JP2004243272A - Catalyst for purification of exhaust gas - Google Patents
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Abstract
Description
【０００１】
【発明の属する技術分野】
本発明は排気ガス浄化触媒に係り、更に詳細には、リーン域の窒素酸化物（ＮＯｘ）浄化性能及び低温域の炭化水素（ＨＣ）浄化性能に優れた排気ガス浄化触媒に関する。
【０００２】
【従来の技術】
従来から、リーン域でＮＯｘを浄化する触媒は種々提案されており、例えば白金（Ｐｔ）とランタン（Ｌａ）を多孔質担体に担持した触媒（例えば、特許文献１参照。）に代表されるように、リーン域でＮＯｘを吸着し、理論空燃比（ストイキ）からリッチ域の時にＮＯｘを放出させて浄化する触媒が知られている。
また、低温域でＨＣを浄化する触媒としては、ゼオライトを含む触媒が提案されている（例えば、特許文献２参照。）。
更に、２層以上の積層構造を備え、表層と内層の比較において、貴金属含有比とアルカリ土類金属含有比の双方とも表層が大きい触媒が提案されている（例えば、特許文献３参照。）。
【０００３】
【特許文献１】
特開平５−１６８８６０号公報
【特許文献２】
特開平１１−４７５９６号公報
【特許文献３】
特開２００１−１２９４０３号公報
【０００４】
【発明が解決しようとする課題】
上述したＮＯｘ吸着浄化触媒とＨＣ吸着浄化触媒とを組み合わせて、ＮＯｘ吸着浄化機能とＨＣ吸着浄化機能の両機能を併有する排気ガス浄化触媒を作成できれば便利であるが、かかる両機能を１つの触媒に兼備させようとすると、以下のような不具合を生じ実現が困難であった。
【０００５】
即ち、ＨＣ吸着材であるゼオライトを含むゼオライト層上に、ＮＯｘ吸着材であるアルカリ金属等と貴金属を含む貴金属層を積層し、下層のゼオライト層で低温ＨＣの吸収し、上層の貴金属層でＮＯｘの吸着、浄化、及び下層から脱離したＨＣの浄化を行わせることが考えられるが、この場合、まず、ハニカム担体などの一体構造型担体にゼオライト層をコートし、次いで、貴金属を含むアルミナ層をコートし、更にアルカリ金属等を含浸させて触媒を製造する必要がある。
【０００６】
しかし、かかる製造法によれば、アルカリ金属等がゼオライト層にかなり吸収されてしまい、そのＨＣ吸着機能が十分には発揮できなくなる。これに対し、炭酸バリウム（ＢａＣＯ３）のような固体のアルカリ金属化合物を用い、これと貴金属とアルミナを混合してスラリーを調製し、このスラリーをゼオライト層にコートすることが考えられ、この手法によって、確かにゼオライト層へのアルカリ金属等の吸収の大部分は抑制できる。
しかしながら、この手法によれば、アルカリ金属化合物として固体のものを用いるので、アルカリ金属の結晶子径が大きくなってしまい、リーン域におけるＮＯｘ吸着性能が悪化するという問題点があった。
【０００７】
一方、ゼオライトは嵩高く、ＨＣ吸着浄化性能を十分に発揮できるだけの量のゼオライトをハニカム担体上にコートしようとすると、担体のセルが目詰まりして排圧が高くなってしまうので、ゼオライト層上には僅かな量のＮＯｘ浄化層しか設けることができず、十分なＮＯｘ浄化性能を発揮することができなくなる。
更に、上述した表層と内層の比較において、貴金属含有比とアルカリ土類金属含有比の双方とも表層が大きい触媒は、ゼオライト層がないので低温域でのＨＣの浄化ができず、また内層の厚みが記載されていないので、表層からのアルカリ土類金属の溶出を抑制できるかが不明であった。
【０００８】
本発明は、このような従来技術の有する課題に鑑みてなされたものであり、その目的とするところは、ＨＣ浄化性能及びＮＯｘ浄化性能を兼備した排気ガス浄化触媒を提供することにある。
【０００９】
【課題を解決するための手段】
本発明者らは、上記目的を達成すべく鋭意検討を重ねた結果、ＨＣ吸着層、ＮＯｘを吸着・浄化し且つＨＣを浄化する吸着浄化内層及び吸着浄化外層の三層構造とし、吸着浄化内層及び外層に含まれるアルカリ金属等の含有量を適切に調整し、吸着浄化内層の厚さを所定範囲に調整することなどにより、上記目的が達成できることを見出し、本発明を完成するに至った。
【００１０】
即ち、本発明の排気ガス浄化触媒は、一体構造型担体に、ＨＣ吸着層、ＮＯｘを吸着・浄化し且つＨＣを浄化する吸着浄化内層及び外層をこの順で積層して成る。
かかるＨＣ吸着層はゼオライトを含有し、吸着浄化内層及び外層は、白金（Ｐｔ）、ロジウム（Ｒｈ）及びパラジウム（Ｐｄ）から成る群より選ばれた少なくとも１種の貴金属と、アルカリ金属及び／又はアルカリ土類金属と、耐火性無機酸化物を含有する。
また、本発明の排気ガス浄化触媒では、吸着浄化内層に含まれるアルカリ金属及び／又はアルカリ土類金属と吸着浄化外層に含まれるアルカリ金属及び／又はアルカリ土類金属とのモル比が１／１〜１／５０であり、吸着浄化内層の層の厚さが１０〜２００μｍである。
【００１１】
【発明の実施の形態】
以下、本発明の排気ガス浄化触媒について詳細に説明する。なお、本明細書において、「％」は特記しない限り質量百分率を表すものとする。
【００１２】
上述の如く、本発明の排気ガス浄化触媒は、一体構造型担体に、ＨＣを吸脱着するＨＣ吸着層、ＮＯｘを吸着・浄化し且つＨＣを浄化する吸着浄化内層及び外層をこの順で積層して成り、該ＨＣ吸着層がゼオライトを含有し、該吸着浄化内層及び外層が、Ｐｔ、Ｒｈ又はＰｄ及びこれらの任意の組合せに係る貴金属と、アルカリ金属及びアルカリ土類金属の一方又は双方と、耐火性無機酸化物を含有する。
このような、ＨＣ吸着層（ゼオライト層）、ＮＯｘを吸着・浄化し且つＨＣを浄化する吸着浄化内層及び外層をこの順でハニカム担体に担持した構造を備えることにより、低温域でのＨＣ浄化とリーン域でのＮＯｘ浄化を１つの触媒で行なうことができる。
なお、一体構造型担体としては、例えばコーディエライトなどのセラミックスやフェライト系ステンレスなどの金属等の耐熱性材料から成るモノリス担体やハニカム担体が用いられる。
【００１３】
また、本発明の排気ガス浄化触媒では、吸着浄化内層に含まれるアルカリ金属及びアルカリ土類金属の一方又は双方と吸着浄化外層に含まれるアルカリ金属及びアルカリ土類金属の一方又は双方とのモル比（内層／外層）が１／１〜１／５０であることを要する。
吸着浄化内層と外層のアルカリ金属及びアルカリ土類金属の一方又は双方の含有モル比が１／１より吸着浄化内層の方が大きいと、ＨＣ脱離後の浄化性能が低下する可能性があり、１／５０より吸着浄化内層の方が小さいと、吸着浄化内層及び外層としての効果が薄れ、吸着浄化性能が低下する可能性がある。
【００１４】
更に、吸着浄化内層の層の厚さは１０〜２００μｍであることを要する。
吸着浄化内層の層の厚さが１０μｍ未満では、触媒調整時において、吸着浄化外層からのアルカリ金属、アルカリ土類金属のＨＣ吸着層（ゼオライト層）への溶出を十分には抑制できず、ゼオライト層へアルカリ金属、アルカリ土類金属が溶出することによるＨＣ吸着性能の低下を引き起こす可能性があり、吸着浄化内層の層の厚さが２００μｍを超えると、吸着浄化内層及び外層が厚くなりすぎ、ＨＣのゼオライト層への拡散を阻害するためＨＣ吸着性能が低下する可能性がある。
【００１５】
また、本発明の触媒では、ＨＣ吸着層と（吸着浄化内層と吸着浄化外層の和）のコート層重量比は２／１〜１／１０であることが好ましい。
ＨＣ吸着層と（吸着浄化内層と吸着浄化外層の和）のコート層重量比が２／１より大きく、ＨＣ吸着層重量が多いと、脱離してきたＨＣを吸着浄化内層及び外層で浄化しきれない可能性がある。一方、コート重量比が１／１０より小さく、（吸着浄化内層と吸着浄化外層の和）のコート層重量が多いと、ＨＣ吸着を阻害する可能性がある。
【００１６】
アルカリ金属及びアルカリ土類金属の一方又は双方が粉末形態で吸着浄化内層及び外層に含まれていることが好ましい。
本発明の排気ガス浄化触媒を作製する際に、アルカリが所定の濃度分布で含まれれば、特に限定されるものではないが、アルカリを溶液で含浸させるとゼオライト層にアルカリがしみ込んでしまうので、粉末形態を保持した状態であるスラリーを用いて担持することが望ましい。
【００１７】
更に、アルカリ金属及びアルカリ土類金属が、吸着浄化内層を基準として０．５〜５％の割合で含まれ、アルカリ金属及びアルカリ土類金属の一方又は双方が、吸着浄化外層を基準として５〜１５％の割合で含まれることが好ましい。
吸着浄化内層においては、含有するアルカリ金属及びアルカリ土類金属の濃度が、０．５％未満ではＮＯｘ吸着性能が低下するため、ＮＯｘ浄化性能が著しく低下し、５％を超えると塩基性が強くなるため、ＨＣ浄化性能が低下する可能性がある。
一方、吸着浄化外層においては、含有するアルカリ金属及びアルカリ土類金属の濃度が、５％未満ではＮＯｘ吸着性能が著しく低下するため、ＮＯｘ浄化性能が低下し、１５％を超えると、耐久によりアルカリ金属、アルカリ土類金属の粒子が成長して基材の細孔閉塞等を引き起こし、ＮＯｘ浄化性能に悪影響を及ぼす可能性がある。
【００１８】
また、使用する耐火性無機酸化物としては、耐熱性があり、貴金属を担持できれば特に限定されるものではなく、代表的にはアルミナ等を挙げることができるが、更にセリウム（Ｃｅ）又はランタン（Ｌａ）を担持したアルミナであることが望ましい。アルミナにＣｅやＬａを担持することにより、排気ガス浄化触媒の浄化性能を向上することができる。
【００１９】
更に、本発明の触媒では、吸着浄化内層の貴金属がＰｔとＰｄから成り、吸着浄化外層の貴金属がＲｈから成ることが望ましい。
ＨＣ吸着層から脱離したＨＣの浄化にはアルカリ金属及びアルカリ土類金属の含有比が小さい層、即ち吸着浄化内層にＨＣ浄化性能に優れた貴金属、即ちＰｔやＰｄが含有されることが有効である。また、吸着浄化外層には、特にＲｈを担持することによりＮＯｘ浄化性能を向上することができる。
【００２０】
【実施例】
以下、本発明を実施例及び比較例により更に詳細に説明するが、本発明はこれら実施例に限定されるものではない。
【００２１】
（実施例１）
酢酸バリウム溶液の中にアルミナを投入して室温で１時間撹拌し、次いで、１２０℃で一昼夜乾燥し、更に６００℃で１時間焼成した。これに２％テトラアンミン白金水酸塩溶液（ｐＨ＝１０．５）を含浸し、次いで、１２０℃で一昼夜乾燥し、更に４００℃で１時間焼成し粉末を得た。これを粉末Ａとした。
酢酸バリウム溶液の中にアルミナを投入して室温で１時間撹拌し、次いで、１２０℃で一昼夜乾燥し、更に６００℃で１時間焼成した。これに６％テトラアンミン白金水酸塩溶液を含浸し、次いで、１２０℃で一昼夜乾燥し、更に４００℃で１時間焼成し粉末を得た。これを粉末Ｂとした。
アルミナに６％硝酸ロジウム水溶液を含浸し、次いで、１２０℃で一昼夜乾燥し、更に４００℃で１時間焼成し粉末を得た。これを粉末Ｃとした。
【００２２】
βゼオライトを７２０ｇ、シリカゾルを１８０ｇ、水９００ｇを磁性ボールミルに投入して混合粉砕しスラリーを得た。これを第１触媒スラリーとした。
粉末Ａを５５５．３ｇ、アルミナを２５．２ｇ、アルミナゾルを３０ｇ、水６１０ｇを磁性ボールミルに投入して混合粉砕しスラリーを得た。これを第２触媒スラリーとした。
粉末Ｃを３１８．６ｇ、粉末Ｂを４５３．６ｇ、アルミナを３７．８ｇ、アルミナゾルを９０ｇ、水９００ｇを磁性ボールミルに投入して混合粉砕しスラリーを得た。これを第３触媒スラリーとした。
【００２３】
第１触媒スラリーをコーディエライト質モノリス担体（触媒容量：１．２Ｌ、セル数：４００セル／平方インチ）に付着させ、空気流にてセル内の余剰のスラリーを取り除いて１３０℃で乾燥し、次いで、４００℃で１時間焼成し触媒を得た。これを触媒ａとした。
第２触媒スラリーを触媒ａに付着させ、空気流にてセル内の余剰のスラリーを取り除いて１３０℃で乾燥し、次いで、４００℃で１時間焼成し触媒を得た。これを触媒ｂとした。
第３触媒スラリーを触媒ｂに付着させ、空気流にてセル内の余剰のスラリーを取り除いて１３０℃で乾燥し、次いで、４００℃で１時間焼成し本例の排気ガス浄化触媒を得た。該触媒の第二層の厚さは５０μｍ、第二層と第三層におけるアルカリ金属等の含有モル比は１／６、第一層と（第二層と第三層の和）のコート量比は１／３、第二層のＢａの濃度は１％、第三層のＢａの濃度は６％であった。
【００２４】
（実施例２）
粉末Ｂの作製工程おいてアルミナにナトリウム（Ｎａ）を担持させ、第二層の層の厚さを１７０μｍとして、表１所定の比率とした以外は、実施例１と同様の操作を繰り返し、本例の排気ガス浄化触媒を得た。
【００２５】
（実施例３）
粉末Ｂの作製工程おいてアルミナにＢａとストロンチウム（Ｓｒ）を担持させ、第二層の層の厚さを１１０μｍとして、表１所定の比率とした以外は、実施例１と同様の操作を繰り返し、本例の排気ガス浄化触媒を得た。
【００２６】
（実施例４）
粉末Ｂの作製工程おいてアルミナにＢａとＮａを担持させ、第二層の層の厚さを１００μｍとして、表１所定の比率とした以外は、実施例１と同様の操作を繰り返し、本例の排気ガス浄化触媒を得た。
【００２７】
（実施例５）
粉末Ａの作製工程おいてアルミナにＢａとマグネシウム（Ｍｇ）を担持させ、第二層の層の厚さを１００μｍとして、表１所定の比率とした以外は、実施例１と同様の操作を繰り返し、本例の排気ガス浄化触媒を得た。
【００２８】
（実施例６）
粉末Ａの作製工程おいてアルミナにＢａとＳｒを担持させ、粉末Ｂの作製工程おいてアルミナにＢａとＳｒを担持させ、粉末Ａを５５５．３ｇ、アルミナを２５．２ｇ、βゼオライトを２２９．５ｇ、アルミナゾル９０ｇ、水９００ｇを磁性ボールミルに投入して混合粉砕し第２触媒スラリーを作製し、第二層の層の厚さを１９０μｍとして、表１所定の比率とした以外は、実施例１と同様の操作を繰り返し、本例の排気ガス浄化触媒を得た。
【００２９】
（比較例１）
第二層の層の厚さを２１０μｍとして、表１所定の比率とした以外は、実施例１と同様の操作を繰り返し、本例の排気ガス浄化触媒を得た。
【００３０】
（比較例２）
粉末Ｂの作製工程おいてアルミナにＢａを担持させず、第二層の層の厚さを１００μｍとして、表１所定の比率とした以外は、実施例１と同様の操作を繰り返し、本例の排気ガス浄化触媒を得た。
【００３１】
（比較例３）
第二層の層の厚さを５μｍにして、表１所定の比率とした以外は、実施例１と同様の操作を繰り返し、本例の排気ガス浄化触媒を得た。
上記各例の排気ガス浄化触媒の仕様を表１に示す。
【００３２】
【表１】
［性能評価］
（モデルガス評価試験）
（耐久方法）
排気量４５００ｃｃのエンジンの排気系に上記各例の触媒を装着し、軽油（Ｓ＝１０ｐｐｍ以下）を使用し、触媒入口温度を６５０℃とし、５０時間運転した。
（評価方法）
耐久後の各例の触媒４０ｃｃを用いて、下記条件にてガスを流通させた際のＨＣ減少量を調べ、ＨＣ吸着率を求めた。ＨＣ吸着率は次式▲１▼
（ＨＣ吸着率）＝（ＨＣ減少量）／（触媒へのＨＣ流入量）×１００…▲１▼で算出される。
その後、下記条件にて昇温させるとともにガスを流通させた際のＨＣ排出量を調べ、ＨＣ浄化率を求めた。ＨＣ浄化率は次式▲２▼
（ＨＣ浄化率）＝［（ＨＣ吸着量）−（昇温時のＨＣ排出量）］／（ＨＣ吸着量）×１００…▲２▼で算出される。
（評価条件）
・モデルガス組成 Ｏ２：１０ｖｏｌ％、Ｈ２Ｏ：１０ｖｏｌ％、Ｎ２：バランス
・流量 ６０Ｌ／ｍｉｎ
・初期流通温度 １００℃
・到達温度 ４００℃
・昇温速度 １００℃／ｍｉｎ
・流通時間 ３ｍｉｎ
【００３５】
（エンジン評価試験）
（耐久方法）
排気量４５００ｃｃのエンジンの排気系に上記各例の触媒を装着し、軽油（Ｓ＝１０ｐｐｍ以下）を使用し、触媒入口温度を６５０℃とし、５０時間運転した。
（評価方法）
排気量２５００ｃｃのディーゼルエンジンの排気系に耐久後の上記各例の触媒を装着して、リーン（Ａ／Ｆ＝３０）状態で４０ｓｅｃ、リッチ（Ａ／Ｆ＝１１）状態４ｓｅｃの運転を行い、この区間における排気浄化率（ＮＯｘ転化率）を求めた。このとき、触媒入口温度は２５０℃とした。得られた結果を表２に示す。
【００３６】
【表２】
表２より、本発明の範囲に属する実施例１〜６は、本発明外の比較例１〜３よりもＨＣ吸着層へのアルカリ成分の染み出し量、ＮＯｘ転化率が優れていることが分かる。
また、現時点では、第二層、第三層のアルカリ成分の添加量のバランスという観点から実施例３が最も良好な結果をもたらすものと思われる。
【００３８】
【発明の効果】
以上説明してきたように、本発明によれば、ＨＣ吸着層、ＮＯｘを吸着・浄化し且つＨＣを浄化する吸着浄化内層及び吸着浄化外層の三層構造とし、吸着浄化内層及び外層に含まれるアルカリ金属等の含有量を適切に調整し、吸着浄化内層の厚さを所定範囲に調整することなどとしたため、ＨＣ浄化性能及びＮＯｘ浄化性能を兼備した排気ガス浄化触媒を提供することができる。[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an exhaust gas purifying catalyst, and more particularly, to an exhaust gas purifying catalyst having excellent nitrogen oxide (NOx) purifying performance in a lean region and hydrocarbon (HC) purifying performance in a low temperature region.
[0002]
[Prior art]
Conventionally, various catalysts for purifying NOx in a lean region have been proposed, as exemplified by a catalyst in which platinum (Pt) and lanthanum (La) are supported on a porous carrier (for example, see Patent Document 1). Further, there is known a catalyst which adsorbs NOx in a lean region and releases and purifies NOx in a rich region from a stoichiometric air-fuel ratio (stoichiometric ratio).
As a catalyst for purifying HC in a low temperature range, a catalyst containing zeolite has been proposed (for example, see Patent Document 2).
Further, there has been proposed a catalyst having a laminated structure of two or more layers and having a large surface layer in both the noble metal content ratio and the alkaline earth metal content ratio in comparison between the surface layer and the inner layer (for example, see Patent Document 3).
[0003]
[Patent Document 1]
JP-A-5-168860 [Patent Document 2]
JP-A-11-47596 [Patent Document 3]
JP 2001-129403 A
[Problems to be solved by the invention]
It would be convenient if the above-mentioned NOx adsorption purification catalyst and HC adsorption purification catalyst could be combined to create an exhaust gas purification catalyst having both functions of NOx adsorption purification function and HC adsorption purification function. However, it is difficult to realize the following problems.
[0005]
That is, a noble metal layer containing an alkali metal or the like and a noble metal as a NOx adsorbent is laminated on a zeolite layer containing a zeolite as an HC adsorbent, a low-temperature HC is absorbed by a lower zeolite layer, and NOx is absorbed by an upper noble metal layer. Adsorption, purification, and purification of HC desorbed from the lower layer can be considered. In this case, first, a zeolite layer is coated on a monolithic carrier such as a honeycomb carrier, and then an alumina layer containing a noble metal is applied. And then impregnated with an alkali metal or the like to produce a catalyst.
[0006]
However, according to such a production method, the alkali metal or the like is considerably absorbed in the zeolite layer, and the HC adsorption function cannot be sufficiently exhibited. On the other hand, it is considered that a solid alkali metal compound such as barium carbonate (BaCO 3 ) is mixed with a noble metal and alumina to prepare a slurry, and this slurry is coated on a zeolite layer. Thus, most of the absorption of the alkali metal or the like into the zeolite layer can be suppressed.
However, according to this method, since a solid alkali metal compound is used, the crystallite diameter of the alkali metal becomes large, and there is a problem that the NOx adsorption performance in a lean region is deteriorated.
[0007]
On the other hand, zeolite is bulky, and if an attempt is made to coat the honeycomb carrier with an amount of zeolite that can sufficiently exhibit HC adsorption purification performance, the carrier cells are clogged and the exhaust pressure increases. Can only provide a small amount of NOx purification layer, and cannot exhibit sufficient NOx purification performance.
Furthermore, in the comparison between the surface layer and the inner layer described above, a catalyst having a large surface layer in both the noble metal content ratio and the alkaline earth metal content ratio has no zeolite layer and therefore cannot purify HC in a low temperature region, and has a large thickness of the inner layer. However, it was unclear whether the elution of the alkaline earth metal from the surface layer could be suppressed.
[0008]
The present invention has been made in view of such problems of the related art, and an object of the present invention is to provide an exhaust gas purification catalyst having both HC purification performance and NOx purification performance.
[0009]
[Means for Solving the Problems]
The present inventors have conducted intensive studies to achieve the above object, and as a result, have a three-layer structure of an HC adsorption layer, an adsorption purification inner layer that adsorbs and purifies NOx and purifies HC, and an adsorption purification inner layer. The present inventors have found that the above object can be achieved by appropriately adjusting the content of the alkali metal and the like contained in the outer layer and adjusting the thickness of the inner layer for adsorption and purification to a predetermined range, and have completed the present invention.
[0010]
That is, the exhaust gas purifying catalyst of the present invention is formed by laminating an HC adsorbing layer, an adsorption purifying inner layer for adsorbing and purifying NOx, and purifying HC on an integral structure carrier in this order.
The HC adsorbing layer contains zeolite, and the inner and outer layers for adsorption purification are at least one noble metal selected from the group consisting of platinum (Pt), rhodium (Rh) and palladium (Pd), and an alkali metal and / or Contains alkaline earth metals and refractory inorganic oxides.
Further, in the exhaust gas purifying catalyst of the present invention, the molar ratio of the alkali metal and / or alkaline earth metal contained in the inner layer for adsorption purification to the alkali metal and / or alkaline earth metal contained in the outer layer for adsorption purification is 1/1. １／1/50, and the thickness of the inner layer for adsorption and purification is 10-200 μm.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the exhaust gas purifying catalyst of the present invention will be described in detail. In addition, in this specification, "%" represents a mass percentage unless otherwise specified.
[0012]
As described above, the exhaust gas purifying catalyst of the present invention is obtained by laminating an HC adsorbing layer for adsorbing and desorbing HC, an inner layer for adsorbing and purifying for adsorbing and purifying NOx, and purifying HC on an integral structure type carrier in this order. Wherein the HC adsorption layer contains zeolite, and the adsorption purification inner layer and the outer layer are noble metals according to Pt, Rh or Pd and any combination thereof, and one or both of an alkali metal and an alkaline earth metal, Contains refractory inorganic oxides.
By providing such a structure in which the HC adsorbing layer (zeolite layer), the inner layer for adsorbing and purifying NOx and the outer layer for adsorbing and purifying NOx and purifying HC are supported in this order on the honeycomb carrier, it is possible to purify HC in a low temperature range. NOx purification in the lean region can be performed with one catalyst.
As the monolithic carrier, a monolith carrier or a honeycomb carrier made of a heat-resistant material such as ceramics such as cordierite or a metal such as ferrite stainless steel is used.
[0013]
Further, in the exhaust gas purifying catalyst of the present invention, the molar ratio of one or both of the alkali metal and alkaline earth metal contained in the inner layer of adsorption purification to one or both of the alkali metal and alkaline earth metal contained in the outer layer of adsorption purification is further reduced. (Inner / outer layer) should be 1/1 to 1/50.
If the content ratio of one or both of the alkali metal and alkaline earth metal in the inner layer and the outer layer is larger than 1/1, the purification performance after desorption of HC may decrease, If the inner layer of the adsorption purification is smaller than 1/50, the effect as the inner layer and the outer layer of the adsorption purification is weakened, and the adsorption purification performance may be reduced.
[0014]
Further, the thickness of the inner layer of the adsorption purification layer needs to be 10 to 200 μm.
When the thickness of the inner layer of the adsorption purification is less than 10 μm, the elution of the alkali metal and alkaline earth metal from the outer layer of the adsorption purification into the HC adsorption layer (zeolite layer) cannot be sufficiently suppressed during the preparation of the catalyst. Alkali metal and alkaline earth metal may be eluted into the layer, which may cause a decrease in HC adsorption performance due to elution.If the thickness of the adsorption purification inner layer exceeds 200 μm, the adsorption purification inner layer and the outer layer become too thick, Since the diffusion of HC into the zeolite layer is inhibited, there is a possibility that the HC adsorption performance is reduced.
[0015]
Further, in the catalyst of the present invention, it is preferable that the weight ratio of the HC adsorption layer and the coat layer (sum of the adsorption purification inner layer and the adsorption purification outer layer) is 2/1 to 1/10.
If the weight ratio of the HC adsorption layer and the coat layer (sum of the adsorption purification inner layer and the adsorption purification outer layer) is greater than 2/1 and the HC adsorption layer weight is large, the desorbed HC can be completely purified by the adsorption purification inner layer and the outer layer. May not be. On the other hand, if the coat weight ratio is smaller than 1/10 and the weight of the coat layer (sum of the inner layer for adsorption purification and the outer layer for adsorption purification) is large, HC adsorption may be hindered.
[0016]
Preferably, one or both of the alkali metal and the alkaline earth metal are contained in the inner and outer layers of the adsorption purification in powder form.
When producing the exhaust gas purifying catalyst of the present invention, as long as the alkali is contained in a predetermined concentration distribution, it is not particularly limited, but if the alkali is impregnated with a solution, the alkali soaks into the zeolite layer, It is desirable to carry by using a slurry that is in a state of maintaining a powder form.
[0017]
Further, an alkali metal and an alkaline earth metal are contained at a ratio of 0.5 to 5% based on the inner surface of the adsorption purification, and one or both of the alkali metal and the alkaline earth metal are 5 to 5% based on the outer layer of the adsorption purification. Preferably, it is contained at a rate of 15%.
In the inner layer of adsorption purification, when the concentration of the contained alkali metal and alkaline earth metal is less than 0.5%, the NOx adsorption performance is reduced. Therefore, the NOx purification performance is remarkably reduced. Therefore, the HC purification performance may be reduced.
On the other hand, in the outer layer of adsorption purification, when the concentration of the contained alkali metal and alkaline earth metal is less than 5%, the NOx adsorption performance is remarkably reduced, and the NOx purification performance is reduced. Particles of metals and alkaline earth metals grow and cause pore blockage of the base material and the like, which may adversely affect NOx purification performance.
[0018]
The refractory inorganic oxide to be used is not particularly limited as long as it has heat resistance and can support a noble metal. Representative examples thereof include alumina and the like. Further, cerium (Ce) or lanthanum ( It is desirable to use alumina supporting La). By supporting Ce or La on alumina, the purification performance of the exhaust gas purification catalyst can be improved.
[0019]
Further, in the catalyst of the present invention, it is desirable that the noble metal of the inner layer of adsorption purification is composed of Pt and Pd, and the noble metal of the outer layer of adsorption purification is composed of Rh.
In order to purify HC desorbed from the HC adsorption layer, it is effective that the layer having a small content ratio of alkali metal and alkaline earth metal, that is, the noble metal having excellent HC purification performance, that is, Pt or Pd, is contained in the inner layer of adsorption purification. It is. In addition, NOx purification performance can be improved by supporting Rh particularly on the outer layer of adsorption purification.
[0020]
【Example】
Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples, but the present invention is not limited to these Examples.
[0021]
(Example 1)
Alumina was put into the barium acetate solution, stirred at room temperature for 1 hour, then dried at 120 ° C. for 24 hours, and calcined at 600 ° C. for 1 hour. This was impregnated with a 2% tetraammine platinum hydroxide solution (pH = 10.5), dried at 120 ° C. all day and night, and further calcined at 400 ° C. for 1 hour to obtain a powder. This was designated as powder A.
Alumina was put into the barium acetate solution, stirred at room temperature for 1 hour, then dried at 120 ° C. for 24 hours, and calcined at 600 ° C. for 1 hour. This was impregnated with a 6% tetraammine platinum hydroxide solution, dried at 120 ° C. for 24 hours, and calcined at 400 ° C. for 1 hour to obtain a powder. This was designated as powder B.
Alumina was impregnated with a 6% aqueous solution of rhodium nitrate, dried at 120 ° C. for 24 hours, and calcined at 400 ° C. for 1 hour to obtain a powder. This was designated as powder C.
[0022]
720 g of β zeolite, 180 g of silica sol, and 900 g of water were charged into a magnetic ball mill and mixed and pulverized to obtain a slurry. This was used as a first catalyst slurry.
555.3 g of powder A, 25.2 g of alumina, 30 g of alumina sol, and 610 g of water were charged into a magnetic ball mill and mixed and pulverized to obtain a slurry. This was used as a second catalyst slurry.
318.6 g of powder C, 453.6 g of powder B, 37.8 g of alumina, 90 g of alumina sol, and 900 g of water were charged into a magnetic ball mill and mixed and pulverized to obtain a slurry. This was used as a third catalyst slurry.
[0023]
The first catalyst slurry is attached to a cordierite-based monolithic carrier (catalyst capacity: 1.2 L, number of cells: 400 cells / square inch), excess slurry in the cells is removed by an air stream, and the slurry is dried at 130 ° C. Then, it was calcined at 400 ° C. for 1 hour to obtain a catalyst. This was designated as catalyst a.
The second catalyst slurry was attached to the catalyst a, the excess slurry in the cell was removed with an air stream, dried at 130 ° C., and then calcined at 400 ° C. for 1 hour to obtain a catalyst. This was designated as catalyst b.
The third catalyst slurry was adhered to the catalyst b, the excess slurry in the cell was removed by an air stream, dried at 130 ° C., and then calcined at 400 ° C. for 1 hour to obtain the exhaust gas purifying catalyst of this example. The thickness of the second layer of the catalyst is 50 μm, the molar ratio of the alkali metal and the like in the second layer and the third layer is 、, and the coating amount of the first layer and the (sum of the second layer and the third layer) The ratio was 1/3, the concentration of Ba in the second layer was 1%, and the concentration of Ba in the third layer was 6%.
[0024]
(Example 2)
The same operation as in Example 1 was repeated except that sodium (Na) was supported on alumina in the step of preparing powder B, the thickness of the second layer was 170 μm, and the ratio was set to a predetermined ratio in Table 1. An example exhaust gas purification catalyst was obtained.
[0025]
(Example 3)
The same operation as in Example 1 was repeated except that Ba and strontium (Sr) were supported on alumina in the step of preparing powder B, and the thickness of the second layer was 110 μm and the ratio was set to a predetermined ratio in Table 1. Thus, an exhaust gas purifying catalyst of this example was obtained.
[0026]
(Example 4)
The same operation as in Example 1 was repeated except that Ba and Na were supported on alumina in the process of preparing powder B, the thickness of the second layer was set to 100 μm, and the ratio was set to a predetermined ratio in Table 1. Was obtained.
[0027]
(Example 5)
The same operation as in Example 1 was repeated except that Ba and magnesium (Mg) were supported on alumina in the process of preparing powder A, and the thickness of the second layer was set to 100 μm and the ratio was set to a predetermined ratio in Table 1. Thus, an exhaust gas purifying catalyst of this example was obtained.
[0028]
(Example 6)
Ba and Sr are supported on alumina in the step of preparing powder A, and Ba and Sr are supported on alumina in the step of preparing powder B. 555.3 g of powder A, 25.2 g of alumina, and 229. Example 1 was repeated except that 5 g, 90 g of alumina sol, and 900 g of water were put into a magnetic ball mill and mixed and pulverized to prepare a second catalyst slurry. The thickness of the second layer was 190 μm and the ratio was set to a predetermined ratio in Table 1. The same operation as described above was repeated to obtain the exhaust gas purifying catalyst of this example.
[0029]
(Comparative Example 1)
The same operation as in Example 1 was repeated, except that the thickness of the second layer was set to a predetermined ratio in Table 1 with a thickness of 210 μm, to obtain an exhaust gas purification catalyst of this example.
[0030]
(Comparative Example 2)
The same operation as in Example 1 was repeated except that Ba was not supported on alumina in the process of preparing powder B, and the thickness of the second layer was set to a predetermined ratio in Table 1 by setting the thickness of the second layer to 100 μm. An exhaust gas purification catalyst was obtained.
[0031]
(Comparative Example 3)
The same operation as in Example 1 was repeated, except that the thickness of the second layer was 5 μm and the ratio was set to a predetermined ratio in Table 1, to obtain an exhaust gas purification catalyst of this example.
Table 1 shows the specifications of the exhaust gas purifying catalyst of each of the above examples.
[0032]
[Table 1]
[Performance evaluation]
(Model gas evaluation test)
(Durability method)
The catalyst of each of the above examples was mounted on an exhaust system of an engine having a displacement of 4500 cc, and the operation was performed for 50 hours at a catalyst inlet temperature of 650 ° C. using light oil (S = 10 ppm or less).
(Evaluation method)
Using 40 cc of the catalyst of each example after the endurance, the amount of HC reduction when flowing gas under the following conditions was examined, and the HC adsorption rate was obtained. The HC adsorption rate is given by the following equation (1)
(HC adsorption rate) = (HC reduction amount) / (HC inflow amount to catalyst) × 100 (1)
Thereafter, the temperature was raised under the following conditions and the amount of HC discharged when the gas was circulated was examined to determine the HC purification rate. The HC purification rate is given by the following formula (2)
(HC purification rate) = [(HC adsorption amount) − (HC discharge amount at temperature rise)] / (HC adsorption amount) × 100 (2)
(Evaluation conditions)
Model gas composition O 2: 10vol%, H 2 O: 10vol%, N 2: balance Flow rate 60L / min
・ Initial circulation temperature 100 ℃
・ Achieved temperature 400 ℃
・ Heating rate 100 ℃ / min
・ Distribution time 3min
[0035]
(Engine evaluation test)
(Durability method)
The catalyst of each of the above examples was mounted on the exhaust system of an engine with a displacement of 4500 cc, and was operated for 50 hours at a catalyst inlet temperature of 650 ° C. using light oil (S = 10 ppm or less).
(Evaluation method)
The catalyst of each of the above examples after durability was mounted on the exhaust system of a diesel engine with a displacement of 2500 cc, and operation was performed in a lean (A / F = 30) state for 40 sec and in a rich (A / F = 11) state for 4 sec. The exhaust gas purification rate (NOx conversion rate) in this section was determined. At this time, the catalyst inlet temperature was 250 ° C. Table 2 shows the obtained results.
[0036]
[Table 2]
From Table 2, it can be seen that Examples 1 to 6 belonging to the scope of the present invention are superior to Comparative Examples 1 to 3 excluding the present invention in the amount of the alkaline component leached into the HC adsorption layer and the NOx conversion rate. .
At the present time, it is considered that Example 3 provides the best result from the viewpoint of the balance between the amounts of the alkali components added to the second layer and the third layer.
[0038]
【The invention's effect】
As described above, according to the present invention, the three-layer structure of the HC adsorption layer, the adsorption purification inner layer that adsorbs and purifies NOx, and purifies the HC has a three-layer structure, and the alkali contained in the adsorption purification inner layer and the outer purification layer. Since the content of metals and the like is appropriately adjusted and the thickness of the inner layer of the adsorption purification is adjusted to a predetermined range, it is possible to provide an exhaust gas purification catalyst having both HC purification performance and NOx purification performance.
Claims (6)
ＨＣ吸着層がゼオライトを含有し、
吸着浄化内層及び外層が、白金、ロジウム及びパラジウムから成る群より選ばれた少なくとも１種の貴金属と、アルカリ金属及び／又はアルカリ土類金属と、耐火性無機酸化物を含有し、
吸着浄化内層に含まれるアルカリ金属及び／又はアルカリ土類金属と吸着浄化外層に含まれるアルカリ金属及び／又はアルカリ土類金属とのモル比が１／１〜１／５０であり、
吸着浄化内層の層の厚さが１０〜２００μｍであることを特徴とする排気ガス浄化触媒。An HC adsorption layer, an adsorption purification inner layer for adsorbing and purifying NOx and an HC purification layer and an outer layer are laminated on the monolithic carrier in this order,
HC adsorption layer contains zeolite,
The inner layer and the outer layer for adsorption purification contain at least one noble metal selected from the group consisting of platinum, rhodium and palladium, an alkali metal and / or an alkaline earth metal, and a refractory inorganic oxide;
The molar ratio of the alkali metal and / or alkaline earth metal contained in the adsorption purification inner layer to the alkali metal and / or alkaline earth metal contained in the adsorption purification outer layer is 1/1 to 1/50,
An exhaust gas purifying catalyst, wherein the thickness of the inner layer of the adsorption purification is 10 to 200 μm.
アルカリ金属及び／又はアルカリ土類金属が、吸着浄化外層を基準として５〜１５％の割合で含まれることを特徴とする請求項１〜３のいずれか１つの項に記載の排気ガス浄化触媒。An alkali metal and / or an alkaline earth metal are contained in a ratio of 0.5 to 5% based on the inner layer of the adsorption purification;
The exhaust gas purifying catalyst according to any one of claims 1 to 3, wherein the alkali metal and / or alkaline earth metal is contained in a ratio of 5 to 15% based on the outer layer of the adsorption purification.
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| JP2009052542A (en) * | 2007-08-01 | 2009-03-12 | Nissan Motor Co Ltd | Exhaust emission control system |
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| US9810120B2 (en) | 2007-08-01 | 2017-11-07 | Nissan Motor Co., Ltd. | Exhaust gas purifying system |
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