WO2011118047A1 - 排ガス浄化用触媒 - Google Patents

排ガス浄化用触媒 Download PDF

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WO2011118047A1
WO2011118047A1 PCT/JP2010/055774 JP2010055774W WO2011118047A1 WO 2011118047 A1 WO2011118047 A1 WO 2011118047A1 JP 2010055774 W JP2010055774 W JP 2010055774W WO 2011118047 A1 WO2011118047 A1 WO 2011118047A1
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catalyst layer
nox occlusion
nox
exhaust gas
reduction type
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PCT/JP2010/055774
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English (en)
French (fr)
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新名祐介
高木信之
松原宏幸
今井啓人
鎌田雅也
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トヨタ自動車株式会社
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Priority to US13/636,079 priority Critical patent/US20130065754A1/en
Priority to PCT/JP2010/055774 priority patent/WO2011118047A1/ja
Priority to EP10848439.5A priority patent/EP2551014A4/en
Priority to JP2012506754A priority patent/JPWO2011118047A1/ja
Priority to CN2010800656241A priority patent/CN102811810A/zh
Publication of WO2011118047A1 publication Critical patent/WO2011118047A1/ja

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    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/50—Catalysts, in general, characterised by their form or physical properties characterised by their shape or configuration
    • B01J35/56—Foraminous structures having flow-through passages or channels, e.g. grids or three-dimensional monoliths
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
    • B01D53/34—Chemical or biological purification of waste gases
    • B01D53/92—Chemical or biological purification of waste gases of engine exhaust gases
    • B01D53/94—Chemical or biological purification of waste gases of engine exhaust gases by catalytic processes
    • B01D53/9445—Simultaneously removing carbon monoxide, hydrocarbons or nitrogen oxides making use of three-way catalysts [TWC] or four-way-catalysts [FWC]
    • B01D53/945—Simultaneously removing carbon monoxide, hydrocarbons or nitrogen oxides making use of three-way catalysts [TWC] or four-way-catalysts [FWC] characterised by a specific catalyst
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
    • B01J23/40—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals of the platinum group metals
    • B01J23/42—Platinum
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
    • B01J23/40—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals of the platinum group metals
    • B01J23/44—Palladium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
    • B01J23/54—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36
    • B01J23/56—Platinum group metals
    • B01J23/63—Platinum group metals with rare earths or actinides
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J35/00—Catalysts, in general, characterised by their form or physical properties
    • B01J35/19—Catalysts containing parts with different compositions
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J37/00—Processes, in general, for preparing catalysts; Processes, in general, for activation of catalysts
    • B01J37/02—Impregnation, coating or precipitation
    • B01J37/024—Multiple impregnation or coating
    • B01J37/0244—Coatings comprising several layers
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
    • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
    • F01N3/0807—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents
    • F01N3/0814—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents combined with catalytic converters, e.g. NOx absorption/storage reduction catalysts
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
    • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
    • F01N3/0807—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents
    • F01N3/0828—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by using absorbents or adsorbents characterised by the absorbed or adsorbed substances
    • F01N3/0842—Nitrogen oxides
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N3/00—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust
    • F01N3/08—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous
    • F01N3/10—Exhaust or silencing apparatus having means for purifying, rendering innocuous, or otherwise treating exhaust for rendering innocuous by thermal or catalytic conversion of noxious components of exhaust
    • F01N3/105—General auxiliary catalysts, e.g. upstream or downstream of the main catalyst
    • F01N3/106—Auxiliary oxidation catalysts
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/10—Noble metals or compounds thereof
    • B01D2255/102—Platinum group metals
    • B01D2255/1021—Platinum
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/10—Noble metals or compounds thereof
    • B01D2255/102—Platinum group metals
    • B01D2255/1023—Palladium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/10—Noble metals or compounds thereof
    • B01D2255/102—Platinum group metals
    • B01D2255/1025—Rhodium
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/20—Metals or compounds thereof
    • B01D2255/202—Alkali metals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/20—Metals or compounds thereof
    • B01D2255/204—Alkaline earth metals
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/90—Physical characteristics of catalysts
    • B01D2255/902—Multilayered catalyst
    • B01D2255/9022—Two layers
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01D—SEPARATION
    • B01D2255/00—Catalysts
    • B01D2255/90—Physical characteristics of catalysts
    • B01D2255/91—NOx-storage component incorporated in the catalyst
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
    • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
    • B01J23/00—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00
    • B01J23/38—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals
    • B01J23/54—Catalysts comprising metals or metal oxides or hydroxides, not provided for in group B01J21/00 of noble metals combined with metals, oxides or hydroxides provided for in groups B01J23/02 - B01J23/36
    • B01J23/56—Platinum group metals
    • B01J23/58—Platinum group metals with alkali- or alkaline earth metals
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N2510/00—Surface coverings
    • F01N2510/06—Surface coverings for exhaust purification, e.g. catalytic reaction
    • F01N2510/068—Surface coverings for exhaust purification, e.g. catalytic reaction characterised by the distribution of the catalytic coatings
    • F01N2510/0682—Surface coverings for exhaust purification, e.g. catalytic reaction characterised by the distribution of the catalytic coatings having a discontinuous, uneven or partially overlapping coating of catalytic material, e.g. higher amount of material upstream than downstream or vice versa
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F01—MACHINES OR ENGINES IN GENERAL; ENGINE PLANTS IN GENERAL; STEAM ENGINES
    • F01N—GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR MACHINES OR ENGINES IN GENERAL; GAS-FLOW SILENCERS OR EXHAUST APPARATUS FOR INTERNAL COMBUSTION ENGINES
    • F01N2510/00—Surface coverings
    • F01N2510/06—Surface coverings for exhaust purification, e.g. catalytic reaction
    • F01N2510/068—Surface coverings for exhaust purification, e.g. catalytic reaction characterised by the distribution of the catalytic coatings
    • F01N2510/0684—Surface coverings for exhaust purification, e.g. catalytic reaction characterised by the distribution of the catalytic coatings having more than one coating layer, e.g. multi-layered coatings
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02T—CLIMATE CHANGE MITIGATION TECHNOLOGIES RELATED TO TRANSPORTATION
    • Y02T10/00—Road transport of goods or passengers
    • Y02T10/10—Internal combustion engine [ICE] based vehicles
    • Y02T10/12—Improving ICE efficiencies

Definitions

  • the present invention relates to an exhaust gas purification catalyst, and more particularly to an exhaust gas purification catalyst provided with a NOx occlusion reduction type catalyst layer.
  • NOx occlusion reduction type catalysts are subject to competitive adsorption of HC, CO, and NOx, and it has been difficult to ensure sufficient purification performance.
  • a plurality of NOx occlusion reduction type catalysts having a two-layer coating structure have been reported in JP2009-101252A, etc.
  • both upper and lower layers contain NOx occlusion material (or NOx retention material)
  • HC Further, competitive adsorption of CO and NOx occurs, the active point of the NOx occlusion reduction reaction decreases, and the problem that the NOx occlusion reduction type catalyst cannot be made effective has not been solved.
  • An object of the present invention is to provide an exhaust gas purifying catalyst having a structure capable of preventing the competitive adsorption of HC, CO, and NOx and efficiently utilizing the NOx occlusion reduction type catalyst.
  • the exhaust gas purifying catalyst of the present invention comprises, on a base material, at least one NOx storage material selected from alkali metals or alkaline earth metals, and Pt and / or Rh.
  • a NOx occlusion reduction type catalyst layer containing, An oxidation catalyst layer supporting Pt and / or Pd is provided on the NOx occlusion reduction type catalyst layer.
  • the length of the oxidation catalyst layer is 25 to 60% of the length of the NOx occlusion reduction type catalyst layer when viewed in the direction of the exhaust gas flow.
  • the oxidation catalyst layer has a thickness of 20 to 40 ⁇ m.
  • FIG. 1 schematically shows HC, CO, and NOx that cause competitive adsorption in a NOx occlusion reduction type catalyst layer in a conventional exhaust gas purification catalyst.
  • FIG. 2 shows a state in which HC and CO are selectively oxidized in the upper oxidation catalyst layer and NOx is selectively occluded and reduced in the lower NOx occlusion reduction type catalyst layer in the exhaust gas purification catalyst of the present invention.
  • 3A and 3B are schematic diagrams respectively showing (A) a configuration image of the catalyst according to the present invention for laboratory evaluation and a conventional catalyst and (B) a configuration image of a catalyst according to the present invention for actual device evaluation.
  • FIG. 4 is a graph showing a test cycle for laboratory evaluation.
  • FIG. 5 is a graph showing NOx occlusion characteristics in the laboratory for the catalysts of the present invention and the conventional example.
  • FIG. 6 is a graph showing NOx occlusion characteristics in an actual machine for the catalyst of the present invention.
  • the upper and lower two layers are NOx occlusion reduction type catalyst layers, and as shown in FIG. 1, competitive adsorption of HC, CO, and NOx occurs, and the active point of the NOx occlusion reduction reaction is
  • the NOx occlusion reduction type catalyst layer could not be effectively used. That is, even if the NOx occlusion reduction catalyst has a sufficient NOx occlusion capacity, the NOx occlusion speed at the occlusion start stage is slow, and the mode emission cannot exhibit sufficient performance. As a countermeasure, even when an oxidation catalyst is arranged upstream of the NOx occlusion reduction catalyst, the NOx occlusion speed is still slow when compared with the same capacity.
  • the active point of the lower NOx occlusion reduction type catalyst layer is determined as NOx occlusion. It can be used effectively for the reduction reaction.
  • the present invention can effectively utilize the lower NOx occlusion reduction type catalyst layer by providing the oxidation catalyst layer on the NOx occlusion reduction type catalyst layer. That is, the upper oxidation catalyst layer through which the exhaust gas first passes selectively oxidizes HC (hydrocarbon) and CO (carbon monoxide) that inhibit the reaction of the NOx storage reduction catalyst.
  • the NOx occlusion speed of the NOx occlusion reduction type catalyst layer becomes maximum.
  • the thickness of the oxidation catalyst layer is 20 to 40 ⁇ m. Within this range, the NOx occlusion speed of the NOx occlusion reduction type catalyst layer becomes maximum.
  • the oxidation catalyst layer that overcoats the upper layer includes Pt and / or Pd having excellent catalytic ability as an oxidation catalyst in order to enhance oxidation performance.
  • Rh that hinders the catalytic activity in the lean combustion gas is not added, and no NOx storage material that affects the noble metal activity is added.
  • the NOx occlusion reduction type catalyst layer includes Pt and / or Rh and a NOx occlusion material. In particular, Rh and NOx occlusion material are added only to the lower NOx occlusion reduction type catalyst layer.
  • DOC represents an oxidation catalyst layer
  • NSR represents a NOx occlusion reduction type catalyst layer
  • DOC oxidation catalyst layer
  • the catalyst size is 35 cc (full length 50 mm) for the laboratory and 14 L (full length 110 mm) for the actual machine.
  • the catalyst was subjected to simple durability at 700 ° C. ⁇ 27 h in an electric furnace.
  • Three types of catalysts having the coating specifications shown in Table 1 were subjected to the test with the following constitution.
  • FIG. 6 summarizes the results of the actual machine NOx storage test.
  • the overcoat ratio was 55% (constant), and the overcoat amount was changed in the range of 24 to 72 g / L.
  • the NOx occlusion rate was the highest when the overcoat amount was around 30 g / L.
  • the optimum overcoat amount is in the range of 25 to 35 g / L with 30 g / L as the center. When this is converted into an overcoat thickness with respect to an overcoat ratio of 55%, the optimum overcoat thickness is about 20 to 40 ⁇ m.
  • an exhaust gas purifying catalyst having a structure capable of preventing the competitive adsorption of HC, CO and NOx and efficiently utilizing the NOx occlusion reduction type catalyst.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Materials Engineering (AREA)
  • Organic Chemistry (AREA)
  • Combustion & Propulsion (AREA)
  • Mechanical Engineering (AREA)
  • General Engineering & Computer Science (AREA)
  • Health & Medical Sciences (AREA)
  • Biomedical Technology (AREA)
  • Environmental & Geological Engineering (AREA)
  • Analytical Chemistry (AREA)
  • General Chemical & Material Sciences (AREA)
  • Oil, Petroleum & Natural Gas (AREA)
  • Toxicology (AREA)
  • Catalysts (AREA)
  • Exhaust Gas Treatment By Means Of Catalyst (AREA)
  • Exhaust Gas After Treatment (AREA)

Abstract

　本発明は、ＨＣ、ＣＯ、ＮＯｘの競争吸着を防止して、ＮＯｘ吸蔵還元型触媒を効率的に活用できる 構造を有する排ガス浄化用触媒を提供する。本発明の排ガス浄化用触媒は、基材上に、アルカリ金属ま たはアルカリ土類金属から選択された少なくとも１種のＮＯｘ吸蔵材と、Ｐｔおよび／またはＲｈとを 含むＮＯｘ吸蔵還元型触媒層を備え、前記ＮＯｘ吸蔵還元型触媒層の上に、Ｐｔおよび／またはＰｄを 担持した酸化触媒層を備えたことを特徴とする。

Description

排ガス浄化用触媒
　本発明は、排ガス浄化用触媒に関し、より詳しくはＮＯｘ吸蔵還元型触媒層を備えた排ガス浄化用触媒に関する。
　従来、三元触媒においてＮＯｘ吸蔵還元型触媒はＨＣ、ＣＯ、ＮＯｘの競争吸着が起きるため、十分な浄化性能を確保することが困難であった。
　その解決のために、特開２００９−１０１２５２等に２層コート構造を有するＮＯｘ吸蔵還元型触媒が複数報告されているが、上下２層共にＮＯｘ吸蔵材（あるいはＮＯｘ保持物質）を含むため、ＨＣ、ＣＯ、ＮＯｘの競争吸着が生じ、ＮＯｘ吸蔵還元反応の活性点が減少してしまい、ＮＯｘ吸蔵還元型触媒を有効に出来ないという問題は解消されなかった。
　本発明は、ＨＣ、ＣＯ、ＮＯｘの競争吸着を防止して、ＮＯｘ吸蔵還元型触媒を効率的に活用できる構造を有する排ガス浄化用触媒を提供することを目的とする。
　上記の目的を達成するために、本発明の排ガス浄化用触媒は、基材上に、アルカリ金属またはアルカリ土類金属から選択された少なくとも１種のＮＯｘ吸蔵材と、Ｐｔおよび／またはＲｈとを含むＮＯｘ吸蔵還元型触媒層を備え、
　前記ＮＯｘ吸蔵還元型触媒層の上に、Ｐｔおよび／またはＰｄを担持した酸化触媒層を備えた
ことを特徴とする。
　望ましい形態においては、排ガス流の方向に見て、前記酸化触媒層の長さが前記ＮＯｘ吸蔵還元型触媒層の長さの２５~６０％である。
　望ましい形態においては、前記酸化触媒層の厚さが２０~４０μｍである。
　図１は、従来の排ガス浄化用触媒において、ＮＯｘ吸蔵還元型触媒層で競争吸着を起こすＨＣ、ＣＯ、ＮＯｘを模式的に示す。
　図２は、本発明の排ガス浄化用触媒において、上層の酸化触媒層においてＨＣおよびＣＯが選択的に酸化され、下層のＮＯｘ吸蔵還元型触媒層においてＮＯｘが選択的に吸蔵および還元される状態を模式的に示す。
　図３は、（Ａ）実験室評価用の本発明例および従来例の触媒の構成イメージおよび（Ｂ）実機評価用の本発明例の触媒の構成イメージをそれぞれ示す模式図である。
　図４は、実験室評価用の試験サイクルを示すグラフである。
　図５は、本発明例および従来例の触媒について実験室でのＮＯｘ吸蔵特性を示すグラフである。
　図６は、本発明例の触媒について実機でのＮＯｘ吸蔵特性を示すグラフである。
　従来の２層コート構造では、上下２層ともにＮＯｘ吸蔵還元型触媒層であったため、図１に示すように、ＨＣ、ＣＯ、ＮＯｘの競争吸着が起きてしまい、ＮＯｘ吸蔵還元反応の活性点が減少し、ＮＯｘ吸蔵還元型触媒層を有効に利用できなかった。すなわち、ＮＯｘ吸蔵還元型触媒にＮＯｘの吸蔵容量が十分にあっても、吸蔵開始段階でのＮＯｘ吸蔵速度が遅く、モードエミッションでは十分な性能を発揮できなかった。また、その対策として、ＮＯｘ吸蔵還元型触媒の上流に酸化触媒を配置しても、同容量で比較した場合、やはりＮＯｘ吸蔵速度が遅かった。
　これに対して本発明の特徴として、図２に示すように、まずＨＣ、ＣＯが上層の酸化触媒層で実質的に浄化されるため、下層のＮＯｘ吸蔵還元型触媒層の活性点をＮＯｘ吸蔵還元反応に有効利用できる。
　本発明は、ＮＯｘ吸蔵還元型触媒層の上に、酸化触媒層を設けたことにより、下層のＮＯｘ吸蔵還元型触媒層を有効に利用できる。すなわち、排ガスが最初に通る上層の酸化触媒層が、ＮＯｘ吸蔵還元型触媒の反応を阻害するＨＣ（炭化水素）とＣＯ（一酸化炭素）を選択的に酸化する。したがって、下層のＮＯｘ吸蔵還元型触媒層においては、ＨＣ、ＣＯ、ＮＯｘの競争吸着が実質的に起きず、上層の酸化触媒層を通ってきたＮＯｘの吸蔵および還元が選択的に起きる。
　本発明の排ガス浄化用触媒においては、上層の酸化触媒層と下層のＮＯｘ吸蔵還元型触媒層とを選択的に作用させることにより、酸化により浄化すべきＨＣおよびＣＯと、還元により浄化すべきＮＯｘとの競争吸着を回避し、それぞれに十分な反応サイトを確保して、ＮＯｘ吸蔵還元型触媒の本来の機能を最大限に利用することができる。
　一つの望ましい形態においては、図２に示したように、排ガス流の方向に対して、ＮＯｘ吸蔵還元型触媒層の上流側端部から、ＮＯｘ吸蔵還元型触媒層の長さの２５~６０％を覆うように配置する。この範囲内において、ＮＯｘ吸蔵還元型触媒層のＮＯｘ吸蔵速度が最大になる。
　更に、一つの望ましい形態においては、前記酸化触媒層の厚さが２０~４０μｍである。この範囲内において、ＮＯｘ吸蔵還元型触媒層のＮＯｘ吸蔵速度が最大になる。
　下層のＮＯｘ吸蔵還元型触媒層を形成した後、その上から上層の酸化触媒層をオーバーコートして、上下２層コート構造の触媒を形成する。
　上層にオーバーコートする酸化触媒層は、酸化性能を高めるために、酸化触媒として優れた触媒能を有するＰｔおよび／またはＰｄを備えている。
　酸化触媒層には、希薄燃焼ガスでの触媒活性を妨げるＲｈは添加しないし、貴金属活性に影響を及ぼすＮＯｘ吸蔵材も添加しない。
　ＮＯｘ吸蔵還元型触媒層は、Ｐｔおよび／またはＲｈとＮＯｘ吸蔵材とを含む。特に、ＲｈもＮＯｘ吸蔵材も、下層のＮＯｘ吸蔵還元型触媒層にのみ添加する。
　コージェライト基材上に、表１に示すコート仕様を有する３種類の排ガス浄化用触媒を作製した。以下において、ＤＯＣは酸化触媒層を、ＮＳＲはＮＯｘ吸蔵還元型触媒層をそれぞれ表す。
Figure JPOXMLDOC01-appb-T000001
 　本発明例は、基材上にＮＯｘ吸蔵還元型触媒層（ＮＳＲ）を形成した後に、その上から酸化触媒層（ＤＯＣ）をコートして作製した。
　触媒サイズは、実験室用で容量３５ｃｃ（全長５０ｍｍ）、実機用で容量１４Ｌ（全長１１０ｍｍ）である。
　触媒に電気炉で７００℃×２７ｈの簡易耐久を施した。
　表１のコート仕様の３種類の触媒を下記の構成で試験に供した。構成イメージを図３に示す。
　＜試験触媒の構成＞
（Ａ）実験室評価用の構成（３種）
［１］ＤＯＣ＋［２］ＮＳＲ：タンデム構成（従来例）
　サイズ：ＤＯＣ　容量１０ｃｃ、長さ１４ｍｍ（上流側）
　　　　　ＮＳＲ　容量２５ｃｃ、長さ３６ｍｍ（下流側）
　　　　　　　　　　　　　　　（全長５０ｍｍ）
［２］ＮＳＲ：単独（従来例）
　サイズ：容量３５ｃｃ、長さ（全長）５０ｍｍ
［３］ＮＳＲ上にＤＯＣをオーバーコート（本発明例）
　サイズ：容量３５ｃｃ、長さ（全長）５０ｍｍ
　オーバーコート割合：２７％＝１０ｃｃ（１４ｍｍ（＊））
　　　　　　　　　　　５５％＝２０ｃｃ（２８ｍｍ（＊））
　　　　　　　　　　　８２％＝３０ｃｃ（４２ｍｍ（＊））
　　　　　　　　　　　（＊）ＮＳＲ上流側端部からの長さ
（Ｂ）実機評価用の構成（１種）
［３］ＮＳＲ上にＤＯＣをオーバーコート（本発明例）
　サイズ：容量１４Ｌ、長さ１１０ｍｍ
　オーバーコート割合：５５％＝７．７Ｌ（６０ｍｍ（＊））
　　　　　　　　　　　（＊）ＮＳＲ上流側端部からの長さ
試験条件は下記のとおりであった。
＜試験条件＞
（Ａ）実験室ＮＯｘ吸蔵試験
試験ガス条件：表２に示す。
Figure JPOXMLDOC01-appb-T000002
 　試験サイクル：図４に示す。
　すなわち、初期温度５０℃から４０℃／ｍｉｎで６００℃まで昇温し、６００℃でリッチスパイクＮＯｘ還元（リッチ／リーン＝５ｓｅｃ／５ｓｅｃ）を行なった後、直ちにエア雰囲気で３５０℃まで触媒を冷却して、リーン雰囲気でＮＯｘ吸蔵を行なった。
　（Ｂ）実機ＮＯｘ吸蔵試験
　評価エンジン：ディーゼルエンジン（排気量２．２Ｌ）
　エンジン条件：表３に示す。
Figure JPOXMLDOC01-appb-T000003
 　評価パターン：ＰＭ再生→飽和ＮＯｘ吸蔵量測定
　＜試験結果＞
　（Ａ）オーバーコート割合の検証
　図５に、実験室ＮＯｘ吸蔵試験の結果をまとめて示す。
　本発明により、ＮＳＲ上にＤＯＣをオーバーコートすることにより、従来のＮＳＲ単独およびＤＯＣ／ＮＳＲタンデムに比べてＮＯｘ吸蔵速度が著しく向上した。
　また、オーバーコート割合は下層のＮＳＲ長さの２５％~６０％の範囲でＮＯｘ吸蔵速度が最も高くなる。
　（Ｂ）コート厚さの検証
　図６に、実機ＮＯｘ吸蔵試験の結果をまとめて示す。オーバーコート割合は５５％（一定）であり、オーバーコート量を２４~７２ｇ／Ｌの範囲で変化させた。
　オーバーコート量が３０ｇ／Ｌ付近でＮＯｘ吸蔵速度は最も高くなった。最適オーバーコート量は３０ｇ／Ｌを中心として、２５~３５ｇ／Ｌの範囲である。これを、オーバーコート割合５５％に対して、オーバーコート厚さに換算すると、最適オーバーコート厚さは２０~４０μｍ程度である。
　本発明によれば、ＨＣ、ＣＯ、ＮＯｘの競争吸着を防止して、ＮＯｘ吸蔵還元型触媒を効率的に活用できる構造を有する排ガス浄化用触媒が提供される。

Claims (3)

  1. 基材上に、アルカリ金属またはアルカリ土類金属から選択された少なくとも１種のＮＯｘ吸蔵材と、Ｐｔおよび／またはＲｈとを含むＮＯｘ吸蔵還元型触媒層を備え、
    　前記ＮＯｘ吸蔵還元型触媒層の上に、Ｐｔおよび／またはＰｄを担持した酸化触媒層を備えた
    ことを特徴とする排ガス浄化用触媒。
  2. 請求項１において、前記酸化触媒層は、排ガス流の方向に沿って、前記ＮＯｘ吸蔵還元型触媒層の上流側端部から、該ＮＯｘ吸蔵還元型触媒層の長さの２５~６０％を覆うことを特徴とする排ガス浄化用触媒。
  3. 請求項１または２において、前記酸化触媒層の厚さが２０~４０μｍであることを特徴とする排ガス浄化用触媒。
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