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EP-4574257-B1 - CATALYST FOR EXHAUST GAS PURIFICATION

EP4574257B1EP 4574257 B1EP4574257 B1EP 4574257B1EP-4574257-B1

Inventors

  • TAKASU, Ryosuke
  • ONOE, RYOTA
  • OHASHI, TATSUYA
  • ITO, MASAYA
  • ONOHARA, Yu

Dates

Publication Date
20260506
Application Date
20230824

Claims (6)

  1. A catalyst (100) for exhaust gas purification that is suitable for purifying exhaust gas emitted from an internal combustion engine, the catalyst (100) being disposed in an exhaust passage of the internal combustion engine, the catalyst (100) comprising: a substrate (10); and a catalyst layer (20) formed on the substrate (10), wherein the catalyst layer (20) comprises: a lower layer (22) located on a side of the substrate (10); an upper layer (26) located on a surface layer side; and a middle layer (24) located between the lower layer (22) and the upper layer (26), the upper layer (26) contains Rh, the middle layer (24) contains at least Pt and a NO x storage material, the lower layer (22) includes a lower-layer front portion (22a) located on an upstream side (X1) in a flow direction (F) of the exhaust gas and a lower-layer rear portion (22b) located on a downstream side (X2) in the flow direction (F) of the exhaust gas, when the catalyst (100) is disposed in the exhaust passage, the lower-layer front portion (22a) and the lower-layer rear portion (22b) each contain Pd, and a Pd content (C F ) in the lower-layer front portion (22a) per L of the substrate (10) is greater than a Pd content (C R ) in the lower-layer rear portion (22b) per L of the substrate (10).
  2. The catalyst (100) for exhaust gas purification according to claim 1, wherein a ratio (C F /C R ) of the C F to the C R satisfies the following formula: 1.5 ≤ (C F /C R ) ≤ 3.0.
  3. The catalyst (100) for exhaust gas purification according to claim 1 or 2, wherein a total amount of the Pd in the entire lower layer (22) per L of the substrate (10) is 3.0 g/L or less.
  4. The catalyst (100) for exhaust gas purification according to any one of claims 1 to 3, wherein the lower-layer front portion (22a) and the lower-layer rear portion (22b) each contain an OSC material having an oxygen storage capacity and a non-OSC material having no oxygen storage capacity, a content of the non-OSC material in the lower-layer front portion (22a) per L of the substrate (10) is greater than a content of the non-OSC material in the lower-layer rear portion (22b) per L of the substrate (10), and a content of the OSC material in the lower-layer front portion (22a) per L of the substrate (10) is smaller than a content of the OSC material in the lower-layer rear portion (22b) per L of the substrate (10).
  5. The catalyst (100) for exhaust gas purification according to any one of claims 1 to 4, wherein a coating length (La) of the lower-layer front portion (22a) in the flow direction (F) of the exhaust gas is shorter than a coating length (Lb) of the lower-layer rear portion (22b) in the flow direction (F) of the exhaust gas.
  6. The catalyst (100) for exhaust gas purification according to any one of claims 1 to 5, wherein the coating length (La) of the lower-layer front portion (22a) in the flow direction (F) of the exhaust gas is 30% or more and 60% or less of an entire length (L) of the substrate (10), and the coating length (Lb) of the lower-layer rear portion (22b) in the flow direction (F) of the exhaust gas is 60% or more and 90% or less of the entire length (L) of the substrate (10).

Description

[Technical Field] The present disclosure relates to a catalyst for exhaust gas purification. This application claims the benefit of priority to Japanese Patent Application No. 2022-161921 filed on October 6, 2022. [Background Art] Exhaust gas emitted from internal combustion engines such as automobile engines contains harmful components such as hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx). Conventionally, catalysts for exhaust gas purification including a substrate and a catalyst layer containing a catalyst metal have been used to remove these harmful components. Exhaust gas supplied to the catalyst for exhaust gas purification comes into contact with the catalyst layer, thereby purifying the harmful components. For example, HC and CO in the exhaust gas are oxidized and converted (purified) to water (H2O) and carbon dioxide (CO2), while NOx in the exhaust gas is reduced and converted (purified) to nitrogen (N2). However, when an internal combustion engine is started, a catalyst for exhaust gas purification is not sufficiently warmed up, resulting in low activity of the catalyst metal. Consequently, with harmful components remaining unpurified, the exhaust gas may be emitted until the catalyst metal reaches its predetermined activation temperature. To address this, the air-fuel ratio (A/F) of an air-fuel mixture supplied at the startup of the internal combustion engine may be made more dilute, thus controlling the internal combustion engine to bring it into a lean (oxygen-rich) state, so-called lean start control, to reduce the amount of CO and HC. However, in a lean atmosphere, it is difficult to extract oxygen from NOx, posing a problem in purifying NOx. Therefore, NOx storage-reduction (NSR: NOx Storage-Reduction) catalysts containing a NOx storage material are widely used to suppress NOx emissions in the warm-up process during the lean start control (see Patent Documents 1 and 2). For example, Patent Document 1 discloses an NSR catalyst including three catalyst layers, which are composed of a lower layer containing Pt and/or Pd, a middle layer containing Pt and/or Pd and a NOx storage material, and an upper layer containing Rh. [Citation List] [Patent Literature] [Patent Document 1] Japanese Patent Application Publication No. 2018-143935[Patent Document 2] Japanese Patent Application Publication No. 2016-93760 [Summary of Invention] In a catalyst layer containing a NOx storage material as disclosed in Patent Documents 1 and 2, a NOx storage reaction occurs in the lean atmosphere. That is, since NOx in the exhaust gas is essentially composed of NO, NO is generally first oxidized by the catalyst metal such as Pt to form NO2. The NO2 formed reacts with the NOx storage material (e.g., an alkaline earth metal) to form a nitrate, which is temporarily incorporated into the NOx storage material. Consequently, NOx emissions are suppressed. When the control is switched to a state between stoichiometric (theoretical air-fuel ratio) and rich (fuel-rich) atmospheres, inclusive, the NO2 incorporated in the NOx storage material is desorbed from the catalyst layer and reduced on the catalyst metal using a reducing gas such as HC or CO as a reducing agent. Thus, NOx components in the exhaust gas are converted (purified) to nitrogen (N2). In recent years, emission regulations have become increasingly stringent. In addition, for example, in eco-cars equipped with energy-saving mechanisms, the engine, which is an internal combustion engine, may frequently repeat stopping and starting even during its operation. Therefore, a catalyst for exhaust gas purification including a NOx storage material is required to have further enhanced NOx storage performance in a lean atmosphere, i.e., to further reduce the amount of NOx emissions (emissions). The inventors have proposed a different approach from conventional ones to enhance NOx storage performance in a lean atmosphere. More specifically, they have thought that the NOx storage performance in the lean atmosphere is enhanced by inducing the NOx storage reaction, especially the NO oxidation reaction, at an early stage. That is, to cause the NOx storage reaction described above, it is necessary to oxidize NO, converting it into the state of NO2. However, according to the inventors' study, when CO coexists with NO at this time, the oxidation reaction of CO occurs preferentially, thereby inhibiting the NO oxidation reaction. As a result, it has been found that the formation of NO2 is less likely, causing a delay in inducing the NOx storage reaction. CO is primarily purified by Pd. However, in a catalyst layer such as that disclosed in Patent Document 1, where the NOx storage material is contained in the middle layer and Pd is contained in the lower layer, CO tends to remain particularly in the middle layer, which easily inhibits the NO oxidation reaction. Therefore, the inventors have believed that in order to enhance the NOx storage performance in a lean atmosphe