JP4016420B2

JP4016420B2 - Exhaust gas purification catalyst system

Info

Publication number: JP4016420B2
Application number: JP2002294868A
Authority: JP
Inventors: 伸司山本; 真弘高谷
Original assignee: Nissan Motor Co Ltd
Current assignee: Nissan Motor Co Ltd
Priority date: 2002-10-08
Filing date: 2002-10-08
Publication date: 2007-12-05
Anticipated expiration: 2022-10-08
Also published as: JP2004132193A

Description

【０００１】
【発明の属する技術分野】
本発明は、自動車用エンジンなどの内燃機関から排出される排気ガスを浄化するための触媒システムに係わり、特に内燃機関の始動直後の排気ガス中に含まれる炭化水素（以下、「ＨＣ」と称する）を効率良く浄化することができる排気ガス浄化用触媒システムに関するものである。
【０００２】
【従来の技術】
近年、内燃機関の始動時における低温域で大量に排出されるＨＣ（コールドＨＣ）の浄化を目的に、ＨＣ吸着材にゼオライトを用いたＨＣ吸着型三元触媒（ＨＣ吸着機能付き三元触媒）が開発されている。
このようなＨＣ吸着型三元触媒は、三元触媒がまだ十分に活性化していないエンジン始動時の低温域において、大量に排出されるＨＣをＨＣ吸着材によって一時的に吸着・保持したのち、排気ガス温度の上昇によって三元触媒が活性化した時に、吸着したＨＣを徐々に脱離し、浄化しようとするものであり、このようなＨＣ吸着型三元触媒がコールドＨＣの浄化処理に有効であると考えられてきた。
しかし、ＨＣ吸着型三元触媒が備えている浄化触媒成分あるいはこの触媒の下流側に備えた浄化触媒が十分に活性化する前に、ＨＣ吸着材の昇温によって、当該吸着材に一旦貯蔵されたコールドＨＣが脱離してしまうことから、効率良く浄化することができなかった。
【０００３】
すなわち、従来の合成ゼオライトを主体としたＨＣ吸着材を用いた排ガス浄化用触媒、特に上記のようなＨＣ吸着型三元触媒においては、排ガス中に含まれる水分がゼオライト細孔内の酸点に対する化学吸着を阻害するため、エンジン始動直後の三元触媒が活性化する前に排出されるコールドＨＣの吸着は、ゼオライトの分子篩作用を利用した物理吸着によるものが支配的である。しかし、物理吸着したＨＣは、ゼオライト細孔内での保持力が弱いため、排ガス温度の上昇や排ガス流量の増加などの外的（物理的）な要因によって、容易に脱離するものと推測される。このため、排ガス流路の下流にＨＣ吸着型三元触媒を設置しても、コールドＨＣの吸着及び脱離現象は生じるものの、吸着ＨＣの脱離開始と浄化触媒の活性化とのタイミングが一致せず、吸着ＨＣの脱離浄化処理を効率良く実行するのは極めて困難であった。
【０００４】
従来においては、コールドＨＣの分子径に適した細孔径を有するゼオライトを選択することによって、比較的効率よくコールドＨＣを吸着することは可能であったが、このように物理吸着したＨＣをゼオライト細孔内に長時間留めておくことができないために、浄化触媒が充分に活性化する前に脱離が始まってしまい、所望の浄化処理効率を得ることができないのが実情であった。
【０００５】
さらに、従来においては、ゼオライト骨格構造の耐熱性の向上を図るため、ゼオライト骨格を構成しているシリカ（ＳｉＯ_２）及びアルミナ（Ａｌ_２Ｏ_３）のうちのＡｌを酸溶液で処理して脱離する処理（脱Ａｌ処理）が行われている。しかし、このような脱Ａｌ処理を施したゼオライトは、耐熱性が向上する反面、骨格構造中の酸点であるＡｌのカチオンサイトが減少するため、酸特性が著しく低下し、ゼオライト細孔内に物理吸着したＨＣを保持する特性に著しく欠けるという問題があった。
【０００６】
このため、このようなＨＣ吸着材を用いた排気ガス浄化システムとしては、例えば特開平６−７４０１９号公報、特開平７−１４４１１９号公報、特開平６−１４２４５７号公報、特開平５−５９９４２号公報、特開平７−１０２９５７号公報、特開平９−８５０５６号公報、特開平７−９６１８３号公報、特開平１１−８１９９９号公報等に開示されているものがある。
【０００７】
すなわち、特開平６−７４０１９号公報には、排気流路にバイパスを設け、エンジン始動直後のコールド時に排出されるＨＣをバイパス流路に配置したＨＣ吸着材に一旦吸着させ、その後流路を切り換え、下流の三元触媒が活性化した後に排気ガスの一部をＨＣ吸着触媒に通じ、吸着材から脱離したＨＣを徐々に後段の三元触媒で浄化するシステムが提案されている。
【０００８】
特開平７−１４４１１９号公報においては、コールド時に前段の三元触媒に熱を奪わせて中段のＨＣ吸着触媒の吸着効率を向上する一方、前段の三元触媒が活性化した後は、タンデム配置した中段のＨＣ吸着触媒を介して後段の三元触媒に反応熱を伝熱し易くし、後段の三元触媒による浄化を促進するシステムが提案されている。
【０００９】
また、特開平６−１４２１４５７号公報では、低温域で吸着したＨＣが脱離する際に、脱離ＨＣを含む排気ガスを熱交換器で予熱することによって三元触媒での浄化を促進するコールドＨＣ吸着除去システムが提案されている。
【００１０】
一方、特開平５−５９９４２号公報には、触媒配置とバルブによる排気ガス流路の切り換えによって、ＨＣ吸着材の昇温を緩慢にし、これによって吸着材によるコールドＨＣの吸着効率を向上するシステムが提案されている。
【００１１】
また、特開平７−１０２９５７号公報では、後段の酸化・三元触媒の浄化性能を向上させるために、前段の三元触媒と中段のＨＣ吸着材の間に空気を供給し、後段の酸化・三元触媒の活性化を促進するシステムを提案している。
【００１２】
さらに、特開平９−８５０５６号公報には、ゼオライトからなるＨＣ吸着材とＰｔ−Ｒｈ系触媒成分を含む前段触媒と、ゼオライトからなるＨＣ吸着材とＰｄ系触媒成分と酸化セリウムを含む後段触媒とを同じ容器に充填し、これらの間に２次エアーを供給することによって後段触媒の浄化促進するシステムが提案されている。
【００１３】
そして、特開平７−９６１８３号公報では、ゼオライトと接触する触媒層（ＰｄとＡｌ_２Ｏ_３が主成分）の高温下での劣化抑制の観点から、吸着材層と触媒層との間に多孔質バリヤ層を設ける一方、下層吸着材層の吸着能の低下を抑制する観点から、Ｐｄ担持Ａｌ_２Ｏ_３粒子の平均粒子径が１５〜２５μｍ、耐火性無機質粒子の平均粒子径が５〜１５μｍのものを用いることを提案している。
【００１４】
さらに、特開平１１−８１９９９号公報では、コールド時に吸着材に捕捉されたＨＣの脱離を緩慢にするために、ＨＣ吸着触媒の温度上昇を十分に遅延させるべく、上流三元触媒と下流吸着触媒との間を結ぶ排気通路における管体の肉厚を厚くしたり、担体などを設置したりすることによって、上記排気通路の熱容量を排気マニホールドの熱容量より大きくする方法が提案されている。
【００１５】
【発明が解決しようとする課題】
しかしながら、上記した特開平５−５９９４２号公報や特開平６−７４０１９号公報記載のシステムにおいては、車種や走行パターンなどの諸条件の相違によってコールドＨＣの吸着や脱離時間が大きく変動するため、切り換えバルブの作動タイミングを最適に制御することが困難であり、ＨＣ吸着材からのＨＣ脱離と、後段三元触媒の活性化とのタイミングを常に一致させることができず、浄化処理の効率を十分に向上させることができないという問題がある。
【００１６】
一方、特開平７−１４４１１９号公報や特開平６−１４２１４５７号公報に記載されたシステムでは、ＨＣ吸着材の後段に配置した三元触媒の熱による活性化の促進が不十分となり易く、浄化処理を必ずしも効率良く行うことができないという難点があり、特開平７−１０２９５７号公報や特開平９−８５０５６号公報に提案されたシステムにおいても、２次空気を供給することによる後段の酸化・三元触媒の活性化促進効果にもおのずと限界があり、浄化処理効率の向上幅は必ずしも十分なものとは言えない。
【００１７】
そして、特開平７−９６１８３号公報には、ゼオライト層と触媒層との間にバリヤ層を設けることによる触媒層の劣化抑制が提案されているが、その効果は必ずしも十分とは言えず、三元触媒による脱離ＨＣの浄化処理効率を十分なものとすることができない。
また、特開平１１−８１９９９号公報記載のシステムでは、排気通路の熱容量を大きくして吸着材層の温度上昇を遅らせ、もってコールド時に吸着材に捕捉されたＨＣの脱離が緩慢なものとなるようにしているが、同時に、後段三元触媒の活性化開始も遅れることから、脱離と浄化開始のタイミングを十分に一致させることができず、浄化処理効率を十分に向上させることができないという問題がある。
【００１８】
以上のように、上記公報に開示された各浄化システムにおいては、バルブ切り換え、予熱や空気供給による浄化触媒の活性化促進、吸着材からのＨＣ離脱の緩慢化などによるＨＣ脱離と浄化開始のタイミングを一致させるようにしており、それなりの効果はあるにしても、両者のタイミングを十分に一致させるには到っておらず、いずれのシステムにおいてもＨＣ吸着型三元触媒の浄化触媒成分、あるいはその後段に備えた浄化触媒が活性化する前に、ＨＣ吸着材に一旦吸着されたコールドＨＣが脱離してしまうことがないとは言えず、コールドＨＣを必ずしも効率良く浄化することができないという問題点があり、従来の排気ガス浄化システムにおける課題となっていた。
【００１９】
本発明は、ＨＣ吸着型三元触媒を備えた従来の排気ガス浄化システムにおける上記課題に鑑みてなされたものであって、ＨＣ吸着材層からのＨＣの脱離を遅延化して、触媒成分による浄化開始とのタイミングを十分に一致させることができ、コールドＨＣの浄化効率の向上が可能な排気ガス浄化用触媒システムを提供することを目的としている。
【００２０】
【課題を解決するための手段】
本発明の排気ガス浄化用触媒システムは、排気ガス流路の上流側に三元触媒を配置し、ゼオライトを含有するＨＣ吸着材層と、触媒金属を含有する浄化触媒成分層を担体の多角形セル内にこの順序に積層して成るＨＣ吸着型三元触媒を上記三元触媒の下流側に複数個配置したシステムであって、上記複数個のＨＣ吸着型三元触媒における担体セルの辺数が排気ガス流路の下流側に向けて増加している構成としたことを特徴としている。
【００２１】
【発明の実施の形態】
以下、本発明の排気ガス浄化用触媒システムについて詳細に説明する。
本発明の排気ガス浄化用触媒システムは、排気ガス流路の上流側に三元触媒、その下流側に複数個のＨＣ吸着型三元触媒を配置して成る触媒システムであって、上記ＨＣ吸着型三元触媒が担体の多角形セル内に、ゼオライトを含有するＨＣ吸着材層と触媒金属を含有する浄化触媒成分層とをこの順序に積層した多層構造を有し、これら複数個のＨＣ吸着型三元触媒の担体セルの辺数が排気ガス流路の下流側に向けて増加するように配列されていることから、排気ガス流路の上流側に配置した三元触媒が活性化する前に排出されるコールドＨＣが下流側に配置した複数のＨＣ吸着型三元触媒で分担して吸着されることになり、コールドＨＣの吸着効率の向上と吸着したＨＣの脱離遅延化が図れる。
【００２２】
すなわち、排気ガス流路の前段側と後段側とで昇温プロファイルが異なる２個以上のＨＣ吸着型三元触媒によってコールドＨＣを分担して吸着すること、望ましくは、例えば前段、すなわち最上流のＨＣ吸着型三元触媒でコールドＨＣ排出量の５０％以上を一旦吸着することにより、後段側触媒の吸着負荷が軽減され、前段で未浄化のまま脱離するＨＣを後段側触媒中のゼオライトで効率良く再吸着できるため、システム全体の吸着効率の向上が達成される。
さらに、最上流の三元触媒の活性化開始を速め、その下流に配置したＨＣ吸着型三元触媒に流入するコールドＨＣ量を、該ＨＣ吸着型三元触媒に含有されるゼオライトの飽和吸着容量の７０％以下、望ましくは５０％以下の範囲で捕捉（物理吸着）させることによって、温度が上昇あるいはガス流量が増大した場合にも、ゼオライト細孔内のＨＣ分子の拡散衝突が緩和されるため、脱離の遅延化が図れるものと考えられる。
【００２３】
そして、排気ガス流路に配置された複数のＨＣ吸着型三元触媒が、上流から下流に行くに従って触媒成分を塗布する担体が角（辺）の増えた多角形セル形状（例えば、四角形セルから六角形セル）になるようにしてあるので、これによってコーナー部の角度が大きくなり、スラリーの塗布形状、とりわけ第１層であるＨＣ吸着材層の塗布厚さが平均化され、塗布形態が吸着−保持特性に優れた形状となる。すなわち、塗布量に対して吸着表面積が増すと共に、塗布厚さの極度に薄い部分がなくなることから、急激な加熱によって吸着ＨＣを早期に放出してしまうようなことがなくなる。
したがって、コールドＨＣの吸着効率が向上すると共に、吸着ＨＣの脱離が遅延化され、浄化触媒成分層の温度上昇が緩慢で活性化し難い下流側でも、ＨＣの浄化処理効率が著しく改善されることになる。
【００２４】
なお、三元触媒の下流側に配置される複数個のＨＣ吸着型三元触媒における担体セル形状の具体的配置としては、最上流に位置するＨＣ吸着型三元触媒に用いる担体のセル形状を四角形とし、これよりも下流側の触媒には六角形セルを備えた担体を使用することができる。
すなわち、四角形−六角形（２個配置の場合）、四角形−六角形−六角形（３個配置の場合）、四角形−六角形−六角形−六角形（４個配置の場合）などが考えられるが、下流側触媒ほど温度が低くなり、徒にこれら触媒の数を増したとしても排ガス浄化に実質的に寄与しない触媒を増したに過ぎない結果となることが多いので、四角形及び六角形セルの担体をそれぞれ使用した２個のＨＣ吸着型三元触媒をこの順序に配置することが望ましい。
【００２５】
本発明の排気ガス浄化用触媒システムにおける好適形態としては、上流から下流に行くにしたがって、ＨＣ吸着型三元触媒の触媒成分を塗布する担体のセル形状が角の増えた多角形になり、さらに担体の幾何学的表面積（ＧＳＡ）が増大したものとすることが望ましく、これによって排気ガスとの接触効率が向上することから、前段のＨＣ吸着型三元触媒で未吸着となったコールドＨＣや未浄化のまま排出されるコールドＨＣが後段側のＨＣ吸着型三元触媒によって効率良く吸着されるようになる。このため、後段ＨＣ吸着型三元触媒に流入してくる難吸着性のＨＣ種も効率良く吸着され、浄化触媒層による脱離ＨＣの浄化処理効率が一層改善されることになる。
【００２６】
また、他の好適形態としては、上流から下流に行くにしたがって、触媒成分を塗布する担体のセル形状を角の増えた多角形状とすると共に、セルの角部（コーナー部）と平坦部とのＨＣ吸着材層塗布量の差を小さくすることが望ましい。
すなわち、複数個のＨＣ吸着型三元触媒を配置した排ガス浄化システムにおいては、一般に、上流から下流に行くにしたがって、触媒出口の排気組成が徐々に変化し、下流側の排気ガス中には吸着し難く脱離し易いＨＣ種が残っているため、下流側触媒ほど、ＨＣ吸着材層の塗布形態が吸着−保持特性に優れた形状にすることが望ましい。このため、上記のようにセルの角部と平坦部とのＨＣ吸着材層塗布量の差を小さくすることによって、ＨＣ吸着材層の塗布形態が吸着−保持特性に優れた形状となり、吸着ＨＣの脱離が遅延化し、浄化触媒層の温度上昇が緩慢で活性化し難い下流側でも、ＨＣの浄化処理効率を著しく改善できることになる。
【００２７】
本発明の排気ガス浄化用触媒システムにおけるさらに他の好適形態としては、排気ガス流路の上流から下流に行くにしたがって、ＨＣ吸着型三元触媒において触媒成分を塗布する担体のセル形状を角数の増えた多角形セル形状とし、さらに、下流に行くほどＨＣ吸着材層の塗布量を増やすようになすことができ、これによってＨＣ吸着材層の形態が吸着−保持特性に優れた形状となって吸着ＨＣの脱離が遅延化され、浄化触媒層の温度上昇が緩慢で活性化し難い下流側でも、ＨＣの浄化処理効率がさらに一層改善されることになる。
【００２８】
そして、本発明の排気ガス浄化用触媒システムの別の好適形態においては、上記ＨＣ吸着型三元触媒の表層である浄化触媒成分層に、白金（Ｐｔ），パラジウム（Ｐｄ）及びロジウム（Ｒｈ）から成る群より選ばれた少なくとも１種の金属と、セリウム（Ｃｅ），ジルコニウム（Ｚｒ）及びランタン（Ｌａ）から成る群より選ばれた少なくとも１種を金属換算で１〜１０モル％含むアルミナと、ジルコニウム（Ｚｒ），ネオジウム（Ｎｄ），プラセオジウム（Ｐｒ），イットリウム（Ｙ）及びランタン（Ｌａ）から成る群より選ばれた少なくとも１種を金属換算で１〜５０モル％含むセリウム酸化物を含有させることができ、これによって活性成分であるＰｔ及びＰｄの経時劣化が抑制され、脱離ＨＣの浄化処理効率を高く維持することができると共に、三元触媒としても高い浄化性能が得られることになる。
【００２９】
また、さらなる好適形態としては、上記浄化触媒成分層に、更にセリウム（Ｃｅ），ネオジウム（Ｎｄ），プラセオジウム（Ｐｒ），イットリウム（Ｙ）及びランタン（Ｌａ）から成る群より選ばれた少なくとも１種を金属換算で１〜４０モル％含むジルコニウム酸化物を含有させることが望ましく、これによって活性成分であるＲｈの経時劣化が抑制でき、脱離ＨＣの高い浄化処理効率を維持することができ、三元触媒としてもさらに高い浄化性能を得ることができる。
【００３０】
上記排気ガス浄化用触媒システムのさらなる好適形態としては、上記浄化触媒成分層に、更にアルカリ金属及びアルカリ土類金属から選ばれた少なくとも１種の金属を含有させることが望ましく、これによって、還元雰囲気における浄化活性の向上と共に活性成分の経時劣化の抑制を図ることができ、脱離ＨＣの高い浄化処理効率が維持できると共に、三元触媒としても高い浄化性能が得られることになる。しかも、これらアルカリ金属及びアルカリ土類金属を水に難溶性あるいは不溶性の化合物を用いて浄化触媒成分層に含有させることによって、スラリー中の水分の移動に伴ってスラリー中に溶出した金属イオンのゼオライト細孔内へ浸入を抑制することができ、初期から耐久後まで高い吸着効率を維持できることになる。
【００３１】
【実施例】
以下、本発明を実施例に基づいてさらに具体的に説明する。なお、当該実施例において、「％」は特記しない限り質量百分率を表すものとする。
【００３２】
（触媒１）
ＳｉＯ_２／Ａｌ_２Ｏ_３比が４０のβ−ゼオライト粉末２２５７ｇ、シリカゾル（固形分２０％）１２１５ｇ、純水１０００ｇを磁性ボールミルに投入し、混合粉砕してスラリー液を得た。このスラリー液をコージェライト製担体（セル形状：四角形、セル数：４６．５個／ｃｍ^２，セル壁厚み：０．０１５ｃｍ，ＧＳＡ：２４．１ｃｍ^２／ｃｍ^３，担体容量：１．０Ｌ）に付着させ、空気流にてセル内の余剰のスラリーを取り除いて乾燥し、４００℃で１時間焼成した。そして、このゼオライトを含有する吸着材層のコート量が３５０ｇ／Ｌになるまでコーティング作業を繰り返すことによって、触媒−ａ１を得た。
【００３３】
次に、３モル％のＣｅを含むアルミナ粉末（Ａｌ：９７モル％）に、硝酸パラジウム水溶液を含浸あるいは高速撹拌中で噴霧し、１５０℃で２４時間乾燥した後、４００℃で１時間、次いで、６００℃で１時間焼成し、Ｐｄ担持アルミナ粉末（粉末ａ）を得た。この粉末ａのＰｄ濃度は４．０％であった。
また、１モル％のＬａと３２モル％のＺｒを含有するセリウム酸化物粉末（Ｃｅ：６７モル％）に、硝酸パラジウム水溶液を含浸あるいは高速撹拌中で噴霧し、１５０℃で２４時間乾燥した後、４００℃で１時間、次いで、６００℃で１時間焼成し、Ｐｄ担持セリウム酸化物粉末（粉末ｂ）を得た。この粉末ｂのＰｄ濃度は２．０％であった。
そして、上記Ｐｄ担持アルミナ粉末（粉末ａ）４００ｇ、Ｐｄ担持セリウム酸化物粉末（粉末ｂ）１４１ｇ、硝酸酸性アルミナゾル２４０ｇ（ベーマイトアルミナ１０％に１０％の硝酸を添加することによって得られたゾルで、Ａｌ２Ｏ３換算で２４ｇ）及び炭酸バリウム１００ｇ（ＢａＯとして６７ｇ）を純水２０００ｇと共に磁性ボールミルに投入し、混合粉砕してスラリー液を得た。このスラリー液を上記コート触媒−ａ１に付着させ、空気流にてセル内の余剰のスラリーを取り除いて乾燥し、４００℃で１時間焼成して、コート層重量６６．５ｇ／Ｌを塗布した触媒−ｂを得た。
【００３４】
次に、３モル％のＺｒを含むアルミナ粉末（Ａｌ：９７モル％）に、硝酸ロジウム水溶液を含浸或いは高速撹拌中で噴霧し、１５０℃で２４時間乾燥した後、４００℃で１時間、次いで、６００℃で１時間焼成し、Ｒｈ担持アルミナ粉末（粉末ｃ）を得た。この粉末ｃのＲｈ濃度は２．０％であった。
また、３モル％のＣｅを含むアルミナ粉末（Ａｌ：９７モル％）に、ジニトロジアンミン白金水溶液を含浸或いは高速撹拌中で噴霧し、１５０℃で２４時間乾燥した後、４００℃で１時間、次いで、６００℃で１時間焼成し、Ｐｔ担持アルミナ粉末（粉末ｄ）を得た。この粉末ｄのＰｔ濃度は３．０％であった。
さらに、１モル％のＬａと２０モル％のＣｅを含有するジルコニウム酸化物粉末に、ジニトロジアンミン白金水溶液を含浸或いは高速撹拌中で噴霧し、１５０℃で２４時間乾燥した後、４００℃で１時間、次いで、６００℃で１時間焼成し、Ｐｔ担持アルミナ粉末（粉末ｅ）を得た。この粉末ｅのＰｔ濃度は３．０％であった。
そして、上記Ｒｈ担持アルミナ粉末（粉末ｃ）１１８ｇ、Ｐｔ担持アルミナ粉末（粉末ｄ）１１８ｇ、Ｐｔ担持ジルコニウム酸化物粉末（粉末ｅ）１１８ｇ、硝酸酸性アルミナゾル１６０ｇを磁性ボールミルに投入し、混合粉砕してスラリー液を得た。このスラリー液を上記コート触媒−ｂに付着させ、空気流にてセル内の余剰のスラリーを取り除いて乾燥し、４００℃で１時間焼成して、コート層重量３７ｇ／Ｌを塗布したＨＣ吸着型三元触媒１を得た。
当該触媒１の貴金属担持量は、Ｐｔ：０．７１ｇ／Ｌ、Ｐｄ：１．８８ｇ／Ｌ、Ｒｈ：０．２４ｇ／Ｌであった。
【００３５】
（触媒２）
触媒担体として、セル形状が四角形、セル数：３１．０個／ｃｍ^２、セル壁厚み：０．０２５ｃｍ、ＧＳＡ：１９．０ｃｍ^２／ｃｍ^３、担体容量：１．０Ｌのコージェライト製担体を用いたこと以外は、上記触媒１と同様の操作を繰り返し、ＨＣ吸着材層及び浄化触媒成分層を積層して、本例に係わるＨＣ吸着型三元触媒２を得た。
【００３６】
（触媒３）
触媒担体として、セル形状が六角形、セル数：６２．０個／ｃｍ^２、セル壁厚み：０．０１１ｃｍ、ＧＳＡ：２７．２ｃｍ^２／ｃｍ^３、担体容量：１．０Ｌのコージェライト製担体を用いたこと以外は、上記触媒１と同様の操作を繰り返し、ＨＣ吸着材層及び浄化触媒成分層を積層して、本例に係わるＨＣ吸着型三元触媒３を得た。
【００３７】
（触媒４）
触媒担体として、セル形状が六角形、セル数：７１．３個／ｃｍ^２、セル壁厚み：０．０１１ｃｍ、ＧＳＡ：２８．４ｃｍ^２／ｃｍ^３、担体容量：１．０Ｌのコージェライト製担体を用いたこと以外は、上記触媒１と同様の操作を繰り返し、ＨＣ吸着材層及び浄化触媒成分層を積層して、本例に係わるＨＣ吸着型三元触媒４を得た。
【００３８】
（触媒５）
触媒担体として、セル形状が六角形、セル数：９３．０個／ｃｍ^２、セル壁厚み：０．００８ｃｍ、ＧＳＡ：３３．４ｃｍ^２／ｃｍ^３、担体容量：１．０Ｌのコージェライト製担体を用いたこと以外は、上記触媒１と同様の操作を繰り返して、ＨＣ吸着材層及び浄化触媒成分層を積層し、本例に係わるＨＣ吸着型三元触媒５を得た。
【００３９】
（触媒６）
触媒担体として、セル形状が六角形、セル数：１０７．０個／ｃｍ^２、セル壁厚み：０．０１１ｃｍ、ＧＳＡ：３４．０ｃｍ^２／ｃｍ^３、担体容量：１．０Ｌのコージェライト製担体を用いたこと以外は、上記触媒１と同様の操作を繰り返し、ＨＣ吸着材層及び浄化触媒成分層を積層して、本例に係わるＨＣ吸着型三元触媒６を得た。
【００４０】
（触媒７）
触媒担体として、セル形状が四角形、セル数：９３．０個／ｃｍ^２、セル壁厚み：０．０１０ｃｍ、ＧＳＡ：３４．５ｃｍ^２／ｃｍ^３、担体容量：１．０Ｌのコージェライト製担体を用いたこと以外は、上記触媒１と同様の操作を繰り返して、ＨＣ吸着材層及び浄化触媒成分層を積層し、本例に係わるＨＣ吸着型三元触媒７を得た。
【００４１】
（触媒８）
触媒担体として、セル形状が四角形、セル数：１３９．５個／ｃｍ^２、セル壁厚み：０．００５ｃｍ、ＧＳＡ：４３．６ｃｍ^２／ｃｍ^３、担体容量：１．０Ｌのコージェライト製担体を用いたこと以外は、上記触媒１と同様の操作を繰り返して、ＨＣ吸着材層及び浄化触媒成分層を積層し、本例に係わるＨＣ吸着型三元触媒８を得た。
【００４２】
（触媒９）
触媒担体として、セル形状が三角形、セル数：１９．０個／ｃｍ^２、セル壁厚み：０．００５ｃｍ、ＧＳＡ：１１．１ｃｍ^２／ｃｍ^３、担体容量：１．０Ｌのコージェライト製担体を用いたこと以外は、上記触媒１と同様の操作を繰り返し、ＨＣ吸着材層及び浄化触媒成分層を積層して、本例に係わるＨＣ吸着型三元触媒９を得た。
【００４３】
以上によって得られたＨＣ吸着型三元触媒１〜９の仕様をそれぞれ表１に示す。
【００４４】
【表１】

【００４５】
下記条件による耐久後の上記ＨＣ吸着型三元触媒１〜９をそれぞれ選択して組み合わせ、図１に示す排気ガス浄化用触媒システムの触媒位置１及び２に配置し、下記の条件のもとにＨＣ特性評価を行い、ＨＣ吸着率と脱離ＨＣ浄化率を測定した。その結果を表２に示す。
【００４６】
耐久条件
・エンジン排気量３０００ｃｃ
・燃料ガソリン（日石ダッシュ）
・耐久温度６５０℃
・耐久時間５０時間
車両性能試験
・エンジン日産自動車株式会社製直列４気筒２．０Ｌ
・評価方法北米排ガス試験法のＬＡ４−ＣＨのＡ−ｂａｇ
【００４７】
【表２】

【００４８】
【発明の効果】
以上説明してきたように、本発明の排気ガス浄化用触媒システムは、排気ガス流路の上流側に三元触媒を配置すると共に、その下流側に複数のＨＣ吸着型三元触媒を配置して成る触媒システムにおいて、これら複数個のＨＣ吸着型三元触媒の担体セルの辺数が排気ガス流路の下流側に向けて増加するように配列したものであるから、上流側の三元触媒が活性化する前に排出されるコールドＨＣが下流側に配置された複数のＨＣ吸着型三元触媒により分担して吸着されると共に、下流側のＨＣ吸着型三元触媒における、特にＨＣ吸着材層の塗布形態が吸着−保持特性に優れた形状となることから、コールドＨＣの吸着効率の向上と吸着されたＨＣの脱離遅延化が可能になり、吸着ＨＣの脱離と浄化開始とのタイミングが一致するようになり、コールドＨＣの浄化効率を大幅に向上させることができるという極めて優れた効果がもたらされる。
【図面の簡単な説明】
【図１】本発明の排気ガス浄化用触媒システムの構成例を示す概略図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a catalyst system for purifying exhaust gas discharged from an internal combustion engine such as an automobile engine, and more particularly to a hydrocarbon (hereinafter referred to as “HC”) contained in exhaust gas immediately after the start of the internal combustion engine. The present invention relates to an exhaust gas purifying catalyst system that can efficiently purify gas.
[0002]
[Prior art]
In recent years, HC adsorption type three-way catalyst (three-way catalyst with HC adsorption function) using zeolite as the HC adsorbent for the purpose of purifying HC (cold HC) discharged in large quantities in the low temperature range at the start of the internal combustion engine Has been developed.
Such an HC adsorption type three-way catalyst, after temporarily adsorbing and holding a large amount of HC exhausted by the HC adsorbent in a low temperature range at the time of engine start where the three-way catalyst has not been sufficiently activated, When the three-way catalyst is activated due to an increase in exhaust gas temperature, the adsorbed HC is gradually desorbed and purified, and such an HC adsorption type three-way catalyst is effective for the purification process of cold HC. It has been thought that there is.
However, before the purification catalyst component provided in the HC adsorption type three-way catalyst or the purification catalyst provided on the downstream side of this catalyst is sufficiently activated, it is temporarily stored in the adsorbent by the temperature rise of the HC adsorbent. Since the cold HC was desorbed, it could not be purified efficiently.
[0003]
That is, in a conventional exhaust gas purification catalyst using an HC adsorbent mainly composed of synthetic zeolite, particularly in the above-described HC adsorption type three-way catalyst, the moisture contained in the exhaust gas is related to the acid points in the zeolite pores. In order to inhibit chemisorption, the adsorption of cold HC discharged before the activation of the three-way catalyst immediately after engine startup is predominantly based on physical adsorption utilizing the molecular sieve action of zeolite. However, HC that has been physically adsorbed has a weak holding power in the zeolite pores, so it is assumed that HC easily desorbs due to external (physical) factors such as an increase in exhaust gas temperature and an increase in exhaust gas flow rate. The For this reason, even if an HC adsorption type three-way catalyst is installed downstream of the exhaust gas passage, the adsorption and desorption phenomenon of cold HC occurs, but the timing of the start of desorption of adsorbed HC and the activation of the purification catalyst coincides. Therefore, it is extremely difficult to efficiently perform the desorption purification process of the adsorbed HC.
[0004]
In the past, it was possible to adsorb cold HC relatively efficiently by selecting a zeolite having a pore size suitable for the molecular diameter of cold HC. Since it cannot be kept in the hole for a long time, desorption starts before the purification catalyst is sufficiently activated, and the desired purification treatment efficiency cannot be obtained.
[0005]
Furthermore, conventionally, in order to improve the heat resistance of the zeolite framework structure, silica (SiO ₂ ) And alumina (Al ₂ O ₃ ) Is removed by treating with Al with an acid solution (de-Al treatment). However, the zeolite subjected to such de-Al treatment is improved in heat resistance, but on the other hand, since the cation sites of Al, which are acid points in the skeleton structure, are reduced, the acid characteristics are remarkably lowered, and the zeolite pores are reduced. There was a problem that the characteristic of holding the physically adsorbed HC was remarkably lacking.
[0006]
For this reason, as an exhaust gas purification system using such an HC adsorbent, for example, JP-A-6-74019, JP-A-7-144119, JP-A-6-142457, JP-A-5-59942 are disclosed. There are those disclosed in Japanese Patent Laid-Open No. 7-102957, Japanese Patent Laid-Open No. 9-85056, Japanese Patent Laid-Open No. 7-96183, Japanese Patent Laid-Open No. 11-81999, and the like.
[0007]
That is, in JP-A-6-74019, a bypass is provided in the exhaust passage, and HC discharged at the time of cold immediately after engine start is once adsorbed by the HC adsorbent disposed in the bypass passage, and then the passage is switched. A system has been proposed in which after the downstream three-way catalyst is activated, a part of the exhaust gas is passed through the HC adsorption catalyst, and HC desorbed from the adsorbent is gradually purified by the latter three-way catalyst.
[0008]
In Japanese Patent Application Laid-Open No. 7-144119, while the former three-way catalyst is deprived of heat during cold, the adsorption efficiency of the middle HC adsorption catalyst is improved, and after the former three-way catalyst is activated, the tandem arrangement is performed. There has been proposed a system that facilitates the transfer of reaction heat to the subsequent three-way catalyst through the intermediate HC adsorption catalyst and promotes purification by the subsequent three-way catalyst.
[0009]
Japanese Patent Laid-Open No. 6-142457 discloses a cold which promotes purification with a three-way catalyst by preheating exhaust gas containing desorbed HC with a heat exchanger when HC adsorbed in a low temperature region is desorbed. An HC adsorption removal system has been proposed.
[0010]
On the other hand, JP-A-5-59942 discloses a system for slowing the temperature rise of the HC adsorbent by switching the exhaust gas flow path with a catalyst arrangement and a valve, thereby improving the cold HC adsorption efficiency by the adsorbent. Proposed.
[0011]
Further, in JP-A-7-102957, in order to improve the purification performance of the latter-stage oxidation / three-way catalyst, air is supplied between the former three-way catalyst and the middle HC adsorbent, and the latter-stage oxidation / three-way catalyst is improved. A system that promotes the activation of three-way catalysts is proposed.
[0012]
Further, JP-A-9-85056 discloses a pre-stage catalyst containing an HC adsorbent made of zeolite and a Pt-Rh catalyst component, a post-stage catalyst containing an HC adsorbent made of zeolite, a Pd catalyst component, and cerium oxide. Has been proposed which promotes the purification of the subsequent catalyst by filling the same container with the secondary air.
[0013]
In JP-A-7-96183, a catalyst layer (Pd and Al ₂ O ₃ From the viewpoint of suppressing deterioration of the main component) at a high temperature, a porous barrier layer is provided between the adsorbent layer and the catalyst layer, and from the viewpoint of suppressing a decrease in the adsorption capacity of the lower adsorbent layer, Al ₂ O ₃ It has been proposed to use particles having an average particle size of 15 to 25 μm and refractory inorganic particles having an average particle size of 5 to 15 μm.
[0014]
Furthermore, in Japanese Patent Laid-Open No. 11-81999, in order to slow the desorption of HC trapped by the adsorbent during cold, the upstream three-way catalyst and the downstream adsorption are sufficiently delayed in order to sufficiently delay the temperature rise of the HC adsorption catalyst. There has been proposed a method in which the heat capacity of the exhaust passage is made larger than the heat capacity of the exhaust manifold by increasing the thickness of the tube in the exhaust passage connecting to the catalyst or installing a carrier.
[0015]
[Problems to be solved by the invention]
However, in the systems described in JP-A-5-59942 and JP-A-6-74019, the cold HC adsorption and desorption time varies greatly due to differences in conditions such as the vehicle type and travel pattern. It is difficult to optimally control the operation timing of the switching valve, and the timing of the HC desorption from the HC adsorbent and the activation of the subsequent three-way catalyst cannot always be matched, and the efficiency of the purification process is reduced. There is a problem that it cannot be improved sufficiently.
[0016]
On the other hand, in the systems described in JP-A-7-144119 and JP-A-6-142457, the activation of the three-way catalyst arranged at the subsequent stage of the HC adsorbent tends to be insufficiently promoted, and the purification process is performed. In the systems proposed in Japanese Patent Application Laid-Open Nos. 7-102957 and 9-85056, the subsequent oxidation / ternary process by supplying secondary air is also difficult. The catalyst activation promoting effect is naturally limited, and the improvement in purification efficiency is not necessarily sufficient.
[0017]
JP-A-7-96183 proposes suppression of deterioration of the catalyst layer by providing a barrier layer between the zeolite layer and the catalyst layer, but the effect is not necessarily sufficient. The purification efficiency of desorbed HC by the original catalyst cannot be made sufficient.
Further, in the system described in Japanese Patent Application Laid-Open No. 11-81999, the heat capacity of the exhaust passage is increased to delay the temperature rise of the adsorbent layer, so that the desorption of HC trapped in the adsorbent during cold is slow. However, at the same time, the activation start of the latter three-way catalyst is also delayed, so that the timing of desorption and purification start cannot be sufficiently matched, and the purification processing efficiency cannot be sufficiently improved. There's a problem.
[0018]
As described above, in each purification system disclosed in the above publication, HC desorption and purification start by switching valves, promoting activation of the purification catalyst by preheating or air supply, slowing HC desorption from the adsorbent, etc. Although the timing is matched and there is a certain effect, the timing of the two has not been sufficiently matched, and in either system, the purification catalyst component of the HC adsorption type three-way catalyst, Or, it cannot be said that the cold HC once adsorbed to the HC adsorbent is desorbed before the purification catalyst provided in the subsequent stage is activated, and the cold HC cannot necessarily be efficiently purified. There is a problem, which has been a problem in conventional exhaust gas purification systems.
[0019]
The present invention has been made in view of the above problems in a conventional exhaust gas purification system equipped with an HC adsorption type three-way catalyst, and delays the HC desorption from the HC adsorbent layer so as to depend on the catalyst component. It is an object of the present invention to provide an exhaust gas purification catalyst system that can sufficiently match the timing of the start of purification and can improve the purification efficiency of cold HC.
[0020]
[Means for Solving the Problems]
The exhaust gas purifying catalyst system of the present invention has a three-way catalyst disposed upstream of an exhaust gas flow path, and includes an HC adsorbent layer containing zeolite and a purifying catalyst component layer containing catalytic metal in a polygonal shape of the carrier. A system in which a plurality of HC adsorption type three-way catalysts stacked in this order in a cell are arranged on the downstream side of the three-way catalyst, wherein the number of sides of the carrier cell in the plurality of HC adsorption type three-way catalysts Is configured to increase toward the downstream side of the exhaust gas flow path.
[0021]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the exhaust gas purification catalyst system of the present invention will be described in detail.
An exhaust gas purification catalyst system of the present invention is a catalyst system comprising a three-way catalyst disposed upstream of an exhaust gas passage and a plurality of HC adsorption-type three-way catalysts disposed downstream thereof. The type three-way catalyst has a multilayer structure in which a HC adsorbent layer containing zeolite and a purification catalyst component layer containing catalyst metal are laminated in this order in a polygonal cell of the carrier, and these plural HC adsorption Since the number of sides of the support cell of the type three-way catalyst is arranged so as to increase toward the downstream side of the exhaust gas flow path, before the three-way catalyst arranged on the upstream side of the exhaust gas flow path is activated Thus, the cold HC discharged to the catalyst is shared and adsorbed by a plurality of HC adsorption type three-way catalysts arranged on the downstream side, so that the adsorption efficiency of the cold HC and the desorption delay of the adsorbed HC can be achieved.
[0022]
That is, cold HC is shared and adsorbed by two or more HC adsorption type three-way catalysts having different temperature rising profiles on the upstream side and the downstream side of the exhaust gas flow path. By adsorbing more than 50% of the cold HC emissions with the HC adsorption type three-way catalyst, the adsorption load of the rear catalyst is reduced, and the HC that has been unpurified in the previous stage is removed by the zeolite in the rear catalyst. Since re-adsorption can be performed efficiently, an improvement in the adsorption efficiency of the entire system is achieved.
Furthermore, the start of activation of the most upstream three-way catalyst is accelerated, and the amount of cold HC flowing into the HC adsorption-type three-way catalyst arranged downstream thereof is determined by the saturated adsorption capacity of the zeolite contained in the HC adsorption-type three-way catalyst. By trapping (physical adsorption) within a range of 70% or less, desirably 50% or less, the diffusion collision of HC molecules in the zeolite pores is alleviated even when the temperature rises or the gas flow rate increases. Therefore, it is considered that desorption can be delayed.
[0023]
Then, a plurality of HC adsorption type three-way catalysts arranged in the exhaust gas flow path have a polygonal cell shape (for example, a quadrangular cell) in which the carrier on which the catalyst component is applied increases from upstream to downstream. Hexagonal cells), this increases the angle of the corners, averages the slurry coating shape, especially the coating thickness of the HC adsorbent layer, which is the first layer, and adsorbs the coating form. -A shape having excellent holding characteristics. That is, the adsorption surface area increases with respect to the coating amount, and an extremely thin portion of the coating thickness is eliminated, so that the adsorption HC is not released early by rapid heating.
Therefore, the adsorption efficiency of cold HC is improved, the desorption of adsorbed HC is delayed, and the purification treatment efficiency of HC is significantly improved even on the downstream side where the temperature rise of the purification catalyst component layer is slow and difficult to activate. become.
[0024]
In addition, as a specific arrangement of the carrier cell shape in a plurality of HC adsorption type three-way catalysts arranged on the downstream side of the three-way catalyst, the cell shape of the carrier used for the HC adsorption type three-way catalyst located in the most upstream is used. A carrier having a rectangular shape and a hexagonal cell can be used as the catalyst downstream of the rectangular shape.
That is, quadrangle-hexagon (when two are arranged), quadrangle-hexagon-hexagon (when three are arranged), quadrilateral-hexagon-hexagon-hexagon (when four are arranged), and the like. However, the temperature of the downstream catalyst is lower, and even if the number of these catalysts is increased, it is often the result that only the number of catalysts that do not substantially contribute to exhaust gas purification is increased. It is desirable to arrange two HC adsorption type three-way catalysts using the above-mentioned supports in this order.
[0025]
As a preferred form in the exhaust gas purifying catalyst system of the present invention, the cell shape of the carrier to which the catalyst component of the HC adsorption type three-way catalyst is applied becomes a polygon with increased corners as it goes from upstream to downstream, It is desirable to increase the geometric surface area (GSA) of the carrier, and this improves the contact efficiency with the exhaust gas, so that the cold HC that has not been adsorbed by the preceding HC adsorption type three-way catalyst or Cold HC discharged without purification is efficiently adsorbed by the HC adsorption type three-way catalyst on the rear stage side. For this reason, the hardly adsorbing HC species flowing into the latter-stage HC adsorption type three-way catalyst are also efficiently adsorbed, and the purification efficiency of the desorbed HC by the purification catalyst layer is further improved.
[0026]
Further, as another preferred embodiment, the cell shape of the carrier on which the catalyst component is applied is changed to a polygonal shape with increased corners from the upstream side to the downstream side, and the corner portion (corner portion) of the cell and the flat portion are formed. It is desirable to reduce the difference in the application amount of the HC adsorbent layer.
That is, in an exhaust gas purification system in which a plurality of HC adsorption type three-way catalysts are arranged, generally, the exhaust composition at the catalyst outlet gradually changes from upstream to downstream, and is adsorbed in the downstream exhaust gas. Since the HC species that is difficult to detach and remains, it is desirable that the downstream side catalyst has a shape in which the application form of the HC adsorbent layer is excellent in adsorption-retention characteristics. For this reason, by reducing the difference in the amount of HC adsorbent layer applied between the corner and the flat portion of the cell as described above, the application form of the HC adsorbent layer becomes a shape with excellent adsorption-holding characteristics, and adsorbed HC The HC purification treatment efficiency can be remarkably improved even on the downstream side where the desorption of the catalyst is delayed and the temperature rise of the purification catalyst layer is slow and hardly activated.
[0027]
In another preferred embodiment of the catalyst system for exhaust gas purification according to the present invention, the cell shape of the carrier on which the catalyst component is applied in the HC adsorption type three-way catalyst as it goes from upstream to downstream of the exhaust gas flow path The shape of the HC adsorbent layer can be increased and the shape of the HC adsorbent layer is excellent in adsorption-holding characteristics. As a result, the desorption of adsorbed HC is delayed, and the purification treatment efficiency of HC is further improved even on the downstream side where the temperature rise of the purification catalyst layer is slow and difficult to activate.
[0028]
In another preferred embodiment of the exhaust gas purifying catalyst system of the present invention, platinum (Pt), palladium (Pd) and rhodium (Rh) are formed on the purifying catalyst component layer which is the surface layer of the HC adsorption type three-way catalyst. At least one metal selected from the group consisting of: alumina containing 1 to 10 mol% in terms of metal of at least one selected from the group consisting of cerium (Ce), zirconium (Zr) and lanthanum (La); Cerium oxide containing at least one selected from the group consisting of zirconium (Zr), neodymium (Nd), praseodymium (Pr), yttrium (Y) and lanthanum (La) in terms of metal As a result, the deterioration over time of the active components Pt and Pd can be suppressed, and the purification treatment efficiency of desorbed HC can be maintained high. Rutotomoni, so that high purification performance can be obtained as a three-way catalyst.
[0029]
Further, as a further preferred embodiment, the purification catalyst component layer further includes at least one selected from the group consisting of cerium (Ce), neodymium (Nd), praseodymium (Pr), yttrium (Y) and lanthanum (La). It is desirable to contain a zirconium oxide containing 1 to 40 mol% in terms of metal, which can suppress deterioration over time of Rh, which is an active component, and maintain high purification efficiency of desorbed HC. Even higher purification performance can be obtained as the original catalyst.
[0030]
As a further preferred embodiment of the exhaust gas purification catalyst system, it is desirable that the purification catalyst component layer further contains at least one metal selected from alkali metals and alkaline earth metals, thereby reducing the atmosphere. In addition to improving the purification activity of the catalyst, it is possible to suppress the deterioration of the active components with time, maintain high purification efficiency of desorbed HC, and obtain high purification performance as a three-way catalyst. In addition, by containing these alkali metals and alkaline earth metals in the purification catalyst component layer using a compound that is hardly soluble or insoluble in water, the zeolite of metal ions eluted in the slurry as the moisture in the slurry moves. Infiltration into the pores can be suppressed, and high adsorption efficiency can be maintained from the initial stage to after the endurance.
[0031]
【Example】
Hereinafter, the present invention will be described more specifically based on examples. In the examples, “%” represents mass percentage unless otherwise specified.
[0032]
(Catalyst 1)
SiO ₂ / Al ₂ O ₃ A β-zeolite powder of 2257 g with a ratio of 40, 1215 g of silica sol (solid content 20%), and 1000 g of pure water were put into a magnetic ball mill, mixed and ground to obtain a slurry liquid. This slurry solution was used as a cordierite carrier (cell shape: square, number of cells: 46.5 cells / cm ² Cell wall thickness: 0.015 cm, GSA: 24.1 cm ² / Cm ³ , Carrier volume: 1.0 L), excess slurry in the cell was removed by air flow, dried, and fired at 400 ° C. for 1 hour. And catalyst-a1 was obtained by repeating a coating operation until the coating amount of this adsorbent layer containing a zeolite became 350 g / L.
[0033]
Next, alumina powder containing 3 mol% of Ce (Al: 97 mol%) is impregnated with an aqueous palladium nitrate solution or sprayed at high speed with stirring, dried at 150 ° C. for 24 hours, then at 400 ° C. for 1 hour, and then And calcining at 600 ° C. for 1 hour to obtain Pd-supported alumina powder (powder a). The Pd concentration of this powder a was 4.0%.
After cerium oxide powder (Ce: 67 mol%) containing 1 mol% La and 32 mol% Zr is impregnated with an aqueous solution of palladium nitrate or sprayed at high speed and dried at 150 ° C for 24 hours. And calcining at 400 ° C. for 1 hour and then at 600 ° C. for 1 hour to obtain Pd-supported cerium oxide powder (powder b). The Pd concentration of this powder b was 2.0%.
Then, 400 g of the above Pd-supported alumina powder (powder a), 141 g of Pd-supported cerium oxide powder (powder b), 240 g of nitric acid acidic alumina sol (a sol obtained by adding 10% nitric acid to 10% boehmite alumina, 24 g in terms of Al2O3) and 100 g of barium carbonate (67 g as BaO) were put into a magnetic ball mill together with 2000 g of pure water, mixed and pulverized to obtain a slurry liquid. This slurry was adhered to the coat catalyst-a1, excess slurry in the cell was removed by air flow, dried, calcined at 400 ° C. for 1 hour, and a coat layer weight of 66.5 g / L was applied. -B was obtained.
[0034]
Next, an alumina powder containing 3 mol% of Zr (Al: 97 mol%) is impregnated with an aqueous rhodium nitrate solution or sprayed with high-speed stirring, dried at 150 ° C. for 24 hours, then at 400 ° C. for 1 hour, and then And calcining at 600 ° C. for 1 hour to obtain Rh-supported alumina powder (powder c). The Rh concentration of this powder c was 2.0%.
Also, an alumina powder containing 3 mol% Ce (Al: 97 mol%) is impregnated with a dinitrodiammine platinum aqueous solution or sprayed with high-speed stirring, dried at 150 ° C. for 24 hours, then at 400 ° C. for 1 hour, and then And calcining at 600 ° C. for 1 hour to obtain a Pt-supported alumina powder (powder d). The Pt concentration of this powder d was 3.0%.
Further, a zirconium oxide powder containing 1 mol% La and 20 mol% Ce is impregnated with a dinitrodiammine platinum aqueous solution or sprayed with high-speed stirring, dried at 150 ° C. for 24 hours, and then at 400 ° C. for 1 hour. Subsequently, firing was performed at 600 ° C. for 1 hour to obtain a Pt-supported alumina powder (powder e). The Pt concentration of this powder e was 3.0%.
Then, 118 g of the above Rh-supported alumina powder (powder c), 118 g of Pt-supported alumina powder (powder d), 118 g of Pt-supported zirconium oxide powder (powder e), and 160 g of nitric acid acidic alumina sol were put into a magnetic ball mill, mixed and ground. A slurry liquid was obtained. HC adsorption type in which this slurry liquid was adhered to the above-mentioned coat catalyst-b, excess slurry in the cell was removed by air flow, dried, calcined at 400 ° C. for 1 hour, and coated with a coat layer weight of 37 g / L. A three-way catalyst 1 was obtained.
The supported amount of noble metal of the catalyst 1 was Pt: 0.71 g / L, Pd: 1.88 g / L, and Rh: 0.24 g / L.
[0035]
(Catalyst 2)
As a catalyst carrier, the cell shape is square, the number of cells: 31.0 / cm ² Cell wall thickness: 0.025 cm, GSA: 19.0 cm ² / Cm ³ , Except that a carrier volume: 1.0 L of cordierite carrier was used, the same operation as in the above catalyst 1 was repeated, and an HC adsorbent layer and a purification catalyst component layer were laminated to form an HC adsorption type according to this example. Three-way catalyst 2 was obtained.
[0036]
(Catalyst 3)
As a catalyst carrier, the cell shape is hexagonal, the number of cells: 62.0 cells / cm ² Cell wall thickness: 0.011 cm, GSA: 27.2 cm ² / Cm ³ , Except that a carrier volume: 1.0 L of cordierite support was used, the same operation as in the above catalyst 1 was repeated, and an HC adsorbent layer and a purification catalyst component layer were laminated to form an HC adsorption type according to this example. Three-way catalyst 3 was obtained.
[0037]
(Catalyst 4)
As a catalyst carrier, the cell shape is hexagonal, the number of cells: 71.3 cells / cm ² Cell wall thickness: 0.011 cm, GSA: 28.4 cm ² / Cm ³ , Except that a carrier volume: 1.0 L of cordierite support was used, the same operation as in the above catalyst 1 was repeated, and an HC adsorbent layer and a purification catalyst component layer were laminated to form an HC adsorption type according to this example. A three-way catalyst 4 was obtained.
[0038]
(Catalyst 5)
As a catalyst carrier, the cell shape is hexagonal, the number of cells: 93.0 cells / cm ² Cell wall thickness: 0.008 cm, GSA: 33.4 cm ² / Cm ³ , Carrier capacity: Except for using a 1.0 L cordierite carrier, the same operation as in the above catalyst 1 is repeated, and the HC adsorbent layer and the purification catalyst component layer are laminated, and the HC adsorption type according to this example A three-way catalyst 5 was obtained.
[0039]
(Catalyst 6)
As a catalyst carrier, the cell shape is hexagonal, the number of cells: 107.0 cells / cm ² Cell wall thickness: 0.011 cm, GSA: 34.0 cm ² / Cm ³ , Except that a carrier volume: 1.0 L of cordierite support was used, the same operation as in the above catalyst 1 was repeated, and an HC adsorbent layer and a purification catalyst component layer were laminated to form an HC adsorption type according to this example. A three-way catalyst 6 was obtained.
[0040]
(Catalyst 7)
As a catalyst carrier, the cell shape is square, the number of cells: 93.0 cells / cm ² Cell wall thickness: 0.010 cm, GSA: 34.5 cm ² / Cm ³ , Carrier capacity: Except for using a 1.0 L cordierite carrier, the same operation as in the above catalyst 1 is repeated, and the HC adsorbent layer and the purification catalyst component layer are laminated, and the HC adsorption type according to this example A three-way catalyst 7 was obtained.
[0041]
(Catalyst 8)
As a catalyst carrier, the cell shape is square, the number of cells: 139.5 / cm. ² Cell wall thickness: 0.005 cm, GSA: 43.6 cm ² / Cm ³ , Except that a carrier volume: 1.0 L of cordierite carrier was used, the same operation as in the above catalyst 1 was repeated, and an HC adsorbent layer and a purification catalyst component layer were laminated, and the HC adsorption type according to this example A three-way catalyst 8 was obtained.
[0042]
(Catalyst 9)
As catalyst support, cell shape is triangular, number of cells: 19.0 cells / cm ² Cell wall thickness: 0.005 cm, GSA: 11.1 cm ² / Cm ³ , Except that a carrier volume: 1.0 L of cordierite carrier was used, the same operation as in the above catalyst 1 was repeated, and an HC adsorbent layer and a purification catalyst component layer were laminated to form an HC adsorption type according to this example. A three-way catalyst 9 was obtained.
[0043]
Table 1 shows the specifications of the HC adsorption type three-way catalysts 1 to 9 obtained as described above.
[0044]
[Table 1]

[0045]
The above-mentioned HC adsorption type three-way catalysts 1 to 9 after durability under the following conditions are selected and combined respectively and arranged at catalyst positions 1 and 2 of the exhaust gas purification catalyst system shown in FIG. HC characteristics were evaluated and HC adsorption rate and desorption HC purification rate were measured. The results are shown in Table 2.
[0046]
Endurance conditions
-Engine displacement 3000cc
・ Fuel Gasoline (Nisseki Dash)
・ Durable temperature 650 ℃
・ Durability 50 hours
Vehicle performance test
-Engine Nissan Motor Co., Ltd. Inline 4 cylinder 2.0L
・ Evaluation method LA4-CH A-bag of North American flue gas test method
[0047]
[Table 2]

[0048]
【The invention's effect】
As described above, the exhaust gas purification catalyst system of the present invention has a three-way catalyst arranged upstream of the exhaust gas flow path and a plurality of HC adsorption type three-way catalysts arranged downstream thereof. In this catalyst system, the number of sides of the carrier cells of the plurality of HC adsorption type three-way catalysts is arranged so as to increase toward the downstream side of the exhaust gas flow path. Cold HC discharged before activation is divided and adsorbed by a plurality of HC adsorption type three-way catalysts arranged on the downstream side, and particularly in the HC adsorption type three-way catalyst on the downstream side, particularly the HC adsorbent layer Since the application form of the resin has a shape excellent in adsorption-holding characteristics, it is possible to improve the adsorption efficiency of cold HC and delay the desorption of adsorbed HC, and the timing of desorption of adsorbed HC and the start of purification Will match and Excellent effect is brought about that the purification efficiency of the field HC can be greatly improved.
[Brief description of the drawings]
FIG. 1 is a schematic view showing a configuration example of an exhaust gas purifying catalyst system of the present invention.

Claims

A three-way catalyst is arranged upstream of the exhaust gas flow path, and a hydrocarbon adsorbent layer containing zeolite and a purification catalyst component layer containing catalyst metal are laminated in this order in the polygonal cell of the carrier. An exhaust gas purification catalyst system comprising a plurality of hydrocarbon adsorption type three-way catalysts arranged downstream of the three-way catalyst, wherein the number of sides of the support cells in the plurality of hydrocarbon adsorption type three-way catalysts is an exhaust gas flow An exhaust gas purification catalyst system characterized by increasing toward the downstream side of the passage.

2. The exhaust gas purifying apparatus according to claim 1, wherein the geometric surface area (GSA) of the carrier in the plurality of hydrocarbon adsorption type three-way catalysts increases toward the downstream side of the exhaust gas passage. Catalyst system.

The difference in the coating amount of the hydrocarbon adsorbent layer between the corner portion and the flat portion of the carrier cell in the plurality of hydrocarbon adsorption type three-way catalysts is reduced toward the downstream side of the exhaust gas flow path. The exhaust gas purifying catalyst system according to claim 1 or 2.

The amount of coating of the hydrocarbon adsorbent layer in the plurality of hydrocarbon adsorption type three-way catalysts decreases toward the downstream side of the exhaust gas flow path. The exhaust gas purifying catalyst system according to the item.

In the purification catalyst component layer of the hydrocarbon adsorption type three-way catalyst, at least one metal selected from the group consisting of platinum, palladium and rhodium and at least one selected from the group consisting of cerium, zirconium and lanthanum are contained. Alumina containing 1 to 10 mol% in terms of metal and cerium oxide containing 1 to 50 mol% in terms of metal of at least one selected from the group consisting of zirconium, neodymium, praseodymium, yttrium and lanthanum The exhaust gas purifying catalyst system according to any one of claims 1 to 4.

The purification catalyst component layer further contains a zirconium oxide containing 1 to 40 mol% in terms of metal of at least one selected from the group consisting of cerium, neodymium, praseodymium, yttrium and lanthanum. The exhaust gas purification catalyst system according to claim 5.

The exhaust gas purification catalyst system according to claim 5 or 6, wherein the purification catalyst component layer further contains at least one metal selected from alkali metals and alkaline earth metals.