JP2004124859A

JP2004124859A - Exhaust emission control system

Info

Publication number: JP2004124859A
Application number: JP2002291766A
Authority: JP
Inventors: Shinji Yamamoto; 山本　伸司; Sumiaki Hiramoto; 平本　純章
Original assignee: Nissan Motor Co Ltd
Current assignee: Nissan Motor Co Ltd
Priority date: 2002-10-04
Filing date: 2002-10-04
Publication date: 2004-04-22

Abstract

<P>PROBLEM TO BE SOLVED: To provide an exhaust emission control system with excellent HC purifying efficiency, especially upon starting of an engine. <P>SOLUTION: The exhaust emission control system is composed to have at least two exhaust emission control catalysts having an HC adsorption function and being supported by an integrated structure-type carrier positioned in series on an exhaust emission flow passage. While the HC adsorption purifying catalyst positioned upstream of the exhaust emission flow passage includes at least platinum as a purifying component, the HC adsorption purifying catalyst positioned downstream includes noble metals such as platinum, palladium, and rhodium. An amount of platinum included in the upstream HC adsorption purifying catalyst is larger than that included in the downstream HC adsorption purifying catalyst. <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、排気ガス浄化システムに係り、更に詳細には、内燃機関の始動時や始動直後に排出される炭化水素（ＨＣ）、いわゆるコールドＨＣを効率よく浄化し得る排気ガス浄化システムに関する。
【０００２】
【従来の技術】
従来、上記のようなコールドＨＣの浄化を目的に、ＨＣ吸着浄化触媒を排気ガス流路に対して直列に多段配置した排気ガス浄化システムが提案されている（特開平７−１４４１１９及び特開平９−８５０７８号公報）。
【０００３】
【発明が解決しようとする課題】
しかしながら、ＨＣ吸着浄化触媒を多段配置した従来の排気ガス浄化システムにあっては、排気上流で反応し易いＨＣが多く浄化されるため、排気下流では難燃性ＨＣが多く残存してしまい、更に温度は低下するため、浄化効率が極端に悪化する場合が多かった。
【０００４】
本発明は、このような従来技術の有する課題に鑑みてなされたものであり、その目的とするところは、ＨＣ浄化効率、特にエンジン始動時や始動直後のＨＣ浄化効率に優れた排気ガス浄化システムを提供することにある。
【０００５】
【課題を解決するための手段】
本発明者らは、上記目的を達成すべく鋭意検討を重ねた結果、直列に配置した複数のＨＣ吸着浄化触媒間で白金量を変化させ、難燃性のＨＣを排気上流位置で優先的に浄化し、下流に易燃性ＨＣを多めに流すことにより、上記目的が達成できることを見出し、本発明を完成するに至った。
【０００６】
即ち、本発明の排気ガス浄化システムは、ＨＣ吸着機能を有し一体構造型担体に担持された少なくとも２個の排気ガス浄化触媒を排気ガス流路に直列に配置して構成される。
また、上記排気ガス流路の上流に配置したＨＣ吸着浄化触媒が、浄化成分として少なくとも白金を含む一方で、下流に配置したＨＣ吸着浄化触媒が、浄化成分として白金、パラジウム及びロジウムから成る群より選ばれた少なくとも１種の貴金属を含む。
そして、上流ＨＣ吸着浄化触媒に含まれる白金量が、下流ＨＣ吸着浄化触媒に含まれる白金量よりも多くなるように構成されており、これにより、ＨＣの浄化効率を向上できる。
【０００７】
【発明の実施の形態】
以下、本発明の排気ガス浄化システムについて詳細に説明する。なお、本明細書において、「％」は特記しない限り質量百分率を表すものとする。
上述の如く、本発明の排気ガス浄化システムは、ＨＣ吸着機能を有し一体構造型担体に担持された排気ガス浄化触媒、即ちＨＣ吸着浄化触媒を排気ガス流路に複数個配置した構造を有する。
また、直列配置したＨＣ吸着浄化触媒のうち、排気ガス流路の上流に配置した上流ＨＣ吸着浄化触媒は浄化成分として少なくとも白金を含み、下流に配置した下流ＨＣ吸着浄化触媒は浄化成分として白金、パラジウム又はロジウム及びこれらの任意の組合せに係る貴金属を含む。
ここで、一体構造型担体としては、コーディエライトなどのセラミックスやフェライト系ステンレスなどの金属等の耐熱性材料から成るモノリス担体が用いられる。
【０００８】
また、本発明の排気ガス浄化システムでは、上流ＨＣ吸着浄化触媒に含まれる白金量が、下流ＨＣ吸着浄化触媒に含まれる白金量よりも多くなるように制御されている。
このように、ＨＣ成分のうちでも難燃性のパラフィンの浄化特性に優れる白金を、エンジン始動後迅速に昇温する上流に配置した上流ＨＣ吸着浄化触媒に多めに含有させると、早期に活性化した白金によって難燃成分であるパラフィンを上流で優先的に酸化（浄化）できるので、易燃成分であるオレフィンが多く含まれるＨＣを下流ＨＣ吸着浄化触媒に供給できるようになる。
これにより、下流ＨＣ吸着浄化触媒は易燃性のオレフィンによって着火が早められ、早期に活性化するので、システム全体として良好なＨＣ浄化効率を実現することができるようになる。
【０００９】
なお、本発明においては、上流ＨＣ吸着浄化触媒に担持する白金につき、例えば２種以上の担持濃度を採用して触媒を製造することによって、粒径分布に大小をつけることができ、これにより、更に広範囲のパラフィンに対する浄化適応性を向上できる。
また、オレフィンによる下流ＨＣ吸着浄化触媒の早期着火現象から、下流ＨＣ吸着浄化触媒には、オレフィン浄化特性に優れるパラジウム及び／又はロジウムを含ませることが好ましい。
【００１０】
上述のように、本発明では、早期に温度上昇する部位に白金を存在させて難燃性のパラフィンを浄化除去し、温度上昇が遅い部位に易燃性のオレフィンリッチＨＣを供給することに着目している。
よって、上流ＨＣ吸着浄化触媒にパラジウム及びロジウムの一方又は双方を更に含有させる場合には、早期に温度上昇する部位である、一体構造型担体の排気ガス流方向とほぼ垂直な断面における中央部に、白金を多く存在させることが好ましく、これにより、当該担体全体を早期に暖機して迅速にライト−オフ温度に到達させることが可能となる。
【００１１】
また、本発明において、白金が多く分布している上記垂直断面中央部は、この垂直断面の断面積の１〜２５％を占有することが好ましく、５〜１５％を占有することが更に好ましい。
１％未満では、上流で易燃性ＨＣが多く反応してしまい、下流に難燃性ＨＣが多く残存することになって、浄化効率に悪影響を及ぼす可能性があり、２５％を超えると、易燃性ＨＣの反応効率が低下し過ぎて、下流ＨＣ吸着浄化触媒への浄化分担が多くなるため、ＨＣの未浄化放出を招くことがあり、全体として浄化効率に悪影響を及ぼす可能性がある。
【００１２】
次に、上流ＨＣ吸着浄化触媒は、排気ガス流路において、触媒入口温度の昇温速度が１０℃／ｓｅｃ以上となるような位置に配置することが好ましく、２０℃／ｓｅｃ以上となるような位置に配置することが更に好ましい。
１０℃／ｓｅｃ未満では、白金等の浄化成分の活性化には遅すぎ、浄化効率に悪影響を及ぼす可能性がある。
【００１３】
一方、下流ＨＣ吸着浄化触媒は、触媒入口温度の昇温速度が１〜１０℃／ｓｅｃとなるような位置に配置することが好ましく、５〜１０℃／ｓｅｃとなるような位置に配置することが更に好ましい。
これにより、ＨＣ脱離が早くなり過ぎず、浄化成分の活性化には遅すぎることがなくなるので、下流ＨＣ吸着浄化触媒の浄化効率が更に向上する。
なお、上流ＨＣ吸着浄化触媒と下流ＨＣ吸着浄化触媒の配置位置は、各種モード評価時の温度プロファイルに応じて上記の昇温速度範囲内で適切に制御すればよい。
【００１４】
また、本発明の排気ガス浄化システムにおいては、上流ＨＣ吸着浄化触媒と下流ＨＣ吸着浄化触媒との容量比を、２５：７５〜７５：２５とすることが好ましい。
なお、本発明では、ＨＣ吸着浄化触媒の設置個数は上流及び下流触媒の２個以上であれば特に限定されるものではなく、特に下流ＨＣ吸着浄化触媒は２個以上設置してもよい。この場合は、最上流のＨＣ吸着浄化触媒の容量に対し、下流ＨＣ吸着浄化触媒群のトータル容量を上記の容量比にすればよい。
かかる容量比にすることによって、ＨＣ浄化効率の更なる好適化を行うことができる。
【００１５】
次に、上流及び下流のＨＣ吸着浄化触媒の構造などにつき説明する。
本発明において、かかるＨＣ吸着浄化触媒は、ＨＣの吸着・脱離能と排気ガス、特にＨＣの浄化能を有すれば十分であるが、代表的には、ハニカム担体などの一体構造型担体上に、ＨＣ吸着・脱離を担うＨＣ吸着材層と排気ガス浄化を担う浄化成分層とをこの順でコートして成る。
ここで、かかるＨＣ吸着材層としては、ＨＣ吸着材、具体的にはゼオライトを含有していることが好ましく、特にＳｉ／２Ａｌ比が１０〜１０００のＨ型βゼオライトを含有することが好ましい。
かかるＳｉ／２Ａｌ比を有するβゼオライトは適切なＨＣ吸着量と保持力を確保でき、且つ耐熱性にも優れるので好適である。
【００１６】
また、浄化成分層としては、上記浄化成分（貴金属）を含有するが、これ以外にも、セリウム、ジルコニウム又はランタン及びこれらの任意の組合せに係る金属を金属換算で１〜１０原子％含むアルミナと、ジルコニウム、ネオジム、プラセオジウム、イットリウム又はランタン及びこれらの任意の組合せに係る金属を金属換算で１〜５０原子％含むセリウム酸化物を含有することが望ましい。
かかる助触媒成分を添加したアルミナと、酸素供給源としてのセリウム酸化物とが存在すると、共存する貴金属のＨＣ酸化活性が更に向上するので好適である。
なお、パラジウムに対する活性向上効果が大きいので、貴金属種としてパラジウムが担持されている浄化成分層とすることが望ましい。
【００１７】
更に、上述の浄化成分層に、セリウム、ネオジム又はランタン及びこれらの任意の組合せに係る金属を金属換算で１〜４０原子％含むジルコニウム酸化物を添加することが好ましい。
かかるジルコニウム酸化物を添加することで、上記貴金属元素を活性な状態にすることができる。なお、この効果は白金及びロジウムに対して特に有効なので、このジルコニウム酸化物は白金及びロジウムが含有されている層に用いるのが望ましく、これにより、脱離ＨＣ転化率をいっそう向上できる。
【００１８】
更にまた、浄化成分層には、アルカリ金属、アルカリ土類金属のいずれか一方又は双方を含有させてもよく、これにより、特にパラジウムのＨＣ酸化活性を向上させるとともに、劣化も抑制できるので、特に耐久後の脱離ＨＣ転化率を向上できる。
【００１９】
【実施例】
以下、本発明を実施例及び比較例により更に詳細に説明するが、本発明はこれら実施例に限定されるものではない。
【００２０】
ＨＣ吸着浄化触媒に用いる材料としては、吸着材にＳｉ／２Ａｌ＝２５のβ−ゼオライトを用い、ハニカム担体への塗布量は２００ｇ／Ｌで統一した。
浄化成分層において、白金（Ｐｔ）及びパラジウム（Ｐｄ）担時用アルミナ基材にはセリウム（Ｃｅ）、ジルコニウム（Ｚｒ）含有アルミナ（各３モル含有）を、ロジウム（Ｒｈ）担時用アルミナ基材にはジルコニウム３モル含有アルミナを使用した。
また、白金及びパラジウム担時用基材として、上記アルミナのほかにジルコニウム１０モル含有セリアとセリウム２０モル含有ジルコニアを併用した。トータルの貴金属使用量は３ｇ／Ｌで一定とした。
更に、最上流（ＨＣ吸着浄化触媒群の上流）には三元触媒Ｐｄ／Ｒｈ＝３ｇ／Ｌ−５／１（０．７Ｌ）を一律に配置した。
【００２１】
（実施例１）
排気上流における触媒入口昇温速度２０℃／ｓｅｃの位置に、Ｐｔ１．２５ｇ／Ｌ、Ｐｄ１．５ｇ／Ｌ、Ｒｈ０．２５ｇ／Ｌ、貴金属比としてＰｔ／Ｐｄ／Ｒｈ＝５／６／１の上流ＨＣ吸着浄化触媒（容量０．５Ｌ）を配置した。更にその下流の触媒入口昇温速度３℃／ｓｅｃの位置に、Ｐｄ２．５ｇ／Ｌ、Ｒｈ０．５ｇ／Ｌ、貴金属比としてＰｄ／Ｒｈ＝５／１（容量０．９Ｌ）の下流ＨＣ吸着浄化触媒を配置し、本例の排気ガス浄化システムを構築した。
【００２２】
（実施例２）
上流ＨＣ吸着浄化触媒を触媒入口昇温速度１２℃／ｓｅｃの位置に配置し、容量を０．９Ｌに変更し、更に下流ＨＣ吸着浄化触媒をＰｔ／Ｐｄ／Ｒｈ＝１／８／３のＨＣ吸着浄化触媒（容量０．５Ｌ）とした以外は、実施例１と同様の構成を採用し、本例の排気ガス浄化システムを構築した。
【００２３】
（実施例３）
上流及び下流ＨＣ吸着浄化触媒の容量を共に０．７Ｌに変更し、更に下流ＨＣ吸着浄化触媒を触媒入口昇温速度８℃／ｓｅｃの位置に配置した以外は実施例１と同様の構成を採用し、本例の排気ガス浄化システムを構築した。
【００２４】
（実施例４）
上流ＨＣ吸着浄化触媒のＰｔ配置を担体中心部（全断面積に対する面積率として５％）に変更した以外は実施例１と同様の構成を採用し、本例の排気ガス浄化システムを構築した。
【００２５】
（実施例５）
Ｐｔ中心配置の面積率を５％から２０％に変更した以外は実施例４と同様の構成を採用し、本例の排気ガス浄化システムを構築した。
上記各例の排気ガス浄化システムの仕様を表１に示す。
【００２６】
【表１】

【００２７】
（比較例１）
排気上流における触媒入口昇温速度８℃／ｓｅｃの位置に、Ｐｄ２．５ｇ／Ｌ、Ｒｈ０．５ｇ／Ｌ、貴金属比としてＰｄ／Ｒｈ＝５／１の上流ＨＣ吸着浄化触媒（容量０．５Ｌ）を配置した以外は、実施例１と同様の構成を採用し、本例の排気ガス浄化システムを構築した。
【００２８】
（比較例２）
排気上流における触媒入口昇温速度８℃／ｓｅｃの位置に、Ｐｄ２．５ｇ／Ｌ、Ｒｈ０．５ｇ／Ｌ、貴金属比としてＰｄ／Ｒｈ＝５／１の上流ＨＣ吸着浄化触媒（容量０．５Ｌ）を配置し、下流ＨＣ吸着浄化触媒を触媒入口昇温速度０．５℃／ｓｅｃの位置に、Ｐｔ１．２５ｇ／Ｌ、Ｐｄ１．５ｇ／Ｌ、Ｒｈ０．２５ｇ／Ｌ、貴金属比としてＰｔ／Ｐｄ／Ｒｈ＝５／６／１（容量０．９Ｌ）のＨＣ吸着浄化触媒を配置した以外は、実施例１と同様の構成を採用し、本例の排気ガス浄化システムを構築した。
上記各例の排気ガス浄化システムの仕様を表１に示す。
【００２９】
［性能評価］
下記条件にて、各例のシステムにおけるＨＣ吸着浄化触媒を急速劣化させ、そのサンプルを車両評価し、ＨＣ浄化性能を比較した。得られた結果を表２に示す。
（耐久条件）
エンジン排気量　　　　３０００ｃｃ
燃料　　　　　　　　　ガソリン（日石ダッシュ）
触媒入口ガス温度　　　７５０℃
耐久時間　　　　　　　１００時間
（評価条件）
評価車両　　　　　　　日産自動車製直列４気筒　１８００ｃｃ
評価モード　　　　　　ＬＡ４−Ａｂａｇ
コールドＨＣ低減率（表２中「ＨＣ低減率」）は、ＬＡ４−Ａｂａｇ、０〜２５０秒区間におけるＨＣ排出量の低減率を表しており、次式▲１▼
［（Ｅｎｇ．ＯｕｔのＨＣ排出量）−（Ｓｙｓｔｅｍ−ＯｕｔのＨＣ排出量）］／（Ｅｎｇ．ＯｕｔのＨＣ排出量）×１００・・・▲１▼から算出される。
【００３０】
【表２】

【００３１】
表２より、本発明の範囲に属する実施例１〜５の排気ガス浄化システムは、本発明外の比較例１及び２よりもＨＣ浄化効率が優れていることが分かる。
また、現時点では、昇温特性と上流側触媒のＰｔ配置とのバランスの観点から、実施例５が最も良好な結果をもたらすものと思われる。
【００３２】
【発明の効果】
以上説明してきたように、本発明によれば、直列に配置した複数のＨＣ吸着浄化触媒間で白金量を変化させ、難燃性のＨＣを排気上流位置で優先的に浄化し、下流に易燃性ＨＣを多めに流すこととしたため、ＨＣ浄化効率、特にエンジン始動時や始動直後のＨＣ浄化効率に優れた排気ガス浄化システムを提供することができる。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an exhaust gas purification system, and more particularly, to an exhaust gas purification system capable of efficiently purifying hydrocarbons (HC), so-called cold HC, discharged at the time of starting or immediately after starting an internal combustion engine.
[0002]
[Prior art]
Conventionally, for the purpose of purifying the cold HC as described above, an exhaust gas purification system in which HC adsorption purification catalysts are arranged in multiple stages in series with the exhaust gas flow path has been proposed (Japanese Patent Application Laid-Open Nos. Hei 7-144119 and Hei 9). -85078).
[0003]
[Problems to be solved by the invention]
However, in a conventional exhaust gas purification system in which HC adsorption purification catalysts are arranged in multiple stages, a large amount of HC that reacts easily is purified upstream of the exhaust gas, so that a large amount of flame-retardant HC remains downstream of the exhaust gas. Since the temperature decreases, the purification efficiency often deteriorates extremely.
[0004]
SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the related art, and an object of the present invention is to provide an exhaust gas purification system that is excellent in HC purification efficiency, in particular, HC purification efficiency at the time of engine start or immediately after start. Is to provide.
[0005]
[Means for Solving the Problems]
The present inventors have conducted intensive studies to achieve the above object, and as a result, changed the amount of platinum between a plurality of HC adsorption and purification catalysts arranged in series, thereby giving priority to flame retardant HC at the exhaust upstream position. The present inventors have found that the above object can be achieved by purifying the fuel and flowing a large amount of flammable HC downstream, thereby completing the present invention.
[0006]
That is, the exhaust gas purifying system of the present invention is configured by arranging at least two exhaust gas purifying catalysts having an HC adsorbing function and carried on an integral structure type carrier in an exhaust gas flow path.
Further, the HC adsorption purification catalyst arranged upstream of the exhaust gas flow path contains at least platinum as a purification component, while the HC adsorption purification catalyst arranged downstream is selected from the group consisting of platinum, palladium and rhodium as purification components. It contains at least one noble metal selected.
The amount of platinum contained in the upstream HC adsorption / purification catalyst is configured to be larger than the amount of platinum contained in the downstream HC adsorption / purification catalyst, whereby the HC purification efficiency can be improved.
[0007]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the exhaust gas purification system of the present invention will be described in detail. In addition, in this specification, "%" represents a mass percentage unless otherwise specified.
As described above, the exhaust gas purification system of the present invention has a structure in which a plurality of exhaust gas purification catalysts having an HC adsorption function and supported on an integrally-structured carrier, that is, a plurality of HC adsorption purification catalysts are arranged in an exhaust gas passage. .
Further, among the HC adsorption purification catalysts arranged in series, the upstream HC adsorption purification catalyst arranged upstream of the exhaust gas flow path contains at least platinum as a purification component, and the downstream HC adsorption purification catalyst arranged downstream is platinum as a purification component. Noble metals include palladium or rhodium and any combination thereof.
Here, a monolithic carrier made of a heat-resistant material such as ceramics such as cordierite or a metal such as ferritic stainless steel is used as the monolithic carrier.
[0008]
Further, in the exhaust gas purification system of the present invention, the amount of platinum contained in the upstream HC adsorption purification catalyst is controlled to be larger than the amount of platinum contained in the downstream HC adsorption purification catalyst.
As described above, when platinum, which has excellent purification properties of flame-retardant paraffin among HC components, is contained in an upstream HC adsorption purification catalyst arranged upstream, which rapidly rises in temperature after the engine is started, the catalyst is quickly activated. The paraffin, which is a flame retardant component, can be preferentially oxidized (purified) upstream by the platinum, so that HC containing a large amount of olefin, which is a flammable component, can be supplied to a downstream HC adsorption purification catalyst.
Thus, the ignition of the downstream HC adsorption / purification catalyst is accelerated by the flammable olefin, and the downstream HC adsorption / purification catalyst is activated early, so that good HC purification efficiency can be realized as a whole system.
[0009]
In the present invention, for the platinum supported on the upstream HC adsorption purification catalyst, for example, by adopting two or more kinds of supported concentrations to produce the catalyst, the particle size distribution can be made large and small. Further, purification adaptability to a wide range of paraffin can be improved.
Further, it is preferable that the downstream HC adsorption / purification catalyst contains palladium and / or rhodium, which is excellent in olefin purification characteristics, from the early ignition phenomenon of the downstream HC adsorption / purification catalyst due to olefin.
[0010]
As described above, in the present invention, attention is paid to supplying flammable olefin-rich HC to a portion where the temperature rises slowly by purifying and removing flame-retardant paraffin by making platinum exist in a portion where the temperature rises early. are doing.
Therefore, in the case where one or both of palladium and rhodium are further contained in the upstream HC adsorption / purification catalyst, the temperature rises early, and the central part in the cross-section substantially perpendicular to the exhaust gas flow direction of the monolithic carrier is used. It is preferable that a large amount of platinum be present, whereby the entire carrier can be quickly warmed up to quickly reach the light-off temperature.
[0011]
In the present invention, the central portion of the vertical section in which a large amount of platinum is distributed preferably occupies 1 to 25% of the cross-sectional area of the vertical section, and more preferably occupies 5 to 15%.
If it is less than 1%, a lot of flammable HC will react upstream, and a lot of flammable HC will remain downstream, which may adversely affect the purification efficiency. Since the reaction efficiency of the flammable HC is excessively reduced and the purification sharing to the downstream HC adsorption purification catalyst is increased, unpurified HC may be released, which may adversely affect the purification efficiency as a whole. .
[0012]
Next, the upstream HC adsorption / purification catalyst is preferably disposed at a position in the exhaust gas passage where the rate of temperature rise of the catalyst inlet temperature is 10 ° C./sec or more, and is preferably 20 ° C./sec or more. It is more preferable to arrange them at positions.
If the temperature is lower than 10 ° C./sec, the activation of the purification component such as platinum is too slow, which may adversely affect the purification efficiency.
[0013]
On the other hand, the downstream HC adsorption / purification catalyst is preferably arranged at a position where the temperature rise rate of the catalyst inlet temperature is 1 to 10 ° C./sec, and is preferably arranged at a position where the temperature is 5 to 10 ° C./sec. Is more preferred.
As a result, the desorption of HC does not become too fast and the activation of the purification component is not too late, so that the purification efficiency of the downstream HC adsorption purification catalyst is further improved.
Note that the arrangement positions of the upstream HC adsorption purification catalyst and the downstream HC adsorption purification catalyst may be appropriately controlled within the above-mentioned temperature increasing speed range according to the temperature profiles at the time of various mode evaluations.
[0014]
In the exhaust gas purification system of the present invention, it is preferable that the capacity ratio between the upstream HC adsorption purification catalyst and the downstream HC adsorption purification catalyst is 25:75 to 75:25.
In the present invention, the number of installed HC adsorption / purification catalysts is not particularly limited as long as it is two or more of the upstream and downstream catalysts, and in particular, two or more downstream HC adsorption / purification catalysts may be installed. In this case, the total capacity of the downstream HC adsorption / purification catalyst group may be set to the above-mentioned capacity ratio with respect to the capacity of the most upstream HC adsorption / purification catalyst.
With such a capacity ratio, the HC purification efficiency can be further optimized.
[0015]
Next, the structure of the upstream and downstream HC adsorption / purification catalysts will be described.
In the present invention, such an HC adsorption / purification catalyst only needs to have the ability to adsorb and desorb HC and the ability to purify exhaust gas, particularly HC, but it is typically sufficient to use a catalyst having a monolithic structure such as a honeycomb carrier. Further, an HC adsorbent layer for HC adsorption / desorption and a purification component layer for exhaust gas purification are coated in this order.
Here, the HC adsorbent layer preferably contains an HC adsorbent, specifically, a zeolite, and particularly preferably contains an H-type zeolite having a Si / 2Al ratio of 10 to 1,000.
Beta zeolite having such a Si / 2Al ratio is preferable because it can secure an appropriate amount of HC adsorption and holding power and has excellent heat resistance.
[0016]
The purifying component layer contains the purifying component (noble metal), but also contains alumina containing 1 to 10 atomic% in terms of metal in terms of cerium, zirconium or lanthanum and any combination thereof. It is desirable to contain a cerium oxide containing 1 to 50 atomic% in terms of metal in terms of metal, zirconium, neodymium, praseodymium, yttrium or lanthanum and any combination thereof.
The presence of alumina to which such a co-catalyst component is added and cerium oxide as an oxygen supply source are preferable because the HC oxidation activity of the coexisting noble metal is further improved.
In addition, since the activity improvement effect with respect to palladium is large, it is desirable to use a purification component layer in which palladium is supported as a noble metal species.
[0017]
Further, it is preferable to add zirconium oxide containing 1 to 40 atomic% of metal related to cerium, neodymium or lanthanum and any combination thereof to the above-mentioned purification component layer.
By adding such a zirconium oxide, the noble metal element can be activated. Since this effect is particularly effective for platinum and rhodium, it is desirable to use this zirconium oxide in a layer containing platinum and rhodium, whereby the conversion of desorbed HC can be further improved.
[0018]
Furthermore, the purification component layer may contain one or both of an alkali metal and an alkaline earth metal, and thereby, particularly, the HC oxidation activity of palladium can be improved, and deterioration can be suppressed. It is possible to improve the desorption HC conversion rate after durability.
[0019]
【Example】
Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.
[0020]
As a material used for the HC adsorption purification catalyst, β-zeolite with Si / 2Al = 25 was used as the adsorbent, and the amount of application to the honeycomb carrier was unified at 200 g / L.
In the purification component layer, cerium (Ce) and zirconium (Zr) -containing alumina (each containing 3 moles) are used as an alumina base for supporting platinum (Pt) and palladium (Pd), and an alumina base for supporting rhodium (Rh). Alumina containing 3 mol of zirconium was used as the material.
In addition, as the base material for supporting platinum and palladium, ceria containing 10 mol of zirconium and zirconia containing 20 mol of cerium were used in addition to the above alumina. The total amount of noble metal used was kept constant at 3 g / L.
Further, the three-way catalyst Pd / Rh = 3 g / L-5 / 1 (0.7 L) was uniformly arranged at the uppermost stream (upstream of the HC adsorption purification catalyst group).
[0021]
(Example 1)
Pt 1.25 g / L, Pd 1.5 g / L, Rh 0.25 g / L, noble metal ratio Pt / Pd / Rh = 5/6/1 upstream of the catalyst inlet heating rate 20 ° C./sec upstream of the exhaust An HC adsorption purification catalyst (0.5 L capacity) was provided. Further downstream, at the catalyst inlet heating rate of 3 ° C./sec, downstream HC adsorption purification of Pd 2.5 g / L, Rh 0.5 g / L, and a precious metal ratio of Pd / Rh = 5/1 (capacity 0.9 L). The catalyst was arranged, and the exhaust gas purification system of this example was constructed.
[0022]
(Example 2)
The upstream HC adsorption / purification catalyst is disposed at a position at a catalyst inlet temperature increasing rate of 12 ° C./sec, the capacity is changed to 0.9 L, and the downstream HC adsorption / purification catalyst is further converted to Pt / Pd / Rh = 1/8/3 HC Except for using an adsorption purification catalyst (capacity: 0.5 L), the same configuration as in Example 1 was adopted, and an exhaust gas purification system of this example was constructed.
[0023]
(Example 3)
The same configuration as in Example 1 is adopted except that the capacity of both the upstream and downstream HC adsorption / purification catalysts is changed to 0.7 L, and the downstream HC adsorption / purification catalyst is arranged at a position at a catalyst inlet temperature increase rate of 8 ° C./sec. Then, an exhaust gas purification system of this example was constructed.
[0024]
(Example 4)
An exhaust gas purification system of this example was constructed by adopting the same configuration as that of Example 1 except that the Pt arrangement of the upstream HC adsorption purification catalyst was changed to the center portion of the carrier (5% as an area ratio with respect to the total cross-sectional area).
[0025]
(Example 5)
The same configuration as in Example 4 was adopted except that the area ratio of the Pt center arrangement was changed from 5% to 20%, and the exhaust gas purification system of this example was constructed.
Table 1 shows the specifications of the exhaust gas purification system of each of the above examples.
[0026]
[Table 1]

[0027]
(Comparative Example 1)
An upstream HC adsorption / purification catalyst (Pd: 2.5 g / L, Rh: 0.5 g / L, noble metal ratio: Pd / Rh = 5/1) (capacity: 0.5 L) at a catalyst inlet heating rate of 8 ° C./sec upstream of the exhaust gas The exhaust gas purification system of the present example was constructed by adopting the same configuration as that of the example 1 except that was disposed.
[0028]
(Comparative Example 2)
An upstream HC adsorption / purification catalyst (Pd: 2.5 g / L, Rh: 0.5 g / L, noble metal ratio: Pd / Rh = 5/1) (capacity: 0.5 L) at a catalyst inlet heating rate of 8 ° C./sec upstream of the exhaust gas And the downstream HC adsorption / purification catalyst is placed at a position at a catalyst inlet heating rate of 0.5 ° C./sec with Pt 1.25 g / L, Pd 1.5 g / L, Rh 0.25 g / L, and Pt / Pd / Except that a HC adsorption purification catalyst of Rh = 5/6/1 (capacity 0.9 L) was provided, the same configuration as that of Example 1 was adopted, and the exhaust gas purification system of this example was constructed.
Table 1 shows the specifications of the exhaust gas purification system of each of the above examples.
[0029]
[Performance evaluation]
Under the following conditions, the HC adsorption purification catalyst in the system of each example was rapidly deteriorated, the sample was evaluated by a vehicle, and the HC purification performance was compared. Table 2 shows the obtained results.
(Durability conditions)
Engine displacement 3000cc
Fuel Gasoline (Nisseki dash)
Catalyst inlet gas temperature 750 ℃
Endurance time 100 hours (evaluation conditions)
Evaluation vehicle Nissan in-line 4-cylinder 1800cc
Evaluation mode LA4-Abag
The cold HC reduction rate ("HC reduction rate" in Table 2) represents the LA4-Abag, the reduction rate of the HC emission in the section of 0 to 250 seconds, and is expressed by the following equation (1).
It is calculated from [(HC emission amount of Eng.Out) − (HC emission amount of System-Out)] / (HC emission amount of Eng.Out) × 100 (1).
[0030]
[Table 2]

[0031]
From Table 2, it can be seen that the exhaust gas purification systems of Examples 1 to 5 belonging to the scope of the present invention have higher HC purification efficiency than Comparative Examples 1 and 2 outside the present invention.
At the present time, from the viewpoint of the balance between the temperature rise characteristics and the Pt arrangement of the upstream side catalyst, it is considered that Example 5 provides the best result.
[0032]
【The invention's effect】
As described above, according to the present invention, the platinum amount is changed between a plurality of HC adsorption / purification catalysts arranged in series, and the flame-retardant HC is preferentially purified at the exhaust upstream position, and is easily downstream. Since a large amount of flammable HC is allowed to flow, it is possible to provide an exhaust gas purification system that is excellent in HC purification efficiency, particularly in HC purification efficiency at the time of engine start or immediately after start.

Claims

An exhaust gas purification system comprising at least two exhaust gas purification catalysts having an HC adsorption function and supported on an integrally-structured carrier arranged in series in an exhaust gas flow path.
The HC adsorption purification catalyst disposed upstream of the exhaust gas flow path contains at least platinum as a purification component,
The HC adsorption purification catalyst disposed downstream contains at least one noble metal selected from the group consisting of platinum, palladium and rhodium as a purification component,
An exhaust gas purification system, wherein the amount of platinum contained in the upstream HC adsorption purification catalyst is larger than the amount of platinum contained in the downstream HC adsorption purification catalyst.

The upstream HC adsorption purification catalyst further contains palladium and / or rhodium as a purification component,
2. The exhaust gas purification system according to claim 1, wherein a large amount of platinum is distributed in a central portion of the monolithic carrier in a cross section substantially perpendicular to the exhaust gas flow direction. 3.

3. The exhaust gas purification system according to claim 2, wherein the central portion of the vertical section in which a large amount of platinum is distributed occupies 1 to 25% of the cross-sectional area of the vertical section.

The exhaust gas according to any one of claims 1 to 3, wherein the upstream HC adsorption / purification catalyst is arranged at a position where a temperature increase rate of the catalyst inlet temperature is 10 ° C / sec or more. Gas purification system.

The downstream HC adsorption / purification catalyst is disposed at a position where the rate of temperature rise of the catalyst inlet temperature is 1 to 10 ° C./sec, according to any one of claims 1 to 4, characterized in that: Exhaust gas purification system.

The capacity ratio between the upstream HC adsorption purification catalyst and the downstream HC adsorption purification catalyst is
The exhaust gas purification system according to any one of claims 1 to 5, wherein the ratio is 25:75 to 75:25.

The upstream and downstream HC adsorption / purification catalysts have an HC adsorbent layer containing zeolite, and the zeolite is composed of H-type β zeolite having a Si / 2Al ratio of 10 to 1,000. An exhaust gas purification system according to any one of the preceding claims.

The upstream and downstream HC adsorption purification catalyst has a purification component layer containing the purification component,
This purifying component layer is selected from the group consisting of alumina containing 1 to 10 atomic% in terms of metal, and at least one metal selected from the group consisting of cerium, zirconium and lanthanum, and the group consisting of zirconium, neodymium, praseodymium, yttrium and lanthanum. The exhaust gas purification system according to any one of claims 1 to 7, further comprising a cerium oxide containing 1 to 50 atomic% of at least one kind of metal in terms of metal.

The said purifying component layer contains a zirconium oxide containing 1 to 40 atomic% in metal conversion of at least one metal selected from the group consisting of cerium, neodymium and lanthanum. Exhaust gas purification system.

The exhaust gas purification system according to claim 8 or 9, wherein the purification component layer further contains an alkali metal and / or an alkaline earth metal.