JP4498579B2 - Exhaust gas purification catalyst and method for producing the same - Google Patents

Description

本実施形態において最も特徴的な構成は、上記セラミック担体１５を実質的に構成してなるセル壁１２の表面、とくに該セル壁１２を構成する各リン酸ジルコニウム粒子４の表面を希土類酸化物含有アルミナ薄膜３で被覆することにある。このことをもっと正確に言うと、該セル壁１２を構成しているリン酸ジルコニウム焼結体の各リン酸ジルコニウム粒子４を対象として、それぞれのリン酸ジルコニウム粒子４の表面を個別に、各種の方法によって希土類酸化物含有アルミナ薄膜３にて被覆したことにある。
【００３７】
なお、図１７（ｂ）は、前記セル壁１２表面に一様に、ウォッシュコート法によって触媒コート層２を被覆形成した従来技術を示したもので、図３（ａ），（ｂ）は、本実施形態で用いているセラミック担体１５の説明図である。セル壁１２を構成する各リン酸ジルコニウム粒子４の表面に、希土類酸化物含有アルミナ薄膜３（以下、この「希土類酸化物含有アルミナ薄膜３」を、単に「アルミナ薄膜３」と略記する）が個別に被覆された状態のものを示している。
【００３８】
このように、本実施形態にかかる触媒活性成分において特徴的な構成である（アルミナ薄膜３）は、従来のように、単に排気ガスのセル壁１２の壁面を触媒コート層２で一様に被覆したものではない。たとえば、従来のように、セル壁１２を一様に被覆すると、リン酸ジルコニウム粒子４間の間隙が封塞され、目封じされて通気性を阻害することになる。これに対し、本実施形態で用いるセラミック担体１５の場合、セル壁１２を構成している各リン酸ジルコニウム粒子４の表面を、個別にアルミナ薄膜３にて被覆した構造にしてある。
【００３９】
従って、本実施形態については、セル壁１２自体の気孔，即ち各リン酸ジルコニウム粒子４間に生じた間隙を完全に塞ぐようなことなく、気孔は気孔としてそのまま維持されることになるから、従来の触媒コート層２に比べると圧力損失が著しく小さい。しかも、耐熱性にも優れ、さらにはアルミナ薄膜３が各リン酸ジルコニウム粒子４自体を個別に被覆しているので、例えば、洗浄に当たって該薄膜がセル壁１２から剥落するようなことがなく、耐洗浄性に優れたものになる。さらに、排気ガスが触媒活性成分に接触する面積が大きくなる。よって、排気ガス中のＣＯやＨＣの酸化を促進することができる。
【００４０】
そこで以下に、本実施形態にかかる触媒活性成分に用いられる担持膜としてのアルミナ薄膜３の圧力損失特性、耐熱性、耐洗浄性ならびに再生特性について説明する。
【００４１】
アルミナ薄膜３の圧力損失特性について；
一般に、セル壁１２を排気ガスが通過するときの圧力損失特性は、次のように考えられる。即ち、前記セラミック担体１５をディーゼル排気ガスが通過するときの圧力損失は、図４のように示すことができる。この場合、抵抗ΔＰ１、ΔＰ２、ΔＰ３はそれぞれフィルタのセル構造に依存するものであって、ディーゼルパティキュレートの堆積等の時間経過によらない一定の値Δｐｉ＝（ΔＰ１＋ΔＰ２＋ΔＰ３）であり、初期圧力損失という。また、ΔＰ４は堆積したディーゼルパティキュレートを通過するときの抵抗であり、初期圧力損失の２〜３倍以上の値となる。
【００４２】
１４／２００のセル構造をもつセラミック担体１５の比表面積は８．９３１ｃｍ²／ｃｍ³であり、セラミック担体１５の密度は０．６７５ｇ／ｃｍ³であるので、セル壁１２の比表面積は０．００１３ｍ²／ｇとなる。一方、セル壁１２内の細孔比表面積は、水銀ポロシメーターの測定によると０．１２ｍ²／ｇであり、約５０〜１００倍の表面積をもつ。このことは、同じ重量のアルミナ薄膜３をセル壁１２の表面に形成する場合、単にセル壁１２の表面を一様に覆うように被覆するよりも、このセル壁１２を構成している各リン酸ジルコニウム粒子４の表面を個別に被覆した方が、同じ効果を得るためのアルミナ薄膜３の厚みを１／５０〜１／１００にすることができる。
【００４３】
即ち、ウォッシュコートのような従来技術の下でアルミナ薄膜３を一様に形成する場合、触媒活性成分の活性に必要な３mass％程度のアルミナを被覆するには、アルミナ薄膜３の厚みは５０μｍが必要である。このときの圧力損失は、セル壁１２内を通過する抵抗ΔＰ３に加え、アルミナ薄膜３を通過する抵抗が増加する。さらに、開口が小さくなりΔＰ１も大きくなる。そのため、アルミナをコートをしていないフィルタに比較して圧力損失か著しく大きくなり、その傾向は、フィルタにパティキュレートが堆積した場合に、より一層顕著になる。
【００４４】
この点、本実施形態において用いるセラミック担体１５のように、セル壁１２を構成するリン酸ジルコニウム粒子４の表面にアルミナをコートする場合、触媒活性成分の活性化に必要な３mass％程度のアルミナ薄膜３にするには、その厚みは最大でも０．５μｍ程度である。このときの圧力損失の増加は、抵抗ΔＰ３をわずかに増加させるが、その他の圧力損失は実質的に無視できる程度であり、従来のウォッシュコートによって形成されるアルミナ層に比べると、圧力損失特性は飛躍的に向上する。
【００４５】
このアルミナ薄膜３の耐熱性について；
一般に、アルミナは高い比表面積を有し、触媒活性成分の担持膜として好適である。特に、より高温で安定に作動する耐熱性の高い触媒１０の開発が望まれている現在、それに伴って、アルミナ薄膜３についても、より高い耐熱性が要求されている。
【００４６】
この点について本実施形態においては、アルミナの耐熱性を向上させるべく、（１）各アルミナ粒子の形状を小繊維状にすると共に、（２）セリア（酸化セリウム）等の希土類酸化物を含有させることにした。
【００４７】
とくに、前者（１）の構成を採用することにより、各アルミナ粒子問の接点を減らすことができ、焼結速度の低下を通じて粒成長を抑制し、もって比表面積を大きくすることができ、ひいては耐熱性が向上する。
【００４８】
即ち、本実施形態においては、セラミック担体１５の各リン酸ジルコニウム粒子４の表面を覆うアルミナ薄膜３は、各アルミナ粒子のミクロ断面形状が小繊維状か林立した植毛構造を呈している。それ故に隣接するアルミナ小繊維の互いの接触点が減少するため、著しく耐熱性が向上するのである。
【００４９】
次に、（２）について、セリア等の添加によっても耐熱性は改善される。その理由は、アルミナ薄膜３を構成する結晶粒子の表面に新しく化合物を形成し、アルミナ粒子同士の成長を妨げる効果によるものである。
【００５０】
アルミナ薄膜３の耐洗浄性について；
セル壁１２の表面に堆積したパティキュレートの主体はカーボンであり、これは、燃焼等の方法により酸化除去できる。ところが燃焼後も灰分として残る物質がある。このような物質は、中和剤あるいは潤滑剤等としての役割を持たすために、エンジンオイル中に添加してあるＣａ，Ｍｇ，Ｚｎ等の化合物が酸化されたり、硫酸塩になったりしたものがある。また、あらかじめ、燃料中にＣｅＯ₂やＣｕＯ等のカーボン燃焼のために混入してある触媒活性成分がパティキュレートと一緒に堆積したものがある。これらの灰分は、車両の長時間走行に伴って堆積していき、フィルタの圧力損失を増加させていくので、高圧水等による洗浄が必要である。このとき３０ｋｇ／ｃｍ²以上の圧力で洗浄すると灰分を完全に除去できる。
【００５１】
この点に関し、セル壁１２の表面にウォッシュコートによって形成した従来のアルミナ薄膜の場合、セル壁１２の表面全体に物理吸着による厚いコート層があるため、上記洗浄時に剥離することが多い。これに対し、本実施形態において用いる前記担持膜（アルミナ薄膜３）では、アルミナがセラミック担体１５を構成する各リン酸ジルコニウム粒子４の表面に薄く個別に被覆されており、しかも化学的にも結合していることから、リン酸ジルコニウム粒子個々と硬く密着した状態となっている。従って、密着性が高く、それ故に洗浄に対する抵抗も高く、被膜としての耐久性が強力である。
【００５２】
アルミナ薄膜３の再生特性について；
本実施形態においては、上記アルミナ薄膜３は、その中にセリア（ＣｅＯ₂）やランタナ（Ｌａ₂Ｏ₃）の如き希土類酸化物を、Ａ１₂Ｏ₃に対して１０〜８０mass％程度、好ましくは２０〜４０mass％程度添加して、アルミナ薄膜３の表面や内部にこれらの酸化物を均一分散させたものである。
【００５３】
アルミナ薄膜３中にセリア等を添加すると（好ましくはＰｔ等の触媒活性成分と共に添加することの方が望ましい）、セリアのもつ酸素濃度調節作用により、排気ガス中への酸素の供給を活発にして、フィルタに付着した“すす（ディーゼルパティキュレート）”の燃焼除去効率が向上し、ひいては触媒１０の再生率が著しく向上することになる。また、触媒１０の耐久性を向上させることができる。
【００５４】
即ち、セリア等の希土類酸化物は、アルミナの耐熱性を向上させるだけではなく、触媒表面での酸素濃度を調節する役割も果たす。一般に、排気ガス中に存在する炭化水素や一酸化炭素は酸化反応により、またＮＯｘは、還元反応により除去されるが、排気ガス組成は燃料のリッチ域とリーン域との間で絶えず変動しているため、触媒表面の作用雰囲気も激しく変動することになる。ところで、触媒に添加されるセリアは、Ｃｅ³⁺とＣｅ⁴⁺の酸化還元電位が比較的小さく、次式２ＣｅＯ₂⇔Ｃｅ₂Ｏ₃＋１／２Ｏ₂の反応が可逆的に進行する。即ち、排気ガスがリッチ域になると上記の反応は右に進行して雰囲気中に酸素を供給するが、逆にリーン域になると左に進行して雰囲気中の余剰酸素を吸蔵する。このようにして、雰囲気中の酸素濃度を調節することにより、該セリアは、炭化水素や一酸化炭素あるいはＮＯｘを効率よく除去できる空燃比の幅を広げる作用を担う。
【００５５】
図１２（ａ）は、セリア（ＣｅＯ₂）を添加していないアルミナ薄膜３の酸化速度のメカニズムを説明するものである。これに対して図１２（ｂ）は、セリアを添加した場合のアルミナ薄膜３の酸化速度のメカニズムを説明するものである。同図に示すように、ＣｅＯ₂が存在しない触媒は、排気ガス中の酸素を活性化することにより、すす（煤）を酸化させる。この反応は、流体中の酸素を活性化させなければならないために効率が悪い。
【００５６】
一方、ＣｅＯ₂が存在する触媒については、次のような反応；
ＣｅＯ₂⇔ＣｅＯ₂―_x＋ｘ／２Ｏ₂
によって、酸素が供給される。つまり、雰囲気中に吐き出された酸素及び排気ガス中の酸素は、触媒活性成分（貴金属）によって活性化されてすす（カーボン）と反応し、ＣＯ₂となる（ＣｅＯ₂―_xは、酸化して元のＣｅＯ₂に戻る）。また、ＣｅＯ₂とすすは、直接接触するために、吐き出される酸素量は少量であっても、このすすを効率よく酸化できるのである。
【００５７】
しかも、この場合のＣｅＯ₂は、触媒活性成分（貴金属）を担持することによりＯＳＣ（酸素貯蔵機能）を増大させる。というのは、触媒活性成分は、排気ガス中の酸素を活性化し、貴金属近傍のＣｅＯ₂表面の酸素も活性化するため、前記ＯＳＣが増大するのである。
【００５８】
また、図１３，図１４は、アルミナ薄膜３中へのセリア等希土類酸化物の添加効果について、触媒活性成分をＰｔ、助触媒をＣｅＯ₂、サポート材を針状Ａｌ₂Ｏ₃とした触媒１０（実施形態）、Ｐｔ／針状Ａｌ₂Ｏ₃（比較例）及びＰｔ／Ａｌ₂Ｏ₃（ウォッシュコート）触媒の再生特性に関して実験した結果を示すものである。この実験は、すすが付着したディーゼルパティキュレートフィルタ（ＤＰＦ，全長＝１５０ｍｍ）を電気炉中に収容して６５０℃に加熱する一方、１１００ｒｐｍ，３．９Ｎｍのディーゼルエンジンを接続し、その排気ガス（３５０℃）を該フィルタに導入したときのフィルタ温度（導入口より１４５ｍｍ位置での測温）の推移（図１３）、及び再生（燃焼）速度（上昇温度ΔＴと経過時間Δｔの比、図１４）を調べたものである。
【００５９】
図１３に示すように、従来例（ウォッシュコートによって形成された触媒コート層２）はＯ₂が律速になって５０sec−７００℃でピーク温度を迎え、そして比較例（セリア（助触媒）なし）でもＯ₂が律速になって、８０sec−８００℃でピーク温度を迎えるが、本実施形態例では４５sec−９００℃という速い速度で高いピーク温度を迎えており、すすの酸化除去効率が高く、高い再生率を示していることがわかる。このことはまた、図１４の再生（燃焼）速度の差としても明白に現れている。
【００６０】
また、図１５は、再生率そのものを比較したものであるが、本実施形態（セリア含有触媒）の効果が際立っていることが明らかである。ＤＰＦは、排気ガス中のすすを濾過する。そのため、ＤＰＦ内にはすすが堆積する。その堆積したすすを除去する行為を再生と呼ぶ。そこで、再生したすす重量と堆積したすす重量との比を百分率で表し、これを再生率と定義する。
【００６１】
なお、上記希土類酸化物については、上述した例にある単独酸化物（ＣｅＯ₂）の他に、例えば、希土類元素とジルコニウムとの複合酸化物を用いることがより好ましい。それは、希土類酸化物中にジルコニウム酸化物を含有していることで、該希土類酸化物の粒成長の抑制を通じて酸素濃度の制御特性が向上するからであると考えられる。
【００６２】
ジルコニウムとの複合酸化物の形態をとる前記希土類酸化物は、その粒子径を１〜３０ｎｍ程度にすることが好ましく、より好ましくは２〜２０ｎｍの大きさにすることがよい。その理由は、粒子径が１ｎｍ未満の複合酸化物は製造上困難である。一方、粒子径が３０ｎｍ超になると、粒子がシンタリングしやすくなり、粒子表面積が小さくなり、ひいては排気ガスとの接触面積が小さくなって、活性が弱まるという問題が残るからである。しかも、排気ガス通過時の圧力損失も大きくなると懸念される。
【００６３】
アルミナ薄膜３の構造について；
図５は、各リン酸ジルコニウム粒子４の表面にアルミナ薄膜３を被覆したセラミック担体１５と、セル壁１２の表面にアルミナの膜を均一に被覆した担体（従来技術）とを拡大して示す図である。各リン酸ジルコニウム粒子４の表面に針状（小繊維状）のアルミナが林立して、あたかも図３（ｂ）に示すような植毛構造を呈している。アルミナ薄膜３を被覆する対象がリン酸ジルコニウム粒子４であるため、アルミナを針状に形成しやくなっている。
【００６４】
かかるアルミナ薄膜３の構造、即ち、各リン酸ジルコニウム粒子４の表面を被覆することによって形成されたアルミナ薄膜３の結晶構造には、γ一Ａｌ₂Ｏ₃、δ−Ａｌ₂Ｏ₃、θ−Ａｌ₂Ｏ₃のうち少なくとも１つが含まれている。アルミナ薄膜３を構成する小繊維突起状アルミナの直径は、２〜５０ｎｍであり、長さが２０〜３００ｎｍで全長／直径の比が５〜５０の形状を有するものである。そして、このような構造からなるアルミナ薄膜３の厚みは０．５μｍ以下で、アルミナ基準のアルミナの比表面積は、５０〜３００ｍ²／ｇであることが好ましい。
【００６５】
ここで言うアルミナ薄膜３の厚みとは、リン酸ジルコニウム粒子４の表面から小繊維突起状のアルミナの最遠部までの距離の平均である。なお、アルミナの直径は５〜２０ｎｍがより望ましく、全長／直径の比は１０〜３０がより望ましい。
【００６６】
上記小繊維突起状アルミナ薄膜３の特性を上記のように限定する理由は、小繊維突起状アルミナの長さは２０ｎｍよりも小さいと必要な比表面積を確保することがむずかしくなり、一方、３００ｎｍよりも大きいと構造的にもろくなるからである。また、直径については、これが２ｎｍより小さいと貴金属等の触媒活性成分の大きさと同等以下となり、触媒担持層として機能しなくなり、一方、５０ｎｍより大きくなると望ましい大きさの比表面積の確保が難しくなるからである。また、全長／直径の比については、この比が５より小さいと必要な比表面積を確保することが難しく、一方、５０より大きくなると構造的にもろくなり、洗浄作業等により小繊維状突起が折れる場合が生じるからである。
【００６７】
また、アルミナ薄膜３の比表面積について、上記のように限定する理由は、５０ｍ²／ｇより小さいと小繊維突起状アルミナのシンタリングが過剰に進むため耐久性が劣るからである。一方、比表面積が３００ｍ²／ｇより大きくなると小繊維突起状アルミナが微細になりすぎることを意味する。いわゆる触媒担持層として機能しなくなるか、構造的にもろくなる。なお、好ましい比表面積は５０〜２００ｍ²／ｇの範囲である。
【００６８】
次に、上記のようなセラミック担体１５において、担持膜となるアルミナ薄膜３の量は、アルミナ比率で０．１〜１５mass％が好ましい。この理由は、０．１mass％より小さいと耐熱性向上効果が小さく、一方１５mass％より大きいと圧力損失が増大し、フィルタ機能が低下するからである。より好ましくは１〜４mass％である。
【００６９】
上記のリン酸ジルコニウム粒子４の各々の表面がアルミナ薄膜３にてそれぞれ個別に被覆され、あたかもセラミック担体１５の表面がアルミナ薄膜（担持膜）３にて完全に被覆されているかの如き様相を呈する上記セラミック担体１５に対し、触媒活性成分である貴金属元素、元素周期表のVIａ族及び元素周期表のVIII族の中から選ばれる元素を担持させている。この元素を具体的にあげると、白金（Ｐｔ）、パラジウム（Ｐｄ）、ロジウム（Ｒｈ）、ニッケル（Ｎｉ）、コバルト（Ｃｏ）、モリブデン（Ｍｏ）、タングステン（Ｗ）、セリウム（Ｃｅ）、銅（Ｃｕ）、バナジウム（Ｖ）、鉄（Ｆｅ）、金（Ａｕ）、銀（Ａｇ）等がある。
【００７０】
従って、触媒活性成分を貴金属元素としてのＰｔ、Ａｕ、Ａｇ、Ｃｕ、元素周期表VIａ族の元素としてのＭｏ、Ｗ、元素周期表VIII族の元素としてのＦｅ、Ｃｏ、Ｐｄ、Ｒｈ、Ｎｉ、それら以外の周期表の元素としてのＶ、Ｃｅの中から選ばれる少なくとも１つの単体または化合物をアルミナ薄膜３に担持させてもよい。
【００７１】
例えば、化合物として前記元素の組み合わせによる二元系合金や三元系合金が用いられる。これらの合金は、上述したように助触媒として作用するセリアやランタナの如き希土類酸化物とともに用いた方が有利である。こうした触媒１０は被毒劣化（鉛被毒，燐被毒，硫黄被毒）が少なく、かつ熱劣化も小さいので耐久性に優れる。なお、上記元素の組み合わせによる合金以外にも、他の元素との組み合わせによる化合物（酸化物、窒化物又は炭化物）であってもよい。
【００７２】
ちなみに、二元系合金としては、Ｐｔ／Ｐｄ、Ｐｔ／Ｒｈ、Ｐｔ／Ｎｉ、Ｐｔ／Ｃｏ、Ｐｔ／Ｍｏ、Ｐｔ／Ｗ、Ｐｔ／Ｃｅ、Ｐｔ／Ｃｕ、Ｐｔ／Ｖ、Ｐｔ／Ｆｅ、Ｐｔ／Ａｕ、Ｐｔ／Ａｇ、Ｐｄ／Ｒｈ、Ｐｄ／Ｎｉ、Ｐｄ／Ｃｏ、Ｐｄ／Ｍｏ、Ｐｄ／Ｗ、Ｐｄ／Ｃｅ、Ｐｄ／Ｃｕ、Ｐｄ／Ｖ、Ｐｄ／Ｆｅ、Ｐｄ／Ａｕ、Ｐｄ／Ａｇ、Ｒｈ／Ｎｉ、Ｒｈ／Ｃｏ、Ｒｈ／Ｍｏ、Ｒｈ／Ｗ、Ｒｈ／Ｃｅ、Ｒｈ／Ｃｕ、Ｒｈ／Ｖ、Ｒｈ／Ｆｅ、Ｒｈ／Ａｕ、Ｒｈ／Ａｇ、Ｎｉ／Ｃｏ、Ｎｉ／Ｍｏ、Ｎｉ／Ｗ、Ｎｉ／Ｃｅ、Ｎｉ／Ｃｕ、Ｎｉ／Ｖ、Ｎｉ／Ｆｅ、Ｎｉ／Ａｕ、Ｎｉ／Ａｇ、Ｃｏ／Ｍｏ、Ｃｏ／Ｗ、Ｃｏ／Ｃｅ、Ｃｏ／Ｃｕ、Ｃｏ／Ｖ、Ｃｏ／Ｆｅ、Ｃｏ／Ａｕ、Ｃｏ／Ａｇ、Ｍｏ／Ｗ、Ｍｏ／Ｃｅ、Ｍｏ／Ｃｕ、Ｍｏ／Ｖ、Ｍｏ／Ｆｅ、Ｍｏ／Ａｕ、Ｍｏ／Ａｇ、Ｗ／Ｃｅ、Ｗ／Ｃｕ、Ｗ／Ｖ、Ｗ／Ｆｅ、Ｗ／Ａｕ、Ｗ／Ａｇ、Ｃｅ／Ｃｕ、Ｃｅ／Ｖ、Ｃｅ／Ｆｅ、Ｃｅ／Ａｕ、Ｃｅ／Ａｇ、Ｃｕ／Ｖ、Ｃｕ／Ｆｅ、Ｃｕ／Ａｕ、Ｃｕ／Ａｇ、Ｖ／Ｆｅ、Ｖ／Ａｕ、Ｖ／Ａｇ、Ｆｅ／Ａｕ、Ｆｅ／Ａｇ、Ａｕ／Ａｇがある。
【００７３】
また、三元系合金としては、Ｐｔ／Ｐｄ／Ｒｈ、Ｐｔ／Ｐｄ／Ｎｉ、Ｐｔ／Ｐｄ／Ｃｏ、Ｐｔ／Ｐｄ／Ｍｏ、Ｐｔ／Ｐｄ／Ｗ、Ｐｔ／Ｐｄ／Ｃｅ、Ｐｔ／Ｐｄ／Ｃｕ、Ｐｔ／Ｐｄ／Ｖ、Ｐｔ／Ｐｄ／Ｆｅ、Ｐｔ／Ｐｄ／Ａｕ、Ｐｔ／Ｐｄ／Ａｇ、Ｐｔ／Ｒｈ／Ｎｉ、Ｐｔ／Ｒｈ／Ｃｏ、Ｐｔ／Ｒｈ／Ｍｏ、Ｐｔ／Ｒｈ／Ｗ、Ｐｔ／Ｒｈ／Ｃｅ、Ｐｔ／Ｒｈ／Ｃｕ、Ｐｔ／Ｒｈ／Ｖ、Ｐｔ／Ｒｈ／Ｆｅ、Ｐｔ／Ｒｈ／Ａｕ、Ｐｔ／Ｒｈ／Ａｇ、Ｐｔ／Ｎｉ／Ｃｏ、Ｐｔ／Ｎｉ／Ｍｏ、Ｐｔ／Ｎｉ／Ｗ、Ｐｔ／Ｎｉ／Ｃｅ、Ｐｔ／Ｎｉ／Ｃｕ、Ｐｔ／Ｎｉ／Ｖ、Ｐｔ／Ｎｉ／Ｆｅ、Ｐｔ／Ｎｉ／Ａｕ、Ｐｔ／Ｎｉ／Ａｇ、Ｐｔ／Ｃｏ／Ｍｏ、Ｐｔ／Ｃｏ／Ｗ、Ｐｔ／Ｃｏ／Ｃｅ、Ｐｔ／Ｃｏ／Ｃｕ、Ｐｔ／Ｃｏ／Ｖ、Ｐｔ／Ｃｏ／Ｆｅ、Ｐｔ／Ｃｏ／Ａｕ、Ｐｔ／Ｃｏ／Ａｇ、Ｐｔ／Ｍｏ／Ｗ、Ｐｔ／Ｍｏ／Ｃｅ、Ｐｔ／Ｍｏ／Ｃｕ、Ｐｔ／Ｍｏ／Ｖ、Ｐｔ／Ｍｏ／Ｆｅ、Ｐｔ／Ｍｏ／Ａｕ、Ｐｔ／Ｍｏ／Ａｇ、Ｐｔ／Ｗ／Ｃｅ、Ｐｔ／Ｗ／Ｃｕ、Ｐｔ／Ｗ／Ｖ、Ｐｔ／Ｗ／Ｆｅ、Ｐｔ／Ｗ／Ａｕ、Ｐｔ／Ｗ／Ａｇ、Ｐｔ／Ｃｅ／Ｃｕ、Ｐｔ／Ｃｅ／Ｖ、Ｐｔ／Ｃｅ／Ｆｅ、Ｐｔ／Ｃｅ／Ａｕ、Ｐｔ／Ｃｅ／Ａｇ、Ｐｔ／Ｃｕ／Ｖ、Ｐｔ／Ｃｕ／Ｆｅ、Ｐｔ／Ｃｕ／Ａｕ、Ｐｔ／Ｃｕ／Ａｇ、Ｐｔ／Ｖ／Ｆｅ、Ｐｔ／Ｖ／Ａｕ、Ｐｔ／Ｖ／Ａｇ、Ｐｔ／Ｆｅ／Ａｕ、Ｐｔ／Ｆｅ／Ａｇ、Ｐｔ／Ａｕ／Ａｇがある。
【００７４】
これらの触媒活性成分をアルミナ薄膜３に担持するには、種々の方法が考えられるが、本実施形態に有利に適合する方法としては、含浸法，例えば蒸発乾固法、平衡吸着法、インシピアント・ウェットネス法あるいはスプレー法が適用できる。なかでもインシピアント・ウェットネス法が有利である。この方法は、所定量の触媒活性成分を含む水溶液をセラミック担体１５に向けて少しずつ滴下し、担体表面が均一にわずかに濡れはじめた状態（インシピアント：Incipient）となった時点で、触媒活性成分がセラミック担体１５の孔中に含浸するのを停止させ、その後、乾燥、焼成する方法である。即ち、セラミック担体１５の表面にビュレットや注射器を用いて触媒活性成分含有溶液を滴下することによって行う。触媒活性成分の担持量は、その溶液の濃度を調節することによって行う。
【００７５】
次に、触媒１０の製造方法について説明する。
本実施形態にかかる触媒１０の製造方法の特徴は、上記リン酸ジルコニウム含有セラミック担体１５の凹凸表面に、ゾル−ゲル法によって希土類酸化物を含有するアルミナ薄膜３を形成することにある。特に溶液の浸漬によるセル壁１２を形成するリン酸ジルコニウム粒子４の各々の表面に対し、希土類酸化物含有アルミナ薄膜３をそれぞれ個別に被覆する。そして仮焼成の後に、熱水処理工程を経ることにより、前記アルミナ薄膜３のミクロ断面構造をセリア等が分散しているアルミナの小繊維を林立させたような植毛構造を呈するアルミナ薄膜（担持膜）３に変化させ、次いで、そのアルミナ薄膜３の表面に所定量の触媒活性成分を吸着させて固定化（担持）させる点にある。
【００７６】
以下に各工程（（１）セラミック担体１５の形成、（２）触媒活性成分の担持）について詳しく説明する。
（１）リン酸ジルコニウム含有セラミック担体１５へのアルミナ薄膜３の被覆ａ．予備処理工程
この工程では、リン酸ジルコニウム粒子４の各々の表面に、アルミナとの化学的な結合を助成するために必要な量のリン酸ジルコニウムを提供すべく、加熱して酸化する処理を行う。
ｂ．溶液含浸工程
この工程では、セル壁１２を構成する各リン酸ジルコニウム粒子４の表面にそれぞれ、アルミニウムと希土類元素とを含有する金属化合物の溶液、たとえば、硝酸アルミニウムと硝酸セリウムとの混合水溶液等を用いてゾル−ゲル法により含浸させることにより、希土類酸化物含有アルミナ薄膜３を被覆する処理を行う。
【００７７】
上記混合水溶液のうち、アルミニウム含有化合物の溶液については、出発金属化合物としては、金属無機化合物と金属有機化合物とが用いられる。金属無機化合物としては、Ａｌ（ＮＯ₃）₃、ＡｌＣｌ₃、ＡｌＯＣｌ、ＡｌＰＯ₄、Ａｌ₂（ＳＯ₄）₃、Ａｌ₂Ｏ₃、Ａｌ（ＯＨ）₃、Ａｌ等が用いられる。なかでも特に、Ａｌ（ＮＯ₃）₃やＡｌＣｌ₃は、アルコール、水等の溶媒に溶解しやすく扱い易いので好適である。
【００７８】
金属有機化合物の例としては、金属アルコキシド、金属アセチルアセトネート、金属カルボキシレートがある。具体例としてはＡｌ（ＯＣＨ₃）₃、Ａｌ（ＯＣ₂Ｈ₃）₃、Ａｌ（ｉｓｏ−ＯＣ₃Ｈ₇）₃等がある。
【００７９】
一方、上記混合水溶液のうち、セリウム含有化合物の溶液については、Ｃｅ（ＮＯ₃）₃、ＣｅＣｌ₃、Ｃｅ₂（ＳＯ₄）₃、ＣｅＯ₂、Ｃｅ（ＯＨ）₃、Ｃｅ₂（ＣＯ₃）₃等が用いられる。
【００８０】
上記混合溶液の溶媒としては、水、アルコール、ジオール、多価アルコール、エチレングリコール、エチレンオキシド、トリエタノールアミン、キシレン等のうち選ばれる少なくとも１つ以上を混合して使う。
【００８１】
また、溶液を作製するときに触媒活性成分としては、塩酸、硫酸、硝酸、酢酸、フッ酸を加えることもある。さらに、アルミナ薄膜３の耐熱性を向上させるために、希土類酸化物の他に、Ｃｅ、Ｌａ、Ｐｒ、Ｎｄ、Ｂａ、Ｃａ、Ｌｉ、Ｋ、Ｓｒ、Ｓｉ、Ｚｒの中から選ばれる少なくとも１つの単体または酸化物以外の化合物（硝酸塩、塩化物、硫酸塩、水酸化物又は炭酸塩）を、出発原料に添加してもよい。
【００８２】
本実施形態において、好ましい金属化合物の例としては、Ａｌ（ＮＯ₃）₃及びＣｅ（ＮＯ₃）₃をあげることができるが、これらは比較的低温で溶媒に溶解し、原料溶液の作製が容易である。また、好ましい溶媒の例としては、１，３−ブタンジオールを推奨する。推奨の第１の理由は、粘度が適当であり、ゲル状態でリン酸ジルコニウム粒子４上に適当な厚みのゲル膜をつけることが可能だからである。第２の理由は、この溶媒は、溶液中で金属アルコキシドを形成するので酸素・金属・酸素の結合からなる金属酸化物重合体、すなわち金属酸化物ゲルの前駆体を形成しやすいからである。
【００８３】
かかるＡｌ（ＮＯ₃）₃の量は、１０〜５０mass％であることが望ましい。１０mass％未満だと触媒活性成分の活性を長時間維持するだけの表面積をもつアルミナ量を担持することができず、一方、５０mass％より多いと溶解時に発熱量が多くゲル化しやすくなるからである。
【００８４】
また、Ｃｅ（ＮＯ₃）₃の量は１〜３０mass％であることが好ましい。その理由は、１mass％未満だとすすの酸化を促進することができず、３０mass％より多いと焼成後ＣｅＯ₂の粒成長が起こるからである。
【００８５】
一方、Ａｌ（ＮＯ₃）₃とＣｅ（ＮＯ₃）₃との配合割合は、１０：２とすることが好ましい。その理由は、Ａｌ（ＮＯ₃）₃をリッチにすることにより、焼成後のＣｅＯ₂粒子の分散度を向上できるからである。
【００８６】
上記金属化合物の含浸溶液を作製するときの温度は、５０〜１３０℃が望ましい。５０℃未満だと溶質の溶解度が低いからであり、一方１３０℃より高いと反応が急激に進行しゲル化に至るため、塗布溶液として使用できないからである。攪拌時間は１〜９時間が望ましい。この理由は、前記範囲内では溶液の粘度が安定しているからである。
【００８７】
上記のセリウム含有金属化合物（Ａｌ（ＮＯ₃）₃及びＣｅ（ＮＯ₃）₃）については、上述した例の他、ジルコニウムとの複合酸化物または固溶体を生成させるために、ジルコニウム源として、例えばＺｒＯ（ＮＯ₃）₂やＺｒＯ₂を用いる。そして、これらを水やエチレングリコールに溶解して混合溶液とし、その混合溶液に含浸させた後、乾燥，焼成することにより、前記複合酸化物を得るようにすることが好ましい。
【００８８】
本実施形態において重要なことは、上記のようにして調整した金属化合物の溶液を、セル壁１２内の各リン酸ジルコニウム粒子４間の間隙である全ての気孔内に行き渡らせて侵入させることである。そのために、例えば、容器内にセラミック担体１５を入れて前記金属化合物溶液を満たして脱気する方法や、セラミック担体１５の一方から該溶液を流し込み、他方より脱気する方法等を採用することが好ましい。この場合、脱気する装置としては、アスピレータの他に真空ポンプ等を用いるとよい。このような装置を用いると、セル壁１２内の気孔内の空気を抜くことができ、ひいては各リン酸ジルコニウム粒子４の表面に上記金属化合物の溶液をまんべんなく行さ渡らせることができる。
ｃ．乾燥工程
この工程では、ＮＯ₂等の揮発成分を蒸発除去し、溶液をゲル化してリン酸ジルコニウム粒子４の表面に固定すると同時に、余分の溶液を除去する処理であって、１２０〜１７０℃×２時間程度の加熱を行う。それは、加熱温度が１２０℃よりも低いと揮発成分が蒸発し難く、一方１７０℃よりも高いとゲル化した膜厚が不均一になる。
ｄ．仮焼成工程
この工程では、残留成分を除去して、アモルファスのアルミナ薄膜３を形成するための仮焼成の処理を行う。具体的には、３００〜５００℃の温度に加熱することが望ましい。仮燃成の温度が３００℃より低いと残留有機物を除去し難く、一方５００℃より高いとＡｌ₂Ｏ₃が結晶化し、その後の熱水処理により、小繊維突起状のべーマイトが形成できなくなるからである。
ｅ．熱水処理工程
この工程では、上述した本実施形態に特有の構造のアルミナ薄膜３を形成するため、仮焼成したセラミック担体１５を熱水中へ浸漬する処理を行う。このような熱水処理を行うと、その直後にアモルファスアルミナ薄膜３表面の粒子が解膠作用を受けてゾル状態で溶液中に放出され、また水和によって生じたべ一マイト粒子が小繊維状突起となって凝縮し、解膠に対して安定な状態をつくる。
【００８９】
即ち、この熱水処理により、各リン酸ジルコニウム粒子４の表面に個別に付着した希土類酸化物含有アルミナは、小繊維状（針状粒子）となって林立し、いわゆる植毛構造を呈して粗い表面となる。それ故に高い比表面積のアルミナ薄膜３が形成される。一般に、アルミナの焼結は主に表面拡散で進行し、α−アルミナに相転移するときに急激に比表面積が減少する。しかし、前記アルミナ粒子にシリカが取り込まれているため、このシリカが熱処理過程においてアルミナの空孔サイトを埋め、あるいは針状粒子表面に移動して表面拡散や粒子間の焼結を抑制すると考えられる。したがって、セラミック担体１５の焼結初期には、針状粒子間の接触点からの焼結による粘性流動機構が支配的であるが、後期ではシリカが針状粒子間の物質移動経路を遮断するためにα−アルミナヘの転移が阻害され、それ以上の焼結が進行せずに高い比表面積を維持するものと考えられる。
【００９０】
上記熱水処理の温度は５０〜１００℃が望ましい。５０℃より低いとアモルファスアルミナ薄膜３の水和が進行せず、小繊維突起状のべーマイトを形成しないからである。一方、１００℃より高いと水が蒸発し、工程を長時間維持しがたい。処理時間については１時間以上が望ましい。１時間より短いとアモルファスアルミナの水和が不十分になるからである。
ｄ．本焼成工程
この工程では、水和によって生じたベーマイトを膜水させてアルミナ結晶とするための処理を行う。好ましい本焼成の温度は５００〜１０００℃であり、好ましい本焼成の温度は５〜２０である。この温度が５００℃より低いと結晶化が進まないからであり、一方、１０００℃よりも高いと、焼結が進行しすぎて、表面積が低下する傾向にあるからである。
【００９１】
（２）触媒活性成分の担持
ａ．溶液調整工程
セラミック担体１５の表面に、図３（ｂ）に示すような植毛構造を有する希土類酸化物含有アルミナ薄膜（担持膜）３を被覆し、そのアルミナ薄膜３の凹凸状表面に対しＰｔ等の触媒活性成分を担持させる。この場合、触媒活性成分の担持量は、Ｐｔ等を含む水溶液をセラミック担体１５の吸水量だけ滴下して含浸させ、表面がわずかに濡れ始める状態になるようにして決定する。
【００９２】
例えば、セラミック担体１５が保持する吸水量というのは、乾燥担体の吸水量測定値を２２．４６mass％とし、この担体の質量が１１０ｇ、容積が０．１６３ｌを有するものであれば、この担体は２４．７９／ｌの水を吸水する。
【００９３】
ここで、Ｐｔの出発物質としては、例えばジニトロジアンミン白金硝酸溶液（［Ｐｔ（ＮＨ₃）₂（ＮＯ₂）₂］ＨＮＯ₃、Ｐｔ濃度４．５３mass％）を使用する。所定の量１．７ｇ／ｌのＰｔを担持させるためには、担体に１．７（ｇ／ｌ）^*０．１６３（ｌ）＝０．２７２ｇのＰｔを担持すれば良いので、蒸留水によりジニトロジアンミン白金硝酸溶液（Ｐｔ濃度４．５３％）を希釈する。即ち、ジニトロジアンミン白金硝酸溶液（Ｐｔ濃度４．５３mass％）／蒸留水の重量比率Ｘ（％）は、Ｘ＝０．２７２（Ｐｔ量ｇ）／２４．７（含水量ｇ）／４．５３（Ｐｔ濃度mass％）で計算され、２４．８mass％となる。
ｂ．液含浸工程
上記のようにして調整した所定量のジニトロジアンミン白金硝酸水溶液を、上記セラミック担体１５の両端面にピペットにて定間隔に滴下する。例えば、片面に４０〜８０滴づつ定間隔に滴下し、セラミック担体１５を覆うアルミナ薄膜３表面にＰｔを均一に分散固定化させる。
ｃ．乾燥、焼成工程
水溶液の滴下が終わったセラミック担体１５は、１１０℃、２時間程度の処理にて乾燥して水分を除去したのち、デシケータの中に移し１時間放置し、電子天秤等を用いて付着量を測定する。次いで、Ｎ₂雰囲気中で、約５００℃−１時間程度の条件の下で焼成を行いＰｔの金属化を図る。
【００９４】
本実施形態にかかる触媒１０は、排気ガス浄化用フィルタとしての用途に用いられ、その１つの用途としては、素通しハニカムセラミック担体１５の例として、ガソリンエンジン用酸化触媒、三元触媒及びディーゼルエンジン用酸化触媒である。他の用途は、ハニカムを市松模様に交互に目封じしたディーゼルパティキュレートフィルタがある。
【００９５】
このディーゼルパティキュレートフィルタ（以下、単に「ＤＰＦ」と略記する）は、それ自体ではパティキュレート（浮遊粒子状物質：ＰＭ）をセル壁１２）で捕集する機能しか持たないが、これに触媒活性成分を担持することにより、排気ガス中の炭化水素、一酸化炭素を酸化することができる。
【００９６】
また、ディーゼル排気ガスのような酸化雰囲気においても、ＮＯｘを還元できるＮＯｘ選択還元型触媒成分や吸蔵型触媒成分を担持すれば、ＮＯｘの還元も可能である。なお、このＤＰＦ中に捕集される前記パティキュレートは、堆積とともに上記ＤＰＦの圧力損失の増加を招くため、通常は燃焼処理等により除去して再生する必要がある。通常のディーゼル排気ガス中に含まれるパティキュレートの主成分であるすす（炭素）の燃焼が開始される温度は約５５０〜６３０℃である。この点、触媒活性成分をＤＰＦに担持すると、そのすすの燃焼反応パスが変わり、エネルギー障壁を低くすることができる。ひいては燃焼温度を３００〜４００℃と大幅に低下させることができ、再生に要するエネルギーを削減でき、いわゆる上述したセリアの作用とも相俟って、再生効率の高いＤＰＦシステムを構築できるようになる。
【００９７】
以上説明したように、本実施形態にかかる触媒１０は、とくにディーゼル排気ガス処理システムに応用することが好ましいと言えるが、それぞれの次のような機能が期待できる。
Ａ．ディーゼル排気ガス用酸化触媒としての機能
（１）排気ガス浄化機能…ＴＨＣ（全炭化水素）、ＣＯの酸化
（２）エンジンの運転を妨げない機能…圧力損失
Ｂ．触媒付きディーゼルパティキュレートフィルタとしての機能
（１）排気ガス浄化機能…すすの燃焼温度、ＴＨＣ，ＣＯの酸化
（２）エンジンの運転を妨げない機能…圧力損失
【００９８】
【実施例】
（実施例１）
この実施例は、セラミック担体１５の表面に被覆形成したセリア含有アルミナ薄膜３についての作用・効果を確認するために行ったものである。表２に示す条件の下に製造したセラミック担体１５（実施例１、比較例１及び比較例２）を、ディーゼル車の排気ガス浄化装置におけるパティキュレートフィルタ（ＤＰＦ）に取付けて浄化試験を行った。この試験によって該フィルタの圧力損失特性、耐熱性、洗浄耐性について調査した。その調査結果を同表の中に示すと共に図６、図７及び図８に示した。
【００９９】
【表２】

（ａ）表２に示すように、パティキュレート（浮遊粒子状物質：ＰＭ）が蓄積する前では実施例１は、アルミナ薄膜３がないときとほとんど同じ圧力損失特性を示し、蓄積後も比較例１及び比較例２に比べると、同じガスを流通させたときの圧力損失は著しく小さいことがわかった。
【０１００】
（ｂ）また図７に示すように、比較例１に比べると、実施例１は、同じ温度で熱処理したときのアルミナ比表面積の低下が小さく、耐熱性に優れていることがわかった。
【０１０１】
（ｃ）また、洗浄耐性については、表２に示すように、実施例１は、比較例１及び比較例２よりも格段に優れることが判明した。
（ｄ）また、図１５は、再生率（再フィルタから除去されたＣ量／再生前のフィルタに付着していたＣ量）を示すものである。セリアを含有するアルミナ薄膜３の場合、４５％ものカーボンが除去されているのに対し、ウォッシュコートアルミナ均一膜の場合、僅か２０％に止まった。
【０１０２】
（実施例２）
この実施例は、ディーゼルパティキュレートフィルタ（ＤＰＦ）に、触媒活性成分として白金（Ｐｔ）をセラミック担体１５に担持させたときの諸特性について試験した結果を示すものである。実施の条件及び特性については表３に示す。その結果を図８、図９、図１０に示した。
【０１０３】
なお、この実施例は、セラミック担体１５のリン酸ジルコニウム粒子４の表面にアルミナ担持膜（８ｇ／ｌ）３を有するものである。参考例は、セラミック担体１５の表面にいかなる担持膜もないものである。比較例は、セラミック担体１５の表面にウォッシュコートによってアルミナ均一膜を形成したものである。
【０１０４】
【表３】

（１）圧力損失特性；
図８に示すように、実施例、参考例、比較例とを比較すると、本実施例は担持膜をもたない参考例の場合とほぼ同じ圧力損失特性を示し、比較例に比べると格段に効果が認められる。
【０１０５】
（２）耐熱性；
図９（ａ），（ｂ）に示すように、実施例と比較例とについて、１２００℃に加熱したときのアルミナ薄膜３の比表面積の推移と、９００℃に加熱したときの平衡温度の推移を比較すると、本実施例の効果が顕著であることがわかる。
【０１０６】
（３）すす燃焼特性；
触媒１０のすすを燃焼させる性能を平衡温度試験法によって評価した。この試験方法は次のような試験である。即ち、試験装置にディーゼルエンジンを設置し、その排気管の途中に触媒（ＤＰＦ）１０を挿入設置し、この状態で運転を開始する。すると、運転時間とともにＤＰＦにはすすが捕集されるため圧力損失が増大する。この場合に、何らかの方法により排気温度を上昇させていくと、ある温度において、すすが堆積する速度とすすの酸化反応速度が平衡する点（平衡温度）が現れると共に、このときの圧力（平衡圧力）が測定できる。この平衡温度、平衡圧力共に低いほど優れた触媒１０であると言える。
【０１０７】
なお、この試験において、排気ガス温度を上昇させる方法としては、ディーゼルエンジンとＤＰＦの間に電気ヒーターを挿入して行った。この方法では、エンジン回転数、負荷を一定にできるので、ディーゼル排気ガスの組成が試験中に変化せず、平衡温度、平衡圧力が精度よく求められるという特長がある。試験条件は、ディーゼルエンジン排気量２７３ｃｃ、回転数１２５０ｒｐｍ、負荷３Ｎｍで定常運転を実施し、供試したフィルタの堆積は□３４×１５０ｍｍＬで０．１６Litterである。
【０１０８】
上記試験の結果を図１０に示す。
図１０中、触媒活性成分を担持しないセラミック担体１５の例を参考例とした。同図よりわかるように、排気ガスの温度上昇とともにフィルタ温度は上昇していくが、５００℃程度で平衡点が見られる。本実施例と比較例とを比較すると、平衡温度は、それぞれ４００℃、４１０℃でわずかな優位差であったが、そのときの平衡圧力は１１ｋＰａ、９．２ｋＰａと２０％近く向上している。
【０１０９】
また、８５０℃−２０時間の酸化雰囲気中でのエージングを実施した後、同様の試験を実施したところ、本実施例はほとんど平衡温度、圧力が劣化しなかったのに対し、比較例では触媒活性成分を担持していないときと同じ状態にまで劣化していた。
（４）ＴＨＣ，ＣＯ浄化率；
この特性は、酸化触媒を評価する場合の一般的方法である。いわゆるＴＨＣ（全炭化水素）のＣＯ₂と水への浄化及びＣＯのＣＯ₂への浄化の温度との関係を調査したものである。この特性は、低温より転換率が高くなる方が優れた触媒システムと言える。測定方法としては、エンジンとフィルタとを用い、フイルタ前後のＴＨＣ及びＣＯの量を排気ガス分析計にて測定し、温度に対する浄化率を測定することにより行った。
【０１１０】
図１１に示すように、本実施例は比較例に対してＣＯ、ＴＨＣのいずれの浄化温度も約３０℃低下しており優れた性能を示す。これは、本実施例はセル壁１２のリン酸ジルコニウム粒子４に均一に触媒活性成分が分散されているため、ウォッシュコートを通過する時間に対して、排気ガスがセル壁１２内を通過する時間は明らかに長く、それだけＰｔの活性点にＣＯ，ＴＨＣが吸着する機会が増したためと考えられる。別の見方をすれば、リン酸ジルコニウム粒子４に均一に触媒活性成分が分散されているため、触媒コート層２に対する排気ガスの接触面積を大きくすることができる。よって、排気ガス中のＣＯやＨＣの酸化を促進することができる。
【０１１１】
次に、特許請求の範囲に記載された技術的思想のほかに、前述した実施形態によって把握される技術的思想を以下に示す。
（１） ４価金属酸性不溶性塩含有セラミック担体の粒子上に、触媒活性成分、助触媒及びサポート材からなる触媒コート層を形成したことを特徴とする排気ガス浄化用触媒。
【０１１２】
（２） ジルコニウム化合物含有セラミック担体の粒子上に、触媒活性成分、助触媒及びサポート材からなる触媒コート層を形成したことを特徴とする排気ガス浄化用触媒。
（３） ４価金属のリン酸塩含有セラミック担体の粒子上に、触媒活性成分、助触媒及びサポート材からなる触媒コート層を形成したことを特徴とする排気ガス浄化用触媒。
【０１１３】
（４） 請求項１〜８、前記（１）〜（３）のいずれかにおいて、前記触媒活性成分は、貴金属元素、元素周期表VIａ族の元素、及び元素周期表VIII族の元素の中から選ばれる元素を含むことを特徴とする排気ガス浄化用触媒。
【０１１４】
（５） 前記（１）〜（４）のいずれかにおいて、前記助触媒は、セリウム（Ｃｅ）、ランタン（Ｌａ）、バリウム（Ｂａ）及びカルシウム（Ｃａ）の中から選ばれる少なくとも１つの単体または化合物からなることを特徴とする排気ガス浄化用触媒。
【０１１５】
（６） 前記（１）〜（５）のいずれかにおいて、前記サポート材は、アルミナ、ジルコニア、チタニア及びシリカの中から選ばれる少なくとも１つを含むことを特徴とする排気ガス浄化用触媒。
【０１１６】
（７） 請求項１〜８、前記（１）〜（６）のいずれかにおいて、前記セラミック担体は、セル壁により区画されている複数の貫通孔を有するハニカム構造であることを特徴とする排気ガス浄化用触媒。
【０１１７】
（８） 請求項１〜８、前記（１）〜（７）のいずれかにおいて、前記セラミック担体は、その両端部が封止体によって市松模様に交互に目封止されていることを特徴とする排気ガス浄化用触媒。
【０１１８】
【発明の効果】
以上詳述したように、本発明によれば、排気ガスの圧力損失が小さくすることができるとともに、機械的強度を向上することができる。また、排気ガス中に含まれるパティキュレートの捕集効率を高めることができる。
【図面の簡単な説明】
【図１】一実施形態における触媒担体の略線図。
【図２】触媒担体の一部分を示す拡大斜視図。
【図３】本実施形態のアルミナ薄膜の概念図。
【図４】圧力損失特性の説明図。
【図５】触媒担体のリン酸ジルコニウム粒子構造を示す拡大模式図。
【図６】（ａ）は気孔径と圧力損失との関係を示すグラフ、（ｂ）は気孔率と圧力損失との関係を示すグラフ、（ｃ）は気孔率とセラミック担体の曲げ強度の関係を示すグラフ。
【図７】実施例１における耐熱性の比較説明図。
【図８】実施例２における圧力損失特性の比較説明図。
【図９】実施例２におけアルミナ薄膜と触媒の耐熱性の比較説明図。
【図１０】実施例２におけるすす燃焼特性の比較説明図。
【図１１】実施例２におけるＴＨＣ、ＣＯ浄化特性の比較説明図。
【図１２】ＣｅＯ₂添加による酸化速度向上のメカニズムを説明する模式図。
【図１３】ＤＰＦの再生特性に影響するすすの酸化特性の比較グラフ。
【図１４】ＤＰＦの再生特性に影響する再生（燃焼）速度の比軟グラフ。
【図１５】ＤＰＦの再生率の比較グラフ。
【図１６】従来技術における触媒担体の略線図。
【図１７】同じくウォッシュコートアルミナ層の概念図。
【符号の説明】
３…アルミナ薄膜、４…リン酸ジルコニウム粒子、１０…触媒、１５…セラミック担体。[0001]
BACKGROUND OF THE INVENTION
  The present invention purifies exhaust gasFor exhaust gas purificationThe present invention relates to a catalyst and a method for producing the same. Specifically, the carbon monoxide (CO) and hydrocarbons (HC) contained in the exhaust gas of diesel engines can be efficiently removed by oxidation and nitrogen oxides (NOx) can be reduced and removed. Small, high collection efficiency of diesel particulates and good regeneration rateFor exhaust gas purificationThe catalyst and its manufacturing method are proposed.
[0002]
[Prior art]
Conventionally, as an exhaust gas purification catalyst for automobiles, for example, there is a catalyst for purifying exhaust gas of a diesel engine. Each cell 101 serving as an exhaust gas passage as shown in FIGS. 16 (a) and 16 (b) is formed in a honeycomb shape with a porous silicon carbide sintered body excellent in heat resistance and thermal conductivity. A catalyst-carrying filter 100 in which the cells 101 are alternately sealed is used. The catalyst-carrying filter 100 is usually connected to the exhaust side of a diesel engine, and particulates (PM: particulate matter), HC, CO, etc. deposited in the filter are usually oxidized and decomposed.
[0003]
As such a catalyst-carrying filter 100, for example, a carrying layer made of γ-alumina is formed on the surface of the cell wall 102 of a heat-resistant ceramic carrier 105 formed in a honeycomb shape, and Pt, Pd, Rh are further formed on the carrying layer. Those carrying a catalytic active component made of a noble metal such as the above are well known.
[0004]
As such a catalyst-carrying filter 100, in Japanese Patent Laid-Open No. 5-68892, a fine powder obtained by adding an inorganic binder to γ-alumina and mixing and pulverizing it is used as a slurry, and this slurry is used as a honeycomb-shaped ceramic carrier. There is one in which the catalyst coat layer 103 is formed by so-called wash-coating, in which the wall surface 105 is uniformly sprayed and coated.
[0005]
[Problems to be solved by the invention]
The catalyst coat layer 103 (wash coat alumina layer) that has been subjected to this prior art, that is, the wash coat, is formed of a thin film that uniformly covers the wall surface of the cell wall 102 as shown in FIG. It has a pore structure as shown in the partially enlarged view shown in b). The pore diameter in such a pore structure is mainly 20 to 500 angstroms, usually 50 to 300 m.²It usually shows a specific surface area of / g. Further, such a catalyst coat layer 103 disperses and supports a catalytic active component such as a noble metal on the surface, so that the specific surface area must be large and a certain thickness (about 50 to 100 μm) is also required.
[0006]
However, the wash-coated catalyst coat layer 103 has a small pore diameter and porosity, and has a high resistance when exhaust gas passes through the catalyst-carrying filter 100. Therefore, there is a problem that the pressure loss is remarkably increased as compared with the ceramic carrier 105 having no catalyst coat layer. The “pressure loss” refers to a value obtained by subtracting the pressure value on the downstream side from the pressure value on the upstream side of the catalyst-carrying filter 100. The greatest factor causing pressure loss is that the exhaust gas undergoes resistance as it passes through the filter. Therefore, when the pressure loss increases, the heat resistance, mechanical strength, and collection efficiency are lowered, resulting in chemical instability.
[0007]
Furthermore, since the wash-coated catalyst coat layer 103 is simply coated uniformly on the surface of the carrier, which is the cell wall 102, the adhesion is poor, and the ash that is deposited when purifying exhaust gas is removed. There was a risk of peeling during cleaning. In addition, as described above, the catalyst coat layer 103 has a pore structure, but the pore diameter is as small as 20 to 500 angstroms. When the catalyst coat layer 103 is exposed to a high temperature for a long time, the sintering proceeds and the phase transition to the α phase causes a specific surface area. There is also a problem that the heat resistance is inferior because of the small size. Furthermore, since the specific surface area is small, the distance between the particles of the catalytically active component supported on the catalyst coat layer 103 is small, so that when the sintering proceeds, the specific surface area becomes smaller and the catalytic action itself. There was a problem of causing a drop in
[0008]
  The present invention has been made in view of the above-mentioned problems, and the object thereof is not only low in exhaust gas pressure loss but also excellent in mechanical strength.For exhaust gas purificationThe object is to provide a catalyst and a method for producing the same.
[0009]
[Means for Solving the Problems]
  In order to solve the above-described problems, the invention according to claim 1 is obtained by dispersing and supporting a catalytically active component on the surface of a tetravalent metal acidic insoluble salt-containing ceramic carrier.For exhaust gas purificationA catalyst, wherein the ceramic support has a surface coated with a thin film of alumina containing a rare earth oxide for each particle unit forming the support, and the rare earth oxide-containing alumina thin filmHas an uneven surface consisting of a flocked structure with small fibersThe gist is that the catalytically active component is supported on the uneven surface.
[0010]
  In the invention according to claim 2, the catalytically active component is dispersedly supported on the surface of the zirconium compound-containing ceramic carrier.For exhaust gas purificationA catalyst, wherein the ceramic support has a surface coated with a thin film of alumina containing a rare earth oxide for each particle unit forming the support, and the rare earth oxide-containing alumina thin filmHas an uneven surface consisting of a flocked structure with small fibersThe gist is that the catalytically active component is supported on the uneven surface.
[0011]
  In the invention according to claim 3, a catalytically active component is dispersedly supported on the surface of a tetravalent metal phosphate-containing ceramic carrier.For exhaust gas purificationA catalyst, wherein the ceramic support has a surface coated with a thin film of alumina containing a rare earth oxide for each particle unit forming the support, and the rare earth oxide-containing alumina thin filmHas an uneven surface consisting of a flocked structure with small fibersThe gist is that the catalytically active component is supported on the uneven surface.
[0012]
  In the invention according to claim 4, according to claim 1,For exhaust gas purificationIn the catalyst, the tetravalent metal acidic insoluble salt-containing ceramic carrier includes any one of a porous body, a fiber molded body, and a pellet molded body containing any one of zirconium phosphate, titanium phosphate, zirconium arsenate, and titanium arsenate. The gist is that it is composed.
[0013]
  In invention of Claim 5, in any one of Claims 1-4,For exhaust gas purificationIn the catalyst, the rare earth oxide-containing alumina thin film covering the surface of each particle in the ceramic support has a micro cross-sectional shape with a diameter of 2 to 50.nm, Length: 20 to 300 nm, having a concavo-convex surface composed of a flocked structure in which small fibers having a length / diameter ratio of 5 to 100 are forested, and the surface has a specific surface area of 50 to 300 m²/ G.
[0014]
  In invention of Claim 6, in any one of Claims 1-5,For exhaust gas purificationIn the catalyst, the alumina thin film containing the rare earth oxide covering the ceramic support is coated at a rate of 0.1 to 15 mass% in terms of the amount of alumina with respect to the support, and the rare earth contained in the alumina thin film The gist is that the amount of the oxide is 10 to 80 mass% with respect to the alumina.
[0015]
  In invention of Claim 7, in any one of Claims 1-6For exhaust gas purificationIn the catalyst, the gist of the rare earth oxide is that at least a part thereof forms a complex oxide with zirconium.
[0016]
  In the invention described in claim 8, it is described in claim 7.For exhaust gas purificationThe gist of the catalyst is that the particle diameter of the complex oxide of rare earth oxide and zirconium is 1 to 30 nm.
[0017]
  In the invention according to claim 9, the surface of each particle constituting the tetravalent metal acidic insoluble salt-containing ceramic carrier isRecord(By forming a rare earth oxide-containing alumina thin film through steps a) to (e), and then supporting a catalytically active component such as a noble metal on the uneven surface of the rare earth oxide-containing alumina thin film.For exhaust gas purificationA catalyst can be produced.
(A) Solution impregnation step: The ceramic support is immersed in a solution of a metal compound containing aluminum and a rare earth oxide.
(B) Drying step: The ceramic carrier is dried by heating.
(C) Temporary firing step: An amorphous alumina thin film is formed by firing the ceramic carrier at a temperature of 300 to 500 ° C. or higher.
(D) Heat treatment step: The ceramic carrier is immersed in hot water at 100 ° C. and then dried.
(E) Main baking process: Main baking is performed at 500 to 1200 ° C.
[0018]
  In the invention according to claim 10, a rare earth oxide-containing alumina thin film is formed on the surface of each particle constituting the tetravalent metal acidic insoluble salt-containing ceramic carrier through the following steps (a) to (f), and then By dispersing and supporting catalytically active components on the uneven surface of rare earth oxide-containing alumina thin filmFor exhaust gas purificationA catalyst can be produced.
(A) Pretreatment step: The ceramic support is heated to a temperature of 1000 ° C. to 1500 ° C. to form a silicide oxide film.
(B) Solution impregnation step: The ceramic support is immersed in a solution of a metal compound containing aluminum and a rare earth oxide.
(C) Drying step: The ceramic support is dried by heating.
(D) Temporary firing step: An amorphous alumina thin film is formed by firing the ceramic carrier at a temperature of 300 to 500 ° C. or higher.
(E) Heat treatment step: The ceramic carrier is immersed in hot water at 100 ° C. and then dried.
(F) Main baking process: Main baking is performed at 500 to 1200 ° C.
[0019]
  In the invention according to claim 11, according to claim 9 or 10,For exhaust gas purificationIn the method for producing a catalyst, the tetravalent metal acidic insoluble salt-containing ceramic carrier includes any one of a porous body, a fiber molded body, and a pellet molded body containing zirconium phosphate, titanium phosphate, zirconium arsenate, and titanium arsenate. The gist is that it is composed.
[0020]
  In invention of Claim 12, in any one of Claims 9-11For exhaust gas purificationIn the method for producing a catalyst, the alumina thin film containing the rare earth oxide covering the ceramic support is coated at a rate of 0.1 to 15 mass% in terms of the amount of alumina with respect to the support, The gist of the present invention is that the amount of the rare earth oxide contained is 10 to 80 mass% with respect to the alumina.
[0021]
  In invention of Claim 13, in any one of Claims 9-12For exhaust gas purificationIn the catalyst production method, the gist of the rare earth oxide is that at least a part thereof forms a complex oxide with zirconium.
[0022]
  In the invention described in claim 14, it is described in claim 13.For exhaust gas purificationThe gist of the catalyst production method is that the particle diameter of the complex oxide of rare earth oxide and zirconium is 1 to 30 nm.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment embodying the present invention will be described in detail with reference to the drawings.
As shown in FIGS. 1 to 3, the catalyst 10 according to the present embodiment includes a ceramic carrier 15 made of a porous sintered body. The ceramic carrier 15 is made of a ceramic containing a tetravalent metal acidic insoluble salt. In this embodiment, zirconium phosphate (Zr (HPO_Four)₂・ NH₂O) is used. A cell wall 12 is formed on the ceramic carrier 15. The catalyst coating layer 2 is individually coated with a predetermined thickness on the surface of the zirconium phosphate particles 4 constituting the cell wall 12.
[0024]
The catalyst coat layer 2 is a support material on which a catalytically active component and a promoter are supported. In the present embodiment, the support material is alumina (Al₂O_Three) 3 (hereinafter referred to as alumina thin film) 3. Besides alumina, zirconia (zirconium dioxide: ZrO₂), Titania (titanium oxide: TiO₂), Silica (silicon oxide: SiO₂) May be arbitrarily changed as long as it includes at least one selected from the above.
[0025]
Specifically, one type of oxide is ZrO.₂TiO₂Or SiO₂There is. As two types of oxides, Al₂O_Three/ ZrO₂, Al₂O_Three/ TiO₂, Al₂O_Three/ SiO₂, ZrO₂/ TiO₂Or ZrO₂/ SiO₂There is. The three types of oxides are Al₂O_Three/ ZrO₂/ TiO₂, Al₂O_Three/ ZrO₂/ SiO₂, Al₂O_Three/ TiO₂/ SiO₂Or ZrO₂/ TiO₂/ SiO₂There is. The four types of oxides are Al₂O_Three/ ZrO₂/ TiO₂/ SiO₂There is.
[0026]
As the tetravalent metal acidic insoluble salt-containing ceramic carrier 15, in addition to zirconium phosphate, for example, titanium phosphate, zirconium arsenate and titanium arsenate can be used. In this way, a wall flow honeycomb type filter as shown in FIGS. 1A, 1B and 2 is formed.
[0027]
Hereinafter, an example in which a zirconium phosphate sintered body is used as the tetravalent metal acidic insoluble salt-containing ceramic carrier 15 will be described.
The ceramic carrier 15 is composed of a zirconium phosphate sintered body having a substantially square cross section in which cells 11 as a plurality of through holes are regularly formed along the axial direction. The cells 11 are separated from each other by a cell wall 12. The opening of each cell 11 is sealed on one end face side by a sealing body 14, and the other end face of the corresponding cell 11 is opened. As a whole, each end face is arranged so that the open portion and the sealing portion each have a checkered pattern. The ceramic carrier 15 is formed with a large number of cells 11 having a square cross section. In other words, these ceramic carriers 15 have a honeycomb structure.
[0028]
The density of the cells 11 is 200 to 350 / square inch. That is, of the many cells 11, about half of the cells 11 are opened at the upstream end surface, and the remaining cells are opened at the downstream end surface. The thickness of the cell wall 12 separating each cell 11 is about 0.4 mm. Is set to
[0029]
  Thus, the ceramic carrier 15 made of zirconium phosphate has a structure partitioned by the porous cell walls 12 as shown in FIG. 3A, and the pores of the porous cell walls 12 are The average pore diameter measured by the mercury intrusion method is in the range of 10 μm to 500 μm. In addition to this range, the average pore diameter of the ceramic carrier 15 is 20μm~ 250μmIt may be changed to an arbitrary value within the range.
[0030]
If the cell wall 12 has such a pore diameter, it is also suitable for collecting fine particulates. That is, by setting the average pore diameter of the cell wall 12 within the above range, diesel particulates can be reliably collected. On the other hand, if the average value of the pore diameter of the cell wall 12 is less than 10 μm, the pressure loss when the exhaust gas passes through the cell wall 12 becomes extremely large as shown in FIG. Can cause. On the other hand, if the average pore diameter exceeds 300 μm, fine particulates cannot be collected efficiently.
[0031]
Moreover, the porosity of the porous cell wall 12 in the ceramic carrier 15 is set to 30 to 70%. Besides this value, the porosity of the ceramic carrier 15 may be changed to any value within the range of 40 to 60%. As shown in FIG. 6B, if the porosity is less than 30%, the pressure loss when the exhaust gas passes through the cell wall 12 becomes extremely large, which may cause the engine to stop. On the other hand, if the porosity exceeds 70%, fine particulates cannot be collected efficiently. Note that the data indicating the pressure loss characteristics in FIGS. 6A and 6B show the case where the exhaust gas flow velocity is 10 m / s.
[0032]
At the same time, as shown in FIG. 6C, when the porosity exceeds 70%, cracks are likely to occur in the ceramic carrier 15 due to a decrease in mechanical strength. That is, the porosity of the ceramic carrier 15 and the mechanical strength are in an inversely proportional relationship. Therefore, if the porosity is made larger than 70% in order to increase the supported amount of the catalyst coat layer 2 on the zirconium phosphate particles 4 of the ceramic support 15, the honeycomb structure cannot be formed.
[0033]
As described above, since the average pore diameter of the ceramic carrier 15 is set to 10 to 500 μm and the porosity is set to 30 to 70%, the pressure loss can be reduced and the mechanical strength can be improved. . At the same time, the collection efficiency of the particulates contained in the exhaust gas can be increased.
[0034]
In the case of producing such a ceramic carrier 15, for example, about 30 parts of zirconium phosphate powder having an average particle diameter of about 0.5 μm is added to 70 parts by weight of zirconium phosphate powder having an average particle diameter of about 10 μm as a raw material. Part by weight, about 6 parts by weight of methylcellulose as a binder with respect to 100 parts by weight of zirconium phosphate powder, and about 25 parts by weight of a dispersion medium composed of an organic solvent and water with respect to 100 parts by weight of zirconium phosphate powder. Use a blended one. Next, the blended raw materials are kneaded and then formed into a honeycomb shape by extrusion, and then a part of the cell 11 is sealed in a checkered pattern. Next, the molded body is dried and degreased, and then fired in an inert atmosphere to obtain a desired ceramic carrier 15.
[0035]
In addition, as shown in the following Table 1, the zirconium phosphate used for a raw material has what has an amorphous nature and what has crystallinity. If amorphous zirconium phosphate is used, the specific surface area of zirconium phosphate can be increased. In this embodiment, α-zirconium phosphate shown in Table 1 is used. In addition to this α-zirconium phosphate, it may be changed to zirconium phosphate having another crystal structure.
[0036]
[Table 1]

In the present embodiment, the most characteristic configuration is that the surface of the cell wall 12 substantially constituting the ceramic support 15, particularly the surface of each zirconium phosphate particle 4 constituting the cell wall 12 contains a rare earth oxide. It is to coat with the alumina thin film 3. More precisely, for each zirconium phosphate particle 4 of the zirconium phosphate sintered body constituting the cell wall 12, the surface of each zirconium phosphate particle 4 is individually divided into various types. That is, it is coated with the rare earth oxide-containing alumina thin film 3 by a method.
[0037]
FIG. 17 (b) shows the prior art in which the catalyst coat layer 2 is uniformly formed on the surface of the cell wall 12 by the wash coat method. FIGS. 3 (a) and 3 (b) It is explanatory drawing of the ceramic support | carrier 15 used by this embodiment. A rare earth oxide-containing alumina thin film 3 (hereinafter, this “rare earth oxide-containing alumina thin film 3” is simply abbreviated as “alumina thin film 3”) is individually provided on the surface of each zirconium phosphate particle 4 constituting the cell wall 12. Fig. 1 shows a state of being coated.
[0038]
As described above, the characteristic structure of the catalytic active component according to the present embodiment (alumina thin film 3) is that the wall surface of the exhaust gas cell wall 12 is simply uniformly coated with the catalyst coating layer 2 as in the prior art. It was n’t. For example, when the cell walls 12 are uniformly coated as in the prior art, the gaps between the zirconium phosphate particles 4 are sealed and plugged, impairing air permeability. On the other hand, in the case of the ceramic carrier 15 used in the present embodiment, the surface of each zirconium phosphate particle 4 constituting the cell wall 12 is individually covered with the alumina thin film 3.
[0039]
Therefore, in the present embodiment, since the pores of the cell wall 12 itself, that is, the pores formed between the zirconium phosphate particles 4 are not completely blocked, the pores are maintained as pores. Compared with the catalyst coat layer 2, the pressure loss is remarkably small. In addition, it is excellent in heat resistance, and furthermore, since the alumina thin film 3 individually coats each zirconium phosphate particle 4 itself, for example, the thin film is not peeled off from the cell wall 12 during cleaning. Excellent cleaning performance. Furthermore, the area where the exhaust gas contacts the catalytically active component increases. Therefore, the oxidation of CO and HC in the exhaust gas can be promoted.
[0040]
Therefore, the pressure loss characteristics, heat resistance, washing resistance and regeneration characteristics of the alumina thin film 3 as a supporting film used for the catalytic active component according to this embodiment will be described below.
[0041]
Pressure loss characteristics of alumina thin film 3;
In general, the pressure loss characteristic when exhaust gas passes through the cell wall 12 is considered as follows. That is, the pressure loss when diesel exhaust gas passes through the ceramic carrier 15 can be shown as in FIG. In this case, the resistances ΔP1, ΔP2, and ΔP3 depend on the cell structure of the filter, and are constant values Δpi = (ΔP1 + ΔP2 + ΔP3) that do not depend on the passage of time such as the accumulation of diesel particulates. . Further, ΔP4 is a resistance when passing through the accumulated diesel particulate, and has a value of 2 to 3 times or more of the initial pressure loss.
[0042]
The specific surface area of the ceramic carrier 15 having a 14/200 cell structure is 8.931 cm.²/ Cm^ThreeThe density of the ceramic carrier 15 is 0.675 g / cm.^ThreeTherefore, the specific surface area of the cell wall 12 is 0.0013 m.²/ G. On the other hand, the pore specific surface area in the cell wall 12 is 0.12 m as measured by a mercury porosimeter.²/ G and has a surface area of about 50 to 100 times. This is because when the alumina thin film 3 having the same weight is formed on the surface of the cell wall 12, each phosphor constituting the cell wall 12 is not simply covered so as to uniformly cover the surface of the cell wall 12. When the surface of the zirconium oxide particles 4 is individually coated, the thickness of the alumina thin film 3 for obtaining the same effect can be reduced to 1/50 to 1/100.
[0043]
That is, when the alumina thin film 3 is uniformly formed under the conventional technique such as wash coating, the thickness of the alumina thin film 3 is 50 μm to cover about 3 mass% of alumina necessary for the activity of the catalytically active component. is necessary. The pressure loss at this time is increased in resistance through the alumina thin film 3 in addition to the resistance ΔP3 through the cell wall 12. Furthermore, the opening is reduced and ΔP1 is also increased. Therefore, the pressure loss is remarkably increased as compared with the filter not coated with alumina, and the tendency becomes more remarkable when particulates are deposited on the filter.
[0044]
In this regard, when alumina is coated on the surface of the zirconium phosphate particles 4 constituting the cell wall 12 as in the ceramic carrier 15 used in the present embodiment, an alumina thin film of about 3 mass% required for activation of the catalytically active component. 3, the thickness is about 0.5 μm at the maximum. The increase in pressure loss at this time slightly increases the resistance ΔP3, but other pressure losses are substantially negligible, and the pressure loss characteristic is lower than that of an alumina layer formed by a conventional washcoat. Improve dramatically.
[0045]
Regarding the heat resistance of the alumina thin film 3;
In general, alumina has a high specific surface area and is suitable as a support film for a catalytically active component. In particular, the development of a catalyst 10 having high heat resistance that operates stably at a higher temperature is desired. Accordingly, the alumina thin film 3 is also required to have higher heat resistance.
[0046]
In this embodiment, in order to improve the heat resistance of alumina in this embodiment, (1) the shape of each alumina particle is made into a small fiber, and (2) a rare earth oxide such as ceria (cerium oxide) is contained. It was to be.
[0047]
In particular, by adopting the former configuration (1), it is possible to reduce the number of contact points of each alumina particle, to suppress grain growth through a decrease in the sintering rate, to increase the specific surface area, and thus to heat resistance. Improves.
[0048]
That is, in the present embodiment, the alumina thin film 3 covering the surface of each zirconium phosphate particle 4 of the ceramic carrier 15 has a flocked structure in which the micro cross-sectional shape of each alumina particle is fibrillar or forested. Therefore, the contact points between adjacent alumina fibrils are reduced, and the heat resistance is remarkably improved.
[0049]
Next, regarding (2), the heat resistance is also improved by adding ceria or the like. The reason is due to the effect that a new compound is formed on the surface of the crystal particles constituting the alumina thin film 3 and hinders the growth of the alumina particles.
[0050]
About the cleaning resistance of the alumina thin film 3;
The main part of the particulates deposited on the surface of the cell wall 12 is carbon, which can be oxidized and removed by a method such as combustion. However, some substances remain as ash after combustion. Such a substance has a role as a neutralizing agent or a lubricant, etc., so that a compound such as Ca, Mg, Zn or the like added to engine oil is oxidized or sulfated. is there. In addition, CeO in the fuel in advance₂And catalytically active components mixed for carbon combustion such as CuO are deposited together with particulates. Since these ash components accumulate as the vehicle travels for a long time and increase the pressure loss of the filter, cleaning with high-pressure water or the like is necessary. At this time 30 kg / cm²Ash can be completely removed by washing at the above pressure.
[0051]
In this regard, in the case of a conventional alumina thin film formed on the surface of the cell wall 12 by a wash coat, since there is a thick coat layer by physical adsorption on the entire surface of the cell wall 12, it is often peeled off during the cleaning. On the other hand, in the supporting film (alumina thin film 3) used in the present embodiment, alumina is thinly coated on the surface of each zirconium phosphate particle 4 constituting the ceramic support 15 and is chemically bonded. Therefore, it is in a state of being in tight contact with the individual zirconium phosphate particles. Therefore, the adhesiveness is high, and therefore the resistance to cleaning is high, and the durability as a coating is strong.
[0052]
Regeneration characteristics of alumina thin film 3;
In the present embodiment, the alumina thin film 3 includes ceria (CeO) therein.₂) Or Lantana (La₂O_Three) Such as A1)₂O_ThreeIn contrast, about 10 to 80 mass%, preferably about 20 to 40 mass% is added, and these oxides are uniformly dispersed on the surface or inside of the alumina thin film 3.
[0053]
When ceria or the like is added to the alumina thin film 3 (preferably added together with a catalytically active component such as Pt), oxygen supply to the exhaust gas is activated by the oxygen concentration adjusting action of ceria. Thus, the combustion removal efficiency of “soot (diesel particulate)” adhering to the filter is improved, and the regeneration rate of the catalyst 10 is significantly improved. Further, the durability of the catalyst 10 can be improved.
[0054]
That is, rare earth oxides such as ceria not only improve the heat resistance of alumina, but also play a role in adjusting the oxygen concentration on the catalyst surface. In general, hydrocarbons and carbon monoxide present in exhaust gas are removed by oxidation reaction, and NOx is removed by reduction reaction, but the exhaust gas composition constantly fluctuates between the rich region and lean region of fuel. Therefore, the working atmosphere on the catalyst surface also fluctuates drastically. By the way, ceria added to the catalyst is Ce.³⁺And Ce⁴⁺The redox potential of is relatively small, and the following formula 2CeO₂⇔Ce₂O_Three+ 1 / 2O₂The reaction proceeds reversibly. That is, when the exhaust gas becomes rich, the above reaction proceeds to the right to supply oxygen into the atmosphere, but conversely, when the exhaust gas reaches the lean area, it proceeds to the left and occludes excess oxygen in the atmosphere. In this way, by adjusting the oxygen concentration in the atmosphere, the ceria serves to expand the range of the air-fuel ratio that can efficiently remove hydrocarbons, carbon monoxide, or NOx.
[0055]
FIG. 12 (a) shows ceria (CeO).₂This explains the mechanism of the oxidation rate of the alumina thin film 3 to which no) is added. In contrast, FIG. 12B illustrates the mechanism of the oxidation rate of the alumina thin film 3 when ceria is added. As shown in the figure, CeO₂The catalyst without the presence of oxygen oxidizes soot (soot) by activating oxygen in the exhaust gas. This reaction is inefficient because oxygen in the fluid must be activated.
[0056]
On the other hand, CeO₂For the catalyst in which is present, the following reaction:
CeO₂⇔CeO₂-_x+ X / 2O₂
To supply oxygen. That is, the oxygen discharged into the atmosphere and the oxygen in the exhaust gas react with soot (carbon) activated by the catalytic active component (noble metal), and CO 2.₂(CeO₂-_xIs oxidized to the original CeO₂Back to). CeO₂Since soot is in direct contact, soot can be oxidized efficiently even if the amount of oxygen exhaled is small.
[0057]
Moreover, CeO in this case₂Increases the OSC (oxygen storage function) by supporting a catalytically active component (noble metal). This is because the catalytically active component activates oxygen in the exhaust gas and CeO near the noble metal.₂Since the surface oxygen is also activated, the OSC increases.
[0058]
13 and 14 show the effect of adding rare earth oxides such as ceria into the alumina thin film 3 with Pt as the catalytic active component and CeO as the promoter.₂Support material is needle-shaped Al₂O_ThreeCatalyst 10 (Embodiment), Pt / Acicular Al₂O_Three(Comparative Example) and Pt / Al₂O_Three(Wash coat) The results of experiments on the regeneration characteristics of the catalyst are shown. In this experiment, a diesel particulate filter (DPF, total length = 150 mm) with soot attached was housed in an electric furnace and heated to 650 ° C., while a 1100 rpm, 3.9 Nm diesel engine was connected and its exhaust gas ( The transition of the filter temperature (measured at 145 mm from the inlet) (FIG. 13) and the regeneration (combustion) speed (ratio of the rising temperature ΔT and the elapsed time Δt, FIG. 14) ).
[0059]
As shown in FIG. 13, the conventional example (catalyst coat layer 2 formed by washcoat) is O₂Became the rate-limiting and reached the peak temperature at 50 sec-700 ° C, and in the comparative example (without ceria (promoter))₂However, in this embodiment, the peak temperature is reached at a high speed of 45 sec-900 ° C., soot oxidation removal efficiency is high, and a high regeneration rate is achieved. You can see that This also clearly appears as a difference in the regeneration (combustion) speed in FIG.
[0060]
FIG. 15 compares the regeneration rates themselves, and it is clear that the effect of this embodiment (ceria-containing catalyst) is outstanding. The DPF filters soot in the exhaust gas. Therefore, soot accumulates in the DPF. The act of removing the accumulated soot is called regeneration. Therefore, the ratio between the regenerated soot weight and the accumulated soot weight is expressed as a percentage, and this is defined as the regenerative ratio.
[0061]
In addition, about the said rare earth oxide, the single oxide (CeO in the example mentioned above).₂For example, it is more preferable to use a complex oxide of a rare earth element and zirconium. This is presumably because the inclusion of zirconium oxide in the rare earth oxide improves the oxygen concentration control characteristics through the suppression of grain growth of the rare earth oxide.
[0062]
  The rare earth oxide in the form of a complex oxide with zirconium preferably has a particle size of about 1 to 30 nm, more preferably 2 to 20 nm.nmIt is good to make it the size. The reason is that a complex oxide having a particle diameter of less than 1 nm is difficult to produce. On the other hand, when the particle diameter exceeds 30 nm, the particles are easily sintered, the surface area of the particles is reduced, and the contact area with the exhaust gas is reduced, resulting in a problem that the activity is weakened. In addition, there is a concern that the pressure loss during exhaust gas passage will also increase.
[0063]
Regarding the structure of the alumina thin film 3;
FIG. 5 is an enlarged view showing a ceramic carrier 15 in which the surface of each zirconium phosphate particle 4 is coated with the alumina thin film 3 and a carrier (in the prior art) in which the surface of the cell wall 12 is uniformly coated with an alumina film. It is. Acicular (small fiber-like) alumina stands on the surface of each zirconium phosphate particle 4 and has a flocking structure as shown in FIG. Since the object which coat | covers the alumina thin film 3 is the zirconium phosphate particle | grains 4, it is easy to form an alumina in needle shape.
[0064]
The structure of the alumina thin film 3, that is, the crystal structure of the alumina thin film 3 formed by coating the surface of each zirconium phosphate particle 4,₂O_Three, Δ-Al₂O_Three, Θ-Al₂O_ThreeAt least one of them. The diameter of the fibril projection-like alumina constituting the alumina thin film 3 is 2 to 50 nm, the length is 20 to 300 nm, and the length / diameter ratio is 5 to 50. The thickness of the alumina thin film 3 having such a structure is 0.5 μm or less, and the specific surface area of the alumina-based alumina is 50 to 300 m.²/ G is preferable.
[0065]
The thickness of the alumina thin film 3 here is an average of the distance from the surface of the zirconium phosphate particles 4 to the farthest portion of the fibril-projected alumina. The diameter of alumina is more desirably 5 to 20 nm, and the ratio of the total length / diameter is more desirably 10 to 30.
[0066]
The reason for limiting the properties of the fibril-projection-like alumina thin film 3 as described above is that if the length of the fibril-projection-like alumina is less than 20 nm, it is difficult to ensure the necessary specific surface area, while from 300 nm. This is because if it is larger, the structure becomes brittle. Also, the diameter is less than or equal to the size of the catalytically active component such as a noble metal if it is smaller than 2 nm, and does not function as a catalyst support layer. On the other hand, if it exceeds 50 nm, it is difficult to secure a specific surface area with a desirable size. It is. As for the ratio of the total length / diameter, if this ratio is less than 5, it is difficult to ensure the required specific surface area. On the other hand, if it exceeds 50, it becomes structurally fragile, and fibrillar protrusions break due to cleaning operations and the like. This is because cases arise.
[0067]
The reason for limiting the specific surface area of the alumina thin film 3 as described above is 50 m.²This is because, if it is less than / g, the sintering of the fibril projection-like alumina proceeds excessively, resulting in poor durability. On the other hand, specific surface area is 300m²When it is larger than / g, it means that the fibril projection alumina becomes too fine. It does not function as a so-called catalyst support layer or is structurally fragile. The preferred specific surface area is 50 to 200 m.²/ G.
[0068]
Next, in the ceramic carrier 15 as described above, the amount of the alumina thin film 3 serving as a supporting film is preferably 0.1 to 15 mass% in terms of alumina ratio. This is because if it is less than 0.1 mass%, the effect of improving heat resistance is small, while if it is more than 15 mass%, the pressure loss increases and the filter function is lowered. More preferably, it is 1-4 mass%.
[0069]
The surface of each of the zirconium phosphate particles 4 is individually coated with the alumina thin film 3 and the surface of the ceramic support 15 is as if completely covered with the alumina thin film (supporting film) 3. The ceramic carrier 15 is loaded with an element selected from a noble metal element which is a catalytically active component, group VIa of the periodic table and group VIII of the periodic table. Specific examples of this element include platinum (Pt), palladium (Pd), rhodium (Rh), nickel (Ni), cobalt (Co), molybdenum (Mo), tungsten (W), cerium (Ce), and copper. (Cu), vanadium (V), iron (Fe), gold (Au), silver (Ag), and the like.
[0070]
Therefore, Pt, Au, Ag, Cu as noble metal elements, Mo and W as elements of group VIa of the periodic table, Fe, Co, Pd, Rh, Ni as elements of group VIII of the periodic table Other than those, at least one simple substance or compound selected from V and Ce as elements of the periodic table may be supported on the alumina thin film 3.
[0071]
For example, a binary alloy or a ternary alloy based on a combination of the above elements is used as the compound. These alloys are advantageously used with rare earth oxides such as ceria and lantana which act as promoters as described above. Such a catalyst 10 is excellent in durability since it has little poisoning deterioration (lead poisoning, phosphorus poisoning, sulfur poisoning) and heat deterioration. In addition to the alloy by a combination of the above elements, a compound (oxide, nitride, or carbide) by a combination with another element may be used.
[0072]
Incidentally, binary alloys include Pt / Pd, Pt / Rh, Pt / Ni, Pt / Co, Pt / Mo, Pt / W, Pt / Ce, Pt / Cu, Pt / V, Pt / Fe, Pt. / Au, Pt / Ag, Pd / Rh, Pd / Ni, Pd / Co, Pd / Mo, Pd / W, Pd / Ce, Pd / Cu, Pd / V, Pd / Fe, Pd / Au, Pd / Ag Rh / Ni, Rh / Co, Rh / Mo, Rh / W, Rh / Ce, Rh / Cu, Rh / V, Rh / Fe, Rh / Au, Rh / Ag, Ni / Co, Ni / Mo, Ni / W, Ni / Ce, Ni / Cu, Ni / V, Ni / Fe, Ni / Au, Ni / Ag, Co / Mo, Co / W, Co / Ce, Co / Cu, Co / V, Co / Fe , Co / Au, Co / Ag, Mo / W, Mo / Ce, Mo / Cu, Mo / V, Mo / Fe, o / Au, Mo / Ag, W / Ce, W / Cu, W / V, W / Fe, W / Au, W / Ag, Ce / Cu, Ce / V, Ce / Fe, Ce / Au, Ce / There are Ag, Cu / V, Cu / Fe, Cu / Au, Cu / Ag, V / Fe, V / Au, V / Ag, Fe / Au, Fe / Ag, and Au / Ag.
[0073]
Further, as ternary alloys, Pt / Pd / Rh, Pt / Pd / Ni, Pt / Pd / Co, Pt / Pd / Mo, Pt / Pd / W, Pt / Pd / Ce, Pt / Pd / Cu , Pt / Pd / V, Pt / Pd / Fe, Pt / Pd / Au, Pt / Pd / Ag, Pt / Rh / Ni, Pt / Rh / Co, Pt / Rh / Mo, Pt / Rh / W, Pt / Rh / Ce, Pt / Rh / Cu, Pt / Rh / V, Pt / Rh / Fe, Pt / Rh / Au, Pt / Rh / Ag, Pt / Ni / Co, Pt / Ni / Mo, Pt / Ni / W, Pt / Ni / Ce, Pt / Ni / Cu, Pt / Ni / V, Pt / Ni / Fe, Pt / Ni / Au, Pt / Ni / Ag, Pt / Co / Mo, Pt / Co / W , Pt / Co / Ce, Pt / Co / Cu, Pt / Co / V, Pt / Co / Fe, Pt / C / Au, Pt / Co / Ag, Pt / Mo / W, Pt / Mo / Ce, Pt / Mo / Cu, Pt / Mo / V, Pt / Mo / Fe, Pt / Mo / Au, Pt / Mo / Ag , Pt / W / Ce, Pt / W / Cu, Pt / W / V, Pt / W / Fe, Pt / W / Au, Pt / W / Ag, Pt / Ce / Cu, Pt / Ce / V, Pt / Ce / Fe, Pt / Ce / Au, Pt / Ce / Ag, Pt / Cu / V, Pt / Cu / Fe, Pt / Cu / Au, Pt / Cu / Ag, Pt / V / Fe, Pt / V / Au, Pt / V / Ag, Pt / Fe / Au, Pt / Fe / Ag, and Pt / Au / Ag.
[0074]
Various methods are conceivable for supporting these catalytically active components on the alumina thin film 3, but as an advantageous method suitable for the present embodiment, an impregnation method such as an evaporation to dryness method, an equilibrium adsorption method, an incipient A wetness method or a spray method can be applied. Of these, the incipient wetness method is advantageous. In this method, an aqueous solution containing a predetermined amount of a catalytically active component is dropped little by little toward the ceramic carrier 15, and when the surface of the carrier begins to be slightly and uniformly wet (Incipient), the catalytically active component Is a method of stopping impregnation in the pores of the ceramic carrier 15 and then drying and firing. That is, it is performed by dropping the catalyst active ingredient-containing solution onto the surface of the ceramic carrier 15 using a burette or a syringe. The amount of the catalytically active component supported is adjusted by adjusting the concentration of the solution.
[0075]
Next, the manufacturing method of the catalyst 10 is demonstrated.
A feature of the manufacturing method of the catalyst 10 according to the present embodiment is that the alumina thin film 3 containing a rare earth oxide is formed on the uneven surface of the zirconium phosphate-containing ceramic support 15 by a sol-gel method. In particular, the rare earth oxide-containing alumina thin film 3 is individually coated on each surface of the zirconium phosphate particles 4 forming the cell wall 12 by immersion of the solution. After the preliminary firing, an alumina thin film (supporting film) having a flocked structure in which the micro cross-sectional structure of the alumina thin film 3 is planted with alumina fibrils in which ceria and the like are dispersed is obtained through a hydrothermal treatment process. 3), and then a predetermined amount of the catalytically active component is adsorbed on the surface of the alumina thin film 3 to be immobilized (supported).
[0076]
Each step ((1) formation of the ceramic carrier 15 and (2) loading of the catalytically active component) will be described in detail below.
(1) Coating of alumina thin film 3 on zirconium phosphate-containing ceramic support 15 a. Pretreatment process
In this step, the surface of each of the zirconium phosphate particles 4 is heated and oxidized to provide an amount of zirconium phosphate necessary to assist chemical bonding with alumina.
b. Solution impregnation process
In this process, a solution of a metal compound containing aluminum and a rare earth element, such as a mixed aqueous solution of aluminum nitrate and cerium nitrate, is used on the surface of each zirconium phosphate particle 4 constituting the cell wall 12. -The process which coat | covers the rare earth oxide containing alumina thin film 3 is performed by making it impregnate by a gel method.
[0077]
Among the above mixed aqueous solutions, for the solution of the aluminum-containing compound, a metal inorganic compound and a metal organic compound are used as the starting metal compound. As a metal inorganic compound, Al (NO_Three)_ThreeAlCl_Three, AlOCl, AlPO_Four, Al₂(SO_Four)_Three, Al₂O_Three, Al (OH)_ThreeAl or the like is used. In particular, Al (NO_Three)_ThreeAnd AlCl_ThreeIs preferable because it is easily dissolved in a solvent such as alcohol or water and is easy to handle.
[0078]
Examples of metal organic compounds include metal alkoxides, metal acetylacetonates, and metal carboxylates. As a specific example, Al (OCH_Three)_Three, Al (OC₂H_Three)_Three, Al (iso-OC_ThreeH₇)_ThreeEtc.
[0079]
On the other hand, among the mixed aqueous solutions, the cerium-containing compound solution is Ce (NO_Three)_Three, CeCl_Three, Ce₂(SO_Four)_Three, CeO₂, Ce (OH)_Three, Ce₂(CO_Three)_ThreeEtc. are used.
[0080]
As the solvent of the mixed solution, at least one selected from water, alcohol, diol, polyhydric alcohol, ethylene glycol, ethylene oxide, triethanolamine, xylene and the like are mixed and used.
[0081]
In preparing the solution, hydrochloric acid, sulfuric acid, nitric acid, acetic acid, and hydrofluoric acid may be added as the catalytically active component. Furthermore, in order to improve the heat resistance of the alumina thin film 3, in addition to the rare earth oxide, at least one selected from Ce, La, Pr, Nd, Ba, Ca, Li, K, Sr, Si, Zr A compound other than a simple substance or an oxide (nitrate, chloride, sulfate, hydroxide or carbonate) may be added to the starting material.
[0082]
In the present embodiment, examples of preferable metal compounds include Al (NO_Three)_ThreeAnd Ce (NO_Three)_ThreeThese can be dissolved in a solvent at a relatively low temperature, and preparation of a raw material solution is easy. Moreover, 1,3-butanediol is recommended as an example of a preferable solvent. The first reason for the recommendation is that the viscosity is appropriate and a gel film having an appropriate thickness can be formed on the zirconium phosphate particles 4 in a gel state. The second reason is that since this solvent forms a metal alkoxide in a solution, it easily forms a metal oxide polymer composed of a combination of oxygen, metal and oxygen, that is, a precursor of a metal oxide gel.
[0083]
Such Al (NO_Three)_ThreeThe amount of is desirably 10 to 50 mass%. If the amount is less than 10 mass%, the amount of alumina having a surface area sufficient to maintain the activity of the catalytically active component for a long time cannot be supported. .
[0084]
Also, Ce (NO_Three)_ThreeThe amount of is preferably 1 to 30 mass%. The reason is that if it is less than 1 mass%, the oxidation of soot cannot be promoted.₂This is because grain growth occurs.
[0085]
On the other hand, Al (NO_Three)_ThreeAnd Ce (NO_Three)_ThreeThe blending ratio is preferably 10: 2. The reason is Al (NO_Three)_ThreeBy enriching, CeO after firing₂This is because the degree of dispersion of the particles can be improved.
[0086]
The temperature at which the metal compound impregnation solution is prepared is preferably 50 to 130 ° C. This is because if the temperature is lower than 50 ° C., the solubility of the solute is low, while if the temperature is higher than 130 ° C., the reaction proceeds rapidly and gelation occurs, so that it cannot be used as a coating solution. The stirring time is desirably 1 to 9 hours. This is because the viscosity of the solution is stable within the above range.
[0087]
The cerium-containing metal compound (Al (NO_Three)_ThreeAnd Ce (NO_Three)_ThreeIn addition to the examples described above, for example, ZrO (NO) is used as a zirconium source in order to produce a complex oxide or solid solution with zirconium._Three)₂And ZrO₂Is used. Then, it is preferable to obtain these composite oxides by dissolving them in water or ethylene glycol to make a mixed solution, impregnating the mixed solution, drying and firing.
[0088]
What is important in the present embodiment is that the solution of the metal compound prepared as described above is made to spread and enter all the pores that are the gaps between the zirconium phosphate particles 4 in the cell wall 12. is there. For this purpose, for example, a method of putting the ceramic carrier 15 in a container and filling the metal compound solution to deaerate, a method of pouring the solution from one of the ceramic carriers 15 and degassing from the other, etc. can be adopted. preferable. In this case, as a device for deaeration, a vacuum pump or the like may be used in addition to the aspirator. If such an apparatus is used, the air in the pores in the cell wall 12 can be removed, and as a result, the solution of the metal compound can be evenly distributed over the surface of each zirconium phosphate particle 4.
c. Drying process
In this process, NO₂This is a process for removing the excess solution at the same time as evaporating and removing the volatile components such as gelling and fixing the solution to the surface of the zirconium phosphate particles 4 and heating at 120 to 170 ° C. for about 2 hours. That is, when the heating temperature is lower than 120 ° C., the volatile component hardly evaporates, while when the heating temperature is higher than 170 ° C., the gelled film thickness becomes non-uniform.
d. Temporary firing process
In this step, a residual component is removed and a temporary firing process is performed to form the amorphous alumina thin film 3. Specifically, it is desirable to heat to a temperature of 300 to 500 ° C. If the pre-combustion temperature is lower than 300 ° C, it is difficult to remove residual organic matter, while if it is higher than 500 ° C, Al₂O_ThreeThis is because crystallization of the boehmite cannot be formed by subsequent hydrothermal treatment.
e. Hot water treatment process
In this step, in order to form the alumina thin film 3 having a structure peculiar to the above-described embodiment, the preliminarily fired ceramic carrier 15 is immersed in hot water. Immediately after such hydrothermal treatment, the particles on the surface of the amorphous alumina thin film 3 are peptized and released into the solution in a sol state, and the baitite particles generated by hydration are fibrillated. Condenses and creates a stable state against peptization.
[0089]
That is, by this hydrothermal treatment, the rare earth oxide-containing alumina individually adhered to the surface of each zirconium phosphate particle 4 is forested in the form of small fibers (acicular particles), exhibiting a so-called flocked structure and a rough surface. It becomes. Therefore, the alumina thin film 3 having a high specific surface area is formed. In general, the sintering of alumina proceeds mainly by surface diffusion, and the specific surface area rapidly decreases when the phase transitions to α-alumina. However, since silica is incorporated in the alumina particles, it is considered that this silica fills the pore sites of alumina in the heat treatment process or moves to the surface of the acicular particles to suppress surface diffusion and sintering between particles. . Therefore, in the early stage of sintering of the ceramic carrier 15, the viscous flow mechanism by the sintering from the contact point between the acicular particles is dominant, but in the latter period, the silica blocks the mass transfer path between the acicular particles. It is considered that the transition to α-alumina is hindered and a high specific surface area is maintained without further sintering.
[0090]
The temperature of the hot water treatment is preferably 50 to 100 ° C. This is because when the temperature is lower than 50 ° C., the hydration of the amorphous alumina thin film 3 does not proceed and the fibril projection-like boehmite is not formed. On the other hand, when the temperature is higher than 100 ° C., water evaporates and it is difficult to maintain the process for a long time. The processing time is preferably 1 hour or longer. This is because the hydration of the amorphous alumina becomes insufficient when it is shorter than 1 hour.
d. Main firing process
In this step, the boehmite generated by hydration is treated with water to form alumina crystals. The preferable firing temperature is 500 to 1000 ° C., and the preferred firing temperature is 5 to 20. This is because if this temperature is lower than 500 ° C., crystallization does not proceed, while if it is higher than 1000 ° C., sintering proceeds too much and the surface area tends to decrease.
[0091]
(2) Supporting catalytically active components
a. Solution adjustment process
The surface of the ceramic carrier 15 is coated with a rare earth oxide-containing alumina thin film (supporting film) 3 having a flocking structure as shown in FIG. 3B, and catalytic activity such as Pt is applied to the uneven surface of the alumina thin film 3. Ingredients are supported. In this case, the amount of the catalytically active component supported is determined such that an aqueous solution containing Pt or the like is dropped and impregnated by the amount of water absorbed by the ceramic carrier 15 so that the surface begins to slightly wet.
[0092]
For example, the water absorption amount retained by the ceramic carrier 15 is that the measured value of the water absorption amount of the dry carrier is 22.46 mass%, the mass of the carrier is 110 g, and the volume is 0.163 l. Absorbs 24.79 / l of water.
[0093]
Here, as a starting material of Pt, for example, dinitrodiammine platinum nitrate solution ([Pt (NH_Three)₂(NO₂)₂] HNO_Three, Pt concentration 4.53 mass%). In order to carry a predetermined amount of 1.7 g / l of Pt, the carrier has 1.7 (g / l).^*Since it is sufficient to support 0.163 (l) = 0.272 g of Pt, dinitrodiammine platinum nitrate solution (Pt concentration: 4.53%) is diluted with distilled water. That is, the weight ratio X (%) of dinitrodiammine platinum nitrate solution (Pt concentration 4.53 mass%) / distilled water is X = 0.272 (Pt amount g) /24.7 (water content g) /4.53. It is calculated by (Pt concentration mass%) and becomes 24.8 mass%.
b. Liquid impregnation process
A predetermined amount of the dinitrodiammine platinum nitric acid aqueous solution prepared as described above is dropped onto the both end faces of the ceramic carrier 15 at regular intervals with a pipette. For example, 40 to 80 drops are dropped on one side at regular intervals, and Pt is uniformly dispersed and fixed on the surface of the alumina thin film 3 covering the ceramic carrier 15.
c. Drying and firing process
After the dropping of the aqueous solution is finished, the ceramic carrier 15 is dried by treatment at 110 ° C. for about 2 hours to remove moisture, then transferred to a desiccator and left for 1 hour, and the amount of adhesion is measured using an electronic balance or the like. To do. Then N₂Pt is metallized by firing in an atmosphere under conditions of about 500 ° C. for about 1 hour.
[0094]
The catalyst 10 according to the present embodiment is used for an application as an exhaust gas purifying filter. As one of the applications, as an example of a transparent honeycomb ceramic carrier 15, an oxidation catalyst for a gasoline engine, a three-way catalyst, and a diesel engine. It is an oxidation catalyst. Another application is a diesel particulate filter in which honeycombs are alternately sealed in a checkered pattern.
[0095]
This diesel particulate filter (hereinafter simply abbreviated as “DPF”) itself has only the function of collecting particulates (floating particulate matter: PM) by the cell wall 12), but this has catalytic activity. By supporting the component, hydrocarbons and carbon monoxide in the exhaust gas can be oxidized.
[0096]
Further, even in an oxidizing atmosphere such as diesel exhaust gas, NOx can be reduced by supporting a NOx selective reduction type catalyst component or an occlusion type catalyst component capable of reducing NOx. Note that the particulates collected in the DPF cause an increase in the pressure loss of the DPF as it accumulates, so it is usually necessary to remove and regenerate it by a combustion process or the like. The temperature at which combustion of soot (carbon), which is a main component of particulates contained in normal diesel exhaust gas, is started is about 550 to 630 ° C. In this regard, when the catalytically active component is supported on the DPF, the soot combustion reaction path is changed, and the energy barrier can be lowered. As a result, the combustion temperature can be greatly reduced to 300 to 400 ° C., the energy required for regeneration can be reduced, and in combination with the so-called ceria action, a DPF system with high regeneration efficiency can be constructed.
[0097]
As described above, it can be said that the catalyst 10 according to this embodiment is preferably applied to a diesel exhaust gas treatment system, but the following functions can be expected.
A. Function as an oxidation catalyst for diesel exhaust gas
(1) Exhaust gas purification function: THC (total hydrocarbons), CO oxidation
(2) Functions that do not interfere with engine operation: Pressure loss
B. Function as a diesel particulate filter with catalyst
(1) Exhaust gas purification function: Soot combustion temperature, oxidation of THC and CO
(2) Functions that do not interfere with engine operation: Pressure loss
[0098]
【Example】
Example 1
This example was performed in order to confirm the action and effect of the ceria-containing alumina thin film 3 formed on the surface of the ceramic support 15. A ceramic carrier 15 manufactured under the conditions shown in Table 2 (Example 1, Comparative Example 1 and Comparative Example 2) was attached to a particulate filter (DPF) in an exhaust gas purification device of a diesel vehicle, and a purification test was conducted. . This test investigated the pressure loss characteristics, heat resistance, and washing resistance of the filter. The results of the investigation are shown in the table and shown in FIGS.
[0099]
[Table 2]

(A) As shown in Table 2, before the accumulation of particulates (floating particulate matter: PM), Example 1 shows almost the same pressure loss characteristics as when the alumina thin film 3 is not present. Compared to 1 and Comparative Example 2, it was found that the pressure loss when the same gas was circulated was extremely small.
[0100]
(B) Moreover, as shown in FIG. 7, compared with the comparative example 1, it turned out that the fall of the alumina specific surface area when Example 1 heat-processes at the same temperature is small, and it is excellent in heat resistance.
[0101]
(C) Further, as shown in Table 2, with respect to cleaning resistance, it was found that Example 1 was significantly superior to Comparative Example 1 and Comparative Example 2.
(D) FIG. 15 shows the regeneration rate (the amount of C removed from the refilter / the amount of C attached to the filter before regeneration). In the case of the alumina thin film 3 containing ceria, as much as 45% of carbon was removed, but in the case of the wash coat alumina uniform film, it was only 20%.
[0102]
(Example 2)
This example shows the results of testing various characteristics when platinum (Pt) is supported on a ceramic carrier 15 as a catalytic active component in a diesel particulate filter (DPF). The implementation conditions and characteristics are shown in Table 3. The results are shown in FIG. 8, FIG. 9, and FIG.
[0103]
In this embodiment, the alumina support film (8 g / l) 3 is provided on the surface of the zirconium phosphate particles 4 of the ceramic support 15. In the reference example, there is no support film on the surface of the ceramic carrier 15. In the comparative example, a uniform alumina film is formed on the surface of the ceramic support 15 by wash coating.
[0104]
[Table 3]

(1) Pressure loss characteristics;
As shown in FIG. 8, when comparing the example, the reference example, and the comparative example, the present example shows almost the same pressure loss characteristic as that of the reference example having no supporting film, and is markedly compared with the comparative example. The effect is recognized.
[0105]
(2) heat resistance;
As shown in FIGS. 9A and 9B, for the examples and comparative examples, the transition of the specific surface area of the alumina thin film 3 when heated to 1200 ° C. and the transition of the equilibrium temperature when heated to 900 ° C. It can be seen that the effect of this example is remarkable.
[0106]
(3) Soot combustion characteristics;
The ability of the catalyst 10 to burn soot was evaluated by an equilibrium temperature test method. This test method is the following test. That is, the diesel engine is installed in the test apparatus, the catalyst (DPF) 10 is inserted and installed in the middle of the exhaust pipe, and the operation is started in this state. Then, the soot is collected in the DPF with the operation time, so that the pressure loss increases. In this case, if the exhaust temperature is raised by some method, a point (equilibrium temperature) at which the soot deposition rate and the soot oxidation reaction rate are balanced at a certain temperature appears, and the pressure at this time (equilibrium pressure) ) Can be measured. It can be said that the lower the equilibrium temperature and the equilibrium pressure, the better the catalyst 10.
[0107]
In this test, as a method of raising the exhaust gas temperature, an electric heater was inserted between the diesel engine and the DPF. This method is characterized in that the engine speed and load can be made constant, so that the composition of diesel exhaust gas does not change during the test, and the equilibrium temperature and the equilibrium pressure are obtained with high accuracy. The test conditions were a diesel engine displacement of 273 cc, a rotational speed of 1250 rpm, and a load of 3 Nm. Steady operation was performed, and the deposition of the tested filter was □ 34 × 150 mmL and 0.16 Litter.
[0108]
The results of the above test are shown in FIG.
In FIG. 10, an example of a ceramic carrier 15 that does not carry a catalytically active component is taken as a reference example. As can be seen from the figure, the filter temperature rises as the exhaust gas temperature rises, but an equilibrium point is seen at about 500 ° C. When this example and the comparative example were compared, the equilibrium temperatures were slightly different at 400 ° C. and 410 ° C., respectively, but the equilibrium pressure at that time was 11 kPa and 9.2 kPa, which was improved by almost 20%. .
[0109]
Further, after performing aging in an oxidizing atmosphere at 850 ° C. for 20 hours, the same test was performed. In this example, the equilibrium temperature and pressure were hardly deteriorated, whereas in the comparative example, the catalytic activity was reduced. It deteriorated to the same state as when no component was supported.
(4) THC, CO purification rate;
This property is a common method for evaluating oxidation catalysts. CO of so-called THC (total hydrocarbons)₂And water purification and CO CO₂The relationship between the temperature and purification temperature is investigated. This characteristic can be said to be an excellent catalyst system when the conversion rate is higher than the low temperature. As a measuring method, using an engine and a filter, the amount of THC and CO before and after the filter was measured with an exhaust gas analyzer, and the purification rate with respect to temperature was measured.
[0110]
As shown in FIG. 11, the present example shows excellent performance because both the purification temperatures of CO and THC are reduced by about 30 ° C. compared to the comparative example. In this embodiment, since the catalytically active component is uniformly dispersed in the zirconium phosphate particles 4 on the cell wall 12, the time for the exhaust gas to pass through the cell wall 12 with respect to the time for passing the washcoat. This is probably due to the fact that the opportunity for adsorption of CO and THC to the active site of Pt increased accordingly. From another viewpoint, since the catalytically active component is uniformly dispersed in the zirconium phosphate particles 4, the contact area of the exhaust gas with respect to the catalyst coat layer 2 can be increased. Therefore, the oxidation of CO and HC in the exhaust gas can be promoted.
[0111]
  Next, in addition to the technical idea described in the claims, the technical idea grasped by the above-described embodiment will be described below.
  (1) A catalyst coat layer comprising a catalytically active component, a promoter and a support material is formed on particles of a tetravalent metal acidic insoluble salt-containing ceramic carrier.For exhaust gas purificationcatalyst.
[0112]
  (2) A catalyst coat layer comprising a catalytically active component, a cocatalyst and a support material is formed on the particles of the zirconium compound-containing ceramic carrier.For exhaust gas purificationcatalyst.
  (3) A catalyst coat layer comprising a catalytically active component, a promoter and a support material is formed on particles of a tetravalent metal phosphate-containing ceramic carrier.For exhaust gas purificationcatalyst.
[0113]
  (4) In any one of claims 1 to 8 and (1) to (3), the catalytically active component is selected from among noble metal elements, elements of group VIa of the periodic table of elements, and elements of group VIII of the periodic table of elements. Characterized by containing selected elementsFor exhaust gas purificationcatalyst.
[0114]
  (5) In any one of the above (1) to (4), the promoter is at least one simple substance selected from cerium (Ce), lanthanum (La), barium (Ba) and calcium (Ca) or It consists of a compoundFor exhaust gas purificationcatalyst.
[0115]
  (6) In any one of (1) to (5), the support material includes at least one selected from alumina, zirconia, titania, and silica.For exhaust gas purificationcatalyst.
[0116]
  (7) In any one of claims 1 to 8 and (1) to (6), the ceramic carrier has a honeycomb structure having a plurality of through holes partitioned by cell walls.For exhaust gas purificationcatalyst.
[0117]
  (8) The ceramic carrier according to any one of claims 1 to 8 and (1) to (7), wherein both ends of the ceramic carrier are alternately plugged in a checkered pattern by a sealing body. DoFor exhaust gas purificationcatalyst.
[0118]
【The invention's effect】
As described above in detail, according to the present invention, the pressure loss of the exhaust gas can be reduced and the mechanical strength can be improved. Moreover, the collection efficiency of the particulates contained in the exhaust gas can be increased.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of a catalyst carrier in one embodiment.
FIG. 2 is an enlarged perspective view showing a part of a catalyst carrier.
FIG. 3 is a conceptual diagram of an alumina thin film according to the present embodiment.
FIG. 4 is an explanatory diagram of pressure loss characteristics.
FIG. 5 is an enlarged schematic view showing the structure of the zirconium phosphate particles of the catalyst carrier.
6A is a graph showing the relationship between the pore diameter and the pressure loss, FIG. 6B is a graph showing the relationship between the porosity and the pressure loss, and FIG. 6C is a relationship between the porosity and the bending strength of the ceramic carrier. Graph showing.
7 is a comparative explanatory view of heat resistance in Example 1. FIG.
8 is a comparative explanatory view of pressure loss characteristics in Example 2. FIG.
9 is a comparative explanatory view of heat resistance of an alumina thin film and a catalyst in Example 2. FIG.
10 is a comparative explanatory view of soot combustion characteristics in Example 2. FIG.
11 is a comparative explanatory view of THC and CO purification characteristics in Embodiment 2. FIG.
FIG. 12 CeO₂The schematic diagram explaining the mechanism of the oxidation rate improvement by addition.
FIG. 13 is a comparative graph of the oxidation characteristics of soot that affect the regeneration characteristics of DPF.
FIG. 14 is a specific soft graph of regeneration (combustion) speed that affects the regeneration characteristics of DPF.
FIG. 15 is a comparative graph of the regeneration rate of DPF.
FIG. 16 is a schematic diagram of a catalyst carrier in the prior art.
FIG. 17 is a conceptual view of a washcoat alumina layer.
[Explanation of symbols]
3 ... Alumina thin film, 4 ... zirconium phosphate particles, 10 ... catalyst, 15 ... ceramic carrier.

Claims (14)

A catalyst for exhaust gas purification comprising a catalytic support component dispersed and supported on the surface of a tetravalent metal acidic insoluble salt-containing ceramic support, wherein the ceramic support has a rare earth oxidation surface for each particle unit forming the support. The rare-earth oxide-containing alumina thin film has an uneven surface composed of a flocked structure in which small fibers stand, and the catalytically active component is supported on the uneven surface. An exhaust gas purifying catalyst characterized by that. An exhaust gas purifying catalyst in which a catalytically active component is dispersedly supported on the surface of a zirconium compound-containing ceramic carrier, wherein the ceramic carrier has a rare earth oxide on each particle unit forming the carrier. The rare earth oxide-containing alumina thin film is coated with an alumina thin film , and has a concavo-convex surface composed of a flocked structure in which fibrils are forested, and the catalytically active component is supported on the concavo-convex surface. An exhaust gas purifying catalyst characterized by. An exhaust gas purifying catalyst comprising a catalytic support component dispersed and supported on the surface of a tetravalent metal phosphate-containing ceramic carrier, wherein the ceramic carrier has a rare earth surface for each particle unit forming the carrier. The rare-earth oxide-containing alumina thin film is coated with an oxide-containing alumina thin film , and has a concavo-convex surface having a flocked structure in which fibrils are forested, and the catalytic active component is present on the concavo-convex surface An exhaust gas purifying catalyst which is supported. The tetravalent metal acidic insoluble salt-containing ceramic carrier is composed of any one of a porous body, a fiber molded body, and a pellet molded body containing any of zirconium phosphate, titanium phosphate, zirconium arsenate, and titanium arsenate. The exhaust gas purifying catalyst according to claim 1, wherein the exhaust gas purifying catalyst is provided. The rare earth oxide-containing alumina thin film covering the surface of each particle in the ceramic support has a micro cross-sectional shape of a diameter of 2 to 50 nm , a length of 20 to 300 nm , and a length / diameter ratio of 5 to 100. The exhaust gas purification according to any one of claims 1 to 4, which has an uneven surface composed of a flocked structure in which fibrils having vegetation are planted, and a specific surface area of the surface is 50 to 300 m ² / g. use catalyst. The alumina thin film containing the rare earth oxide covering the ceramic support is coated at a rate of 0.1 to 15 mass% in terms of the amount of alumina with respect to the support, and the rare earth oxide contained in the alumina thin film is coated. The exhaust gas purifying catalyst according to any one of claims 1 to 5, wherein the amount is a content of 10 to 80 mass% with respect to the alumina. The exhaust gas purifying catalyst according to any one of claims 1 to 6, wherein at least part of the rare earth oxide forms a composite oxide with zirconium. 8. The exhaust gas purifying catalyst according to claim 7, wherein the particle diameter of the composite oxide of rare earth oxide and zirconium is 1 to 30 nm. The surface of each particle constituting the tetravalent metal acid insoluble salts containing ceramic support, under Symbol (a) ~ (e) step;
(A) Solution impregnation step: immersing the ceramic support in a solution of a metal compound containing aluminum and a rare earth oxide;
(B) Drying step: heating and drying the ceramic carrier.
(C) Temporary firing step: An amorphous alumina thin film is formed by heating and firing the ceramic support at a temperature of 300 to 500 ° C. or higher.
(D) Heat treatment step: The ceramic carrier is immersed in hot water at 100 ° C. and then dried.
(E) main baking step: main baking is performed at 500 to 1200 ° C.
A method for producing an exhaust gas purifying catalyst, comprising: forming a rare earth oxide-containing alumina thin film through a step, and then dispersing and supporting a catalytic active component on the uneven surface of the rare earth oxide-containing alumina thin film. On the surface of each particle constituting the tetravalent metal acidic insoluble salt-containing ceramic carrier, the following steps (a) to (f):
(A) Pretreatment step: heating the ceramic support to a temperature of 1000 ° C. to 1500 ° C. to form a silicide oxide film;
(B) solution impregnation step: immersing the ceramic support in a solution of a metal compound containing aluminum and a rare earth oxide;
(C) Drying step: heating and drying the ceramic carrier.
(D) Temporary firing step: An amorphous alumina thin film is formed by heating and firing the ceramic carrier at a temperature of 300 to 500 ° C. or higher.
(E) Heat treatment step: The ceramic carrier is immersed in hot water at 100 ° C. and then dried.
(F) main firing step: main firing at 500 to 1200 ° C.
A method for producing an exhaust gas purifying catalyst, comprising: forming a rare earth oxide-containing alumina thin film through steps, and then dispersing and supporting a catalytic active component on the uneven surface of the rare earth oxide-containing alumina thin film. The tetravalent metal acidic insoluble salt-containing ceramic carrier is composed of a porous body, a fiber molded body, or a pellet molded body containing zirconium phosphate, titanium phosphate, zirconium arsenate and titanium arsenate. The method for producing an exhaust gas purifying catalyst according to claim 9 or 10. The alumina thin film containing the rare earth oxide covering the ceramic support is coated at a rate of 0.1 to 15 mass% in terms of the amount of alumina with respect to the support, and the rare earth oxide contained in the alumina thin film is coated. The method for producing an exhaust gas purifying catalyst according to any one of claims 9 to 11, wherein the amount is a content of 10 to 80 mass% with respect to the alumina. The method for producing an exhaust gas purifying catalyst according to any one of claims 9 to 12, wherein at least part of the rare earth oxide forms a composite oxide with zirconium. . 14. The method for producing an exhaust gas purifying catalyst according to claim 13, wherein the particle diameter of the composite oxide of rare earth oxide and zirconium is 1 to 30 nm.

Images