JP3846139B2 - Exhaust gas purification catalyst and exhaust gas purification method using the same - Google Patents
Exhaust gas purification catalyst and exhaust gas purification method using the same Download PDFInfo
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- JP3846139B2 JP3846139B2 JP2000006033A JP2000006033A JP3846139B2 JP 3846139 B2 JP3846139 B2 JP 3846139B2 JP 2000006033 A JP2000006033 A JP 2000006033A JP 2000006033 A JP2000006033 A JP 2000006033A JP 3846139 B2 JP3846139 B2 JP 3846139B2
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- catalyst
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Images
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- Exhaust Gas After Treatment (AREA)
- Electrical Control Of Air Or Fuel Supplied To Internal-Combustion Engine (AREA)
- Exhaust Gas Treatment By Means Of Catalyst (AREA)
- Solid-Sorbent Or Filter-Aiding Compositions (AREA)
- Catalysts (AREA)
Description
【０００１】
【発明の属する技術分野】
本発明は排ガス中に含まれる一酸化炭素（ＣＯ）や炭化水素（ＨＣ）を酸化するのに必要な量より過剰な酸素が含まれている排ガス中の、ＮＯxを効率よく浄化できる排ガス浄化用触媒およびその触媒を用いた排ガス浄化方法に関する。
【０００２】
【従来の技術】
リーンバーンエンジンにおいて、常時は酸素過剰の燃料リーン条件で燃焼させ、間欠的に燃料ストイキ〜リッチ条件とすることにより排ガスを還元雰囲気としてＮＯxを還元浄化するシステムが開発され、実用化されている。そしてこのシステムに最適な触媒として、リーン雰囲気でＮＯxを吸蔵し、ストイキ〜リッチ雰囲気で吸蔵されたＮＯxを放出するＮＯx吸蔵材を用いたＮＯx吸蔵還元型の排ガス浄化用触媒が開発されている。
【０００３】
例えば特開平５−３１７６５２号公報には、Ｂａなどのアルカリ土類金属とＰｔをアルミナなどの多孔質酸化物担体に担持した排ガス浄化用触媒が提案されている。また特開平６−３１１３９号公報には、Ｋなどのアルカリ金属とＰｔをアルミナなどの多孔質酸化物担体に担持した排ガス浄化用触媒が提案されている。さらに特開平５−１６８８６０号公報には、Ｌａなどの希土類元素とＰｔをアルミナなどの多孔質酸化物担体に担持した排ガス浄化用触媒が提案されている。
【０００４】
このＮＯx吸蔵還元型触媒を用いれば、空燃比をリーン側からパルス状にストイキ〜リッチ側となるように制御することにより、排ガスもリーン雰囲気からパルス状にストイキ〜リッチ雰囲気となる。したがって、リーン側ではＮＯxがＮＯx吸蔵材に吸蔵され、それがストイキ又はリッチ側で放出されてＨＣやＣＯなどの還元性成分と反応して浄化されるため、リーンバーンエンジンからの排ガスであってもＮＯxを効率良く浄化することができる。また排ガス中のＨＣ及びＣＯは、貴金属により酸化されるとともにＮＯxの還元にも消費されるので、ＨＣ及びＣＯも効率よく浄化される。
【０００５】
ところが排ガス中には、燃料中に含まれる硫黄（Ｓ）が燃焼して生成したＳＯxが含まれ、それがリーン雰囲気の排ガス中で貴金属により酸化されて例えばＳＯ3となる。そしてそれがやはり排ガス中に含まれる水蒸気により容易に硫酸となり、これらがＮＯx吸蔵材と反応して亜硫酸塩や硫酸塩が生成し、これによりＮＯx吸蔵材が被毒劣化することが明らかとなった。つまり、このようにＮＯx吸蔵材が亜硫酸塩や硫酸塩となって被毒劣化すると、もはやＮＯxを吸蔵することができなくなり、耐久後のＮＯx浄化性能が低下するという問題である。
【０００６】
【発明が解決しようとする課題】
近年の大気汚染の現状と排ガス規制の強化に鑑みると、従来のＮＯx吸蔵還元型の排ガス浄化用触媒では、耐久後のＮＯx浄化性能が充分ではなく、そのため初期から耐久後まで高いＮＯx浄化性能を示す排ガス浄化用触媒の早期開発が望まれている。
【０００７】
本発明はこのような事情に鑑みてなされたものであり、ＮＯx吸蔵材の硫黄被毒をさらに抑制して、ＮＯx吸蔵還元型の排ガス浄化用触媒の耐久性を一層向上させることを目的とする。
【０００８】
本発明の排ガス浄化用触媒は、多孔質酸化物からなる担体と、該担体に担持された貴金属と、該担体に担持された第１ＮＯｘ吸蔵材とを含んでなり、排ガスの上流側に位置する第１触媒と、多孔質酸化物からなる担体と、該担体に担持された貴金属と、該担体に担持された第２ＮＯｘ吸蔵材とを含んでなり、排ガスの下流側に位置する第２触媒と、からなるＮＯｘ吸蔵還元型の排ガス浄化用触媒であって、前記第１ＮＯｘ吸蔵材はＢａを含み、前記第２ＮＯｘ吸蔵材はＫを含み、該第１ＮＯｘ吸蔵材は該第２ＮＯｘ吸蔵材よりも、硫黄吸着しやすくかつ硫黄離脱しにくい特性を持つことを特徴とする。
【０００９】
一般に、ＮＯx吸蔵還元型触媒において、硫黄被毒は、排ガスの上流側に位置する部分に集中する傾向があり、特にモノリス型触媒（例えば、ハニカム基材に担体をコートし、ハニカム体として形成した触媒）においてはその傾向が強い。アルカリ金属、アルカリ土類金属、希土類元素をＮＯx吸蔵材として用いる場合には、このＮＯx吸蔵材へのＳＯxの吸着は不可避的なものであるが、吸着されたＳＯxは燃料ストイキ〜リッチ雰囲気とすることによって離脱が可能である。ところが、一旦離脱させた場合であっても、下流側で再びＮＯx吸蔵材に吸着するといった現象をも生じ得る。
【００１０】
上記本発明のＮＯx吸蔵還元型触媒では、上流側、下流側の部分に分け、２種類の硫黄吸着・離脱特性（ＳＯxの吸着しやすさおよび離脱のしやすさ）の異なるＮＯx吸蔵材をそれぞれに配している。つまり、より硫黄吸着しやすくかつ硫黄離脱しにくい特性を持つＮＯx吸蔵材を上流側に配することで、燃料リーン雰囲気において優先的に上流側に存在する触媒の部分にＳＯxを吸着させ、間欠的に生じる燃料ストイキ〜リッチ雰囲気において、吸着したＳＯxを離脱還元させるものである。一方、下流側の触媒の部分では、硫黄吸着しにくいＮＯx吸蔵材を配していることから、このストイキ〜リッチ雰囲気において離脱還元されたＳＯxは、下流側の触媒の部分には吸着せずに排気されることとなる。このような作用により、本発明の排ガス浄化用触媒では、硫黄被毒が抑制され耐久性が向上させられる。
【００１１】
なお、ＳＯxとＮＯx吸蔵材との反応は、ＮＯx吸蔵材に吸着したＳＯxが、排ガス中に含まれる水蒸気により容易に亜硫酸、硫酸等となり、これらがＮＯx吸蔵材と反応して亜硫酸塩、硫酸塩等が生成しするように進行するものと考えられる。したがって、「吸着」という語は、単に吸着するのみではなく、吸着後の上記反応をも含むことを意味する。また、「離脱」とは、上記反応が逆に進行することをも含むことを意味する。
【００１２】
さらに、本発明の排ガス浄化方法は、上記本発明の排ガス浄化用触媒を用い、この排ガス浄化用触媒を、間欠的に燃料ストイキ〜リッチ雰囲気となるリーンバーンエンジンからの排ガスに接触させ、燃料リーン雰囲気で該排ガス中に含まれるＮＯxを該第１ＮＯx吸蔵材および該第２ＮＯx吸蔵材に吸蔵させ、燃料ストイキ〜リッチ雰囲気で該第１ＮＯx吸蔵材および該第２ＮＯx吸蔵材から放出されたＮＯxを還元させることを特徴とする。
【００１３】
上記本発明の排ガス浄化用触媒を用い、間欠的に燃料ストイキ〜リッチ雰囲気をつくりだすことで、本発明の排ガス浄化方法は、硫黄被毒による劣化を抑制しつつ、リーンバーンエンジンの排ガスから効率的にＮＯxを浄化させることができる。
【００１４】
【発明の実施の形態】
以下に、本発明の実施の形態について詳細に説明する。
【００１５】
本発明の排ガス浄化用触媒は、排ガスの上流側に位置する第１触媒と、排ガスの下流に位置する第２触媒とからなる。この第１触媒と第２触媒とは、一体に形成されたものであってもよく、また、それぞれ分離して形成されたものであってもよい。つまり、モノリス型の触媒の場合には、１つのモノリス（ハニカム体として形成された触媒）の上流部分と下流部分のそれぞれに異なるＮＯx吸蔵材を担持させて、１つの触媒コンバータに組み込む態様のものでもよく、また、異なるＮＯx吸蔵材を担持させた２つのモノリスを用い、連結して若しくは適当な間隔を隔てて１つの触媒コンバータに組み込む態様のものであってもよい。さらにまた、第１触媒と第２触媒とを別々の触媒コンバータに組み込んで、この２つの触媒コンバータを排ガス流路中に直列に配置させるものであってもよい。ペレット型の触媒の場合は、第１触媒となる複数のペレットと第２触媒となる複数のペレットとをそれぞれ群体とし、両ペレット群を直列に１つの触媒コンバータに組み込む態様のものでもよく、また、それぞれのペレット群を別々の触媒コンバータに組み込んで、この２つの触媒コンバータを排ガス流路中に直列に配置させるものであってもよい。なお、第１触媒または第２触媒のいずれか一方をモノリス型、他方をペレット型にすることも可能である。
【００１６】
第１触媒および第２触媒とも、多孔質酸化物からなる担体と、該担体に担持された貴金属と、該担体に担持されたアルカリ金属、アルカリ土類金属および希土類元素から選ばれる少なくとも１種のＮＯx吸蔵材とを含んで構成される。以下に、これらの構成要素について説明する。
【００１７】
貴金属およびＮＯx吸蔵材を担持する担体は、多孔質酸化物からなり、用いることのできる多孔質酸化物としては、例えば、アルミナ（Ａｌ2Ｏ3）、ゼオライト、シリカ（ＳｉＯ2）、ジルコニア（ＺｒＯ2）、チタニア（ＴｉＯ2）等や、これらを複合させたＴｉＯ2−Ａｌ2Ｏ3、ＳｉＯ2−Ａｌ2Ｏ3、ＺｒＯ2−Ａｌ2Ｏ3等を挙げることができる。また、浄化性能の向上を目的として、セリア（ＣｅＯ2）や、ジルコニアで安定させたセリア（セリア−ジルコニア複合酸化物：ＣｅＯ2−ＺｒＯ2）を添加するものであってもよい。モノリス型の触媒を形成させる場合は、これらの酸化物をスラリー状にして、コージェライト等のセラミックス、耐熱合金等からなるハニカム基材にコートさせて担体とすればよく、また、ペレット型の触媒の場合には、これらの酸化物を所定の大きさの粒状に焼結させて担体とすればよい。
【００１８】
担体に担持させる貴金属は、主に、ＣＯ、ＨＣを酸化させるのと同時に、排ガス中のＮＯxの主成分たるＮＯを酸化させてＮＯ2として、ＮＯx吸蔵材に吸蔵させやすくする役割りを果たす。用いることのできる貴金属としては、Ｐｔ、Ｒｈ、Ｐｄ、Ｉｒ、Ｒｕ等を挙げることができる。触媒の特性に応じ、これらの内の１種または２種以上を、選択的に担持させればよい。担持の方法については特に限定するものでなく、用いる貴金属に応じた既に公知の方法にて行えばよい。上記貴金属の担持量は、例えばモノリス型の触媒を形成させるのであれば、担体体積１リットル当たりに、Ｐｔ、Ｒｄの場合は０．１〜２０ｇとするが好ましく、０．５〜１０ｇとするのがより好ましい。また、Ｒｈの場合は、０．０１〜１０ｇとするのが好ましく、０．０５〜５ｇとするのがより好ましい。
【００１９】
担体に担持させるＮＯx吸蔵材は、燃料リーン雰囲気下でＮＯxを吸蔵し、燃料ストイキ〜リッチ雰囲気下でＮＯxを放出させる役割りを果たし、アルカリ金属、アルカリ土類金属および希土類元素から選ばれる少なくとも１種以上を用いることができる。アルカリ金属としてはＬｉ、Ｎａ、Ｋ、Ｒｂ、Ｃｓが、アルカリ土類金属としてはＢｅ、Ｍｇ、Ｃａ、Ｓｒ、Ｂａが、希土類元素としてはＳｃ、Ｙ、Ｌａ、Ｃｅ、Ｐｒ、Ｎｄ、Ｄｙ、Ｙｂ等がそれぞれ例示できる。
【００２０】
本発明の排ガス浄化用触媒においては、排ガスの上流側に位置する第１触媒と、下流側に位置する第２触媒とで、硫黄吸着・離脱特性の異なるＮＯx吸蔵材を担持させる。つまり第１触媒には、ＳＯxが吸着しやすくかつ離脱しにくい特性を持つ第１のＮＯx吸蔵材を担持させ、第２触媒には、ＳＯxが吸着しにくくかつ離脱しやすい特性を持つ第２のＮＯx吸蔵材を担持させる。なお、第１触媒または第２触媒のそれぞれに２種以上のＮＯx吸蔵材を担持させることもできることから、この「第１ＮＯx吸蔵材」および「第２ＮＯx吸蔵材」とは、それぞれ第１触媒および第２触媒に担持されるＮＯx吸蔵材の総称であることを意味する。したがって、２種以上のＮＯx吸蔵材を担持させた場合、第１ＮＯx吸蔵材を構成する個々のＮＯx吸蔵材のすべてが、第２ＮＯx吸蔵材を構成する個々のＮＯx吸蔵材より、ＳＯxが吸着しやすくかつ離脱しにくい特性を持つ必要はなく、第１ＮＯx吸蔵材および第２ＮＯx吸蔵材それぞれの総合特性として、第１ＮＯx吸蔵材が第２ＮＯx吸蔵材よりもＳＯxが吸着しやすくかつ離脱しにくい特性を有していればよい。
【００２１】
ＮＯx吸蔵材は、アルカリ度が高くＮＯx吸蔵能力に優れるという点を考慮すれば、アルカリ金属、アルカリ土類金属の中から選択するのが望ましい。ＳＯxが吸着しやすくかつ離脱しにくい特性という点では、アルカリ土類金属がアルカリ金属に比較して優れている。このことを考慮すれば、第１ＮＯx吸蔵材にアルカリ土類金属を含ませ、かつ、第２ＮＯx吸蔵材にアルカリ金属を含ませることが望ましい。さらに、１モル当たりのＮＯx、ＳＯx吸着量が多いという理由から、第１ＮＯx吸蔵材に含ませるアルカリ土類金属としては、Ｂａを用いるのが望ましく、また、硫酸塩の分解が非常に容易であるという理由から、第２ＮＯx吸蔵材に含ませるアルカリ金属としては、Ｋを用いるのが望ましい。
【００２２】
また、第１ＮＯx吸蔵材にＢａを用い、第２ＮＯx吸蔵材にＫを用いた場合、ＢａのＳＯx離脱能は約６５０℃にＫのＳＯx離脱能は約６００℃にそれぞれピークをもつことから、エンジン制御により高温ストイキ〜リッチ雰囲気を作り出すことによって、上流側の第１触媒と下流側の第２触媒の温度差から、両触媒ともＳＯx離脱能の大きい温度域を使用できることになり、より一層硫黄被毒が抑制された触媒を構成できるという利点をも有する。
【００２３】
さらに、第１ＮＯx吸蔵材にＢａを用い、第２ＮＯx吸蔵材にＫを用いた場合、ＢａのＮＯx吸蔵能力は約２７０℃の温度条件においてそのピークを示し、ＫのＮＯx吸蔵能力は約４００℃の温度条件においてそのピークを示すことから、触媒全体としてより幅広い温度域において高いＮＯx吸蔵能力が確保できるという利点もある。
【００２４】
なお、第２ＮＯx吸蔵材にＫを用いた場合、高温領域において、このＫはＴｉＯ2と反応しやすい。このことを考慮すれば、第２触媒を構成する担体は、ＺｒＯ2、Ａｌ2Ｏ3、ＭｇＡｌ2Ｏ4、ＣｅＯ2−ＺｒＯ2複合酸化物等のＫと反応しにくい酸化物を単独であるいはこれらの２種以上を混合して構成することが望ましい。このような担体を用いることで、硫黄被毒を受け難い触媒としつつ、ＮＯｘ吸蔵能力をより向上させることができる。
【００２５】
第１触媒および第２触媒における第１ＮＯx吸蔵材および第２ＮＯx吸蔵材の担持方法は、特に限定されるものではなく、担持されるそれぞれのＮＯx吸蔵材に応じた既に公知の方法にて行えばよい。ＮＯx吸蔵材の担持量は、例えばモノリス型触媒の場合、第１触媒、第２触媒とも、担体体積１リットルに対して０．０５〜１モルの範囲とするのが望ましい。担持量が０．０５モルより少ないと、ＮＯx吸蔵能力が小さくＮＯx浄化性能が低下し、担持量が１モルを超える場合は、効果が飽和することに加え、他成分量の低下による不具合が生じる可能性があるからである。
【００２６】
本発明の排ガス浄化触媒をモノリス型触媒として（ハニカム体として）形成する場合、第１触媒および第２触媒はそれぞれの別体となるハニカム体として形成され、その第１触媒と第２触媒とを間隔を隔てて配置することが望ましい。モノリス型触媒を排ガス経路にコンバータとして設置する場合、コンバータ内に１つのハニカム体を設置するときは、ハニカム体の中心部分のみ排ガス流量が多く、その周辺部のガス流量が減少し、周辺部の貴金属およびＮＯx吸蔵材の利用率が低下するという現象が起こり得る。そこで、第１触媒と第２触媒とを別々のハニカム体に形成し、その間隔を隔てて設置すれば、その間隔部において排ガスが乱流となり、背圧が大きくなって排ガスのコンバータ内の通過抵抗が増加することで、排ガスが均一に分散して触媒内を通過することになり、特に第２触媒側では排ガスにその第２触媒に排ガスが接触する時間が長くなる。このことにより、触媒全体のＮＯx吸蔵量がより増加するすることとなる。また、２つのハニカム体の間に間隔を設け触媒を分断することにより、担体１個当たりの体積が減少するため担体の熱容量が下がる。このため、触媒は暖気性に優れるので入りガス温度が低い場合（低温時）でも触媒活性が上昇し、ＮＯx吸蔵量がより増加するすることとなる。
【００２７】
上記第１触媒と第２触媒との間隔は、５ｍｍ以上２５ｍｍ以下とすることが望ましい。この間隔の好適範囲は、後に示す実験により確かめられたものであり、この範囲において、本発明の排ガス浄化触媒は、よりＮＯx吸蔵能力に優れた排ガス浄化触媒となる。
【００２８】
また、第１触媒と第２触媒とを別体のハニカム体とし、間隔を設けて配置する場合において、下流側の第２触媒を上流側の第１触媒のハニカム体より高セルの状態に形成することが望ましい。ハニカム体は、排ガスが通過する多数の導通孔（導通路）を有し、高セルの状態とは、断面において、単位面積当たりの導通孔の数が多いことを意味する。言い換えれば、高セルの状態とは、より細い導通路がよりたくさん存在する状態を意味する。
【００２９】
第２触媒を第１触媒に比べて高セル化すれば、上述した第１触媒と第２触媒との間隔部における排ガスの圧力（背圧）がより上昇し、上記の排ガスが均一に分散して触媒内を通過するという作用、特に第２触媒側では排ガスにその第２触媒に排ガスが接触する時間が長くなるという作用がより大きなものとなる。このことにより、触媒全体のＮＯx吸蔵量がさらに増加するすることとなる。
【００３０】
次に、上記本発明の排ガス浄化用触媒を使用した本発明の排ガス浄化方法について説明する。本発明の排ガス浄化方法は、リーンバーンエンジンからの排ガスを浄化するもので、排ガス流路内に第１触媒が上流側に第２触媒が下流側に位置するように上記排ガス浄化触媒を配置し、間欠的に燃料ストイキ〜リッチ雰囲気となるリーンバーンエンジンからの排ガスに接触させ、燃料リーン雰囲気で該排ガス中に含まれるＮＯxを該第１ＮＯx吸蔵材および該第２ＮＯx吸蔵材に吸蔵させ、燃料ストイキ〜リッチ雰囲気で該第１ＮＯx吸蔵材および該第２ＮＯx吸蔵材から放出されたＮＯxを還元させることを特徴とする。
【００３１】
燃料リーン雰囲気では、排ガス中に含まれるＮＯが触媒上で酸化され、ＮＯ2をはじめとするＮＯxとなり、それがＮＯx吸蔵材に吸蔵されていく。そして間欠的に燃料ストイキ〜リッチ雰囲気とされると、今度は吸蔵されたＮＯxが放出され、それが触媒上でＨＣやＣＯと反応し還元される。
【００３２】
排ガス中に含まれるＳＯxは、主成分をＳＯ2とするもので、このＳＯ2は、ＮＯと同様に、触媒上で酸化されＳＯ3を主成分とするものになる。ＳＯ3は排ガス中のＨ2Ｏと容易に反応して硫酸に変化し、ＮＯx吸蔵材に容易に吸着するものとなる。吸着の方法は、ＮＯx吸蔵材がＢａである場合には、例えばＢａＳＯ4という形になるものと考えられる。ここで、第１ＮＯx吸蔵材はＳＯxの吸着能力が高くかつ離脱性能に劣るであることから、ＳＯxが吸着された第１ＮＯx吸蔵材に吸着し、第１触媒に蓄積される。次いで間欠的に燃料ストイキ〜リッチ雰囲気とされると、今度は吸着していたＳＯxは放出され、それが触媒上でＨＣやＣＯと反応し還元され、ＳＯ2となって下流に向かうこととなる。
【００３３】
本発明の排ガス浄化用触媒では、下流側の第２触媒が有する第２ＮＯx吸蔵材にＳＯxの吸着能力に劣りかつ離脱性能に優るものを用いているため、第１触媒上で放出されたＳＯxは、この第２ＮＯx吸蔵材に吸着することなく排気されることとなる。したがって、上記排ガス浄化用触媒を用いた本発明の排ガス浄化方法では、触媒の硫黄被毒を抑制しつつ、効率的にＮＯxを吸蔵できるため、耐久後であっても、ＮＯx浄化性能が劣化しない排ガス浄化方法となる。
【００３４】
以上、本発明の排ガス浄化用触媒およびそれを用いた本発明の排ガス浄化方法の実施形態について説明したが、上述した実施形態は一実施形態にすぎず、本発明の排ガス浄化用触媒およびそれを用いた本発明の排ガス浄化方法は、上記実施形態を始めとして、当業者の知識に基づいて種々の変更、改良を施した形態で実施することができる。
【００３５】
【実施例】
上記実施形態に基づいて、本発明の排ガス浄化用触媒の一態様である排ガス浄化用触媒を、実施例として作製した。そして、第１触媒と第２触媒との位置関係を逆転させた排ガス浄化用触媒と、触媒全体に均一なＮＯx吸蔵材を担持させた排ガス浄化用触媒とを、比較例として作製し、実施例、比較例の排ガス浄化用触媒に対して耐久促進試験を行い、それぞれの排ガス浄化用触媒の試験後のＮＯx吸蔵量を比較することで、本発明の排ガス浄化用触媒が耐硫黄被毒特性に優れていることを確認した。
【００３６】
さらに、本発明の排ガス浄化用触媒をモノリス型の触媒とした場合において、第１触媒と第２触媒との間隔、第２触媒の担体を構成する多孔質酸化物、第２触媒を構成するハニカム基材のセル数（導通孔数）の変更によるＮＯx吸蔵量への影響を調査すべく、種々の排ガス浄化用触媒を実施例として作製し、これらの触媒に対しても耐久促進試験を行い、それぞれの排ガス浄化用触媒の試験後のＮＯx吸蔵量を比較した。以下、これらついて説明する。
【００３７】
［実施例１］
〈実施例１の排ガス浄化用触媒〉
本排ガス浄化用触媒の第１触媒は、以下のように作製した。
【００３８】
ジルコニア粉末に硝酸ロジウム溶液を用いて担持させ、これを乾燥、焼成してＲｈを担持したジルコニア粉末を得た。Ｒｈの担持量は、Ｒｈ担持ジルコニア粉末を１００ｗｔ％とした場合の０．４２ｗｔ％とした。次いでこのＲｈ担持ジルコニア粉末に、アルミナとチタニアとの混合粉末を混合させた。混合比率は、アルミナ−チタニア混合粉末においてアルミナ：チタニアを重量比で１：１とし、アルミナ−チタニア混合粉末：Ｒｈ担持ジルコニア粉末を重量比で４：１とした。この混合粉末にさらにセリアジルコニア複合酸化物を、アルミナ−チタニア混合粉末：セリアジルコニア複合酸化物が重量比で１０：１となるような割合で、混合した。この粉末を所定の方法でスラリー化し、容量０．６５リットルのセラミック製ハニカムの担体基材にコートし、モノリス担体前駆体を作製した。コート量はモノリス担体前駆体１リットル当たり２７０ｇとした。そして、このモノリス担体前駆体を２５０℃で１５分間乾燥させた後、５００℃で３０分間焼成し、モノリス担体を完成させた。
【００３９】
このモノリス担体を、所定濃度の酢酸バリウム水溶液に浸漬させ、Ｂａを担持させ、２５０℃で１５分間乾燥させた後、５００℃で３０分間焼成した。Ｂａの担持量は、モノリス担体１リットル当たり０．３モルとした。次いで、これを濃度１５ｇ／リットルの重炭酸アンモニウム水溶液に１５分間浸漬した後、１５分間乾燥させた。次にこれにジニトロジアミン白金の硝酸溶液を用いて、Ｐｔを担持させ、３００℃で１５分乾燥、焼成した。Ｐｔの担持量は、モノリス担体１リットル当たり２ｇとした。最後に所定濃度の硝酸リチウム水溶液に浸漬させてリチウムを担持させ、２５０℃で１５分間乾燥した後、５００℃で３０分間焼成して第１触媒を完成させた。Ｌｉの担持量は、モノリス担体１リットル当たり０．１モルとした。なお、本第１触媒は、主たるＮＯx吸蔵材として、Ｂａを担持させたものである。
【００４０】
本排ガス浄化用触媒の第２触媒は、以下のように作製した。
【００４１】
上記第１触媒の作製におけるモノリス担体の完成までは同じ方法で行った。したがって、モノリス担体は上記第１触媒と同じものを使用することとなる。このモノリス担体に、ジニトロジアミン白金の硝酸溶液を用いて、Ｐｔを担持させ、３００℃で１５分乾燥、焼成した。Ｐｔの担持量は、モノリス担体１リットル当たり２ｇとした。次に、これを所定濃度の酢酸カリウムと硝酸リチウムとの混合水溶液に浸漬させてＫおよびＬｉを担持させ、２５０℃で１５分間乾燥した後、５００℃で３０分間焼成して第２触媒を完成させた。ＫおよびＬｉの担持量は、モノリス担体１リットル当たり、Ｋを０．３モル、Ｌｉを０．１モルとした。なお、本第２触媒は、主たるＮＯx吸蔵材として、Ｋを担持させたものである。
【００４２】
上記第１触媒を排ガスの上流側に、上記第２触媒を下流側に位置するように触媒コンバータに組み込むことで実施例１の排ガス浄化用触媒を完成させた。
【００４３】
〈比較例１の排ガス浄化用触媒〉
上記実施例１で作製した第１触媒を排ガスの下流側に、第２触媒を上流側に位置するように触媒コンバータに組み込むようにして完成させた排ガス浄化用触媒である。実施例の排ガス浄化用触媒と異なり、下流側に担持させたＮＯx吸蔵材が、上流側に担持させたＮＯx吸蔵材よりも、硫黄吸着しやすくかつ離脱しにくいものとなっている。
【００４４】
〈比較例２の排ガス浄化用触媒〉
触媒の全域にわたって、均一な硫黄吸着・離脱特性を持つようにＮＯx吸蔵材を担持させた排ガス浄化用触媒である。この触媒は以下のように作製した。
【００４５】
担体基材にコートするスラリーの製造までは、上記実施例１の場合と同様とした。このスラリーを、容量１．３リットルのセラミック製ハニカムの担体基材にコートし、モノリス担体前駆体を作製した。コート量はモノリス担体前駆体１リットル当たり２７０ｇとした。そして、このモノリス担体前駆体を２５０℃で１５分間乾燥させた後、５００℃で３０分間焼成し、モノリス担体を完成させた。
【００４６】
このモノリス担体を、所定濃度の酢酸バリウム水溶液に浸漬させ、Ｂａを担持させ、２５０℃で１５分間乾燥させた後、５００℃で３０分間焼成した。Ｂａの担持量は、モノリス担体１リットル当たり０．１５モルとした。次いで、これを濃度１．５ｇ／リットルの重炭酸アンモニウム水溶液に１５分間浸漬した後、１５分間乾燥させた。次にこれにジニトロジアミン白金の硝酸溶液を用いて、Ｐｔを担持させ、３００℃で１５分乾燥、焼成した。Ｐｔの担持量は、モノリス担体１リットル当たり２ｇとした。最後に、これを所定濃度の酢酸カリウムと硝酸リチウムとの混合水溶液に浸漬させてＫおよびＬｉを担持させ、２５０℃で１５分間乾燥した後、５００℃で３０分間焼成した。ＫおよびＬｉの担持量は、モノリス担体１リットル当たり、Ｋを０．１５モル、Ｌｉを０．１モルとした。これを、触媒コンバータに組み込むことで、比較例２の排ガス浄化用触媒を完成させた。
【００４７】
〈耐硫黄被毒性の評価〉
実施例１、比較例１および比較例２のそれぞれの排ガス浄化用触媒を組み込んだ触媒コンバータを、１．８リットルのリーンバーンエンジンに取り付け、市街走行を模擬したパターンで、高硫黄濃度（硫黄＝２００ｐｐｍ）燃料を用い促進耐久試験を５０時間行った。その後、触媒への入りガス温度が３００℃〜４８０℃におけるそれぞれの排ガス浄化触媒の触媒１個当たりのＮＯx吸蔵量を測定した。測定結果を、図１に示す。
【００４８】
図１から明らかなように、実施例１の排ガス浄化用触媒は、３００〜４８０℃の温度域のすべてにおいて、２つの比較例の排ガス浄化用触媒より、耐久試験後のＮＯx吸蔵量が大きいことが判る。特に、高温側において、良好な結果を示していることが判る。したがって、本発明の排ガス浄化用触媒は、耐硫黄被毒特性に優れた触媒であることが確認できる。
【００４９】
［実施例２］
〈第１触媒および第２触媒の製造〉
上記実施例１の場合と同様の方法により、第１触媒を作製した。なおこのモノリス型第１触媒は、円筒形のモノリス担体から構成されているが、このモノリス担体は、直径１０３ｍｍφ、長さ７５ｍｍであり、ハニカム体壁面の厚さは６ミル（１ミルは約２５μｍ）、セル数（導通孔（導通路）の数）は約４００セルのものとした。
【００５０】
上記実施例１の場合と同様の方法により、第２触媒を作製した。なおこのモノリス型第１触媒は、円筒形のモノリス担体から構成されいるが、このモノリス担体も、上記第１触媒のモノリス担体と同様、直径１０３ｍｍφ、長さ７５ｍｍであり、ハニカム体壁面の厚さは６ミル、セル数は約４００セルのものである。そして、この第２触媒をモノリス(１)とした。
【００５１】
次に、第２触媒の担体を構成する多孔質酸化物の種類を変更し、種々の第２触媒を作製した。モノリス(１)の製造方法において（実施例１の場合の第２触媒の製造方法において）、チタニア粉末をジルコニア粉末に置換えたて作製した第２触媒をモノリス(２)と、以下同様に、それぞれ、チタニア粉末をアルミナ粉末に置換えたものをモノリス(３)と、チタニア粉末をジルコニア粉末とアルミナ粉末との重量比１：１の混合粉末に置換えたものをモノリス(４)と、チタニア粉末をスピネル（ＭｇＡｌ2Ｏ4）粉末に置換えたものをモノリス(５)と、チタニア粉末をスピネル粉末とジルコニア粉末との重量比１：１の混合粉末に置換えたものをモノリス(６)とした。
【００５２】
次に、モノリス担体のセル数を変更して、２種の第２触媒を作製した。上記モノリス(１)の製造において、ハニカム体壁面の厚さが３ミルで、６００セルのモノリス担体を用いた第２触媒をモノリス(７)とし、ハニカム体壁面の厚さが２ミルで、９００セルのモノリス担体を用いた第２触媒をモノリス(８)とした。
【００５３】
上記第１触媒および第２触媒である上記モノリス(１)〜モノリス(８)の仕様について、下記表１にまとめて示す。
【００５４】
【表１】
上記第１触媒を上流側に、モノリス(１)〜モノリス(８)から選ばれる第２触媒の一つを下流側に配置して、種々の実施例２の排ガス浄化用触媒を作製した。まず、上記第１触媒とモノリス(１)とを組み合わせ、両者の間に間隔を設けずに両者を配置した排ガス浄化用触媒を実施例２−１の排ガス浄化用触媒とし、同様にそれぞれ、両者の間に５ｍｍの間隔を設けて両者を配置したものを実施例２−２の、１０ｍｍの間隔を設けたものを実施例２−３の、１５ｍｍの間隔を設けたものを実施例２−４の、２０ｍｍの間隔を設けたものを実施例２−５の、２５ｍｍの間隔を設けたものを実施例２−６の、３０ｍｍの間隔を設けたものを実施例２−７の排ガス浄化用触媒とした。
【００５６】
次いで、上記第１触媒を上流側に、担体を構成する多孔質酸化物を種々に変更した第２触媒であるモノリス(２)〜モノリス(６)のいずれかを、第１触媒との間隔を設けずに下流側に、それぞれ配置した排ガス浄化用触媒を作製した。そして、モノリス(２)を用いた排ガス浄化用触媒を実施例２−８の排ガス浄化用触媒とし、同様にそれぞれ、モノリス(３)を用いたものを実施例２−９の、モノリス(４)を用いたものを実施例２−１０の、モノリス(５)を用いたものを実施例２−１１の、モノリス(６)を用いたものを実施例２−１２の排ガス浄化用触媒とした。
【００５７】
また、上記第１触媒を上流側に、その第１触媒とセル数の異なる第２触媒であるモノリス(７)、モノリス(８)のいずれかを、第１触媒との間隔を設けずに下流側に、それぞれ配置した排ガス浄化用触媒を作製した。そして、モノリス(７)を用いた排ガス浄化用触媒を実施例２−１３の排ガス浄化用触媒とし、モノリス(８)を用いたものを実施例２−１４の排ガス浄化用触媒とした。
【００５８】
さらに、上記第１触媒を上流側に、担体を構成する多孔質酸化物を変更した第２触媒であるモノリス(４)を下流側に用いた排ガス浄化用触媒においても、第１触媒と第２触媒の間隔の影響を確認すべく、両者の間に５ｍｍの間隔を設けて両者を配置したものを作製し、これを実施例２−１５の排ガス浄化用触媒とした。同様に、１０ｍｍの間隔を設けたものを実施例２−１６の、１５ｍｍの間隔を設けたものを実施例２−１７の、２０ｍｍの間隔を設けたものを実施例２−１８の、２５ｍｍの間隔を設けたものを実施例２−１９の、３０ｍｍの間隔を設けたものを実施例２−２０の排ガス浄化用触媒とした。
【００５９】
さらにまた、上記第１触媒を上流側に、セル数の異なる第２触媒であるモノリス(７)を下流側に用いた排ガス浄化用触媒においても、第１触媒と第２触媒の間隔の影響を確認すべく、両者の間に５ｍｍの間隔を設けて両者を配置したものを作製し、これを実施例２−２１の排ガス浄化用触媒とした。同様に、１０ｍｍの間隔を設けたものを実施例２−２２の、１５ｍｍの間隔を設けたものを実施例２−２３の、２０ｍｍの間隔を設けたものを実施例２−２４の、２５ｍｍの間隔を設けたものを実施例２−２５の、３０ｍｍの間隔を設けたものを実施例２−２６の排ガス浄化用触媒とした。
【００６０】
上記実施例２−１〜実施例２−２２の排ガス浄化用触媒の仕様について、下記表２にまとめて示す。
【００６１】
【表２】
〈ＮＯx吸蔵能力の評価〉
上記実施例２のそれぞれの排ガス浄化用触媒を組み込んだ触媒コンバータを、上記実施例１の場合に行ったのと、同一の条件で促進耐久試験を行い、触媒への入りガス温度が４００℃におけるそれぞれの排ガス浄化用触媒のＮＯx吸蔵量を測定した。なお、実施例２−１の排ガス浄化用触媒は、実施例１の排ガス浄化用触媒とその構成が近似しており、ＮＯx吸蔵量の絶対値は把握できている。そのため、ここからのＮＯx吸蔵量については、任意単位（arbitrary unit：a.u）で表すものとする。
【００６３】
まず、第１触媒と第２触媒と間隔の大きさとＮＯx吸蔵量との関係について評価すべく、実施例２−１〜実施例２−７の排ガス浄化用触媒のＮＯx吸蔵量をグラフにして図２に示す。図２から明らかなように、第１触媒と第２触媒との間にある程度の間隔を設けて両者を配置することにより、両者に間隔を設けずに配置する場合と比較して、ＮＯx吸蔵量が増加することが確認できる。また、図２から、本実施例の排ガス浄化用触媒においては、その間隔を５ｍｍ以上２５ｍｍ以下とすることで、より大きなＮＯx吸蔵量の増加効果が得られることも確認できる。
【００６４】
次いで、第２触媒の担体を構成する多孔質酸化物の種類とＮＯx吸蔵量との関係について評価すべく、実施例２−１および実施例２−８〜実施例２−１２の排ガス浄化用触媒のＮＯx吸蔵量をグラフにして図３に示す。図３から判るように、実施例２−１の排ガス浄化用触媒に比較して、実施例２−８〜実施例２−１２の排ガス浄化用触媒は大きなＮＯx吸蔵能力を有する。実施例２−１の排ガス浄化用触媒では、第２触媒の担体を構成する酸化物としてチタニアを含み、これがＮＯx吸蔵材であるＫとの反応性に富むことより、ＮＯx吸蔵能力が若干低下するものと考えられる。したがって、第２ＮＯx吸蔵材としてＫを用いた場合、第２触媒の担体はチタニアを含まずに構成することが望ましいことが確認できる。
【００６５】
さらに、第２触媒のセル数とＮＯx吸蔵量との関係について評価すべく、実施例２−１および実施例２−１３〜実施例２−１４の排ガス浄化用触媒のＮＯx吸蔵量をグラフにして図４に示す。この図から判るように、第２触媒を第１触媒に比べて高セル化することにより、ＮＯx吸蔵能力が向上することが確認できる。
【００６６】
また、第２触媒の担体を構成する多孔質酸化物の種類を変更した場合または第２触媒を高セル化した場合において、第１触媒と第２触媒と間隔の大きさとＮＯx吸蔵量との関係について評価すべく、実施例２−１０および実施例２−１５〜実施例２−２０の排ガス浄化用触媒のＮＯx吸蔵量をグラフにして図５に、実施例２−１３および実施例２−２１〜実施例２−２６の排ガス浄化用触媒のＮＯx吸蔵量をグラフにして図６に、それぞれ示す。これらの図から判るように、いずれの場合も、第１触媒と第２触媒との間にある程度の間隔を設けて両者を配置することにより、両者に間隔を設けずに配置する場合と比較して、ＮＯx吸蔵量が増加することが確認できる。また、いずれの場合も、その間隔を５ｍｍ以上２５ｍｍ以下とすることで、より大きなＮＯx吸蔵量の増加効果が得られることが確認できる。
【００６７】
【発明の効果】
本発明の排ガス浄化用触媒は、ＮＯx吸蔵還元型の排ガス浄化用触媒を、排ガスの上流側に位置する第１触媒と下流側に位置する第２触媒とに分け、第１触媒に担持させるＮＯx吸蔵材を、第２触媒に担持させるＮＯx吸蔵材よりも硫黄吸着しやすくかつ硫黄離脱しにくい特性を持つように構成するものである。このような構成としたことにより、本発明の排ガス浄化用触媒は、硫黄被毒が抑制され、耐久後もＮＯx吸蔵能力の低下の少ない触媒となる。さらに、第１触媒と第２触媒とを間隔を設けて配置することにより、ＮＯx吸蔵能力のより高い排ガス浄化用触媒とすることが可能になる。
【００６８】
また、本発明の排ガス浄化方法は、上記本発明の排ガス浄化用触媒を用い、間欠的に燃料ストイキ〜リッチ雰囲気となるようなリーンバーンエンジンからの排ガスを浄化する方法であり、この方法によれば、硫黄被毒を抑制させつつ、排ガス中のＮＯxを効率よく浄化させることができる。
【図面の簡単な説明】
【図１】 実施例１および比較例１〜比較例２の排ガス浄化用触媒の促進耐久試験後のＮＯx吸蔵量を示す。
【図２】 実施例２の排ガス浄化用触媒において、第１触媒と第２触媒と間隔の大きさとＮＯx吸蔵量との関係を示す。
【図３】 実施例２の排ガス浄化用触媒において、第２触媒の担体を構成する多孔質酸化物の種類とＮＯx吸蔵量との関係を示す。
【図４】 実施例２の排ガス浄化用触媒において、第２触媒のセル数とＮＯx吸蔵量との関係を示す。
【図５】 実施例２の排ガス浄化用触媒において、第２触媒の担体を構成する多孔質酸化物の種類を変更した場合の、第１触媒と第２触媒と間隔の大きさとＮＯx吸蔵量との関係を示す。
【図６】 実施例２の排ガス浄化用触媒において、第２触媒を高セル化した場合の、第１触媒と第２触媒と間隔の大きさとＮＯx吸蔵量との関係を示す。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to NO in exhaust gas containing oxygen in excess of the amount necessary to oxidize carbon monoxide (CO) and hydrocarbon (HC) contained in the exhaust gas. x The present invention relates to an exhaust gas purification catalyst capable of efficiently purifying gas and an exhaust gas purification method using the catalyst.
[0002]
[Prior art]
In lean burn engines, combustion is always performed under fuel lean conditions with excess oxygen, and the exhaust gas is reduced to NO in the reducing atmosphere by intermittently changing to fuel stoichiometric to rich conditions. x Has been developed and put into practical use. And as an optimal catalyst for this system, NO in a lean atmosphere x NO, stored in a stoichiometric to rich atmosphere x NO release x NO using occlusion material x Occupational reduction type exhaust gas purification catalysts have been developed.
[0003]
For example, JP-A-5-317652 proposes an exhaust gas purifying catalyst in which an alkaline earth metal such as Ba and Pt are supported on a porous oxide carrier such as alumina. Japanese Patent Laid-Open No. 6-31139 proposes an exhaust gas purification catalyst in which an alkali metal such as K and Pt are supported on a porous oxide carrier such as alumina. Further, JP-A-5-168860 proposes an exhaust gas purification catalyst in which a rare earth element such as La and Pt are supported on a porous oxide carrier such as alumina.
[0004]
This NO x If the storage reduction catalyst is used, the exhaust gas is changed from a lean atmosphere to a stoichiometric to rich atmosphere in a pulsed manner by controlling the air-fuel ratio from the lean side to the stoichiometric to rich side. Therefore, NO on the lean side x Is NO x Even if it is exhaust gas from a lean burn engine, it is stored in the storage material and released on the stoichiometric or rich side to be purified by reacting with reducing components such as HC and CO. x Can be efficiently purified. HC and CO in the exhaust gas are oxidized by noble metals and NO. x HC and CO are also efficiently purified.
[0005]
However, in the exhaust gas, SO generated by burning sulfur (S) contained in the fuel. x It is oxidized by noble metals in the exhaust gas in a lean atmosphere, for example SO Three It becomes. And it is also easily converted into sulfuric acid by water vapor contained in the exhaust gas, and these are NO. x Reacts with the occlusion material to produce sulfite and sulfate. x It became clear that the occlusion material deteriorated by poisoning. That is, NO like this x When the storage material becomes sulfite or sulfate and deteriorates poisoning, it is no longer NO x Can no longer be stored, NO after durability x This is a problem that the purification performance is lowered.
[0006]
[Problems to be solved by the invention]
In view of the current state of air pollution in recent years and the tightening of exhaust gas regulations, conventional NO x For exhaust gas purification catalysts of storage reduction type, x The purification performance is not sufficient, so it is high NO from the initial stage to after the endurance x There is a demand for early development of an exhaust gas purification catalyst exhibiting purification performance.
[0007]
The present invention has been made in view of such circumstances, and NO. x NO further suppresses sulfur poisoning of occlusion materials x The object is to further improve the durability of the storage-reduction type exhaust gas purification catalyst.
[0008]
The exhaust gas purifying catalyst of the present invention comprises a support made of a porous oxide, a noble metal supported on the support, and a support supported on the support. First 1 NOx occlusion material, a first catalyst located upstream of the exhaust gas, a carrier made of porous oxide, a noble metal supported on the carrier, and a carrier supported on the carrier First A NOx occlusion reduction type exhaust gas purifying catalyst comprising a second catalyst located downstream of the exhaust gas, wherein the first NOx occlusion material contains Ba, and the second NOx occlusion material The material contains K, and the first NOx occlusion material is characterized in that it has a characteristic that it is more likely to adsorb sulfur and is less likely to desorb sulfur than the second NOx occlusion material.
[0009]
In general, NO x In the occlusion reduction type catalyst, sulfur poisoning tends to concentrate on the portion located upstream of the exhaust gas, particularly in a monolith type catalyst (for example, a catalyst formed as a honeycomb body by coating a carrier on a honeycomb substrate). The tendency is strong. NO alkali metals, alkaline earth metals, rare earth elements x When used as an occlusion material, this NO x SO to occlusion material x Adsorption is inevitable, but the adsorbed SO x Can be detached by making the fuel stoichiometric to rich atmosphere. However, even if it is once separated, NO again on the downstream side x A phenomenon such as adsorption to the occlusion material may also occur.
[0010]
NO of the present invention x The storage reduction catalyst is divided into upstream and downstream parts, and two types of sulfur adsorption / desorption characteristics (SO x Easy adsorption and disengagement) x Occupation material is arranged for each. In other words, NO has a characteristic that it is more likely to adsorb sulfur and is less likely to desorb sulfur x By placing the occlusion material upstream, the SO part preferentially exists upstream in the fuel lean atmosphere. x In a fuel stoichiometric to rich atmosphere that adsorbs x It is a thing which makes a withdrawal reduction. On the other hand, in the catalyst portion on the downstream side, it is difficult to adsorb sulfur. x Since the occlusion material is arranged, SO is released and reduced in this stoichiometric to rich atmosphere. x Is exhausted without being adsorbed by the downstream catalyst portion. By such an effect | action, in the exhaust gas purification catalyst of this invention, sulfur poisoning is suppressed and durability is improved.
[0011]
In addition, SO x And NO x Reaction with occlusion material is NO x SO adsorbed on the storage material x Is easily converted to sulfurous acid, sulfuric acid, etc. by the water vapor contained in the exhaust gas. x It is considered that the reaction proceeds with the occluding material so that sulfite, sulfate and the like are produced. Therefore, the term “adsorption” means not only adsorption but also includes the above reaction after adsorption. In addition, “withdrawal” means that the reaction proceeds in reverse.
[0012]
Furthermore, the exhaust gas purification method of the present invention uses the exhaust gas purification catalyst of the present invention, and this exhaust gas purification catalyst is brought into contact with the exhaust gas from a lean burn engine that is intermittently in a fuel stoichiometric to rich atmosphere, so that the fuel lean NO contained in the exhaust gas in the atmosphere x The first NO x Occlusion material and the second NO x The first NO is stored in a fuel stoichiometric to rich atmosphere. x Occlusion material and the second NO x NO released from occlusion material x It is characterized by reducing.
[0013]
By using the exhaust gas purifying catalyst of the present invention and intermittently creating a fuel stoichiometric to rich atmosphere, the exhaust gas purifying method of the present invention is efficient from the exhaust gas of a lean burn engine while suppressing deterioration due to sulfur poisoning. NO x Can be purified.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail.
[0015]
The exhaust gas purifying catalyst of the present invention comprises a first catalyst located upstream of the exhaust gas and a second catalyst located downstream of the exhaust gas. The first catalyst and the second catalyst may be formed integrally, or may be formed separately from each other. In other words, in the case of a monolith type catalyst, NO is different for each of the upstream portion and the downstream portion of one monolith (catalyst formed as a honeycomb body). x The storage material may be supported and incorporated in one catalytic converter, or a different NO. x Two monoliths carrying the occlusion material may be used and connected to one catalytic converter at an appropriate interval. Furthermore, the first catalyst and the second catalyst may be incorporated in separate catalytic converters, and the two catalytic converters may be arranged in series in the exhaust gas flow path. In the case of a pellet type catalyst, a plurality of pellets serving as the first catalyst and a plurality of pellets serving as the second catalyst may be grouped together, and both pellet groups may be incorporated in one catalytic converter in series. Each of the pellet groups may be incorporated into separate catalytic converters, and the two catalytic converters may be arranged in series in the exhaust gas flow path. Note that either the first catalyst or the second catalyst can be a monolith type, and the other can be a pellet type.
[0016]
Each of the first catalyst and the second catalyst includes at least one selected from a support made of a porous oxide, a noble metal supported on the support, an alkali metal, an alkaline earth metal, and a rare earth element supported on the support. NO x Containing occlusion material. Hereinafter, these components will be described.
[0017]
Precious metals and NO x The carrier supporting the occlusion material is made of a porous oxide, and examples of the porous oxide that can be used include alumina (Al 2 O Three ), Zeolite, silica (SiO 2 ), Zirconia (ZrO 2 ), Titania (TiO 2 ), Etc., and TiO that combines these 2 -Al 2 O Three , SiO 2 -Al 2 O Three , ZrO 2 -Al 2 O Three Etc. can be mentioned. In addition, for the purpose of improving purification performance, ceria (CeO 2 ) And ceria stabilized with zirconia (ceria-zirconia composite oxide: CeO) 2 -ZrO 2 ) May be added. In the case of forming a monolithic catalyst, these oxides may be made into a slurry and coated on a honeycomb substrate made of ceramics such as cordierite, heat-resistant alloy, etc., and used as a carrier. In such a case, these oxides may be sintered to a predetermined size to form a carrier.
[0018]
The noble metal supported on the carrier mainly oxidizes CO and HC and at the same time NO in exhaust gas. x NO which is the main component of NO 2 As NO x It plays the role of making it easy to occlude in the occlusion material. Examples of noble metals that can be used include Pt, Rh, Pd, Ir, and Ru. According to the characteristics of the catalyst, one or more of these may be selectively supported. The supporting method is not particularly limited, and may be performed by a known method according to the noble metal used. For example, if a monolithic catalyst is to be formed, the amount of the noble metal supported is preferably 0.1 to 20 g in the case of Pt and Rd per liter of the carrier volume, and preferably 0.5 to 10 g. Is more preferable. Moreover, in the case of Rh, it is preferable to set it as 0.01-10g, and it is more preferable to set it as 0.05-5g.
[0019]
NO to be supported on the carrier x The storage material is NO in a fuel lean atmosphere. x Occluded, NO in fuel stoichiometric rich atmosphere x It is possible to use at least one selected from alkali metals, alkaline earth metals and rare earth elements. Li, Na, K, Rb, Cs as the alkali metal, Be, Mg, Ca, Sr, Ba as the alkaline earth metal, Sc, Y, La, Ce, Pr, Nd, Dy as the rare earth element Yb and the like can be exemplified.
[0020]
In the exhaust gas purifying catalyst of the present invention, the NO adsorption and separation characteristics differ between the first catalyst located upstream of the exhaust gas and the second catalyst located downstream. x Load occlusion material. In other words, the first catalyst has SO x Is the first NO that has the characteristics of being easily adsorbed and difficult to separate x The occlusion material is supported, and the second catalyst has SO. x Is the second NO with the characteristics that it is difficult to adsorb and easily x Load occlusion material. Two or more kinds of NO are added to each of the first catalyst and the second catalyst. x Since the occlusion material can be supported, this “first NO. x "Occlusion material" and "2nd NO x “Occlusion material” means NO supported on the first catalyst and the second catalyst, respectively. x It means that it is a general term for occlusion materials. Therefore, two or more kinds of NO x When NO is stored, the first NO x The individual NOs that make up the storage material x All of the occlusion material is the second NO x The individual NOs that make up the storage material x From storage material, SO x Does not need to have the property of being easily adsorbed and difficult to be removed. x Occlusion material and second NO x As the total characteristics of each occlusion material, the first NO x Occlusion material is 2nd NO x SO more than occlusion material x As long as it has a characteristic of being easily adsorbed and difficult to separate.
[0021]
NO x The occlusion material has high alkalinity and NO x In view of the excellent occlusion ability, it is desirable to select from alkali metals and alkaline earth metals. SO x Alkaline earth metals are superior to alkali metals in that they are easy to adsorb and difficult to desorb. Considering this, the first NO x Alkaline earth metal is included in the occlusion material and the second NO x It is desirable to include an alkali metal in the occlusion material. In addition, NO per mole x , SO x 1st NO because of the large amount of adsorption x As the alkaline earth metal to be included in the occlusion material, it is desirable to use Ba, and since the decomposition of sulfate is very easy, the second NO. x As the alkali metal contained in the occlusion material, it is desirable to use K.
[0022]
Also, the first NO x Ba is used as the occlusion material and the second NO x When K is used as the storage material, Ba SO x The release ability is about 650 ° C with K SO x Since the separation ability has a peak at about 600 ° C., by creating a high-temperature stoichiometric to rich atmosphere by engine control, both catalysts have SO temperature from the temperature difference between the upstream first catalyst and the downstream second catalyst. x It becomes possible to use a temperature range with a large ability to leave, and there is an advantage that a catalyst in which sulfur poisoning is further suppressed can be configured.
[0023]
In addition, the first NO x Ba is used as the occlusion material and the second NO x When K is used as the occlusion material, Ba NO x The storage capacity shows its peak at a temperature of about 270 ° C. x Since the storage capacity shows its peak at a temperature condition of about 400 ° C., the NO. x There is also an advantage that the storage capacity can be secured.
[0024]
2nd NO x When K is used as the occlusion material, this K is TiO at high temperatures. 2 It is easy to react with. In consideration of this, the carrier constituting the second catalyst is ZrO. 2 , Al 2 O Three , MgAl 2 O Four , CeO 2 -ZrO 2 It is desirable to form an oxide that does not easily react with K, such as a complex oxide, alone or in a mixture of two or more thereof. By using such a carrier, the NOx occlusion ability can be further improved while making the catalyst difficult to receive sulfur poisoning.
[0025]
First NO in the first catalyst and the second catalyst x Occlusion material and second NO x The loading method of the occlusion material is not particularly limited, and each loaded NO is supported. x What is necessary is just to perform by the already well-known method according to the occlusion material. NO x For example, in the case of a monolithic catalyst, the loading amount of the occlusion material is desirably in the range of 0.05 to 1 mol for both the first catalyst and the second catalyst with respect to 1 liter of the carrier volume. If the supported amount is less than 0.05 mol, NO x NO occlusion capacity x This is because, when the purification performance is reduced and the loading amount exceeds 1 mol, the effect is saturated, and in addition, there may be a problem due to a decrease in the amount of other components.
[0026]
When the exhaust gas purification catalyst of the present invention is formed as a monolithic catalyst (as a honeycomb body), the first catalyst and the second catalyst are formed as separate honeycomb bodies, and the first catalyst and the second catalyst are combined. It is desirable to arrange them at intervals. When a monolithic catalyst is installed as a converter in the exhaust gas path, when one honeycomb body is installed in the converter, the exhaust gas flow rate is large only in the central part of the honeycomb body, the gas flow rate in the peripheral part is reduced, and the peripheral part Precious metals and NO x A phenomenon that the utilization rate of the occlusion material decreases may occur. Therefore, if the first catalyst and the second catalyst are formed in separate honeycomb bodies and are spaced apart from each other, the exhaust gas becomes a turbulent flow at the interval, and the back pressure increases and the exhaust gas passes through the converter. By increasing the resistance, the exhaust gas is uniformly dispersed and passes through the catalyst. In particular, on the second catalyst side, the time for the exhaust gas to contact the second catalyst becomes longer. As a result, the total catalyst NO x The amount of occlusion will increase further. Further, by providing a gap between the two honeycomb bodies to divide the catalyst, the volume per carrier is reduced, so that the heat capacity of the carrier is lowered. For this reason, since the catalyst is excellent in warming property, the catalytic activity increases even when the inlet gas temperature is low (at low temperature), and NO x The amount of occlusion will increase further.
[0027]
The distance between the first catalyst and the second catalyst is preferably 5 mm or more and 25 mm or less. The preferable range of this interval has been confirmed by experiments shown later. In this range, the exhaust gas purification catalyst of the present invention is more NO. x It becomes an exhaust gas purification catalyst with excellent storage capacity.
[0028]
In addition, when the first catalyst and the second catalyst are formed as separate honeycomb bodies and arranged with a space therebetween, the downstream second catalyst is formed in a higher cell state than the upstream first catalyst honeycomb body. It is desirable to do. The honeycomb body has a large number of conduction holes (conduction paths) through which exhaust gas passes, and the high cell state means that the number of conduction holes per unit area is large in the cross section. In other words, the high cell state means a state where there are more narrow conductive paths.
[0029]
If the second catalyst has a higher cell than the first catalyst, the pressure (back pressure) of the exhaust gas at the interval between the first catalyst and the second catalyst described above increases, and the exhaust gas is uniformly dispersed. Thus, the action of passing through the catalyst, in particular, the action of increasing the time during which the exhaust gas contacts the exhaust gas on the second catalyst side becomes larger. As a result, the total catalyst NO x The amount of occlusion will further increase.
[0030]
Next, the exhaust gas purification method of the present invention using the exhaust gas purification catalyst of the present invention will be described. The exhaust gas purification method of the present invention purifies exhaust gas from a lean burn engine. The exhaust gas purification catalyst is arranged in the exhaust gas flow path so that the first catalyst is located upstream and the second catalyst is located downstream. NO is contained in the exhaust gas in a fuel lean atmosphere by intermittently contacting the exhaust gas from a lean burn engine that becomes a fuel stoichiometric to rich atmosphere. x The first NO x Occlusion material and the second NO x The first NO is stored in a fuel stoichiometric to rich atmosphere. x Occlusion material and the second NO x NO released from occlusion material x It is characterized by reducing.
[0031]
In the fuel lean atmosphere, NO contained in the exhaust gas is oxidized on the catalyst, and NO 2 NO including NO x And that is NO x It is stored in the storage material. And if the fuel stoichiometric to rich atmosphere is intermittently made, this time the stored NO x Is released and reacts with HC and CO on the catalyst to be reduced.
[0032]
SO contained in exhaust gas x Is the main component 2 This SO 2 Is oxidized on the catalyst and SO Three Is the main component. SO Three Is H in the exhaust gas 2 Reacts easily with O to turn into sulfuric acid, NO x It will be easily adsorbed by the storage material. The adsorption method is NO x When the occlusion material is Ba, for example, BaSO Four It is thought that it becomes the form. Where the first NO x The storage material is SO x Because of its high adsorption capacity and inferior release performance, SO x NO is adsorbed x Adsorbed on the storage material and accumulated in the first catalyst. Next, when the fuel stoichiometric to rich atmosphere is intermittently established, this time the adsorbed SO x Is released, and it reacts with HC and CO on the catalyst and is reduced. 2 And going downstream.
[0033]
In the exhaust gas purifying catalyst of the present invention, the second NO of the downstream second catalyst. x SO as a storage material x SO2 released on the first catalyst is used because it is inferior in adsorption capacity and superior in separation performance. x This second NO x It will be exhausted without adsorbing to the occlusion material. Therefore, in the exhaust gas purification method of the present invention using the exhaust gas purification catalyst, NO is efficiently reduced while suppressing sulfur poisoning of the catalyst. x NO can be stored even after endurance. x This is an exhaust gas purification method in which the purification performance does not deteriorate.
[0034]
As mentioned above, although the embodiment of the exhaust gas purification catalyst of the present invention and the exhaust gas purification method of the present invention using the same has been described, the embodiment described above is only one embodiment, the exhaust gas purification catalyst of the present invention and The exhaust gas purification method of the present invention that has been used can be implemented in various modified and improved forms based on the knowledge of those skilled in the art, including the above-described embodiment.
[0035]
【Example】
Based on the said embodiment, the exhaust gas purification catalyst which is one aspect | mode of the exhaust gas purification catalyst of this invention was produced as an Example. An exhaust gas purifying catalyst in which the positional relationship between the first catalyst and the second catalyst is reversed, and a uniform NO over the entire catalyst. x An exhaust gas purifying catalyst carrying an occlusion material was prepared as a comparative example, and an endurance acceleration test was performed on the exhaust gas purifying catalyst of the example and the comparative example, and the NO after the test of each exhaust gas purifying catalyst was performed. x By comparing the amount of occlusion, it was confirmed that the exhaust gas purifying catalyst of the present invention was excellent in sulfur poisoning resistance.
[0036]
Further, when the exhaust gas purifying catalyst of the present invention is a monolith type catalyst, the distance between the first catalyst and the second catalyst, the porous oxide constituting the carrier of the second catalyst, and the honeycomb constituting the second catalyst NO by changing the number of base cells (conduction holes) x In order to investigate the effect on the amount of occlusion, various exhaust gas purification catalysts were prepared as examples, and durability promotion tests were also conducted on these catalysts. x The amount of occlusion was compared. These will be described below.
[0037]
[Example 1]
<Exhaust gas purification catalyst of Example 1>
The first catalyst of the present exhaust gas purification catalyst was produced as follows.
[0038]
The zirconia powder was supported using a rhodium nitrate solution, dried and fired to obtain zirconia powder supporting Rh. The amount of Rh supported was 0.42 wt% when the Rh supported zirconia powder was 100 wt%. Next, a mixed powder of alumina and titania was mixed with the Rh-supported zirconia powder. The mixing ratio of alumina: titania in the alumina-titania mixed powder was 1: 1 by weight, and the ratio of alumina-titania mixed powder: Rh-supported zirconia powder was 4: 1. The mixed powder was further mixed with a ceria zirconia composite oxide in such a ratio that the weight ratio of alumina-titania mixed powder: ceria zirconia composite oxide was 10: 1. This powder was slurried by a predetermined method and coated on a carrier substrate of a ceramic honeycomb having a capacity of 0.65 liter to produce a monolith carrier precursor. The coating amount was 270 g per liter of monolith support precursor. The monolith carrier precursor was dried at 250 ° C. for 15 minutes and then calcined at 500 ° C. for 30 minutes to complete the monolith carrier.
[0039]
This monolith support was immersed in an aqueous barium acetate solution having a predetermined concentration, supported with Ba, dried at 250 ° C. for 15 minutes, and then calcined at 500 ° C. for 30 minutes. The supported amount of Ba was 0.3 mol per liter of monolith support. Next, this was immersed in an aqueous solution of ammonium bicarbonate having a concentration of 15 g / liter for 15 minutes and then dried for 15 minutes. Next, Pt was supported on this using a nitric acid solution of dinitrodiamine platinum, and dried and fired at 300 ° C. for 15 minutes. The amount of Pt supported was 2 g per liter of monolith support. Finally, it was immersed in a predetermined concentration of lithium nitrate aqueous solution to carry lithium, dried at 250 ° C. for 15 minutes, and then calcined at 500 ° C. for 30 minutes to complete the first catalyst. The amount of Li supported was 0.1 mol per liter of monolith support. The first catalyst is the main NO x Ba is supported as an occlusion material.
[0040]
The second catalyst of the exhaust gas purification catalyst was produced as follows.
[0041]
The same method was used until the completion of the monolith support in the production of the first catalyst. Therefore, the same monolith carrier as the first catalyst is used. The monolith carrier was supported with Pt using a nitric acid solution of dinitrodiamine platinum, dried and calcined at 300 ° C. for 15 minutes. The amount of Pt supported was 2 g per liter of monolith support. Next, this is immersed in a mixed aqueous solution of potassium acetate and lithium nitrate at a predetermined concentration to support K and Li, dried at 250 ° C. for 15 minutes, and then calcined at 500 ° C. for 30 minutes to complete the second catalyst. I let you. The supported amounts of K and Li were 0.3 mol of K and 0.1 mol of Li per liter of monolith support. The second catalyst is the main NO x As the occlusion material, K is supported.
[0042]
The exhaust gas purification catalyst of Example 1 was completed by incorporating the first catalyst into the catalytic converter so that the first catalyst was located upstream of the exhaust gas and the second catalyst was located downstream.
[0043]
<Exhaust gas purifying catalyst of Comparative Example 1>
The exhaust gas purifying catalyst completed by incorporating the first catalyst produced in Example 1 into the catalytic converter so that the first catalyst is located on the downstream side of the exhaust gas and the second catalyst is located on the upstream side. Unlike the exhaust gas purifying catalyst of the example, NO carried on the downstream side x NO stored on the upstream side by the occlusion material x It is easier to adsorb sulfur and harder to separate than the occlusion material.
[0044]
<Exhaust gas purification catalyst of Comparative Example 2>
NO to have uniform sulfur adsorption / desorption characteristics throughout the catalyst x An exhaust gas purifying catalyst carrying an occlusion material. This catalyst was prepared as follows.
[0045]
The process up to the production of the slurry for coating the carrier substrate was the same as in Example 1. The slurry was coated on a ceramic honeycomb carrier substrate having a capacity of 1.3 liters to produce a monolithic carrier precursor. The coating amount was 270 g per liter of monolith support precursor. The monolith carrier precursor was dried at 250 ° C. for 15 minutes and then calcined at 500 ° C. for 30 minutes to complete the monolith carrier.
[0046]
This monolith support was immersed in an aqueous barium acetate solution having a predetermined concentration, supported with Ba, dried at 250 ° C. for 15 minutes, and then calcined at 500 ° C. for 30 minutes. The amount of Ba supported was 0.15 mol per liter of monolith support. Next, this was immersed in an aqueous solution of ammonium bicarbonate having a concentration of 1.5 g / liter for 15 minutes and then dried for 15 minutes. Next, Pt was supported on this using a nitric acid solution of dinitrodiamine platinum, and dried and fired at 300 ° C. for 15 minutes. The amount of Pt supported was 2 g per liter of monolith support. Finally, this was immersed in a mixed aqueous solution of potassium acetate and lithium nitrate at a predetermined concentration to carry K and Li, dried at 250 ° C. for 15 minutes, and then baked at 500 ° C. for 30 minutes. The supported amounts of K and Li were 0.15 mol of K and 0.1 mol of Li per liter of monolith support. By incorporating this into the catalytic converter, the exhaust gas purifying catalyst of Comparative Example 2 was completed.
[0047]
<Evaluation of sulfur resistance>
The catalytic converter incorporating the exhaust gas purifying catalysts of Example 1, Comparative Example 1 and Comparative Example 2 was attached to a 1.8 liter lean burn engine, and a high sulfur concentration (sulfur = 200 ppm) fuel was used for an accelerated durability test for 50 hours. Thereafter, NO per catalyst of each exhaust gas purification catalyst when the gas entering the catalyst is 300 ° C. to 480 ° C. x The occlusion amount was measured. The measurement results are shown in FIG.
[0048]
As is apparent from FIG. 1, the exhaust gas purifying catalyst of Example 1 is more NO. After the endurance test than the two exhaust gas purifying catalysts in the temperature range of 300 to 480 ° C. x It can be seen that the amount of occlusion is large. In particular, it can be seen that good results are shown on the high temperature side. Therefore, it can be confirmed that the exhaust gas purifying catalyst of the present invention is a catalyst excellent in sulfur poisoning resistance.
[0049]
[Example 2]
<Production of first catalyst and second catalyst>
A first catalyst was produced in the same manner as in Example 1 above. The monolithic first catalyst is composed of a cylindrical monolithic carrier. This monolithic carrier has a diameter of 103 mmφ and a length of 75 mm, and the honeycomb wall surface has a thickness of 6 mils (1 mil is about 25 μm). ), The number of cells (number of conduction holes (conduction paths)) was about 400 cells.
[0050]
A second catalyst was produced by the same method as in Example 1 above. The monolithic first catalyst is composed of a cylindrical monolithic carrier. The monolithic carrier, like the monolithic carrier of the first catalyst, has a diameter of 103 mmφ and a length of 75 mm, and has a thickness of the honeycomb body wall surface. Is 6 mils and the number of cells is about 400 cells. This second catalyst was designated as monolith (1).
[0051]
Next, the type of the porous oxide constituting the carrier of the second catalyst was changed to produce various second catalysts. In the production method of the monolith (1) (in the production method of the second catalyst in the case of Example 1), the second catalyst produced by replacing the titania powder with the zirconia powder was designated as the monolith (2), and so on. Monolith (3) in which titania powder is replaced with alumina powder, monolith (4) in which titania powder is replaced with a mixed powder of 1: 1 weight ratio of zirconia powder and alumina powder, and titania powder in spinel (MgAl 2 O Four ) Monolith (5) was obtained by replacing the powder with monolith (5), and monolith (6) was obtained by replacing the titania powder with a mixed powder of spinel powder and zirconia powder in a weight ratio of 1: 1.
[0052]
Next, the number of cells of the monolith support was changed to produce two types of second catalysts. In the production of the monolith (1), the honeycomb body wall thickness is 3 mils, the second catalyst using a 600-cell monolith support is the monolith (7), the honeycomb body wall thickness is 2 mils, 900 The second catalyst using the cell monolith support was designated as monolith (8).
[0053]
The specifications of the monolith (1) to monolith (8), which are the first catalyst and the second catalyst, are summarized in Table 1 below.
[0054]
[Table 1]
Various exhaust gas purification catalysts of Example 2 were prepared by arranging the first catalyst on the upstream side and one of the second catalysts selected from monolith (1) to monolith (8) on the downstream side. First, an exhaust gas purification catalyst in which the first catalyst and the monolith (1) are combined and both are disposed without providing an interval therebetween is used as the exhaust gas purification catalyst of Example 2-1, and both Example 5-2 with a distance of 5 mm between them, Example 2-2 with a distance of 10 mm, Example 2-3 with a distance of 10 mm, Example 2-4 with a distance of 15 mm Exhaust gas purifying catalyst of Example 2-5, with an interval of 20 mm, Example 2-6 with an interval of 25 mm, and Exhaust gas purification catalyst of Example 2-7 with an interval of 30 mm It was.
[0056]
Next, any one of the monolith (2) to monolith (6), which is the second catalyst in which the porous oxide constituting the support is variously changed, is set to the upstream side of the first catalyst, and the distance from the first catalyst is set. Exhaust gas purifying catalysts arranged on the downstream side without being provided were prepared. The exhaust gas purification catalyst using the monolith (2) was used as the exhaust gas purification catalyst of Example 2-8. Similarly, the monolith (3) using the monolith (3) was the monolith (4) of Example 2-9. A catalyst for purifying exhaust gas of Example 2-10, a catalyst using Monolith (5) for Example 2-11, and a catalyst using Monolith (6) were used for Example 2-12.
[0057]
Also, the first catalyst is placed upstream, and either the monolith (7) or the monolith (8), which is a second catalyst having a different number of cells from the first catalyst, is placed downstream without providing an interval from the first catalyst. Exhaust gas purifying catalysts arranged on the sides were prepared. The exhaust gas purification catalyst using the monolith (7) was used as the exhaust gas purification catalyst of Example 2-13, and the exhaust gas purification catalyst using the monolith (8) was used as the exhaust gas purification catalyst of Example 2-14.
[0058]
Further, in the exhaust gas purification catalyst using the monolith (4), which is the second catalyst obtained by changing the porous oxide constituting the carrier, on the upstream side, the first catalyst and the second catalyst. In order to confirm the influence of the catalyst interval, a catalyst was prepared in which a gap of 5 mm was provided between them, and this was used as the exhaust gas purifying catalyst of Example 2-15. Similarly, those with a 10 mm spacing are of Example 2-16, those with a spacing of 15 mm are of Example 2-17, those with a spacing of 20 mm are of 25 mm of Example 2-18. The catalyst with the interval was used as the exhaust gas purifying catalyst of Example 2-19, and the catalyst with the interval of 30 mm was used as the exhaust gas purifying catalyst of Example 2-20.
[0059]
Furthermore, in the exhaust gas purification catalyst using the first catalyst on the upstream side and the monolith (7), which is the second catalyst having a different number of cells, on the downstream side, the influence of the interval between the first catalyst and the second catalyst is affected. In order to confirm, the thing which provided the space | interval of 5 mm between both and has arrange | positioned both was produced, and this was made into the exhaust gas purification catalyst of Example 2-21. Similarly, those with a 10 mm spacing are of Example 2-22, those with a spacing of 15 mm are of Example 2-23, and those with a spacing of 20 mm are of 25 mm of Example 2-24. The catalyst with an interval was used in Example 2-25, and the catalyst with an interval of 30 mm was used as the exhaust gas purifying catalyst in Example 2-26.
[0060]
The specifications of the exhaust gas purifying catalysts of Examples 2-1 to 2-22 are summarized in Table 2 below.
[0061]
[Table 2]
<NO x Evaluation of storage capacity>
The catalytic converter incorporating each exhaust gas purifying catalyst of Example 2 was subjected to an accelerated durability test under the same conditions as in Example 1, and the gas entering the catalyst was at 400 ° C. NO of each exhaust gas purification catalyst x The occlusion amount was measured. The exhaust gas purifying catalyst of Example 2-1 is similar in configuration to the exhaust gas purifying catalyst of Example 1, and NO. x The absolute value of the occlusion amount is known. Therefore, NO from here x The occlusion amount is expressed in an arbitrary unit (au).
[0063]
First, the distance between the first catalyst and the second catalyst and NO x In order to evaluate the relationship with the amount of occlusion, NO in the exhaust gas purifying catalyst of Example 2-1 to Example 2-7. x The amount of occlusion is shown as a graph in FIG. As is clear from FIG. 2, by disposing both with a certain distance between the first catalyst and the second catalyst, compared to the case where the both are disposed without any gap, the NO is reduced. x It can be confirmed that the amount of occlusion increases. Further, from FIG. 2, in the exhaust gas purifying catalyst of the present example, by setting the interval to 5 mm or more and 25 mm or less, a larger NO. x It can also be confirmed that the effect of increasing the amount of occlusion is obtained.
[0064]
Next, the type of porous oxide constituting the support of the second catalyst and NO x In order to evaluate the relationship with the amount of occlusion, NO of the exhaust gas purifying catalysts of Example 2-1 and Example 2-8 to Example 2-12 x The amount of occlusion is shown as a graph in FIG. As can be seen from FIG. 3, the exhaust gas purifying catalysts of Example 2-8 to Example 2-12 are larger in NO than the exhaust gas purifying catalyst of Example 2-1. x Has storage capacity. In the exhaust gas purifying catalyst of Example 2-1, titania is included as an oxide constituting the carrier of the second catalyst, and this is NO. x Because it is highly reactive with K, an occlusion material, NO x It is thought that the occlusion capacity is slightly reduced. Therefore, the second NO x In the case where K is used as the occlusion material, it can be confirmed that the carrier of the second catalyst is preferably constituted without containing titania.
[0065]
Furthermore, the number of cells of the second catalyst and NO x In order to evaluate the relationship with the amount of occlusion, NO of the exhaust gas purifying catalysts of Example 2-1 and Example 2-13 to Example 2-14 x The occlusion amount is shown as a graph in FIG. As can be seen from this figure, by making the second catalyst higher in cell than the first catalyst, NO. x It can be confirmed that the storage capacity is improved.
[0066]
In addition, when the type of the porous oxide constituting the support of the second catalyst is changed or when the second catalyst is made to have a high cell, the distance between the first catalyst and the second catalyst and the NO. x In order to evaluate the relationship with the amount of occlusion, NO in the exhaust gas purifying catalysts of Example 2-10 and Example 2-15 to Example 2-20 x FIG. 5 is a graph showing the amount of occlusion. FIG. 5 shows the NO of the exhaust gas purifying catalysts of Example 2-13 and Example 2-21 to Example 2-26. x The amount of occlusion is shown as a graph in FIG. As can be seen from these figures, in both cases, a certain distance is provided between the first catalyst and the second catalyst, so that both are arranged without a gap between them. NO x It can be confirmed that the amount of occlusion increases. Moreover, in any case, by setting the interval to 5 mm or more and 25 mm or less, a larger NO. x It can be confirmed that the effect of increasing the amount of occlusion is obtained.
[0067]
【The invention's effect】
The exhaust gas purifying catalyst of the present invention is NO. x The NOx to be supported by the first catalyst is divided into the first catalyst located on the upstream side of the exhaust gas and the second catalyst located on the downstream side of the exhaust gas purification catalyst of the storage reduction type. x NO supported on the second catalyst x It is configured to have a characteristic that it is more easily adsorbed with sulfur than an occlusion material and is less likely to desorb sulfur. By adopting such a configuration, the exhaust gas purifying catalyst of the present invention suppresses sulfur poisoning and is NO even after durability. x It becomes a catalyst with little decrease in storage capacity. Furthermore, by arranging the first catalyst and the second catalyst at an interval, NO NO x An exhaust gas purifying catalyst having a higher storage capacity can be obtained.
[0068]
Further, the exhaust gas purification method of the present invention is a method for purifying exhaust gas from a lean burn engine that intermittently becomes a fuel stoichiometric to rich atmosphere using the exhaust gas purification catalyst of the present invention. NO in the exhaust gas while suppressing sulfur poisoning x Can be efficiently purified.
[Brief description of the drawings]
FIG. 1 shows NO after an accelerated endurance test of exhaust gas purifying catalysts of Example 1 and Comparative Examples 1 and 2. x Indicates the amount of occlusion.
FIG. 2 shows an exhaust gas purifying catalyst according to Example 2, in which the distance between the first catalyst and the second catalyst and the NO x The relationship with the amount of occlusion is shown.
3 shows the types of porous oxides and NO that constitute the carrier of the second catalyst in the exhaust gas purifying catalyst of Example 2. FIG. x The relationship with the amount of occlusion is shown.
4 shows the number of cells of the second catalyst and NO in the exhaust gas purifying catalyst of Example 2. FIG. x The relationship with the amount of occlusion is shown.
FIG. 5 shows the exhaust gas purifying catalyst of Example 2, in which the first catalyst and the second catalyst, the size of the gap, and NO when the kind of the porous oxide constituting the carrier of the second catalyst is changed. x The relationship with the amount of occlusion is shown.
FIG. 6 shows the exhaust gas purifying catalyst of Example 2 when the second catalyst has a high cell size and the distance between the first catalyst and the second catalyst and NO. x The relationship with the amount of occlusion is shown.
Claims (6)
多孔質酸化物からなる担体と、該担体に担持された貴金属と、該担体に担持された第２ＮＯｘ吸蔵材とを含んでなり、排ガスの下流側に位置する第２触媒と、
からなるＮＯｘ吸蔵還元型の排ガス浄化用触媒であって、
前記第１ＮＯｘ吸蔵材はＢａを含み、前記第２ＮＯｘ吸蔵材はＫを含み、該第１ＮＯｘ吸蔵材は該第２ＮＯｘ吸蔵材よりも、硫黄吸着しやすくかつ硫黄離脱しにくい特性を持つことを特徴とする排ガス浄化用触媒。A first catalyst located on the upstream side of exhaust gas, comprising a support made of a porous oxide, a noble metal supported on the support, and a first NOx storage material supported on the support;
A second catalyst located on the downstream side of the exhaust gas, comprising a support made of a porous oxide, a noble metal supported on the support, and a second NOx occlusion material supported on the support;
NOx occlusion reduction type exhaust gas purification catalyst comprising:
The first NOx occlusion material contains Ba, the second NOx occlusion material contains K, and the first NOx occlusion material has characteristics that it is easier to adsorb sulfur and is less likely to desorb sulfur than the second NOx occlusion material. Exhaust gas purification catalyst.
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| JP4161478B2 (en) * | 1999-08-25 | 2008-10-08 | トヨタ自動車株式会社 | Exhaust gas purification catalyst |
| JP4144174B2 (en) * | 2000-10-25 | 2008-09-03 | トヨタ自動車株式会社 | Exhaust gas purification device |
| JP2003038936A (en) * | 2001-07-30 | 2003-02-12 | Toyota Motor Corp | Exhaust gas purifying apparatus |
| JP4290391B2 (en) * | 2002-06-07 | 2009-07-01 | ヴァルティオン テクンニィルリネン ツッツキムスケスクス | Method and apparatus for catalytic removal of nitrogen oxides |
| JP4534749B2 (en) * | 2004-12-20 | 2010-09-01 | 株式会社豊田中央研究所 | NOx storage material, method for supporting the same, and NOx storage reduction catalyst |
| US20090081087A1 (en) | 2005-03-29 | 2009-03-26 | Yanmar Co., Ltd. | Exhaust gas purifier |
| JP2007125524A (en) * | 2005-11-07 | 2007-05-24 | Toyota Motor Corp | Exhaust gas purifier |
| JP5216189B2 (en) * | 2005-12-22 | 2013-06-19 | 株式会社キャタラー | Exhaust gas purification catalyst |
| JP2009248057A (en) * | 2008-04-10 | 2009-10-29 | Toyota Motor Corp | Catalyst for exhaust gas purification |
| JP2010104910A (en) * | 2008-10-30 | 2010-05-13 | Toyota Motor Corp | S (sulfur)-storage catalyst |
| JP5589320B2 (en) * | 2009-08-17 | 2014-09-17 | マツダ株式会社 | Exhaust gas purification catalyst and method for producing the same |
| JP5620722B2 (en) * | 2010-06-18 | 2014-11-05 | 三浦工業株式会社 | Combustion device and catalyst |
| JP7167614B2 (en) * | 2018-10-04 | 2022-11-09 | 三菱自動車工業株式会社 | Exhaust gas purifier |
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| JP3436427B2 (en) * | 1994-10-21 | 2003-08-11 | 株式会社豊田中央研究所 | Exhaust gas purification catalyst and exhaust gas purification method |
| JPH0985093A (en) * | 1995-09-26 | 1997-03-31 | Toyota Motor Corp | Catalyst for cleaning exhaust gas |
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| JPH11104493A (en) * | 1997-10-02 | 1999-04-20 | Nissan Motor Co Ltd | Catalyst for purifying exhaust gas and its use |
| JP3459037B2 (en) * | 1998-06-22 | 2003-10-20 | 日産自動車株式会社 | Exhaust gas purification equipment |
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| US10036293B2 (en) | 2015-12-11 | 2018-07-31 | Hyundai Motor Company | Apparatus for purifying exhaust gas |
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