less, and is formed by the same method as the first exhaust gas purification catalyst layer.

[0083] <Oxygen storage material>

[0084] The ratio of the base point amount (mmol-CO2 / m 2 ) to the specific surface area (m 2 / g) of the oxygen storage material included in the second exhaust gas purification catalyst layer is 4.50×10 -5 or less.

[0085] The ratio of the base point amount (mmol-CO2 / m 2 ) to the specific surface area (m 2 / g) of the oxygen storage material included in the second exhaust gas purification catalyst layer may be 4.50×10 -5 or less, 4.00×10 -5 or less, 3.50×10 -5 or less, or 3.00×10 -5 or less, and may be 0.50×10 -5 or more, 1.00×10 -5 or more, 1.50×10 -5 or more, or 2.00×10 -5 or more.

[0086] The specific surface area of the oxygen storage material can be 40.0 m 2 / g to 110.0 m 2 / g. The specific surface area of the oxygen storage material can be 40.0 m 2 / g or more, 50.0 m 2 / g or more, 60.0 m 2 / g or more, or 70.0 m 2 / g or more, and can be 110.0 m 2 / g or less, 100.0 m 2 / g or less, 90.0 m 2 / g or less, or 80.0 m 2 / g or less.

[0087] The oxygen storage material can be an oxide containing Ce, La, Pr, or a combination thereof. More specifically, the oxygen storage material can be CeO2, LaO2, PrO2, or a combination thereof.

[0088] The oxygen storage material can have a fluorite structure. The oxygen storage material having a fluorite structure can be a structure in which the (111) plane is exposed on its surface.

[0089] In the oxygen storage material having a fluorite structure, oxygen atoms are exposed on the (111) plane, and metal atoms are located below it, that is, not exposed. Therefore, by increasing the exposure of the (111) plane, the metal atoms that become basic points can be reduced, and thus, the amount of basic points per unit specific surface area (mmol-CO2 / m 2 ) of the oxygen storage material relative to the specific surface area (m 2 / g) is 4.50×10 -5 or less.

[0090] Furthermore, such an oxygen storage material can be manufactured, for example, by hydrothermal synthesis or the like.

[0091] For example, when the oxygen storage material is CeO2, it can be manufactured by dissolving Ce(NO3)3·6H2O well in a 10 L aqueous solution containing Na3PO4, adjusting the pH value of the solution to 4, then standing at 170 °C for 12 hours for hydrothermal synthesis, then drying it at 150 °C, and finally, firing it in air at 500 °C.

[0092] Furthermore, the manufacturing method of the oxygen storage material is not limited to the above method.

[0093] Exhaust Gas Purification Method

[0094] The exhaust gas purification method of the present disclosure is a method for purifying the exhaust gas discharged from an internal combustion engine, which includes: purifying the exhaust gas discharged from the internal combustion engine through a first exhaust gas purification catalyst layer; and further purifying the exhaust gas purified by the first exhaust gas purification catalyst layer through a second exhaust gas purification catalyst layer. Here, the second exhaust gas purification catalyst layer contains an oxygen storage material, and the amount of basic sites per unit specific surface area (mmol-CO2 / m 2 ) of the oxygen storage material relative to the specific surface area (m 2 / g) is 4.50×10 -5 or less.

[0095] The exhaust gas purification system used in the exhaust gas purification method of the present disclosure can, for example, use the exhaust gas purification system described in the above-mentioned "Exhaust Gas Purification System", but is not limited to such a structure.

[0096] In the exhaust gas purification method of the present disclosure, the first exhaust gas purification catalyst layer more preferably contains an oxygen storage material in which the amount of basic sites per unit specific surface area (mmol-CO2 / m 2 ) relative to the specific surface area (m 2 / g) is greater than 4.50×10 -5 .

[0097] As described in the above-mentioned "Exhaust Gas Purification System", in the first exhaust gas purification catalyst layer, an oxygen storage material in which the amount of basic sites per unit specific surface area (mmol-CO2 / m 2 ) relative to the specific surface area (m 2 / g) is greater than 4.50×10 -5 is used. At the same time, in the second exhaust gas purification catalyst layer, an oxygen storage material in which the amount of basic sites per unit specific surface area (mmol-CO2 / m 2 ) relative to the specific surface area (m 2 / g) is 4.50×10 -5 or less is used. Thus, high exhaust gas purification performance is exhibited in the first exhaust gas purification catalyst layer, and the amount of HC flowing in the second exhaust gas purification catalyst layer is reduced, thereby being able to further reduce HC poisoning and coking in the second exhaust gas purification catalyst layer.

[0098] "Examples 1-1, 1-2, and 2-1 to 2-6 and Comparative Examples 1 to 5"

[0099] "<Example 1-1>"

[0100] 100 parts of Ce(NO3)3·6H2O was fully dissolved in 10 L of an aqueous solution containing 0.37 g of Na3PO4 to buffer the pH. The pH of the solution was confirmed to be 4. The solution was transferred to a stirred corrosion-resistant reactor and stirred only for a certain initial time, then left standing at 170 °C for 12 hours of hydrothermal synthesis. After hydrothermal synthesis, the obtained solid components were centrifuged or filtered by a conventional method, washed with distilled water, and a decrease in the amount of Na was confirmed. It was dried at 150 °C and calcined in air at 500 °C. The obtained CeO2 powder was used as the oxygen storage material of Example 1-1.

[0101] The specific surface area (m 2 / g), basic site amount (mmol-CO2 / g), basic site amount per unit specific surface area (mmol-CO2 / m 2 ) and the ratio of the basic site amount per unit specific surface area to the specific surface area of the oxygen storage material of Example 1-1 are shown in Table 1 below.

[0102] <Example 1-2>

[0103] Except for changing the hydrothermal conditions, the oxygen storage material of Example 1-2 was prepared in the same manner as in Example 1-1.

[0104] The specific surface area (m 2 / g), basic site amount (mmol-CO2 / g), basic site amount per unit specific surface area (mmol-CO2 / m 2 ) and the ratio of the basic site amount per unit specific surface area to the specific surface area of the oxygen storage material of Example 1-2 are shown in Table 1 below.

[0105] <Example 2-1>

[0106] 100 parts of Ce(NO3)3·6H2O was fully dissolved in 10 L of an aqueous solution containing 0.37 g of Na3PO4 to buffer the pH. While diluting it 2-fold with distilled water, nitric acid was added to adjust the pH to 2. The hydrothermal synthesis was carried out at a lower temperature of 120 °C for 18 hours. After hydrothermal synthesis, the obtained solid components were centrifuged or filtered by a conventional method, washed with distilled water, and a decrease in the amount of Na was confirmed. It was dried at 150 °C and calcined in air at 500 °C. The obtained nanorod CeO2 powder was used as the oxygen storage material of Example 2-1.

[0107] The specific surface area (m 2 / g), basic site amount (mmol-CO2 / g), basic site amount per unit specific surface area (mmol-CO2 / m 2 ) and the ratio of the basic site amount per unit specific surface area to the specific surface area of the oxygen storage material of Example 2-1 are shown in Table 1 below.

[0108] <Example 2-2 to 2-6>

[0109] The oxygen storage materials of Examples 2-2 to 2-6 were prepared in the same manner as in Example 2-1, except that the hydrothermal conditions were changed.

[0110] The specific surface area (m 2 / g), the amount of basic sites (mmol-CO2 / g), the amount of basic sites per unit specific surface area (mmol-CO2 / m 2 ), and the ratio of the amount of basic sites per unit specific surface area to the specific surface area of the oxygen storage materials of Examples 2-2 to 2-6 are shown in Table 1 below.

[0111] <Comparative Examples 1 to 5>

[0112] As the oxygen storage materials of Comparative Examples 1 to 5, CeO2 powder produced by a manufacturing method of the related art was used.

[0113] The specific surface area (m 2 / g), the amount of basic sites (mmol-CO2 / g), the amount of basic sites per unit specific surface area (mmol-CO2 / m 2 ), and the ratio of the amount of basic sites per unit specific surface area to the specific surface area of the oxygen storage materials of Comparative Examples 1 to 5 are shown in Table 1 below. Furthermore, the specific surface area of the oxygen storage material was measured by the B.E.T. method in accordance with JIS Z-8830. In addition, the amount of basic sites of the oxygen storage material was determined based on the amount of carbon dioxide desorbed per 1 g of the carrier obtained by temperature-programmed desorption of carbon dioxide.

[0114] Table 1

[0115]

[0116] As shown in Table 1 and Figure 2 , in the oxygen storage materials of Examples 1-1, 1-2, and 2-1 to 2-6, the ratio of the amount of basic sites per unit specific surface area to the specific surface area is 0.52×10 -5 to 4.04×10 -5 . In contrast, in the oxygen storage materials of Comparative Examples 1 to 5, the ratio of the amount of basic sites per unit specific surface area to the specific surface area is 4.60×10 -5 to 10.48×10 -5 .

[0117] <Measurement of Carbon Deposition Amount>

[0118] For the oxygen storage materials of Example 2-4 and Comparative Example 3, they were mixed with Pt as metal catalyst particles and ZrO2 particles as catalyst carriers to form an exhaust gas purification catalyst. The slurry in which the exhaust gas purification catalyst was dispersed was coated on a substrate and dried to form an exhaust gas purification catalyst layer on the substrate.

[0119] Using a low-inertia dynamometer (manufactured by Meidensha), a gasoline engine with a displacement of 0.660 L and a series of three cylinders, to which the exhaust purification catalyst layer is installed on the exhaust pipe, is operated in accordance with the WLTC-H mode test sequence.

[0120] After a predetermined time, the carbon deposition amount at a position of a predetermined length from the upstream side of the exhaust purification catalyst layer is measured using a visible spectrophotometer.

[0121] The measurement results are shown in Figure 3 .

[0122] Figure 3 is a coordinate diagram comparing the carbon deposition amounts in the exhaust purification catalyst layers using the oxygen storage materials of Examples 2-4 and Comparative Example 3. Furthermore, in Figure 3 , the black circles represent the exhaust purification catalyst layers using the oxygen storage materials of Examples 2-4, and the white circles represent the results when using the exhaust purification catalyst layer of Comparative Example 3.

[0123] As Figure 3 shown, in the case of using the oxygen storage materials of Examples 2-4 and in the case of using the oxygen storage material of Comparative Example 3, the position where carbon deposition starts is around 40 mm from the upstream side of the exhaust purification catalyst layer.

[0124] However, if the two are compared, the carbon deposition amount in the exhaust purification catalyst layer using the oxygen storage materials of Examples 2-4 is low. This difference is significant as the flow direction goes downstream of the exhaust purification catalyst layer.

[0125] <Measurement of NMHC emissions>

[0126] In the same test as the above “<Measurement of carbon deposition amount>”, the NMHC (non-methane hydrocarbons) discharged is measured using an exhaust gas analysis device (manufactured by HORIBA).

[0127] Figure 4 is a coordinate diagram comparing the NMHC emissions in the exhaust purification catalyst layers using the oxygen storage materials of Examples 2-4 and Comparative Example 3.

[0128] As Figure 4 shown, in the case of using the oxygen storage materials of Examples 2-4 and in the case of using the oxygen storage material of Comparative Example 3, NMHC is detected after about 1 minute of stable operation time.

[0129] In the case of using the oxygen storage materials of Examples 2-4, after about 5 minutes of stable operation time, the NMHC emissions become flat, and even after about 10 minutes, they are much lower than the limit allowable value of the on-board diagnostic system (OBD).

[0130] In contrast, in the case of using the oxygen storage material of Comparative Example 3, the OBD limit allowable value was exceeded at the time point of about 5 minutes.

[0131] <Measurement of Reactivity of Carbon>

[0132] The oxygen storage materials of Examples 2-4 and Comparative Example 3 after the same test as the above-mentioned "Measurement of Carbon Precipitation Amount" were heated in air, and the reactivity of the carbon precipitated near 90 mm from the upstream side of the exhaust gas purification catalyst layer with oxygen in the air was measured using a visible spectrophotometer.

[0133] Figure 5 It is a coordinate diagram comparing the reactivities of the carbon precipitated in the exhaust gas purification catalyst layers using the oxygen storage materials of Examples 2-4 and Comparative Example 3.

[0134] As Figure 5 shown, in the case of using the oxygen storage materials of Examples 2-4, the amount of carbon precipitation began to decrease at a temperature slightly higher than 500 °C.

[0135] In contrast, in the case of using the oxygen storage material of Comparative Example 3, the amount of carbon precipitation did not decrease until reaching around 650 °C.

Claims

1. An exhaust gas purification system, characterized in that, It has a first exhaust gas purification catalyst layer and a second exhaust gas purification catalyst layer. The second exhaust gas purification catalyst layer is located downstream of the first exhaust gas purification catalyst layer. The first exhaust gas purification catalyst layer is used to purify the exhaust gas discharged from the internal combustion engine, and the second exhaust gas purification catalyst layer is used to further purify the exhaust gas purified by the first exhaust gas purification catalyst layer. The first exhaust gas purification catalyst layer contains an oxygen storage material, and the ratio of the amount of basic sites per unit specific surface area of the oxygen storage material to the specific surface area is greater than 4.50×10 -5 , The second exhaust gas purification catalyst layer contains an oxygen storage material, and the ratio of the amount of base points per unit specific surface area of the oxygen storage material to the specific surface area is 0.50×10 -5 or more and 4.50×10 -5 or less. Among them, the alkali point amount of the oxygen storage material is obtained as the CO2 desorption amount per 1 g of the oxygen storage material measured by temperature-programmed desorption of carbon dioxide, and the unit of the alkali point amount per unit specific surface area is mmol-CO2 / m 2 , and the unit of the specific surface area is m 2 / g.

2. The exhaust gas purification system according to claim 1, wherein The specific surface area of the oxygen storage material is 40.0 m 2 / g to 110.0 m 2 / g.

3. The exhaust gas purification system according to claim 1 or 2, characterized in that, The oxygen storage material is an oxide containing Ce, La, Pr, or a combination thereof.

4. The exhaust gas purification system according to claim 1 or 2, characterized in that, The oxygen storage material is CeO2, LaO2, PrO2, or a combination thereof.

5. The exhaust gas purification system according to claim 1 or 2, characterized in that, The oxygen storage material has a fluorite structure.

6. The exhaust gas purification system according to claim 1 or 2, characterized in that, The second exhaust gas purification catalyst layer further contains catalyst metal particles.

7. The exhaust gas purification system according to claim 6, characterized in that, The catalyst metal particles are particles of Pt, Pd, or Rh.

8. An exhaust gas purification method for purifying the exhaust gas discharged from an internal combustion engine, characterized in that it includes: The exhaust gas flows through the exhaust gas purification system in the order of the first exhaust gas purification catalyst layer and the second exhaust gas purification catalyst layer. The exhaust gas discharged from the internal combustion engine is purified by the first exhaust gas purification catalyst layer, and the exhaust gas purified by the first exhaust gas purification catalyst layer is further purified by the second exhaust gas purification catalyst layer, and The first exhaust gas purification catalyst layer contains an oxygen storage material, and the ratio of the amount of basic sites per unit specific surface area of the oxygen storage material to the specific surface area is greater than 4.50×10 -5 , The second exhaust gas purification catalyst layer contains an oxygen storage material, and the ratio of the amount of basic sites per unit specific surface area of the oxygen storage material to the specific surface area is 0.50×10 -5 or more and 4.50×10 -5 or less. Among them, the alkali point amount of the oxygen storage material is obtained as the amount of CO2 desorbed per 1 g of the oxygen storage material measured by temperature-programmed desorption of carbon dioxide, and the unit of the alkali point amount per unit specific surface area is mmol-CO2 / m 2 , and the unit of the specific surface area is m 2 / g.

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