JP2004114015A - Exhaust gas cleaning catalyst and method of evaluating its cleaning capacity - Google Patents
Exhaust gas cleaning catalyst and method of evaluating its cleaning capacity Download PDFInfo
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- JP2004114015A JP2004114015A JP2002285206A JP2002285206A JP2004114015A JP 2004114015 A JP2004114015 A JP 2004114015A JP 2002285206 A JP2002285206 A JP 2002285206A JP 2002285206 A JP2002285206 A JP 2002285206A JP 2004114015 A JP2004114015 A JP 2004114015A
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- Exhaust Gas After Treatment (AREA)
Abstract
Description
【０００１】
【発明の属する技術分野】
本発明は、自動車エンジンなどの内燃機関から排出される排ガスを効率よく浄化できる排ガス浄化用触媒と、その浄化能の評価方法に関する。
【０００２】
【従来の技術】
自動車の排気系には酸化触媒，三元触媒，ＮＯｘ 吸蔵還元型触媒などの排ガス浄化用触媒が搭載され、排ガス中のＨＣ，ＣＯ，ＮＯｘ などの有害成分を浄化している。これらの排ガス浄化用触媒はＰｔなどの貴金属を触媒金属として担持し、その触媒作用によりＨＣ，ＣＯを酸化して浄化し、ＮＯｘ を還元して浄化している。
【０００３】
ところがＰｔなどの触媒金属は、低温域では触媒活性が発現されず、所定の活性化温度以上で触媒活性を発現する。そのため始動時などにおいては、排ガス温度が触媒金属の活性化温度まで上昇するまでの間は排ガス中の有害成分を浄化することが困難である。しかしながら近年では、環境問題意識の高まりから排ガス規制がさらに強化され、排ガス浄化用触媒においては、始動時からできるだけ短時間で触媒金属を活性化させることが求められている。
【０００４】
そこで、できるだけ高温の排ガスが排ガス浄化用触媒に流入するように、エンジン直下に排ガス浄化用触媒を配置することが行われている。また排ガス浄化用触媒は外筒に収納された状態で配置されるので、排ガス浄化用触媒と外筒との間に断熱材を介在させて排ガス浄化用触媒が冷却されにくくすることも行われている。このように構成することにより、高温の排ガスが直ちに排ガス浄化用触媒に流入し、また熱伝導によって排ガス浄化用触媒の熱が外部に逃げるのが防止されるため、排ガスの熱を利用して排ガス浄化用触媒を加熱することができ、暖機が促進される。
【０００５】
一方、排ガス浄化用触媒側の対策としては、排ガス流入側に触媒金属を多く担持させることや、触媒金属を微細に高分散担持して活性点を増大させることなどが行われている。
【０００６】
しかしながらいずれの対策も一長一短があり、始動時などの低温域からできるだけ短時間で触媒金属を活性化させて早期に浄化活性を発現させることを満足させるには至っていない。
【０００７】
また排ガス浄化用触媒の浄化能の指標としては、５０％浄化温度が一般に用いられている。例えば酸化触媒によるＨＣの浄化率は、低温域ではゼロであるが、温度が上昇するにつれて上昇し、排ガス温度が ４００℃以上ではほぼ １００％の浄化率となる。そこでＨＣの浄化率が５０％になる温度を測定してＨＣ５０％浄化温度とし、ＨＣ５０％温度が低いほど低温域におけるＨＣ浄化能が高いと評価している。
【０００８】
しかしながら、温度を横軸に、浄化率を縦軸にとって実際の浄化挙動をプロットした場合に、ある温度で急激に立ち上がる曲線を描くものと、温度上昇とともになだらかにＨＣ浄化率が上昇するものの比較などでは、５０％浄化温度による評価では低温域における浄化能を正しく評価することは困難であった。
【０００９】
【特許文献１】特許第３２３５６４０号
【特許文献２】特開平０９−２９９７９７号
【特許文献３】特開平０８−０３８８９８号
【００１０】
【発明が解決しようとする課題】
本発明はこのような事情に鑑みてなされたものであり、低温域からできるだけ短時間で触媒金属を活性化でき低温域における浄化活性が高い排ガス浄化用触媒を提供するとともに、その低温域における浄化活性を正確に評価できる評価方法を提供することを目的とする。
【００１１】
【課題を解決するための手段】
上記課題を解決する本発明の排ガス浄化用触媒の特徴は、基材と、基材の表面に形成された触媒担持層と、触媒担持層に担持された貴金属と、を含んでなる排ガス浄化用触媒であって、触媒担持層は、熱容量をＣ１ 、熱伝導率をλ１ としたとき、Ｃ１ ２／λ１ ≦０．０４を満たす酸化物よりなることにある。
【００１２】
上記排ガス浄化用触媒において、触媒担持層と基材の間には、熱容量をＣ２ 、熱伝導率をλ２ としたとき、Ｃ２ ２×λ２ ≦１０を満たす酸化物よりなる下層を有することが望ましく、下層の酸化物はさらにＣ２ ２／λ２ ≦０．０４を満たすことが望ましい。また触媒担持層には、ＣｅＯ２−ＺｒＯ２複合酸化物を含むことが望ましい。
【００１３】
そして本発明の浄化能の評価方法の特徴は、基材と、基材の表面に形成された触媒担持層と、触媒担持層に担持された貴金属と、を含んでなる排ガス浄化用触媒の浄化能の評価方法であって、触媒担持層を構成する酸化物の熱容量をＣ１ 、熱伝導率をλ１ としたとき、Ｃ１ ２／λ１ ≦０．０４のときに浄化能が高いと評価することにある。
【００１４】
本発明の評価方法において、触媒担持層と基材の間には、熱容量がＣ２ 、熱伝導率がλ２ の酸化物よりなる下層を備え、Ｃ２ ２×λ２ ≦１０のときにさらに浄化能が高いと評価することも好ましい。
【００１５】
【発明の実施の形態】
本発明にいう熱容量Ｃは次元がＪ／ｇ・Ｋ（Ｊ：ジュール，ｇ：グラム，Ｋ：ケルビン）で表されるものであり、熱伝導率λは次元がＷ／ｍ・Ｋ（Ｗ：ワット，ｍ：メートル，Ｋ：ケルビン）で表されるものである。
【００１６】
本発明の排ガス浄化用触媒では、熱容量をＣ１ 、熱伝導率をλ１ としたとき、Ｃ１ ２／λ１ ≦０．０４を満たす酸化物よりなる触媒担持層を有している。この酸化物は熱伝導率λ１ が高く、熱容量Ｃ１ が小さいため、排ガスの熱エネルギーが触媒担持層に効率よく吸収され、排ガスによって速やかに貴金属の活性化温度まで暖機される。したがって始動時などにおいても触媒活性が発現するまでの時間が短縮され、その結果、浄化能が向上する。Ｃ１ ２ ／λ１ ＞０．０４では、上記作用効果の発現が困難である。
【００１７】
触媒担持層に含まれる酸化物は、２５℃における熱容量Ｃ１ が ０．２〜 １．２Ｊ／ｇ・Ｋの範囲が好ましく、２５℃における熱伝導率λ１ が ０．８〜７０Ｗ／ｍ・Ｋの範囲が好ましい。Ｃ１ ＜ ０．２Ｊ／ｇ・Ｋであったり、λ１ ＞７０Ｗ／ｍ・Ｋであるような酸化物には、触媒担持層として適するものが存在しない。またＣ１ ＞ １．２Ｊ／ｇ・Ｋであったり、λ１ ＜ ０．８Ｗ／ｍ・Ｋであるような酸化物では、Ｃ１ ２／λ１ ≦０．０４を満たすことが困難である。
【００１８】
Ｃ１ ２／λ１ ≦０．０４を満たす酸化物の種としては、アルミナ，ジルコニア，チタニア，セリア，シリカ−アルミナ，シリカなどの単体あるいは複合酸化物から選ばれるものを単独で、あるいは複数種類混合して用いることができる。複数種混合した場合には、混合物の熱容量及び熱伝導率がＣ１ ２／λ１ ≦０．０４を満たせばよい。中でもＣｅＯ２−ＺｒＯ２複合酸化物を含むことが特に好ましい。ＣｅＯ２−ＺｒＯ２複合酸化物は酸素吸蔵放出能が高く、その安定性にも優れているため、ＨＣ及びＣＯの浄化活性が特に向上する。そしてＣ１ ２／λ１ ≦０．０４を満たすＣｅＯ２−ＺｒＯ２複合酸化物を含むことで、低温域におけるＨＣ及びＣＯの浄化能が高く、その高い浄化能を長期間維持することが可能となる。
【００１９】
ＣｅＯ２−ＺｒＯ２複合酸化物は、触媒担持層に２０重量％以上含まれることが望ましい。この酸化物の含有量が２０重量％未満であると、酸素吸蔵放出能が不足して排ガス雰囲気の変動を吸収することが困難となりＨＣ及びＣＯの浄化能が低下する。
【００２０】
基材としては、コーディエライトなどの耐熱性セラミックス製、あるいは金属製のものを用いることができ、その形状はペレット状、ハニカム状、フォーム状など従来用いられているものを用いることができる。
【００２１】
排ガス浄化用触媒の暖機性には、触媒担持層の形成量も関係する。したがって触媒担持層は、基材の１リットルあたり １００〜 ３００ｇ形成することが望ましい。触媒担持層の形成量がこの範囲より少ないと、暖機性には貢献するものの、貴金属の担持密度が増大するため高温耐熱時に貴金属の粒成長が顕著となり耐熱性が低下する。また触媒担持層の形成量がこの範囲より多くなると、暖機性が低下するとともに、通気抵抗が増大してしまう。
【００２２】
触媒担持層に担持される貴金属としては、Ｐｔ，Ｒｈ，Ｐｄ，Ｉｒなどから選択して用いることができ、単独で担持してもよいし複数種担持することもできる。その担持量は、基材１Ｌあたり ０．１〜１０ｇ程度と従来と同様でよい。またＮＯｘ 吸蔵還元型触媒であれば、貴金属と共にアルカリ金属，アルカリ土類金属及び希土類元素から選択されるＮＯｘ 吸蔵材が担持される。
【００２３】
触媒担持層と基材の間には、熱容量をＣ２ 、熱伝導率をλ２ としたとき、Ｃ２ ２×λ２ ≦１０を満たす酸化物よりなる下層を有することが望ましい。このように熱容量が少なくかつ熱伝導率が高い酸化物よりなる下層を有することにより、触媒担持層の熱エネルギーが基材に奪われるのを抑制でき、かつ下層自体も暖機性に優れるため、コート層全体の暖機性を高く維持することができる。なお下層を構成する酸化物も、Ｃ２ ２／λ２ ≦０．０４を満たすことが特に好ましい。
【００２４】
上記のように二層構造のコート層とする場合、下層のコート量は基材の１リットルあたり３０〜 ２００ｇとすることが望ましい。下層の形成量がこの範囲より少ないと上記作用の発現が困難となり、下層の形成量がこの範囲より多くなると、暖機性が低下するようになる。なお前述した理由と同様の理由により、下層と触媒担持層の合計で基材の１リットルあたり １００〜 ３００ｇとすることが望ましい。
【００２５】
下層のＣ２ ２×λ２ ≦１０を満たす酸化物としては、アルミナ，ジルコニア，チタニア，セリア，シリカ−アルミナ，シリカなどの単体あるいは複合酸化物から選ばれるものを単独で、あるいは複数種類混合して用いることができる。複数種混合した場合には、混合物の熱容量及び熱伝導率がＣ２ ２×λ２ ≦１０を満たせばよい。中でもＣｅＯ２−ＺｒＯ２複合酸化物を含むことが特に好ましい。ＣｅＯ２−ＺｒＯ２複合酸化物は酸素吸蔵放出能が高く、その安定性にも優れているため、ＨＣ及びＣＯの浄化活性が特に向上する。そしてＣ２ ２×λ２ ≦１０を満たすＣｅＯ２−ＺｒＯ２複合酸化物を含むことで、低温域におけるＨＣ及びＣＯの浄化能が高く、その高い浄化能を長期間維持することが可能となる。
【００２６】
ＣｅＯ２−ＺｒＯ２複合酸化物は、下層に２０重量％以上含まれることが望ましい。この酸化物の含有量が２０重量％未満であると、酸素吸蔵放出能が不足して排ガス雰囲気の変動を吸収することが困難となりＨＣ及びＣＯの浄化能が低下する。
【００２７】
下層には貴金属を担持しなくてもよいが、触媒担持層と同様に貴金属を担持することが好ましい。このようにすれば触媒担持層の貴金属の担持密度が増大するのを抑制でき、貴金属の粒成長を抑制することができる。
【００２８】
そして本発明の浄化能の評価方法では、触媒担持層を構成する酸化物の熱容量をＣ１ 、熱伝導率をλ１ としたとき、Ｃ１ ２／λ１ ≦０．０４のときに浄化能が高いと評価している。この評価方法を用いることにより、従来の５０％浄化温度を指標とした評価方法に比べて、始動時など低温域から短時間の間の浄化能をより正確に評価することができ、実際のエミッションをより正確に推定することができる。したがって排ガスやモデルガスを用いて実験することなく排ガス浄化用触媒の浄化能を評価することができ、排ガス浄化用触媒の開発期間を大きく短縮することが可能となる。
【００２９】
触媒担持層を構成する酸化物の熱容量Ｃ１ と熱伝導率λ１ を測定するには、触媒担持層を掻き取り、比熱測定装置などを用いて行うことができる。
【００３０】
【実施例】
以下、実施例及び比較例により本発明を具体的に説明する。
【００３１】
（実施例１）
Ａｌ２Ｏ３粉末 １００重量部と、ＣｅＯ２−ＺｒＯ２複合酸化物粉末（モル比Ｃｅ／Ｚｒ＝６５／３５）５０重量部と、バインダーとしてのアルミナゾル（アルミナ含有量７０重量％）５重量部、イオン交換水 １００重量部を混合し、ボールミルにて湿式粉砕してスラリーを調製した。
【００３２】
一方、コーディエライト製のハニカム基材（体積 １．１Ｌ、セル密度 ４００セル／ｉｎ２ ）を用意し、上記スラリーに浸漬後に引き上げて余分なスラリーを吸引除去した後、 ２５０℃で１時間乾燥し ５００℃で１時間焼成してコート層を形成した。コート層はハニカム基材１Ｌあたり ２００ｇ形成された。
【００３３】
このコート層を掻き取り、レーザフラッシュ法で２５℃における熱伝導率（λ１ ）を、乾燥試料の重量と比熱より２５℃における熱容量（Ｃ１ ）を測定した。そしてＣ１ ２／λ１ を算出し、結果を触媒Ａとして表１に示す。
【００３４】
このコート層をもつハニカム基材を、所定濃度のジニトロジアンミン白金水溶液に浸漬し、引き上げて余分な水滴を吹き払った後、 ２５０℃で１時間乾燥してコート層にＰｔを担持した。次いで所定濃度の硝酸ロジウム水溶液に浸漬し、引き上げて余分な水滴を吹き払った後、 ２５０℃で１時間乾燥してコート層にＲｈを担持した。Ｐｔの担持量はコート層に対して ０．７重量％であり、Ｒｈの担持量はコート層に対して ０．２重量％である。こうして触媒Ａを調製した。
【００３５】
Ａｌ２Ｏ３粉末とＣｅＯ２−ＺｒＯ２複合酸化物粉末との混合比を重量比で１：１としたこと以外は触媒Ａと同様にして、触媒Ｂを調製した。また触媒Ａと同様にして熱容量（Ｃ１ ）と熱伝導率（λ１ ）を測定した後にＣ１ ２／λ１ を算出し、結果を表１に示す。
【００３６】
ＣｅＯ２−ＺｒＯ２複合酸化物粉末として、モル比Ｃｅ／Ｚｒ＝５０／５０のものを用い、 Ａｌ２Ｏ３粉末とＣｅＯ２−ＺｒＯ２複合酸化物粉末との混合比を重量比で１：１としたこと以外は触媒Ａと同様にして、触媒Ｃを調製した。また触媒Ａと同様にして熱容量（Ｃ１ ）と熱伝導率（λ１ ）を測定した後にＣ１ ２／λ１ を算出し、結果を表１に示す。
【００３７】
ＣｅＯ２−ＺｒＯ２複合酸化物粉末として、モル比Ｃｅ／Ｚｒ＝２５／７５のものを用い、 Ａｌ２Ｏ３粉末とＣｅＯ２−ＺｒＯ２複合酸化物粉末との混合比を重量比で１：２としたこと以外は触媒Ａと同様にして、触媒Ｄを調製した。また触媒Ａと同様にして熱容量（Ｃ１ ）と熱伝導率（λ１ ）を測定した後にＣ１ ２／λ１ を算出し、結果を表１に示す。
【００３８】
ＣｅＯ２−ＺｒＯ２複合酸化物粉末として、モル比Ｃｅ／Ｚｒ＝２５／７５のものを用い、 Ａｌ２Ｏ３粉末とＣｅＯ２−ＺｒＯ２複合酸化物粉末との混合比を重量比で１：２としたこと以外は触媒Ａと同様にして、触媒Ｅを調製した。また触媒Ａと同様にして熱容量（Ｃ１ ）と熱伝導率（λ１ ）を測定した後にＣ１ ２／λ１ を算出し、結果を表１に示す。
【００３９】
ＣｅＯ２−ＺｒＯ２複合酸化物粉末として、モル比Ｃｅ／Ｚｒ＝２５／７５のものを用い、 Ａｌ２Ｏ３粉末とＣｅＯ２−ＺｒＯ２複合酸化物粉末との混合比を重量比で１：３としたこと以外は触媒Ａと同様にして、触媒Ｆを調製した。また触媒Ａと同様にして熱容量（Ｃ１ ）と熱伝導率（λ１ ）を測定した後にＣ１ ２／λ１ を算出し、結果を表１に示す。
【００４０】
ＣｅＯ２−ＺｒＯ２複合酸化物粉末として、モル比Ｃｅ／Ｚｒ＝１５／８５のものを用い、 Ａｌ２Ｏ３粉末とＣｅＯ２−ＺｒＯ２複合酸化物粉末との混合比を重量比で１：３としたこと以外は触媒Ａと同様にして、触媒Ｇを調製した。また触媒Ａと同様にして熱容量（Ｃ１ ）と熱伝導率（λ１ ）を測定した後にＣ１ ２／λ１ を算出し、結果を表１に示す。
【００４１】
【表１】
上記のようにして得られた触媒をそれぞれ排気量３Ｌのガソリンエンジンの排気系に搭載し、ストイキ雰囲気で燃焼された排ガスを触媒入りガス温度 ９００℃で５０時間流通させるエージングを行った。そしてエージング後に上記エンジンを２０００ ｒｐｍ、吸気管負圧 ４０ｋＰａ、Ａ／Ｆ＝１４．６の条件にて運転し、始動時から各触媒のＨＣ浄化率を温度と共に測定して、５０％ＨＣ浄化温度を算出した。各触媒のＣ１ ２／λ１ に対する５０％ＨＣ浄化温度の結果を図１に示す。
【００４３】
一方、上記エンジンをＬＡ＃４モードで運転し、始動時から １２０秒間に排出されたＨＣ量を測定した。結果を図２に示す。
【００４４】
図２から、触媒担持層のＣ１ ２／λ１ が小さい触媒ほど始動時から １２０秒間に排出されたＨＣ量が少なく、排出されたＨＣ量はＣ１ ２／λ１ の増加量に対してほぼ直線的に増加していることが明らかであり、Ｃ１ ２／λ１ ≦０．０４の範囲が好ましい値を示していることがわかる。しかし図１では、ＨＣ５０％浄化温度の序列とＣ１ ２／λ１ の序列は全く一致していないことから、始動直後の浄化能を評価するには、ＨＣ５０％浄化温度を指標とすることは好ましくなく、Ｃ１ ２／λ１ を指標とすることが望ましいことが明らかである。
【００４５】
（実施例２）
ハニカム基材１Ｌあたり １００ｇのコート量としたこと以外は実施例１と同様のコート層（下層）が形成された、Ｐｔの担持前の各コート付き基材（Ａ〜Ｇ）を用意した。この下層の、５０℃における熱容量（Ｃ２ ）と熱伝導率（λ２ ）は実施例１の各触媒Ａ〜Ｇと同様であり、それから算出されたＣ２ ２×λ２ を表２に示す。
【００４６】
次に、触媒Ｂの調製時に用いた、 Ａｌ２Ｏ３粉末とＣｅＯ２−ＺｒＯ２複合酸化物粉末との混合比を重量比で１：１とした混合粉末の所定量に、ジニトロジアンミン白金と硝酸ロジウムが所定濃度で溶解した水溶液の所定量とアルミナゾルの所定量を混合してスラリーを調製した。そして各コート付き基材（Ａ〜Ｇ）をこのスラリーに浸漬し、引き上げて余分なスラリーを吸引除去した後、 ２５０℃で１時間乾燥し５００℃で１時間焼成して、下層の表面にそれぞれ触媒担持層を形成した。触媒担持層はハニカム基材１Ｌあたりそれぞれ １００ｇ形成され、Ｐｔの担持量は触媒担持層と下層の合計量に対して ０．７重量％であり、Ｒｈの担持量は触媒担持層と下層の合計量に対して ０．２重量％である。こうして触媒Ａ’〜Ｇ’を調製した。触媒担持層の５０℃における熱容量（Ｃ１ ）と熱伝導率（λ１ ）は、実施例１の触媒Ｂと同様である。
【００４７】
【表２】
得られた触媒Ａ’〜Ｇ’について、実施例１と同様にしてエージングを行い、実施例１と同様にしてＨＣ５０％浄化温度を測定した。結果を図３に示す。また実施例１と同様にして、始動時から １２０秒間に排出されたＨＣ量を測定し、結果を図４に示す。
【００４９】
図４から、下層のＣ２ ２×λ２ が小さい触媒ほど始動時から １２０秒間に排出されたＨＣ量が少なく、排出されたＨＣ量はＣ２ ２×λ２ の増加量に対してほぼ直線的に増加していることが明らかであり、Ｃ２ ２×λ２ ≦１０の範囲が好ましい値を示していることがわかる。しかし図３では、ＨＣ５０％浄化温度の序列とＣ２ ２×λ２ の序列は一致しておらず、Ｃ２ ２×λ２ の序列に無関係にほぼ一定となっていることから、始動直後の浄化能を評価するには、ＨＣ５０％浄化温度を指標とすることは好ましくなく、Ｃ２ ２×λ２ を指標とすることが望ましいことが明らかである。
【００５０】
【発明の効果】
すなわち本発明の排ガス浄化用触媒によれば、始動時などの低温域において触媒活性が発現するまでの時間が短縮され、その結果、浄化能が向上する。
【００５１】
また本発明の浄化能の評価方法によれば、従来の５０％浄化温度を指標とした評価方法に比べて、始動時など低温域から短時間の間の浄化能をより正確に評価することができ、実際のエミッションをより正確に推定することができる。したがって排ガスやモデルガスを用いて実験することなく排ガス浄化用触媒の浄化能を評価することができ、排ガス浄化用触媒の開発期間を大きく短縮することが可能となる。
【図面の簡単な説明】
【図１】実施例１におけるＣ２ ２／λ２ とＨＣ５０％浄化温度との関係を示すグラフである。
【図２】実施例１におけるＣ２ ２／λ２ と始動時〜 １２０秒間のＨＣ排出量との関係を示すグラフである。
【図３】実施例２におけるＣ２ ２×λ２ とＨＣ５０％浄化温度との関係を示すグラフである。
【図４】実施例２におけるＣ２ ２×λ２ と始動時〜 １２０秒間のＨＣ排出量との関係を示すグラフである。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an exhaust gas purifying catalyst capable of efficiently purifying exhaust gas discharged from an internal combustion engine such as an automobile engine, and a method for evaluating the purifying ability thereof.
[0002]
[Prior art]
Oxidation catalyst in an exhaust system of an automobile, a three-way catalyst, an exhaust gas purifying catalyst such as the NO x storage reduction catalyst is mounted, and purify HC in exhaust gas, CO, harmful components such as NO x. The noble metal such as those of the exhaust gas purifying catalyst is Pt supported as a catalyst metal, HC, by oxidizing CO and purified by the catalytic action, and purifying by reducing NO x.
[0003]
However, catalytic metals such as Pt do not exhibit catalytic activity in a low temperature range, and exhibit catalytic activity at a predetermined activation temperature or higher. Therefore, at the time of starting or the like, it is difficult to purify harmful components in the exhaust gas until the exhaust gas temperature rises to the activation temperature of the catalytic metal. However, in recent years, exhaust gas regulations have been further strengthened due to increased awareness of environmental issues, and it has been demanded for an exhaust gas purifying catalyst to activate the catalytic metal in as short a time as possible from the start.
[0004]
Therefore, an exhaust gas purifying catalyst is disposed immediately below the engine so that the exhaust gas having the highest possible temperature flows into the exhaust gas purifying catalyst. In addition, since the exhaust gas purifying catalyst is arranged in a state housed in the outer cylinder, it is also possible to interpose a heat insulating material between the exhaust gas purifying catalyst and the outer cylinder to make it difficult for the exhaust gas purifying catalyst to be cooled. I have. With this configuration, the high-temperature exhaust gas immediately flows into the exhaust gas purification catalyst, and the heat of the exhaust gas purification catalyst is prevented from escaping to the outside by heat conduction. The purification catalyst can be heated, and warm-up is promoted.
[0005]
On the other hand, as a countermeasure on the exhaust gas purifying catalyst side, a large amount of a catalytic metal is supported on an exhaust gas inflow side, and an active point is increased by finely dispersing and supporting a catalytic metal finely.
[0006]
However, each of these measures has advantages and disadvantages, and it has not yet been satisfied that the catalytic metal is activated in a short time as possible from a low temperature range such as at the time of start-up, so that the purification activity can be quickly expressed.
[0007]
Also, as an index of the purification ability of the exhaust gas purification catalyst, a 50% purification temperature is generally used. For example, the purification rate of HC by the oxidation catalyst is zero in a low temperature range, but increases as the temperature rises. When the exhaust gas temperature is 400 ° C. or higher, the purification rate becomes almost 100%. Therefore, the temperature at which the HC purification rate becomes 50% is measured and set as the
[0008]
However, when plotting the actual purification behavior with the temperature on the horizontal axis and the purification rate on the vertical axis, a comparison is made between a curve that suddenly rises at a certain temperature and a curve in which the HC purification rate rises gently with increasing temperature. Then, it was difficult to correctly evaluate the purifying ability in a low temperature range by the evaluation based on the 50% purification temperature.
[0009]
[Patent Document 1] Japanese Patent No. 3235640 [Patent Document 2] Japanese Patent Application Laid-Open No. 09-299797 [Patent Document 3] Japanese Patent Application Laid-Open No. 08-038889 [0010]
[Problems to be solved by the invention]
The present invention has been made in view of such circumstances, and provides an exhaust gas purifying catalyst which can activate a catalytic metal from a low temperature range in a short time as possible and has a high purification activity in a low temperature range, and also has a purification function in the low temperature range. An object of the present invention is to provide an evaluation method capable of accurately evaluating an activity.
[0011]
[Means for Solving the Problems]
The feature of the exhaust gas purifying catalyst of the present invention that solves the above problems is that the exhaust gas purifying catalyst comprises a base material, a catalyst supporting layer formed on the surface of the base material, and a noble metal supported on the catalyst supporting layer. a catalyst, catalyst supporting layer, C 1 heat capacity, when the thermal conductivity was lambda 1, is to become an oxide satisfying C 1 2 / λ 1 ≦ 0.04 .
[0012]
In the exhaust gas purifying catalyst, a lower layer made of an oxide that satisfies C 2 2 × λ 2 ≦ 10 is provided between the catalyst supporting layer and the base material, where the heat capacity is C 2 and the thermal conductivity is λ 2. it is desirable, oxides underlayers it is desirable to further satisfy C 2 2 / λ 2 ≦ 0.04 . Further, it is desirable that the catalyst supporting layer contains a CeO 2 -ZrO 2 composite oxide.
[0013]
The method for evaluating the purification ability of the present invention is characterized in that the purification of an exhaust gas purification catalyst comprising a base material, a catalyst support layer formed on the surface of the base material, and a noble metal supported on the catalyst support layer. a method for evaluating ability, C 1 the heat capacity of the oxide constituting the catalyst carrier layer, when the thermal conductivity was lambda 1, the high purification performance when the C 1 2 / λ 1 ≦ 0.04 To evaluate.
[0014]
In the evaluation method of the present invention, between the catalyst-carrying layer and the substrate, the heat capacity C 2, comprises a lower thermal conductivity is made of oxide of lambda 2, further when the C 2 2 × λ 2 ≦ 10 It is also preferable to evaluate that the purification ability is high.
[0015]
BEST MODE FOR CARRYING OUT THE INVENTION
In the present invention, the heat capacity C has a dimension expressed by J / g · K (J: joule, g: gram, K: Kelvin), and the thermal conductivity λ has a dimension of W / m · K (W: Watts, m: meters, K: kelvin).
[0016]
The exhaust gas purifying catalyst of the present invention, C 1 heat capacity, when the thermal conductivity was lambda 1, and a catalyst supporting layer made of an oxide that satisfies C 1 2 / λ 1 ≦ 0.04 . Since this oxide has a high thermal conductivity λ 1 and a small heat capacity C 1 , the thermal energy of the exhaust gas is efficiently absorbed by the catalyst supporting layer, and the exhaust gas quickly warms up to the activation temperature of the noble metal. Therefore, even at the time of starting or the like, the time until the catalytic activity is developed is shortened, and as a result, the purification performance is improved. In C 1 2 / λ 1> 0.04 , the expression of the advantageous effects is difficult.
[0017]
The oxide contained in the catalyst supporting layer preferably has a heat capacity C 1 at 25 ° C. in the range of 0.2 to 1.2 J / g · K, and a thermal conductivity λ 1 at 25 ° C. of 0.8 to 70 W / m · K. The range of K is preferred. There is no oxide suitable for the catalyst-supporting layer in oxides having C 1 <0.2 J / g · K or λ 1 > 70 W / m · K. The or a C 1> 1.2J / g · K , the oxide such that λ 1 <0.8W / m · K , it is difficult to satisfy C 1 2 / λ 1 ≦ 0.04 .
[0018]
The seeds oxide satisfying C 1 2 / λ 1 ≦ 0.04 , alumina, zirconia, titania, ceria, silica - alumina, those selected from single or composite oxides such as silica alone or in a plurality of types, They can be used in combination. When mixed multiple species, heat capacity and thermal conductivity of the mixture should satisfy C 1 2 / λ 1 ≦ 0.04 . Among them, it is particularly preferable to include a CeO 2 —ZrO 2 composite oxide. Since the CeO 2 -ZrO 2 composite oxide has a high oxygen storage / release capability and is excellent in stability, the activity of purifying HC and CO is particularly improved. Then, by including the CeO 2 -ZrO 2 composite oxide satisfying C 1 2 / λ 1 ≦ 0.04 , can purification performance of HC and CO in low-temperature region is high, maintains the high purification performance long term It becomes.
[0019]
CeO 2 -ZrO 2 composite oxide is preferably contained in the catalyst supporting layer 20% by weight or more. If the content of this oxide is less than 20% by weight, the oxygen storage / release capability is insufficient, making it difficult to absorb fluctuations in the exhaust gas atmosphere, and the HC and CO purification capabilities are reduced.
[0020]
As the base material, a heat-resistant ceramic material such as cordierite or a metal material can be used, and a conventionally used material such as a pellet, a honeycomb, or a foam can be used.
[0021]
The warming property of the exhaust gas purifying catalyst also depends on the amount of the catalyst supporting layer formed. Therefore, it is desirable to form the catalyst supporting layer in an amount of 100 to 300 g per liter of the substrate. If the formation amount of the catalyst supporting layer is less than this range, it contributes to the warm-up property, but the supporting density of the noble metal increases, so that the grain growth of the noble metal becomes remarkable at high temperature heat resistance and the heat resistance decreases. On the other hand, when the amount of the catalyst-supporting layer is larger than this range, the warm-up property is reduced and the airflow resistance is increased.
[0022]
The noble metal supported on the catalyst supporting layer can be selected from Pt, Rh, Pd, Ir, and the like, and can be used alone or in combination. The supporting amount may be about 0.1 to 10 g per liter of the substrate, which is the same as the conventional one. Also, if the NO x storage-reduction catalyst, an alkali metal with the precious metal, the NO x storage material selected from alkaline earth metals and rare earth elements is supported.
[0023]
Between the catalyst-carrying layer and the substrate, the heat capacity C 2, when the thermal conductivity was lambda 2, it is desirable to have a lower layer made of an oxide that satisfies C 2 2 × λ 2 ≦ 10 . By having the lower layer made of an oxide having a low heat capacity and a high thermal conductivity as described above, the heat energy of the catalyst supporting layer can be suppressed from being taken away by the base material, and the lower layer itself has excellent warm-up properties. Warmability of the entire coat layer can be kept high. It is particularly preferable that the oxide constituting the lower layer also satisfies C 2 2 / λ 2 ≦ 0.04.
[0024]
When the coating layer has a two-layer structure as described above, the coating amount of the lower layer is desirably 30 to 200 g per liter of the base material. When the formation amount of the lower layer is smaller than this range, it is difficult to exhibit the above-mentioned effect, and when the formation amount of the lower layer is larger than this range, the warm-up property is reduced. For the same reason as described above, the total of the lower layer and the catalyst supporting layer is desirably 100 to 300 g per liter of the base material.
[0025]
As the oxide satisfying C 2 2 × λ 2 ≦ 10 in the lower layer, an oxide selected from simple or composite oxides such as alumina, zirconia, titania, ceria, silica-alumina, and silica may be used alone or in combination of two or more. Can be used. When mixed multiple species, heat capacity and thermal conductivity of the mixture should satisfy C 2 2 × λ 2 ≦ 10 . Among them, it is particularly preferable to include a CeO 2 —ZrO 2 composite oxide. Since the CeO 2 -ZrO 2 composite oxide has a high oxygen storage / release capability and is excellent in stability, the activity of purifying HC and CO is particularly improved. By including the CeO 2 -ZrO 2 composite oxide satisfying C 2 2 × λ 2 ≦ 10, the purifying ability of HC and CO in a low temperature region is high, and the high purifying ability can be maintained for a long time. .
[0026]
It is desirable that the CeO 2 -ZrO 2 composite oxide be contained in the lower layer in an amount of 20% by weight or more. If the content of this oxide is less than 20% by weight, the oxygen storage / release capability is insufficient, making it difficult to absorb fluctuations in the exhaust gas atmosphere, and the HC and CO purification capabilities are reduced.
[0027]
The lower layer need not carry a noble metal, but preferably carries a noble metal in the same manner as the catalyst carrying layer. By doing so, it is possible to suppress an increase in the noble metal loading density in the catalyst supporting layer, and to suppress the grain growth of the noble metal.
[0028]
And in the evaluation method of the purification ability of the present invention, purification performance at the time of the C 1 to the heat capacity of the oxide constituting the catalyst carrier layer, when the thermal conductivity was λ 1, C 1 2 / λ 1 ≦ 0.04 Rate is high. By using this evaluation method, it is possible to more accurately evaluate the purification performance in a short time from a low temperature region, such as at the time of starting, as compared with a conventional evaluation method using a 50% purification temperature as an index. Can be more accurately estimated. Therefore, it is possible to evaluate the purifying ability of the exhaust gas purifying catalyst without conducting an experiment using the exhaust gas or the model gas, and it is possible to greatly shorten the development period of the exhaust gas purifying catalyst.
[0029]
The measurement of the heat capacity C 1 and the thermal conductivity λ 1 of the oxide constituting the catalyst supporting layer can be performed by scraping the catalyst supporting layer and using a specific heat measuring device or the like.
[0030]
【Example】
Hereinafter, the present invention will be specifically described with reference to Examples and Comparative Examples.
[0031]
(Example 1)
100 parts by weight of Al 2 O 3 powder, 50 parts by weight of CeO 2 -ZrO 2 composite oxide powder (molar ratio Ce / Zr = 65/35), and 5 parts by weight of alumina sol (alumina content 70% by weight) as a binder And 100 parts by weight of ion-exchanged water were mixed and wet-pulverized with a ball mill to prepare a slurry.
[0032]
On the other hand, a cordierite honeycomb substrate (volume: 1.1 L, cell density: 400 cells / in 2 ) was prepared, pulled up after dipping in the above slurry, suctioned off excess slurry, and dried at 250 ° C. for 1 hour. Then, it was baked at 500 ° C. for 1 hour to form a coat layer. The coating layer was formed in an amount of 200 g per liter of the honeycomb substrate.
[0033]
The coated layer was scraped off, and the thermal conductivity (λ 1 ) at 25 ° C. was measured by a laser flash method, and the heat capacity (C 1 ) at 25 ° C. was measured from the weight and specific heat of the dried sample. Then calculate the C 1 2 / lambda 1, shown in Table 1 the results as catalyst A.
[0034]
The honeycomb substrate having the coat layer was immersed in an aqueous solution of dinitrodiammine platinum having a predetermined concentration, pulled up to blow off excess water droplets, and dried at 250 ° C. for 1 hour to carry Pt on the coat layer. Next, the coated layer was immersed in a predetermined concentration of rhodium nitrate aqueous solution, pulled up and blown off excess water droplets, and dried at 250 ° C. for 1 hour to carry Rh on the coat layer. The supported amount of Pt was 0.7% by weight based on the coat layer, and the supported amount of Rh was 0.2% by weight based on the coat layer. Thus, catalyst A was prepared.
[0035]
A catalyst B was prepared in the same manner as the catalyst A, except that the mixing ratio of the Al 2 O 3 powder and the CeO 2 -ZrO 2 composite oxide powder was 1: 1 by weight. The heat capacity in the same manner as catalyst A (C 1) and the thermal conductivity (lambda 1) to calculate the C 1 2 / lambda 1 after measuring and the results are shown in Table 1.
[0036]
The CeO 2 -ZrO 2 composite oxide powder having a molar ratio of Ce / Zr = 50/50 was used, and the mixing ratio of the Al 2 O 3 powder and the CeO 2 -ZrO 2 composite oxide powder was 1: Except having changed to 1, it carried out similarly to catalyst A, and prepared catalyst C. The heat capacity in the same manner as catalyst A (C 1) and the thermal conductivity (lambda 1) to calculate the C 1 2 / lambda 1 after measuring and the results are shown in Table 1.
[0037]
A CeO 2 -ZrO 2 composite oxide powder having a molar ratio of Ce / Zr = 25/75 was used, and the mixing ratio of the Al 2 O 3 powder and the CeO 2 -ZrO 2 composite oxide powder was 1: A catalyst D was prepared in the same manner as the catalyst A except that the catalyst D was 2. The heat capacity in the same manner as catalyst A (C 1) and the thermal conductivity (lambda 1) to calculate the C 1 2 / lambda 1 after measuring and the results are shown in Table 1.
[0038]
A CeO 2 -ZrO 2 composite oxide powder having a molar ratio of Ce / Zr = 25/75 was used, and the mixing ratio of the Al 2 O 3 powder and the CeO 2 -ZrO 2 composite oxide powder was 1: A catalyst E was prepared in the same manner as the catalyst A except that the catalyst was set to 2. The heat capacity in the same manner as catalyst A (C 1) and the thermal conductivity (lambda 1) to calculate the C 1 2 / lambda 1 after measuring and the results are shown in Table 1.
[0039]
A CeO 2 -ZrO 2 composite oxide powder having a molar ratio of Ce / Zr = 25/75 was used, and the mixing ratio of the Al 2 O 3 powder and the CeO 2 -ZrO 2 composite oxide powder was 1: A catalyst F was prepared in the same manner as the catalyst A, except that the catalyst was set to 3. The heat capacity in the same manner as catalyst A (C 1) and the thermal conductivity (lambda 1) to calculate the C 1 2 / lambda 1 after measuring and the results are shown in Table 1.
[0040]
The CeO 2 -ZrO 2 composite oxide powder having a molar ratio of Ce / Zr = 15/85 was used, and the mixing ratio of the Al 2 O 3 powder and the CeO 2 -ZrO 2 composite oxide powder was 1: A catalyst G was prepared in the same manner as the catalyst A except that the catalyst was set to 3. The heat capacity in the same manner as catalyst A (C 1) and the thermal conductivity (lambda 1) to calculate the C 1 2 / lambda 1 after measuring and the results are shown in Table 1.
[0041]
[Table 1]
Each of the catalysts obtained as described above was mounted on an exhaust system of a gasoline engine having a displacement of 3 L, and aging was performed in which exhaust gas burned in a stoichiometric atmosphere was allowed to flow at a catalyst-containing gas temperature of 900 ° C. for 50 hours. After aging, the engine was operated under the conditions of 2000 rpm, negative pressure of the intake pipe of 40 kPa, and A / F = 14.6. From the start, the HC purification rate of each catalyst was measured together with the temperature. Was calculated. The results of 50% HC purification temperature for C 1 2 / lambda 1 of each catalyst shown in FIG.
[0043]
On the other hand, the engine was operated in the LA # 4 mode, and the amount of HC discharged for 120 seconds from the start was measured. FIG. 2 shows the results.
[0044]
From Figure 2, C 1 2 / λ HC amount less discharged to 1 120 seconds from the start smaller catalyst of the catalyst-carrying layer, HC amount discharged for increase of C 1 2 / lambda 1 it is clear that increases substantially linearly, it is understood that the scope of C 1 2 / λ 1 ≦ 0.04 indicates a preferred value. However, in Figure 1, since the order of hierarchy and C 1 2 / lambda 1 of HC50% purification temperature not at all coincide, to evaluate purification performance immediately after starting is to an index HC50% purification temperature not preferred, it is apparent that it is desirable to index the C 1 2 / λ 1.
[0045]
(Example 2)
Except that the coating amount was 100 g per liter of the honeycomb base material, each coated base material (A to G) before carrying Pt was prepared in which the same coating layer (lower layer) as in Example 1 was formed. The lower layer, shown heat capacity at 50 ° C. and (C 2) thermal conductivity (lambda 2) is the same as the catalyst A~G of Example 1, the C 2 2 × lambda 2 therefrom is calculated in Table 2 .
[0046]
Next, dinitrodiammine platinum and dinitrodiammineplatinum were added to a predetermined amount of the mixed powder used in the preparation of the catalyst B, in which the mixing ratio of the Al 2 O 3 powder and the CeO 2 -ZrO 2 composite oxide powder was 1: 1 by weight. A predetermined amount of an aqueous solution in which rhodium nitrate was dissolved at a predetermined concentration and a predetermined amount of alumina sol were mixed to prepare a slurry. Then, each coated substrate (A to G) is immersed in this slurry, pulled up, and the excess slurry is removed by suction. Then, it is dried at 250 ° C. for 1 hour and baked at 500 ° C. for 1 hour. A catalyst supporting layer was formed. The catalyst supporting layer is formed in an amount of 100 g per liter of the honeycomb substrate. The amount of Pt supported is 0.7% by weight based on the total amount of the catalyst supporting layer and the lower layer, and the amount of Rh supported is the total of the catalyst supporting layer and the lower layer. It is 0.2% by weight based on the amount. Thus, catalysts A ′ to G ′ were prepared. The heat capacity (C 1 ) and the thermal conductivity (λ 1 ) of the catalyst supporting layer at 50 ° C. are the same as those of the catalyst B of Example 1.
[0047]
[Table 2]
The obtained catalysts A ′ to G ′ were aged in the same manner as in Example 1, and the
[0049]
From Figure 4, lower C 2 2 × lambda HC amount less discharged to 2 120 seconds from the start smaller catalyst, HC amount discharged is substantially linear with increasing amounts of C 2 2 × lambda 2 manner it is apparent that has increased, it can be seen that the range of C 2 2 × λ 2 ≦ 10 indicates a preferred value. However, in FIG. 3, the order of the
[0050]
【The invention's effect】
That is, according to the exhaust gas purifying catalyst of the present invention, the time required for the catalytic activity to be exhibited in a low temperature range such as at the time of starting is reduced, and as a result, the purifying ability is improved.
[0051]
Further, according to the purification ability evaluation method of the present invention, it is possible to more accurately evaluate the purification ability in a short time from a low temperature region, such as at the start, as compared with the conventional evaluation method using a 50% purification temperature as an index. It is possible to estimate the actual emission more accurately. Therefore, it is possible to evaluate the purifying ability of the exhaust gas purifying catalyst without conducting an experiment using the exhaust gas or the model gas, and it is possible to greatly shorten the development period of the exhaust gas purifying catalyst.
[Brief description of the drawings]
FIG. 1 is a graph showing a relationship between C 2 2 / λ 2 and an
FIG. 2 is a graph showing the relationship between C 2 2 / λ 2 and the amount of HC discharged from the time of starting to 120 seconds in Example 1.
FIG. 3 is a graph showing a relationship between C 2 2 × λ 2 and an
FIG. 4 is a graph showing the relationship between C 2 2 × λ 2 and the amount of HC discharged from the time of starting up to 120 seconds in Example 2.
Claims (6)
該触媒担持層は、熱容量をＣ１ 、熱伝導率をλ１ としたとき、Ｃ１ ２／λ１ ≦０．０４を満たす酸化物よりなることを特徴とする排ガス浄化用触媒。A base material, a catalyst support layer formed on the surface of the base material, and a noble metal supported on the catalyst support layer, an exhaust gas purification catalyst comprising:
The catalyst carrying layer, C 1 heat capacity, when the thermal conductivity was lambda 1, an exhaust gas purifying catalyst characterized by comprising an oxide satisfying C 1 2 / λ 1 ≦ 0.04 .
該触媒担持層を構成する酸化物の熱容量をＣ１ 、熱伝導率をλ１ としたとき、Ｃ１ ２／λ１ ≦０．０４のときに浄化能が高いと評価することを特徴とする浄化能の評価方法。A substrate, a catalyst supporting layer formed on the surface of the substrate, and a noble metal supported on the catalyst supporting layer, a method for evaluating the purification performance of an exhaust gas purifying catalyst comprising:
C 1 the heat capacity of the oxide constituting the catalyst carrier layer, when the thermal conductivity was lambda 1, and evaluating the purifying ability when C 1 2 / λ 1 ≦ 0.04 is high Evaluation method of purification ability.
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| JP2003200062A (en) * | 2001-10-26 | 2003-07-15 | Denso Corp | Catalyst for vehicle |
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| JPS63258648A (en) * | 1987-04-15 | 1988-10-26 | Mazda Motor Corp | Catalyst for purifying exhaust gas |
| JPH06269682A (en) * | 1993-03-22 | 1994-09-27 | Nissan Motor Co Ltd | Electric heating type catalyst for purifying exhaust gas |
| JPH08229395A (en) * | 1995-02-24 | 1996-09-10 | Mazda Motor Corp | Exhaust gas purifying catalyst |
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