JP3855267B2 - Exhaust gas purification catalyst and method for producing the same - Google Patents
Exhaust gas purification catalyst and method for producing the same Download PDFInfo
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- JP3855267B2 JP3855267B2 JP2002342084A JP2002342084A JP3855267B2 JP 3855267 B2 JP3855267 B2 JP 3855267B2 JP 2002342084 A JP2002342084 A JP 2002342084A JP 2002342084 A JP2002342084 A JP 2002342084A JP 3855267 B2 JP3855267 B2 JP 3855267B2
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Images
Description
【０００１】
【発明の属する技術分野】
本発明は、ディーゼルエンジンからの排ガスなど、パティキュレートを含む排ガスを浄化する排ガス浄化用触媒及びその製造方法に関する。
【０００２】
【従来の技術】
ガソリンエンジンについては、排ガスの厳しい規制とそれに対処できる技術の進歩とにより、排ガス中の有害成分は確実に減少されてきている。しかし、ディーゼルエンジンについては、有害成分がパティキュレート（粒子状物質：炭素微粒子、サルフェート等の硫黄系微粒子、高分子量炭化水素微粒子等、以下ＰＭという）として排出されるという特異な事情から、規制も技術の進歩もガソリンエンジンに比べて遅れている。
【０００３】
現在までに開発されているディーゼルエンジン用排ガス浄化装置としては、大きく分けてトラップ型の排ガス浄化フィルタ（ウォールフロー）と、オープン型の排ガス浄化装置（ストレートフロー）とが知られている。このうちトラップ型の排ガス浄化フィルタとしては、セラミック製の目封じタイプのハニカム体（ディーゼルＰＭフィルタ（以下ＤＰＦという））が知られている。このＤＰＦは、セラミック排ガス浄化フィルタのセルの開口部の両端を例えば交互に市松状に目封じしてなるものであり、排ガス下流側で目詰めされた流入側セルと、流入側セルに隣接し排ガス上流側で目詰めされた流出側セルと、流入側セルと流出側セルを区画するフィルタ隔壁とよりなり、フィルタ隔壁の細孔で排ガスを濾過してＰＭを捕集することで排出を抑制するものである。
【０００４】
しかしＤＰＦでは、ＰＭの堆積によって圧損が上昇するため、何らかの手段で堆積したＰＭを定期的に除去して再生する必要がある。そこで従来は、圧損が上昇した場合にバーナあるいは電気ヒータ等で堆積したＰＭを燃焼させることでＤＰＦを再生することが行われている。しかしながらこの場合には、ＰＭの堆積量が多いほど燃焼時の温度が上昇し、それによる熱応力でＤＰＦが破損する場合もある。
【０００５】
そこで近年では、ＤＰＦのフィルタ隔壁の表面にアルミナなどからコート層を形成し、そのコート層に白金（Pt）などの触媒金属を担持した触媒層をもつ連続再生式ＤＰＦが開発されている。この連続再生式ＤＰＦによれば、捕集されたＰＭが触媒金属の触媒反応によって酸化燃焼するため、捕集と同時にあるいは捕集に連続して燃焼させることでＤＰＦを再生することができる。そして触媒反応は比較的低温で生じること、及び捕集量が少ないうちに燃焼できることから、ＤＰＦに作用する熱応力が小さく破損が防止されるという利点がある。
【０００６】
このような連続再生式ＤＰＦとして、例えば特開平09−220423号公報には、フィルタ隔壁の気孔率が40〜65％で、平均孔径が５〜35μｍであり、触媒層を構成する多孔質酸化物はフィルタ隔壁の平均孔径より小さい粒径のものが90重量％以上を占めている構成のものが開示されている。このような高比表面積の多孔質酸化物をコートすることにより、フィルタ隔壁の表面だけでなく細孔の内表面にまで触媒層を形成することができる。またコート量を一定とすれば触媒層の厚さを薄くすることができるので、圧損の増大を抑制することができる。
【０００７】
また特開平06−159037号公報には、上記触媒層にさらにNOx 吸蔵材を担持した連続再生式ＤＰＦが記載されている。このようにすればNOx 吸蔵材にNOx を吸蔵することができ、軽油などの還元剤を噴霧することで吸蔵されたNOx を還元して浄化することが可能となる。
【０００８】
【特許文献１】
特開平09−220423号
【特許文献２】
特開平06−159037号
【０００９】
【発明が解決しようとする課題】
ところが従来のＤＰＦにおいては、排ガス中にＰＭが多量に含まれると堆積するＰＭ量が急増して排気圧損が上昇し、これを抑制しようとしてフィルタ隔壁の気孔率を高めるとフィルタ隔壁をすり抜けるＰＭが増加してＰＭの捕集率が低下する。つまり排気圧損とＰＭの捕集率とは背反事象の関係にあるため、気孔率の制御のみでは両方を満足させることが困難であった。
【００１０】
また、フィルタ隔壁の細孔の内表面にも触媒層を形成した連続再生式ＤＰＦにおいては、フィルタ隔壁の細孔が触媒層によって閉塞し、その結果、排気圧損の増大、ＰＭ酸化能の低下などの不具合が生じている。そして細孔が閉塞されない程度に触媒層のコート量を抑制すると、浄化性能が低下するとともに、担持密度が高くなるために高温耐久時に白金などが粒成長して活性が低下するという問題がある。
【００１１】
本発明はこのような事情に鑑みてなされたものであり、十分な量の触媒層を形成するとともに、圧損の増大を抑制することを目的とする。
【００１５】
【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、排ガス下流側で目詰めされた流入側セルと、流入側セルに隣接し排ガス上流側で目詰めされた流出側セルと、流入側セルと流出側セルを区画するフィルタ隔壁と、フィルタ隔壁に形成された触媒層とを含み、フィルタ隔壁の細孔のうち孔径が所定値より大きい大細孔に偏って触媒層が形成されていることにある。
【００１６】
そして上記排ガス浄化用触媒を製造できる本発明の製造方法の特徴は、排ガス下流側で目詰めされた流入側セルと、流入側セルに隣接し排ガス上流側で目詰めされた流出側セルと、流入側セルと流出側セルを区画するフィルタ隔壁と、よりなる複数のセルを含む排ガス浄化フィルタのフィルタ隔壁の細孔のうち孔径が所定値以下の小細孔に偏って可燃性物質を充填し充填排ガス浄化フィルタとする充填工程と、充填排ガス浄化フィルタのフィルタ隔壁に少なくとも多孔質酸化物と貴金属とを含む触媒層を形成する触媒層形成工程と、フィルタ隔壁の細孔に充填されている可燃性物質を焼失させる焼失工程と、を含むことにある。
【００１７】
上記製造方法において、充填工程では、孔径が前記所定値より大きい大細孔に充填された可燃性物質を物理的に除去する除去工程を行うことが望ましい。
【００２６】
【発明の実施の形態】
フィルタ隔壁に触媒層を形成するには、アルミナなどの担体粉末のスラリーをウォッシュコートし、それを焼成した後にPtなどを担持する方法が採用される。そして特開平9-220423号公報に記載されているように、フィルタ隔壁の平均孔径より小さい粒径のものが90重量％以上を占めるような担体粉末をコートすることにより、フィルタ隔壁の表面だけでなく内部細孔の内表面にまで触媒層を形成することができる。またコート量を一定とすれば触媒層の厚さを薄くすることができるので、圧損の増大を抑制することができる。
【００２７】
ところが担体粉末は、粒径が細かいために孔径の大小問わず内部細孔内に侵入し、孔径の小さな小細孔から順次詰まっていく。そのままではフィルタ隔壁に開口する表面空孔の開口面積が小さくなり圧損が上昇してしまう。そこでスラリーをコートした後に、吸引あるいはエアブローによって余分なスラリーを内部細孔から排出させることが行われている。このようにすることで、小細孔からスラリーを除去することが困難であっても、孔径の大きな大細孔からはスラリーが排出され、内部細孔の内表面にコート層が形成されるとともに内部は空洞となり表面空孔の開口面積も確保できるため圧損を低くすることができる。
【００２８】
連続再生式ＤＰＦによるＰＭの浄化は、内部細孔内に捕集されたＰＭが触媒層のPtなどと接触することで酸化浄化される原理によって行われる。しかし上記したように内部細孔の大部分が孔径の大きな大細孔である場合には、ＰＭは大細孔を通過することとなるために、その内表面に形成されている触媒層との衝突頻度が低い。そのためフィルタ隔壁をすり抜けるＰＭが多くなり、ＰＭの浄化性能が低くなってしまう。
【００２９】
一方、コート量を少なくして小細孔の閉塞を抑制し小細孔をＰＭの捕集場とすれば、ＰＭの捕集率が向上しＰＭの浄化性能が向上する。しかし触媒層の厚さが薄くなるために、白金などの担持量を同等とすると担持密度が高くなりすぎ、高温耐久時に粒成長して活性が低下するという不具合がある。
【００３０】
そこで本発明の排ガス浄化用触媒では、フィルタ隔壁の内部細孔のうち、孔径が所定値より大きい大細孔に偏って触媒層を形成している。ＰＭの捕集に特に寄与するのは孔径が20〜40μｍの内部細孔であるので、所定値としては40μｍ以下の値が好ましく、20μｍ以下の値を採用することが特に好ましい。例えば所定値を20μｍとすれば、孔径が20μｍ以下の小細孔には触媒層が形成されておらず閉塞されていないので、小細孔が排ガスの通路となり初期圧損を低くすることができる。また小細孔にＰＭが堆積したとしても、孔径が20μｍより大きな大細孔によって圧損を低く維持することができる。そして20μｍより大きな大細孔には触媒層が形成されているので、その大細孔に捕集されたＰＭは触媒層と接触して効率よく燃焼浄化される。
【００３１】
本発明の排ガス浄化用触媒を製造するには、先ず排ガス下流側で目詰めされた流入側セルと、流入側セルに隣接し排ガス上流側で目詰めされた流出側セルと、流入側セルと流出側セルを区画するフィルタ隔壁と、よりなる複数のセルを含む排ガス浄化フィルタを用意する。従来のＤＰＦをそのまま用いてもよいし、場合によっては、両端で目詰めされていないストレートセルを含む排ガス浄化フィルタを用いてもよい。
【００３２】
次に、排ガス浄化フィルタのフィルタ隔壁の内部細孔のうち、孔径が所定値以下の小細孔に偏って可燃性物質を充填する。可燃性物質としては、カーボン粉末，セルロース粉末，澱粉，樹脂粉末などの固体有機物粉末、あるいは界面活性剤，液状樹脂，アクリルオリゴマー，有機可塑剤，有機物を有機溶媒に溶解した有機溶液などの液状有機物を用いることができる。可燃性物質として固体有機物粉末を用いる場合は、平均粒径が前記所定値以下の粉末を用いることによって孔径が所定値以下の小細孔に偏って充填することができる。
【００３３】
また可燃性物質として液状有機物を用いる場合は、液状有機物はほぼ全ての内部細孔に充填されるが、孔径が小さな細孔では表面張力によって充填状態が維持され、孔径が大きな細孔からは流出しやすいので、孔径が所定値以下の小細孔に偏って充填することができる。
【００３４】
なお充填工程では、充填された可燃性物質を物理的に除去する除去工程を行うことが望ましい。例えば吸引あるいはエアブローを行うことで、孔径が所定値より大きい大細孔に充填されている可燃性物質をより確実に除去することができる。また充填排ガス浄化フィルタを振動させて流出を促進させたり、回転させて遠心力によって除去することもできる。
【００３５】
孔径が所定値以下の小細孔に可燃性物質が偏って充填された充填排ガス浄化フィルタは、続いて触媒層形成工程において触媒層が形成される。この触媒層は、多孔質酸化物に触媒金属を担持してなるものであり、多孔質酸化物としては Al2O3、ZrO2、CeO2、TiO2、SiO2などの酸化物あるいはこれらの複数種からなる複合酸化物を用いることができる。
【００３６】
この触媒層は、フィルタ隔壁の表面ばかりでなく可燃性物質が充填されていない孔径が前記所定値を超える大細孔の内表面にも形成される。
【００３７】
このフィルタ隔壁に形成されている触媒層は、 100〜 200ｇ／Ｌのコート量とするのが望ましい。コート量が 100ｇ／Ｌ未満では触媒金属の粒成長による耐久性の低下が避けられず、 200ｇ／Ｌを超えると圧損が高くなりすぎて実用的ではない。
【００３８】
触媒層を形成するには、酸化物粉末あるいは複合酸化物粉末をアルミナゾルなどのバインダ成分及び水とともにスラリーとし、そのスラリーをフィルタ隔壁に付着させた後に焼成すればよい。スラリーをフィルタ隔壁に付着させるには通常の浸漬法を用いることができるが、エアブローあるいは吸引によって細孔内に入ったスラリーの余分なものを除去することが望ましい。
【００３９】
触媒層に担持される触媒金属としては、触媒反応によってＰＭの酸化を促進するものであれば用いることができるが、少なくともPt、Rh、Pdなどの白金族の貴金属から選ばれた一種あるいは複数種を担持することが好ましい。さらにNOx 吸蔵材を担持することも好ましい。貴金属の担持量は、排ガス浄化フィルタの体積１リットルあたり２〜８ｇの範囲とすることが好ましい。担持量がこれより少ないと活性が低すぎて実用的でなく、この範囲より多く担持しても活性が飽和するとともにコストアップとなってしまう。
【００４０】
また貴金属を担持するには、貴金属の硝酸塩などを溶解した溶液を用い、吸着担持法、吸水担持法などによって酸化物粉末あるいは複合酸化物粉末からなるコート層に担持すればよい。また酸化物粉末あるいは複合酸化物粉末に予め貴金属を担持しておき、その触媒粉末を用いて触媒層を形成することもできる。
【００４１】
触媒層に担持できるNOx 吸蔵材としては、Ｋ，Na，Cs，Liなどのアルカリ金属、Ba，Ca，Mg，Srなどのアルカリ土類金属、あるいはSc，Ｙ，Pr，Ndなどの希土類元素から選択して用いることができる。中でもNOx 吸蔵能に長けたアルカリ金属及びアルカリ土類金属の少なくとも一種を用いることが望ましい。
【００４２】
このNOx 吸蔵材の担持量は、排ガス浄化フィルタの体積１リットルあたり0.25〜0.45モルの範囲とすることが好ましい。担持量がこれより少ないと活性が低すぎて実用的でなく、この範囲より多く担持すると貴金属を覆って活性が低下するようになる。
【００４３】
またNOx 吸蔵材を担持するには、酢酸塩、硝酸塩などを溶解した溶液を用い、吸水担持法などによってコート層に担持すればよい。また酸化物粉末あるいは複合酸化物粉末に予めNOx 吸蔵材を担持しておき、その粉末を用いて触媒層を形成することもできる。
【００４４】
次に、フィルタ隔壁の内部細孔に充填されている可燃性物質を焼失させる焼失工程を行う。可燃性物質が焼失することで、触媒層が形成されていない孔径が前記所定値以下の小細孔が空洞となって開口し、使用時に排ガス通路として機能するので圧損の増大を抑制することができる。この焼失工程は、コートされたスラリーの焼成と同時に行ってもよいし、スラリーの焼成とは別に行ってもよい。
【００４５】
なお可燃性物質として金属イオンを含むものを用いれば、焼失後にもフィルタ隔壁に金属が残留し、それが触媒として作用する場合もある。
【００４６】
【実施例】
以下、試験例、実施例及び比較例により本発明を具体的に説明する。
【００４７】
（試験例）
フィルタ隔壁の細孔分布が異なるコーディエライト製ＤＰＦ基材（２Ｌ）を複数種類用意し、そのフィルタ隔壁の表面空孔分布と内部細孔分布をそれぞれ測定した。表面空孔分布は顕微鏡写真の画像解析によって測定し、内部細孔分布は水銀ポロシメータによって測定した。
【００４８】
次に、平均粒径１〜３μｍのアルミナ粉末を主とするスラリーを各ＤＰＦ基材にウォッシュコートし、余分なスラリーを吸引除去した。次いで 110℃で乾燥後 450℃で焼成してコート層を形成した。コート層は各ＤＰＦ基材１リットルあたり 150ｇ形成された。その後所定濃度のジニトロジアンミン白金水溶液の所定量をコート層に含浸させ、 120℃で１時間乾燥後 500℃で１時間焼成して、コート層にPtを担持し触媒層を形成した。Ptの担持量は各ＤＰＦ基材１リットルあたり２ｇである。
【００４９】
得られた複数の排ガス浄化用触媒をそれぞれケース内に詰め、触媒コンバータ化した。これらをそれぞれエンジンベンチにて２Ｌ直噴ディーゼルエンジンの排気系に取付け、2000rpm ×30Nmで運転した時の、３時間後の排気圧損とＰＭ捕集率をそれぞれ測定した。なおＰＭ捕集率は、入りガス中のＰＭ量と出ガス中のＰＭ量の比率から算出した。そして表面空孔分布又は内部細孔分布に応じて測定値を整理し、結果を図１〜図10に示す。
【００５０】
先ず表面空孔の割合と排気圧損との関係について見ると、図１及び図２から、フィルタ隔壁の表面空孔のうち、孔径が40μｍ以下の表面空孔の割合と圧損との間には負の相関関係があり、孔径が40μｍ以下の表面空孔の割合が高くなるほど圧損が低くなって、孔径が40μｍ以下の表面空孔が50％以上であれば圧損は 15kPa以下となると考えられる。一方、孔径が40μｍを超える表面空孔の割合と圧損との間には、相関関係は認められない。
【００５１】
また表面空孔の割合とＰＭ捕集率との関係について見ると、図３及び図４から、フィルタ隔壁の表面空孔のうち、孔径が40μｍ以下の表面空孔の割合とＰＭ捕集率との間には正の相関関係があり、孔径が40μｍ以下の表面空孔の割合が高くなるほどＰＭ捕集率が高くなり、孔径が40μｍ以下の表面空孔が50％以上であればＰＭ捕集率はほぼ 100％になると認められる。一方、孔径が40μｍを超える表面空孔の割合と圧損との間には相関関係は認められない。
【００５２】
したがって孔径が40μｍ以下の表面空孔の合計開口面積が全表面空孔の合計開口面積の50％以上を占めるようにすれば、圧損は 15kPa以下と低く、ＰＭ捕集率はほぼ 100％となることが明らかである。
【００５３】
次に内部細孔の割合と排気圧損との関係について見ると、図５〜図７より、フィルタ隔壁の内部細孔のうち、孔径が20〜40μｍの内部細孔の割合と圧損との間には負の相関関係があり、孔径が20〜40μｍの内部細孔の割合が50％以上であれば圧損を数 kPaときわめて小さくすることができる。しかし孔径が20μｍ未満の内部細孔の割合と圧損との間には正の相関関係が認められ、孔径が20μｍ未満の内部細孔を多くするほど圧損が増大してしまう。一方、孔径が40μｍを超える内部細孔の割合と圧損との間には相関関係は認められない。
【００５４】
また内部細孔の割合とＰＭ捕集率との関係について見ると、図８〜図10より、フィルタ隔壁の内部細孔のうち、孔径が20〜40μｍの内部細孔の割合とＰＭ捕集率との間には正の相関関係があり、孔径が20〜40μｍの内部細孔の割合が50％以上であればＰＭ捕集率をほぼ 100％とすることができる。しかし孔径が20μｍ未満又は40μｍを超える内部細孔の割合とＰＭ捕集率との間には負の相関関係が認められ、孔径が20μｍ未満又は40μｍを超える内部細孔を多くするほどＰＭ捕集率が低下してしまう。
【００５５】
したがって孔径が20〜40μｍの内部細孔が全内部細孔の50％以上を占めるようにすれば、圧損は数 kPa以下と低く、ＰＭ捕集率はほぼ 100％とすることができる。
【００５６】
なお図１〜図10のデータから、表面空孔分布又は内部細孔の分布が圧損又はＰＭ捕集率に影響を及ぼす度合い（寄与率）を計算し、結果を図11に示す。なお計算方法は、多変量解析手法の重回帰分析に従った。
【００５７】
図11より、表面空孔の分布の方が圧損及びＰＭ捕集率に及ぼす影響が大きく、特にＰＭ捕集率には表面空孔の分布が大きく寄与していることがわかる。
【００５８】
（実施例１）
２リットルのコーディエライト製ＤＰＦ基材を用意し、平均粒径10μｍのカーボン粉末が１ｇ／hrの流量で流れている大気雰囲気中に配置して１時間処理した。カーボンを含む空気は流入側セルに流入し、フィルタ隔壁を通過して流出側セルから流出するので、その間にカーボン粉末がフィルタ隔壁の細孔内に捕集される。
【００５９】
図12に示すように、カーボン粉末２の平均粒径は10μｍであるので、フィルタ隔壁１の孔径が10μｍを超える細孔10はすり抜けやすく、孔径が10μｍ以下の細孔11には徐々に堆積してその細孔11を充填する。
【００６０】
次に、平均粒径１〜３μｍのアルミナ粉末を主とするスラリーをウォッシュコートし、余分なスラリーを吸引除去した。図13に示すように、スラリー３はフィルタ隔壁１の孔径が10μｍ以下の細孔11には入らず、カーボン粉末２が充填されていない孔径が10μｍを超える細孔10の内表面には付着している。
【００６１】
次いで 110℃で乾燥後 450℃で焼成してコート層４を形成した。コート層４は基材１リットルあたり 150ｇ形成された。またこの際に、細孔11に充填されていたカーボン粉末２が焼失し、図14に示すようにフィルタ隔壁１の孔径が10μｍ以下の細孔11にはコート層４が形成されず、孔径が10μｍを超える細孔10の表面に偏ってコート層４が形成される。
【００６２】
次いで所定濃度のジニトロジアンミン白金水溶液の所定量を含浸させ、 120℃で１時間乾燥後 500℃で１時間焼成して、コート層にPtを担持し触媒層を形成した。Ptの担持量は基材１リットルあたり２ｇである。
【００６３】
上記のようにして得られた本実施例の排ガス浄化用触媒では、図14に示すように孔径が10μｍより大きい細孔10に偏ってコート層４が形成され、孔径が10μｍ以下の細孔11にはコート層４が形成されず閉塞が防止されている。したがって細孔10及び細孔11ともに排ガスが通過可能であり、初期圧損が低い。また細孔11にＰＭが堆積したとしても、孔径が10μｍより大きい細孔10によって開口面積が確保でき、圧損の増大が防止されている。
【００６４】
そしてコート量を多くしても、孔径が10μｍ以下の細孔11が閉塞されることがないので初期圧損の増大を抑制でき、コート層を厚くすることでPtの担持密度が低くなるため高温耐久時の粒成長を抑制することができ耐久性が向上する。
【００６５】
（実施例２）
カーボン粉末に代えて、平均粒径10μｍのアクリル粒子を用いたこと以外は実施例１と同様にして排ガス浄化用触媒を調製した。
【００６６】
（実施例３）
実施例１と同様のＤＰＦを用意し、ヘキサデシルベンゼンスルホン酸ナトリウムを含浸させた。ヘキサデシルベンゼンスルホン酸ナトリウムは、フィルタ隔壁の全ての細孔に含浸するが、孔径が大きな細孔からは流出しやすく、孔径が小さな細孔には表面張力によって充填状態が維持される。
【００６７】
その後、実施例１と同様にしてコート層を形成し、細孔内に充填されていたヘキサデシルベンゼンスルホン酸ナトリウムはその際に焼失したので、孔径が小さな細孔にはコート層が形成されず、孔径が大きな細孔の表面に偏ってコート層が形成された。そして実施例１と同様にPtを担持した。
【００６８】
（実施例４）
実施例１と同様のＤＰＦを用意し、オクチル酸セリウムの軽油溶液を含浸させ、その後、基材の流出側端面から２KPa の負圧によりエア吸引を２分間行った。オクチル酸セリウムの軽油溶液は、フィルタ隔壁の全ての細孔に含浸するが、エア吸引によって孔径が大きな細孔からは流出しやすく、孔径が小さな細孔には表面張力によって充填状態が維持される。
【００６９】
その後、実施例１と同様にしてコート層を形成し、細孔内に充填されていたオクチル酸セリウムの軽油溶液はその際に焼失したので、孔径が小さな細孔にはコート層が形成されず、孔径が大きな細孔の表面に偏ってコート層が形成された。そして実施例１と同様にPtを担持した。
【００７０】
（比較例１）
実施例１と同様のＤＰＦを用意し、実施例１と同様にコート層を形成し、同様にPtを担持した。本比較例では、ほとんど全ての細孔にスラリーが充填されたため、孔径が10μｍ以下の細孔が閉塞されている恐れがある。
【００７１】
（比較例２）
平均粒径が10μｍのカーボン粉末に代えて、平均粒径が50μｍのカーボン粉末を用いたこと以外は実施例１と同様にして排ガス浄化用触媒を調製した。本比較例では、孔径が50μｍ以下の細孔には触媒層がほとんど形成されていない。
【００７２】
（比較例３）
エア吸引を行わなかったこと以外は実施例４と同様にして排ガス浄化用触媒を調製した。本比較例ではエア吸引を行わなかったため、オクチル酸セリウムの軽油溶液はほとんど全部の細孔内に充填され、ほとんど全部の細孔内に触媒層が形成されない。したがって触媒層は大部分がフィルタ隔壁の表面に形成されたと考えられる。
【００７３】
（比較例４）
実施例１と同様のＤＰＦを用意し、ポリビニルアルコール水溶液（濃度20重量％）を含浸させ、その後水洗した。そして実施例１と同様にコート層を形成し、Ptを担持した。本比較例では、ポリビニルアルコール水溶液はフィルタ隔壁の全ての細孔に充填され、ほとんど全部の細孔内に触媒層が形成されない。したがって触媒層は大部分がフィルタ隔壁の表面に形成されたと考えられる。
【００７４】
＜試験・評価＞
実施例１と比較例１の排ガス浄化用触媒、及び実施例１で用いたＤＰＦのみについて、フィルタ隔壁の細孔の容積率を水銀ポロシメーターによって測定した。結果を図15に示す。図15からわかるように、実施例１の触媒では孔径の小さな細孔の細孔容積がＤＰＦと同様に大きい。これは、孔径の小さな細孔が触媒層によって閉塞されなかったことを意味している。
【００７５】
それぞれの排ガス浄化用触媒のフィルタ隔壁の内部細孔分布と表面平均細孔開口径（細孔開口部の平均径）をそれぞれ測定し、結果を表１に示す。なお細孔分布は水銀ポロシメーターによって測定し、表面平均細孔開口径はＳＥＭ（走査型電子顕微鏡）写真を画像処理することによって測定した。
【００７６】
また、それぞれの排ガス浄化用触媒をケース内に詰め、触媒コンバータ化した。これらをそれぞれエンジンベンチにて２Ｌ直噴ディーゼルエンジンの排気系に取付け、2000rpm ×30Nmで運転した時の、３時間後の圧力損失とＰＭ除去率を測定した。結果を表１に示す。なおＰＭ除去率は、排ガス浄化用触媒の重量増加分を測定し、それをエンジンから排出された総ＰＭ量から減じた残りを除去されたＰＭとし、その総ＰＭ量に対する割合を算出した。
【００７７】
【表１】
表１より、比較例１，２の触媒では20〜40μｍの細孔割合が低く、ＰＭの捕集に最適な細孔の多くが触媒層によって閉塞されているため、ＰＭ除去率が低くなっている。また表面平均細孔開口径は実施例より大きいにも関わらず、圧損は実施例より大きい。これは20〜40μｍの細孔割合が低いこと、つまり孔径が小さな細孔が触媒層によって閉塞されていることに起因している。
【００７９】
また比較例３，４の触媒では、20〜40μｍの細孔割合が比較的高いため、ＰＭ除去率が比較例１，２に比べて高い。しかし表面平均細孔開口径がきわめて小さく、そのため圧損がきわめて大きくなっている。これは、コート層形成時にほとんどの細孔が可燃性物質によって閉塞されているために、コート層が細孔を覆うように形成されたものと考えられる。
【００８０】
しかし各実施例の排ガス浄化用触媒では、高いＰＭ除去率と低い圧損とが両立し、これは20〜40μｍの内部細孔の割合が高く、かつ表面平均細孔開口径も十分に大きいことに起因していることが明らかである。20〜40μｍの内部細孔の割合は35％以上が好ましく、40％以上が特に好ましい。また、表面平均細孔開口径は10〜60μｍが好ましく、20〜40μｍが特に好ましい。
【００８２】
【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、十分な量の触媒層を形成しても圧損の増大が抑制されている。したがって触媒金属の担持密度を低くすることができ、粒成長の抑制により耐久性が向上する。
【００８３】
そして本発明の製造方法によれば、所定値より大きい大細孔に偏って触媒層を確実に形成することができ、本発明の排ガス浄化用触媒を容易にかつ安定して製造することができる。
【図面の簡単な説明】
【図１】40μｍ以下の表面空孔の割合と圧損の関係を示すグラフである。
【図２】40μｍ超の表面空孔の割合と圧損の関係を示すグラフである。
【図３】40μｍ以下の表面空孔の割合とＰＭ捕集率の関係を示すグラフである。
【図４】40μｍ超の表面空孔の割合とＰＭ捕集率の関係を示すグラフである。
【図５】20〜40μｍの内部細孔の割合と圧損の関係を示すグラフである。
【図６】20μｍ未満の内部細孔の割合と圧損の関係を示すグラフである。
【図７】40μｍ超の内部細孔の割合と圧損の関係を示すグラフである。
【図８】20〜40μｍの内部細孔の割合とＰＭ捕集率の関係を示すグラフである。
【図９】20μｍ未満の内部細孔の割合とＰＭ捕集率の関係を示すグラフである。
【図１０】40μｍ超の内部細孔の割合とＰＭ捕集率の関係を示すグラフである。
【図１１】表面空孔分布と内部細孔分布が圧損とＰＭ捕集率に影響する寄与率を示すグラフである。
【図１２】本発明の一実施例における充填工程後のフィルタ隔壁の模式的断面図である。
【図１３】本発明の一実施例におけるスラリー含浸後のフィルタ隔壁の模式的断面図である。
【図１４】本発明の一実施例における焼失工程後のフィルタ隔壁の模式的断面図である。
【図１５】孔径と細孔の容積率との関係を示すグラフである。
【符号の説明】
１：フィルタ隔壁 ２：カーボン粉末 ３：スラリー ４：コート層
10：孔径が10μｍを超える細孔 11：孔径が10μｍ以下の細孔[0001]
BACKGROUND OF THE INVENTION
The present invention relates to exhaust gas containing particulates such as exhaust gas from a diesel engine.Exhaust gas purification catalyst to be purifiedAnd a manufacturing method thereof.
[0002]
[Prior art]
As for gasoline engines, harmful components in exhaust gas have been steadily reduced due to strict regulations on exhaust gas and advances in technology that can cope with it. However, because diesel engines emit harmful components as particulates (particulate matter: carbon fine particles, sulfur fine particles such as sulfate, high molecular weight hydrocarbon fine particles, etc., hereinafter referred to as PM), regulations are also restricted. Technological progress is also slow compared to gasoline engines.
[0003]
As exhaust gas purification devices for diesel engines that have been developed to date, a trap type exhaust gas purification filter (wall flow) and an open type exhaust gas purification device (straight flow) are known. Among these, as a trap-type exhaust gas purification filter, a ceramic plug-type honeycomb body (diesel PM filter (hereinafter referred to as DPF)) is known. The DPF is formed by alternately sealing both ends of the opening of the cell of the ceramic exhaust gas purification filter in a checkered pattern, for example, and is adjacent to the inflow side cell clogged on the exhaust gas downstream side and the inflow side cell. It consists of an outflow side cell clogged upstream of the exhaust gas and a filter partition that partitions the inflow side cell and the outflow side cell, and controls exhaust by filtering the exhaust gas through the pores of the filter partition and collecting PM. To do.
[0004]
However, in the DPF, the pressure loss increases due to the accumulation of PM. Therefore, it is necessary to periodically remove and regenerate PM accumulated by some means. Therefore, conventionally, when the pressure loss increases, the DPF is regenerated by burning PM accumulated by a burner or an electric heater. However, in this case, as the amount of accumulated PM increases, the temperature at the time of combustion rises, and the DPF may be damaged by the thermal stress caused thereby.
[0005]
Therefore, in recent years, a continuous regeneration type DPF has been developed in which a coating layer is formed from alumina or the like on the surface of the filter partition wall of the DPF, and the coating layer has a catalyst layer carrying a catalyst metal such as platinum (Pt). According to this continuous regeneration type DPF, the collected PM is oxidized and combusted by the catalytic reaction of the catalytic metal. Therefore, the DPF can be regenerated by combusting simultaneously with the collection or continuously in the collection. Since the catalytic reaction occurs at a relatively low temperature and can be burned while the amount collected is small, there is an advantage that the thermal stress acting on the DPF is small and damage is prevented.
[0006]
As such a continuously regenerating DPF, for example, Japanese Patent Laid-Open No. 09-220423 discloses a porous oxide constituting a catalyst layer having a filter partition wall porosity of 40 to 65% and an average pore diameter of 5 to 35 μm. Discloses a structure in which a particle size smaller than the average pore size of the filter partition accounts for 90% by weight or more. By coating such a high specific surface area porous oxide, the catalyst layer can be formed not only on the surface of the filter partition wall but also on the inner surface of the pores. Further, if the coating amount is constant, the thickness of the catalyst layer can be reduced, so that an increase in pressure loss can be suppressed.
[0007]
Japanese Patent Laid-Open No. 06-159037 further discloses NO in the catalyst layer.x A continuous regeneration type DPF carrying an occlusion material is described. This way NOx NO in the occlusion materialx NO can be stored by spraying a reducing agent such as light oil.x Can be reduced and purified.
[0008]
[Patent Document 1]
JP 09-220423
[Patent Document 2]
JP 06-159037 A
[0009]
[Problems to be solved by the invention]
However, in the conventional DPF, if a large amount of PM is contained in the exhaust gas, the amount of accumulated PM increases rapidly and the exhaust pressure loss rises. If the porosity of the filter partition is increased to suppress this, the PM that passes through the filter partition is reduced. It increases and PM collection rate decreases. In other words, since the exhaust pressure loss and the PM collection rate are in a contradictory relationship, it is difficult to satisfy both by controlling the porosity alone.
[0010]
Further, in a continuous regeneration type DPF in which a catalyst layer is also formed on the inner surface of the pores of the filter partition walls, the filter partition pores are blocked by the catalyst layer, resulting in an increase in exhaust pressure loss, a decrease in PM oxidation ability, etc. The problem has occurred. If the coating amount of the catalyst layer is suppressed to such an extent that the pores are not blocked, there is a problem that the purification performance is lowered, and the support density is increased, so that platinum and the like grow during high temperature durability and the activity is lowered.
[0011]
The present invention has been made in view of such circumstances.Is enough andAn object is to form an amount of the catalyst layer and suppress an increase in pressure loss.
[0015]
[Means for Solving the Problems]
The exhaust gas purifying catalyst of the present invention that solves the above problemsThe features of the inflow side cell clogged on the exhaust gas downstream side, the outflow side cell clogged on the exhaust gas upstream side adjacent to the inflow side cell, the filter partition wall partitioning the inflow side cell and the outflow side cell, And the catalyst layer is formed so as to be biased toward large pores having a pore diameter larger than a predetermined value among the pores of the filter partition walls.
[0016]
And the characteristics of the production method of the present invention capable of producing the exhaust gas purifying catalyst are characterized in that the inflow side cell clogged on the exhaust gas downstream side, the outflow side cell adjacent to the inflow side cell and clogged on the exhaust gas upstream side, A filter partition that separates the inflow side cell and the outflow side cell, and the pores of the filter partition wall of the exhaust gas purification filter that includes a plurality of cells composed of a plurality of cells, the pore diameter is biased toward small pores of a predetermined value or less and is filled with a combustible substance. A filling step for forming a filled exhaust gas purification filter, a catalyst layer forming step for forming a catalyst layer containing at least a porous oxide and a noble metal on the filter partition wall of the filled exhaust gas purification filter, and a combustible material filled in the pores of the filter partition wall And a burning step for burning off the active substance.
[0017]
In the above manufacturing method, it is desirable to perform a removing step of physically removing the combustible substance filled in the large pores having a pore diameter larger than the predetermined value in the filling step.
[0026]
DETAILED DESCRIPTION OF THE INVENTION
In order to form the catalyst layer on the filter partition wall, a method is adopted in which a slurry of a carrier powder such as alumina is wash-coated, and after firing it, Pt and the like are supported. And, as described in JP-A-9-220423, by coating a carrier powder in which a particle size smaller than the average pore size of the filter partition occupies 90% by weight or more, only the surface of the filter partition The catalyst layer can be formed up to the inner surface of the internal pores. Further, if the coating amount is constant, the thickness of the catalyst layer can be reduced, so that an increase in pressure loss can be suppressed.
[0027]
However, since the carrier powder has a small particle size, the carrier powder penetrates into the internal pores regardless of the size of the pores, and sequentially clogs from the small pores with the small pore sizes. If it is left as it is, the opening area of the surface vacancies opening in the filter partition becomes small, and the pressure loss increases. Therefore, after coating the slurry, excess slurry is discharged from the internal pores by suction or air blow. In this way, even if it is difficult to remove the slurry from the small pores, the slurry is discharged from the large pores having a large pore diameter, and a coat layer is formed on the inner surface of the internal pores. Since the inside becomes a cavity and the opening area of the surface holes can be secured, the pressure loss can be reduced.
[0028]
The purification of PM by the continuous regeneration type DPF is performed on the principle that the PM collected in the internal pores is oxidized and purified by contacting with Pt or the like of the catalyst layer. However, as described above, when most of the internal pores are large pores having a large pore diameter, PM passes through the large pores, so that the catalyst layer formed on the inner surface thereof The collision frequency is low. Therefore, the amount of PM that passes through the filter partition wall increases, and the PM purification performance is lowered.
[0029]
On the other hand, if the coating amount is reduced to prevent the clogging of the small pores and the small pores are used as a PM collection field, the PM collection rate is improved and the PM purification performance is improved. However, since the thickness of the catalyst layer is reduced, if the loading amount of platinum or the like is made equal, the loading density becomes too high, and there is a problem that the activity grows due to grain growth during high temperature durability.
[0030]
Therefore, the exhaust gas purifying catalyst of the present inventionThen, among the internal pores of the filter partition wall, the catalyst layer is formed so as to be biased toward large pores having a pore diameter larger than a predetermined value. Since it is internal pores having a pore diameter of 20 to 40 μm that particularly contributes to the collection of PM, the predetermined value is preferably 40 μm or less, and particularly preferably 20 μm or less. For example, if the predetermined value is 20 μm, since the catalyst layer is not formed and closed on the small pores having a pore diameter of 20 μm or less, the small pores become exhaust gas passages, and the initial pressure loss can be reduced. Even if PM accumulates in the small pores, the pressure loss can be kept low by the large pores having a pore diameter larger than 20 μm. Since the catalyst layer is formed in the large pore larger than 20 μm, the PM collected in the large pore comes into contact with the catalyst layer and is efficiently burned and purified.
[0031]
To manufacture the exhaust gas purifying catalyst of the present invention, first, an inflow side cell clogged on the exhaust gas downstream side, an outflow side cell adjacent to the inflow side cell and clogged on the exhaust gas upstream side, an inflow side cell, An exhaust gas purification filter including a plurality of cells including a filter partition wall that partitions the outflow side cell is prepared. A conventional DPF may be used as it is, or an exhaust gas purification filter including straight cells that are not clogged at both ends may be used.
[0032]
Next, among the internal pores of the filter partition wall of the exhaust gas purification filter, the combustible substance is filled with bias toward small pores having a pore diameter of a predetermined value or less. Combustible substances include solid organic powders such as carbon powder, cellulose powder, starch, and resin powder, or liquid organic substances such as surfactants, liquid resins, acrylic oligomers, organic plasticizers, and organic solutions in which organic substances are dissolved in organic solvents. Can be used. When a solid organic powder is used as the combustible substance, the powder having an average particle diameter of not more than the predetermined value can be used so that the pore diameter is biased toward small pores having a predetermined value or less.
[0033]
When liquid organic substances are used as flammable substances, the liquid organic substances are filled in almost all internal pores. However, in the case of pores with small pore diameters, the filling state is maintained by surface tension, and outflow from pores with large pore diameters. Therefore, it is possible to fill the small pores with a pore diameter of a predetermined value or less.
[0034]
In the filling step, it is desirable to perform a removal step of physically removing the filled combustible substance. For example, by performing suction or air blow, the combustible substance filled in the large pores having a pore diameter larger than a predetermined value can be more reliably removed. Further, the exhaust gas purification filter can be vibrated to promote outflow, or rotated to be removed by centrifugal force.
[0035]
In the filled exhaust gas purification filter in which the flammable substance is biased and filled in small pores having a pore size of a predetermined value or less, a catalyst layer is subsequently formed in the catalyst layer forming step. This catalyst layer is made by supporting a catalyst metal on a porous oxide.2OThree, ZrO2, CeO2, TiO2, SiO2Or a complex oxide composed of a plurality of these oxides can be used.
[0036]
This catalyst layer is formed not only on the surface of the filter partition wall but also on the inner surface of the large pores whose pore diameter not filled with the combustible substance exceeds the predetermined value.
[0037]
It is desirable that the catalyst layer formed on the filter partition wall has a coating amount of 100 to 200 g / L. If the coating amount is less than 100 g / L, a decrease in durability due to grain growth of the catalyst metal is unavoidable, and if it exceeds 200 g / L, the pressure loss becomes too high to be practical.
[0038]
In order to form the catalyst layer, the oxide powder or composite oxide powder is made into a slurry together with a binder component such as alumina sol and water, and the slurry is attached to the filter partition wall and then fired. A normal dipping method can be used to attach the slurry to the filter partition walls, but it is desirable to remove excess slurry that has entered the pores by air blowing or suction.
[0039]
As the catalyst metal supported on the catalyst layer, any catalyst metal can be used as long as it promotes the oxidation of PM by a catalytic reaction, but at least one or more kinds selected from platinum group noble metals such as Pt, Rh, and Pd. Is preferably supported. NOx It is also preferable to carry an occlusion material. The amount of the noble metal supported is preferably in the range of 2 to 8 g per liter of the exhaust gas purification filter. If the loading amount is less than this, the activity is too low to be practical, and if the loading amount exceeds this range, the activity is saturated and the cost is increased.
[0040]
In order to support the noble metal, a solution in which nitrate of noble metal or the like is dissolved may be used and supported on the coat layer made of oxide powder or composite oxide powder by an adsorption support method, a water absorption support method, or the like. Alternatively, a noble metal may be previously supported on the oxide powder or composite oxide powder, and the catalyst layer may be formed using the catalyst powder.
[0041]
NO that can be supported on the catalyst layerx The occlusion material is selected from alkali metals such as K, Na, Cs and Li, alkaline earth metals such as Ba, Ca, Mg and Sr, or rare earth elements such as Sc, Y, Pr and Nd. it can. NOx It is desirable to use at least one of an alkali metal and an alkaline earth metal having excellent occlusion ability.
[0042]
This NOx The amount of occlusion material supported is preferably in the range of 0.25 to 0.45 mol per liter of the exhaust gas purification filter. If the loading is less than this, the activity is too low to be practical, and if the loading is more than this range, the noble metal is covered and the activity decreases.
[0043]
NOx In order to support the occlusion material, a solution in which acetate, nitrate or the like is dissolved may be used and supported on the coat layer by a water absorption support method or the like. In addition, the oxide powder or composite oxide powder is pre-loaded with NO.x It is also possible to form a catalyst layer using the powder by supporting an occlusion material.
[0044]
Next, a burnout process is performed in which the combustible material filled in the internal pores of the filter partition wall is burnt down. By burning off the combustible material, a small pore having a pore size not formed of the catalyst layer is opened as a hollow and functions as an exhaust gas passage during use, thereby suppressing an increase in pressure loss. it can. This burning step may be performed simultaneously with the baking of the coated slurry, or may be performed separately from the baking of the slurry.
[0045]
If a material containing metal ions is used as the combustible substance, the metal may remain in the filter partition wall even after being burned out, which may act as a catalyst.
[0046]
【Example】
Hereinafter, the present invention will be specifically described with reference to test examples, examples and comparative examples.
[0047]
(Test example)
A plurality of cordierite DPF base materials (2L) having different pore distributions of the filter partition walls were prepared, and the surface pore distribution and the internal pore distribution of the filter partition walls were measured. The surface pore distribution was measured by image analysis of micrographs, and the internal pore distribution was measured by a mercury porosimeter.
[0048]
Next, a slurry mainly composed of alumina powder having an average particle diameter of 1 to 3 μm was wash-coated on each DPF substrate, and excess slurry was removed by suction. Next, it was dried at 110 ° C. and baked at 450 ° C. to form a coat layer. The coating layer was formed in an amount of 150 g per liter of each DPF substrate. Thereafter, a predetermined amount of a dinitrodiammine platinum aqueous solution having a predetermined concentration was impregnated into the coat layer, dried at 120 ° C. for 1 hour and then calcined at 500 ° C. for 1 hour, whereby Pt was supported on the coat layer to form a catalyst layer. The amount of Pt supported is 2 g per liter of each DPF substrate.
[0049]
The obtained plurality of exhaust gas purifying catalysts were each packed in a case to form a catalytic converter. These were each attached to the exhaust system of a 2 L direct-injection diesel engine with an engine bench, and the exhaust pressure loss and PM collection rate after 3 hours when operating at 2000 rpm × 30 Nm were measured. The PM collection rate was calculated from the ratio of the amount of PM in the incoming gas to the amount of PM in the outgoing gas. Then, the measured values are arranged according to the surface pore distribution or the internal pore distribution, and the results are shown in FIGS.
[0050]
First, looking at the relationship between the ratio of surface vacancies and exhaust pressure loss, it can be seen from FIGS. 1 and 2 that there is a negative difference between the ratio of surface vacancies with a pore diameter of 40 μm or less and the pressure loss. The pressure loss decreases as the proportion of surface vacancies with a pore diameter of 40 μm or less increases, and the pressure loss is considered to be 15 kPa or less when the surface vacancies with a pore diameter of 40 μm or less are 50% or more. On the other hand, there is no correlation between the ratio of the surface vacancies whose pore diameter exceeds 40 μm and the pressure loss.
[0051]
Further, looking at the relationship between the ratio of the surface vacancies and the PM collection rate, from FIG. 3 and FIG. 4, among the surface vacancies of the filter partition walls, the ratio of the surface vacancies having a pore diameter of 40 μm or less There is a positive correlation, and the higher the ratio of surface vacancies with a pore diameter of 40 μm or less, the higher the PM collection rate. If the surface vacancies with a pore diameter of 40 μm or less are 50% or more, PM collection The rate is recognized to be almost 100%. On the other hand, there is no correlation between the ratio of the surface vacancies whose pore diameter exceeds 40 μm and the pressure loss.
[0052]
Therefore, if the total open area of surface vacancies with a pore diameter of 40 μm or less occupies 50% or more of the total open area of all surface vacancies, the pressure loss is as low as 15 kPa or less and the PM collection rate is almost 100% It is clear.
[0053]
Next, the relationship between the ratio of the internal pores and the exhaust pressure loss is shown in FIGS. 5 to 7 between the ratio of the internal pores having a pore diameter of 20 to 40 μm and the pressure loss among the internal pores of the filter partition wall. Has a negative correlation, and if the proportion of internal pores having a pore diameter of 20 to 40 μm is 50% or more, the pressure loss can be made extremely small as several kPa. However, a positive correlation is recognized between the proportion of internal pores having a pore diameter of less than 20 μm and pressure loss, and the pressure loss increases as the number of internal pores having a pore diameter of less than 20 μm increases. On the other hand, there is no correlation between the proportion of internal pores having a pore diameter exceeding 40 μm and the pressure loss.
[0054]
Also, looking at the relationship between the ratio of the internal pores and the PM collection rate, from FIG. 8 to FIG. 10, the ratio of the internal pores having a pore diameter of 20 to 40 μm and the PM collection rate among the internal pores of the filter partition wall. There is a positive correlation, and if the proportion of internal pores having a pore diameter of 20 to 40 μm is 50% or more, the PM collection rate can be almost 100%. However, there is a negative correlation between the proportion of internal pores with a pore diameter of less than 20 μm or greater than 40 μm and the PM collection rate, and the more internal pores with a pore diameter of less than 20 μm or greater than 40 μm, the more PM is collected. The rate will drop.
[0055]
Therefore, if the internal pores having a pore diameter of 20 to 40 μm occupy 50% or more of all the internal pores, the pressure loss is as low as several kPa and the PM collection rate can be almost 100%.
[0056]
In addition, from the data of FIGS. 1-10, the degree (contribution rate) in which the distribution of surface pores or the distribution of internal pores affects the pressure loss or the PM collection rate is calculated, and the results are shown in FIG. The calculation method followed the multiple regression analysis of the multivariate analysis method.
[0057]
From FIG. 11, it can be seen that the distribution of surface vacancies has a greater effect on the pressure loss and the PM collection rate, and in particular, the distribution of surface vacancies greatly contributes to the PM collection rate.
[0058]
(Example 1)
A 2-liter cordierite DPF base material was prepared, and placed in an air atmosphere in which carbon powder having an average particle size of 10 μm was flowing at a flow rate of 1 g / hr, and treated for 1 hour. The air containing carbon flows into the inflow side cell, passes through the filter partition wall, and flows out from the outflow side cell, so that the carbon powder is collected in the pores of the filter partition wall.
[0059]
As shown in FIG. 12, since the average particle diameter of the
[0060]
Next, a slurry mainly composed of alumina powder having an average particle diameter of 1 to 3 μm was wash-coated, and excess slurry was removed by suction. As shown in FIG. 13, the
[0061]
Next, the
[0062]
Next, a predetermined amount of a dinitrodiammine platinum aqueous solution having a predetermined concentration was impregnated, dried at 120 ° C. for 1 hour and then calcined at 500 ° C. for 1 hour, whereby Pt was supported on the coat layer to form a catalyst layer. The amount of Pt supported is 2 g per liter of base material.
[0063]
In the exhaust gas purifying catalyst of the present example obtained as described above, as shown in FIG. 14, the
[0064]
And even if the coating amount is increased, the pore 11 having a pore diameter of 10 μm or less is not blocked, so that the increase in initial pressure loss can be suppressed, and by increasing the coating layer, the Pt loading density is lowered, resulting in high temperature durability. Grain growth at the time can be suppressed and durability is improved.
[0065]
(Example 2)
An exhaust gas purifying catalyst was prepared in the same manner as in Example 1 except that acrylic particles having an average particle diameter of 10 μm were used in place of the carbon powder.
[0066]
(Example 3)
A DPF similar to that of Example 1 was prepared and impregnated with sodium hexadecylbenzenesulfonate. Sodium hexadecylbenzenesulfonate is impregnated into all pores of the filter partition wall, but easily flows out from pores having a large pore diameter, and the pores having a small pore diameter are maintained in a filled state by surface tension.
[0067]
Thereafter, a coating layer was formed in the same manner as in Example 1, and the sodium hexadecylbenzenesulfonate filled in the pores was burned out at that time, so that the coating layer was not formed in the pores having a small pore diameter. The coat layer was formed biased to the surface of the pore having a large pore diameter. In the same manner as in Example 1, Pt was supported.
[0068]
(Example 4)
The same DPF as in Example 1 was prepared, impregnated with a light oil solution of cerium octylate, and then air suction was performed for 2 minutes from the end surface on the outflow side of the substrate with a negative pressure of 2 KPa. Although the cerium octylate gas oil solution impregnates all the pores of the filter partition wall, it tends to flow out of pores with a large pore size by air suction, and the pores with a small pore size are maintained in a packed state by surface tension. .
[0069]
Thereafter, a coating layer was formed in the same manner as in Example 1, and the light oil solution of cerium octylate that had been filled in the pores was burned off at that time, so that the coating layer was not formed on pores having a small pore diameter. The coat layer was formed biased to the surface of the pore having a large pore diameter. In the same manner as in Example 1, Pt was supported.
[0070]
(Comparative Example 1)
A DPF similar to that in Example 1 was prepared, a coating layer was formed in the same manner as in Example 1, and Pt was supported in the same manner. In this comparative example, since almost all the pores are filled with the slurry, the pores having a pore diameter of 10 μm or less may be blocked.
[0071]
(Comparative Example 2)
An exhaust gas purifying catalyst was prepared in the same manner as in Example 1 except that carbon powder having an average particle size of 50 μm was used instead of carbon powder having an average particle size of 10 μm. In this comparative example, a catalyst layer is hardly formed in pores having a pore diameter of 50 μm or less.
[0072]
(Comparative Example 3)
Exhaust gas purification catalyst was prepared in the same manner as in Example 4 except that air suction was not performed. Since air suction was not performed in this comparative example, the light oil solution of cerium octylate was filled in almost all pores, and no catalyst layer was formed in almost all pores. Therefore, it is considered that most of the catalyst layer was formed on the surface of the filter partition wall.
[0073]
(Comparative Example 4)
A DPF similar to that in Example 1 was prepared, impregnated with an aqueous polyvinyl alcohol solution (
[0074]
<Test and evaluation>
For only the exhaust gas purifying catalyst of Example 1 and Comparative Example 1 and the DPF used in Example 1, the volume ratio of the pores of the filter partition walls was measured with a mercury porosimeter. The results are shown in FIG. As can be seen from FIG. 15, in the catalyst of Example 1, the pore volume of the pores having a small pore diameter is as large as that of the DPF. This means that pores having a small pore diameter were not blocked by the catalyst layer.
[0075]
The internal pore distribution and surface average pore opening diameter (average diameter of the pore openings) of the filter partition walls of each exhaust gas purification catalyst were measured, and the results are shown in Table 1. The pore distribution was measured with a mercury porosimeter, and the surface average pore opening diameter was measured by subjecting a SEM (scanning electron microscope) photograph to image processing.
[0076]
Each exhaust gas purifying catalyst was packed in a case to form a catalytic converter. Each of these was attached to the exhaust system of a 2 L direct injection diesel engine on an engine bench, and the pressure loss and PM removal rate after 3 hours when operating at 2000 rpm × 30 Nm were measured. The results are shown in Table 1. The PM removal rate was determined by measuring the increase in the weight of the exhaust gas purifying catalyst, taking the remaining amount subtracted from the total PM amount discharged from the engine as the removed PM, and calculating the ratio to the total PM amount.
[0077]
[Table 1]
From Table 1, the catalyst of Comparative Examples 1 and 2 has a low pore ratio of 20 to 40 μm, and most of the optimal pores for PM collection are blocked by the catalyst layer, so the PM removal rate is low. Yes. Moreover, although the surface average pore opening diameter is larger than that of the example, the pressure loss is larger than that of the example. This is due to the fact that the proportion of pores of 20 to 40 μm is low, that is, pores having a small pore diameter are blocked by the catalyst layer.
[0079]
Further, in the catalysts of Comparative Examples 3 and 4, since the proportion of pores of 20 to 40 μm is relatively high, the PM removal rate is higher than that of Comparative Examples 1 and 2. However, the surface average pore opening diameter is extremely small, so that the pressure loss is very large. This is presumably because most of the pores were blocked by the combustible substance when the coating layer was formed, so that the coating layer was formed to cover the pores.
[0080]
However, in the exhaust gas purifying catalyst of each Example, a high PM removal rate and a low pressure loss are compatible, which means that the ratio of internal pores of 20 to 40 μm is high and the surface average pore opening diameter is sufficiently large. It is clear that this is the case. The proportion of internal pores of 20 to 40 μm is preferably 35% or more, particularly preferably 40% or more. Further, the surface average pore opening diameter is preferably 10 to 60 μm, particularly preferably 20 to 40 μm.
[0082]
【The invention's effect】
That is, the exhaust gas purifying catalyst of the present inventionAccording to this, even if a sufficient amount of the catalyst layer is formed, an increase in pressure loss is suppressed. Therefore, the supporting density of the catalyst metal can be lowered, and the durability is improved by suppressing the grain growth.
[0083]
According to the production method of the present invention, the catalyst layer can be reliably formed with large pores larger than a predetermined value, and the exhaust gas purifying catalyst of the present invention can be produced easily and stably. .
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the ratio of surface vacancies of 40 μm or less and pressure loss.
FIG. 2 is a graph showing the relationship between the ratio of surface vacancies exceeding 40 μm and pressure loss.
FIG. 3 is a graph showing the relationship between the percentage of surface vacancies of 40 μm or less and the PM collection rate.
FIG. 4 is a graph showing the relationship between the percentage of surface vacancies exceeding 40 μm and the PM collection rate.
FIG. 5 is a graph showing the relationship between the ratio of internal pores of 20 to 40 μm and pressure loss.
FIG. 6 is a graph showing the relationship between the proportion of internal pores of less than 20 μm and pressure loss.
FIG. 7 is a graph showing the relationship between the ratio of internal pores exceeding 40 μm and pressure loss.
FIG. 8 is a graph showing the relationship between the proportion of internal pores of 20 to 40 μm and the PM collection rate.
FIG. 9 is a graph showing the relationship between the proportion of internal pores less than 20 μm and the PM trapping rate.
FIG. 10 is a graph showing the relationship between the proportion of internal pores exceeding 40 μm and the PM trapping rate.
FIG. 11 is a graph showing contribution ratios in which the surface pore distribution and the internal pore distribution influence the pressure loss and the PM collection rate.
FIG. 12 is a schematic cross-sectional view of a filter partition wall after a filling step in one embodiment of the present invention.
FIG. 13 is a schematic cross-sectional view of a filter partition wall after slurry impregnation in one embodiment of the present invention.
FIG. 14 is a schematic cross-sectional view of a filter partition wall after a burnout process in an embodiment of the present invention.
FIG. 15 is a graph showing the relationship between pore diameter and pore volume ratio.
[Explanation of symbols]
1: Filter partition wall 2: Carbon powder 3: Slurry 4: Coat layer
10: Pore with a pore diameter exceeding 10 μm 11: Pore with a pore diameter of 10 μm or less
Claims (4)
該フィルタ隔壁の内部細孔のうち孔径が所定値より大きい大細孔に偏って該触媒層が形成されていることを特徴とする排ガス浄化用触媒。An inflow side cell clogged on the exhaust gas downstream side, an outflow cell adjacent to the inflow side cell and clogged on the exhaust gas upstream side, a filter partition wall partitioning the inflow side cell and the outflow side cell, A catalyst layer formed on the filter partition wall,
An exhaust gas purifying catalyst, wherein the catalyst layer is formed so as to be biased toward large pores having a pore diameter larger than a predetermined value among the internal pores of the filter partition wall.
該充填排ガス浄化フィルタの該フィルタ隔壁に少なくとも多孔質酸化物と貴金属とを含む触媒層を形成する触媒層形成工程と、
該フィルタ隔壁の細孔に充填されている該可燃性物質を焼失させる焼失工程と、を含むことを特徴とする排ガス浄化用触媒の製造方法。An inflow side cell clogged on the exhaust gas downstream side, an outflow side cell adjacent to the inflow side cell and clogged on the exhaust gas upstream side, a filter partition wall that partitions the inflow side cell and the outflow side cell, and A filling step of filling the combustible substance with a pore size biased to a small pore having a pore diameter equal to or smaller than a predetermined value among the internal pores of the filter partition wall of the exhaust gas purification filter including a plurality of cells,
A catalyst layer forming step of forming a catalyst layer containing at least a porous oxide and a noble metal on the filter partition wall of the filled exhaust gas purification filter;
A method for producing an exhaust gas purifying catalyst, comprising: a burning step for burning off the combustible material filled in pores of the filter partition wall.
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| JP2013212500A (en) * | 2012-03-06 | 2013-10-17 | Ngk Insulators Ltd | Honeycomb structure and honeycomb catalyst body |
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| JPWO2005044422A1 (en) | 2003-11-07 | 2007-11-29 | イビデン株式会社 | Honeycomb structure |
| JP4429756B2 (en) * | 2004-02-10 | 2010-03-10 | 株式会社キャタラー | Filter catalyst |
| US7727923B2 (en) | 2004-03-30 | 2010-06-01 | Nikki-Universal Co., Ltd. | Catalyst for exhaust gas purification and method for exhaust gas purification |
| US20080092499A1 (en) * | 2004-09-14 | 2008-04-24 | Ngk Insulators Ltd | Porous Honeycomb Filter |
| JP2006167680A (en) * | 2004-12-20 | 2006-06-29 | Hitachi Metals Ltd | Method for producing ceramic honeycomb filter |
| WO2006070504A1 (en) * | 2004-12-28 | 2006-07-06 | Ibiden Co., Ltd. | Filter and filter aggregate |
| EP2070576B1 (en) | 2006-09-28 | 2014-03-19 | Hitachi Metals, Ltd. | Ceramic honeycomb structure and process for producing ceramic honeycomb structure |
| JP5070173B2 (en) | 2008-09-24 | 2012-11-07 | 本田技研工業株式会社 | Exhaust gas purification filter and manufacturing method thereof |
| JP5518518B2 (en) * | 2010-02-15 | 2014-06-11 | 日本碍子株式会社 | Manufacturing method of honeycomb filter |
| JP5643692B2 (en) * | 2011-03-25 | 2014-12-17 | 日本碍子株式会社 | Honeycomb filter and manufacturing method thereof |
| JP5746986B2 (en) * | 2012-02-03 | 2015-07-08 | 株式会社日本自動車部品総合研究所 | Manufacturing method of exhaust gas purification filter |
| JPWO2013145316A1 (en) * | 2012-03-30 | 2015-08-03 | イビデン株式会社 | Honeycomb filter and method for manufacturing honeycomb filter |
| JP6542690B2 (en) * | 2016-02-12 | 2019-07-10 | トヨタ自動車株式会社 | Method for producing filter catalyst |
| JP2022124800A (en) * | 2021-02-16 | 2022-08-26 | 株式会社キャタラー | Exhaust gas purification catalyst |
| JP2022153941A (en) * | 2021-03-30 | 2022-10-13 | 日本碍子株式会社 | honeycomb structure |
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| JP2013212500A (en) * | 2012-03-06 | 2013-10-17 | Ngk Insulators Ltd | Honeycomb structure and honeycomb catalyst body |
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