JP4012456B2 - Exhaust gas treatment catalyst and exhaust gas treatment method - Google Patents
Exhaust gas treatment catalyst and exhaust gas treatment method Download PDFInfo
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- JP4012456B2 JP4012456B2 JP2002332391A JP2002332391A JP4012456B2 JP 4012456 B2 JP4012456 B2 JP 4012456B2 JP 2002332391 A JP2002332391 A JP 2002332391A JP 2002332391 A JP2002332391 A JP 2002332391A JP 4012456 B2 JP4012456 B2 JP 4012456B2
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Landscapes
- Exhaust Gas Treatment By Means Of Catalyst (AREA)
- Catalysts (AREA)
Description
【０００１】
【発明の属する技術分野】
本発明は、排ガス処理用触媒および排ガス処理方法に関する。詳しくは、ボイラー、ガスタービン、ディーゼルエンジン、ガスエンジンその他の各種燃焼装置等から排出される排ガスに含まれる低濃度ＣＯを効率的に処理する排ガス処理用触媒と、この排ガス処理用触媒を用いた排ガス処理方法とに関する。
【０００２】
【従来の技術】
ボイラー、ガスタービン、ディーゼルエンジン、ガスエンジンなど各種燃焼装置から排出される燃焼排ガス中には、燃焼装置、運転条件などによりそれぞれ異なるが、一般的に未燃燃料由来の揮発性有機化合物、ＣＯやＮＯX、ＳＯXが有害成分として含有されている。これら燃焼装置では、燃焼効率あるいは熱効率を高めるために、また燃焼排ガス中の揮発性有機化合物、ＣＯ、ＮＯXを減じることを目的として、燃焼時における供給空気量を、燃料ガスを完全燃焼させるに必要な理論空気量より多くして燃焼をおこなうことが多い。このような燃焼状態の制御などにより燃焼排ガス中の揮発性有機化合物、ＣＯ、ＮＯXは減少されてきているが、それでもなお、揮発性有機化合物や微量のＣＯが残存しており、このためこれら有害成分を処理する必要があるが、これら燃焼排ガス中には、余剰空気量に対応した多量の酸素および燃焼の結果生成する水蒸気が含まれており、このためその中の揮発性有機化合物や微量ＣＯを酸化して処理するには、これらが含まれていてもなお有効に適用し得る処理触媒および処理方法の開発が必要となる。
【０００３】
これまで、燃料に対して理論空気比に近い条件で燃焼を行う燃焼装置から排出される燃焼排ガスの浄化用としては、この排ガスには酸素はほとんど含まれず、ＮＯX、ＣＯおよび未燃の揮発性有機化合物が含まれているため、例えばＰｔ，Ｒｈ／アルミナ触媒等の三元触媒が用いられ、現に実用化されている。この使用形態としては上記排ガスを約５００〜７００℃の条件下においてＰｔ，Ｒｈ／アルミナ触媒（ハニカム触媒）を通すことにより実施され、これにより排ガス中のＮＯX、ＣＯおよび未燃の揮発性有機化合物を同時に除去するものである。
しかし、この方法で対象とする被処理排ガスは、その由来からして、その中に酸素がほとんど含まれず、しかも処理温度として約５００〜７００℃の条件下において実施することを前提とするため、このＣＯ酸化除去法は、多量の酸素および水蒸気が含まれ、また通常３００〜５００℃程度で排出され、しかも低濃度のＣＯを含有する排ガス中におけるＣＯの酸化処理方法としては直ちに有効に適応することはできない。
【０００４】
一方、温度３００〜５００℃程度で排出され、酸素や水蒸気を多量に含有する排ガス中の低濃度ＣＯを酸化して無害化する方法としては、例えば、特開平７−２４１４６７、特開平７−２４１４６８号公報に希薄燃焼ガスエンジン排ガス中の低濃度ＣＯ酸化除去方法が記載されている。特開平７−２４１４６７号公報では、希薄燃焼ガスエンジン排ガス中の低濃度ＣＯを酸化して無害化する触媒および方法に係るものであるが、この触媒はハニカム担体に担持した白金／アルミナ触媒であって、かつ白金担持量を１．２〜２．５ｇ／リットルとすることを特徴とするというものである。他方、特開平７−２４１４６８号公報は、希薄燃焼ガスエンジン排ガス中の低濃度ＣＯを酸化して無害化する触媒および方法に係るものであるが、触媒としてハニカム担体に担持した白金−パラジウム／アルミナあるいは白金−ロジウム／アルミナ触媒を使用することを特徴とするというものである。
【０００５】
特開平７−２４１４６７号公報記載の技術では、希薄燃焼ガスエンジンから排出される排ガス中の低濃度ＣＯを長期にわたり安定して有効に酸化、除去するためにはハニカム担体にアルミナとともに担持される白金の担持量を１．２〜２．５ｇ／リットルの範囲とすることが必要であり、白金担持量を１ｇ／リットル以下へと低減させた場合、排ガス中の低濃度ＣＯを長期にわたり安定して有効に酸化、除去することができなかった。また、特開平７−２４１４６８号公報記載の技術では、ハニカム担体に担持した白金／アルミナ触媒を白金−パラジウム／アルミナあるいは白金−ロジウム／アルミナ触媒とすることで特開平７−２４１４６７号公報技術にみられた問題点、すなわち、排ガス中の低濃度ＣＯを長期にわたり安定して有効に酸化、除去するために白金担持量を増加させ、ある特定範囲としなければならないという制限を受けることなく白金担持量を選択できるとしている。しかし、そのためには白金と同類の高価な貴金属であるパラジウムおよびロジウムを白金と同量程度担持させる必要があった。
【０００６】
貴金属を担持した低濃度ＣＯ除去触媒では、貴金属の担持量を増やせば、触媒機能が向上することが予想されるが、担持量を増やす分だけ貴金属の材料コストが増大し、経済性の劣るものとなる。しかも、排ガス中にＳＯXが含まれる場合には、ＳＯ2→ＳＯ3転化率が高くなって、ＳＯ3による配管腐食などの問題が発生する。また、排ガス中のＳＯXは触媒性能自体へ影響を及ぼすことが考えられ、低濃度ＣＯ含有排ガスを処理する触媒には排ガス中のＳＯXに対する耐性も有している必要がある。しかし、特開平７−２４１４６７号公報、特開平７−２４１４６８号公報では排ガス中の低濃度ＣＯを酸化・無害化する際のＳＯXの影響に関してはまったく触れられていない。
【０００７】
【発明が解決しようとする課題】
本発明の課題は、前述した触媒成分担持金属酸化物触媒の低濃度ＣＯ含有排ガスに対する処理効率を一層向上させ、特に、触媒成分の担持量を増やすことなく高い処理効率を達成し得るとともに長期にわたり安定して高い処理効率を維持することができる排ガス処理触媒、および、該排ガス処理用触媒を用いた排ガス処理方法を提供することにある。かつ、触媒材料コストを低く抑えた低濃度ＣＯ除去用触媒を提供することにある。
【０００８】
【課題を解決するための手段】
本発明者が種々の元素に関して検討した結果、排ガス中に含まれている低濃度ＣＯを処理するに際し、チタン系酸化物を担体とし、少なくとも一種の貴金属元素からなる触媒成分Ａを含む触媒、または、該触媒成分Ａと必要に応じてさらに周期律表第Ｉ〜III族に含まれる少なくとも１種の元素からなる触媒成分Ｂをも含む触媒に、さらに下記１）〜３）のうちの少なくとも１つの条件を満たせば、処理効率がさらに向上することが判った。また、触媒成分の担持量を増やすことなく、触媒材料コストを低く抑えたうえで、高い処理効率を達成し得るとともに、長期にわたり安定して高い処理効率を維持することができることが判った。
【０００９】
１）水銀圧入法で測定される全細孔容積が０．２０〜０．８０ｃｍ3／ｇである。
２）ＢＥＴ法で測定される比表面積が２０ｍ2／ｇ以上である。
３）前記チタン系酸化物としてアナターゼ型結晶構造を有するチタン系酸化物を含む。
本発明は、これらの知見に基づいて完成されたものである。
すなわち、本発明にかかる排ガス処理用触媒は、低濃度ＣＯ含有排ガスを処理する触媒であって、チタン系酸化物を担体とし、少なくとも１種の貴金属元素からなる触媒成分Ａが担持されてなり、上記１）〜３）のうちの少なくとも１つの条件を満たすことを特徴とする。
【００１０】
より詳しくは、本発明にかかる排ガス処理用触媒は、低濃度ＣＯ含有排ガスを処理する触媒であって、Ｔｉ−Ｗ均密混合物からなるチタン系酸化物を担体とし、少なくとも１種の貴金属元素からなる触媒成分ＡとＣａからなる触媒成分Ｂとが担持されてなり、水銀圧入法で測定される全細孔容積が０．２０〜０．８０ｃｍ３／ｇであることを特徴とする。
また、本発明にかかる排ガス処理方法は、低濃度ＣＯ含有排ガスを処理する方法であって、ＣＯ濃度１００ｐｐｍ以下の低濃度ＣＯ含有排ガスを、空間速度５０，０００Ｈ−１〜５００，０００Ｈ−１の条件で請求項１に記載の排ガス処理用触媒と接触させる工程（ａ）を含む。
【００１１】
【発明の実施の形態】
〔低濃度ＣＯ含有排ガス〕
通常の各種産業装置や設備から排出される低濃度ＣＯ含有排ガスに適用できる。具体的には、ボイラー、ガスタービン、ディーゼルエンジン、ガスエンジン、加熱炉、その他各種工業プロセスの燃焼排ガスが挙げられる。
燃焼排ガスの場合には、燃料由来の成分であるが燃焼されなかった未燃の揮発性化合物が含まれ、環境への悪影響が問題とされている。本発明の排ガス処理触媒は、低濃度ＣＯに加え揮発性化合物を含む排ガスの処理にも有効である。
【００１２】
排ガスは、本発明の排ガス処理触媒による処理工程を行なう前に、各種の排ガス処理が施されている場合がある。したがって、前記供給源から排出された段階の排ガスと、本発明の排ガス処理触媒で排ガス処理する段階の排ガスとは、その成分が異なっている場合がある。
本発明は、従来の排ガス処理触媒では効率的な処理が行ない難い低濃度ＣＯ含有排ガスに有効である。具体的にはＣＯ濃度が１００ｐｐｍ以下の排ガスに適している。
排ガスは、供給源からの排出条件や排ガス処理を行うまでの履歴によって、温度条件や速度が変わる。
【００１３】
〔触媒材料〕
触媒材料としては、基本的には、通常の排ガス処理触媒と共通する材料の中から選択して使用することができる。
＜担体＞
担体としては、チタン系酸化物を使用する。チタン系酸化物は、Ｔｉのみからなる酸化物（酸化チタン）であってもよいし、Ｓｉ、ＡｌおよびＺｒからなる群より選ばれる少なくとも１種の元素とＴｉとからなる複合酸化物であってもよい。また、このような複合酸化物とＴｉのみからなる酸化物（酸化チタン）との混合物であってもよいし、Ｓｉ、ＡｌおよびＺｒからなる群より選ばれる少なくとも１種の元素からなる酸化物とＴｉのみからなる酸化物（酸化チタン）との混合物であってもよい。
【００１４】
上記複合酸化物や上記混合物においては、Ｔｉの含有量は、該複合酸化物または混合物全体中のＴｉと他の元素との合計モル数に対して５〜９５モル％であることが好ましく、より好ましくは、２０〜９５モル％である。
チタン系酸化物からなる担体は、単に触媒成分を担持する機能を担っているだけでなく、チタン系酸化物担体上に担持される触媒成分の担持状態を低濃度ＣＯの除去に対して好適な状態に保ち、その排ガス処理機能を高める働きに寄与している。その結果、触媒成分の担持量を増加させることなく高いＣＯ除去性能が得られる。また、従来から使用されてきたアルミナ担体などと違い、チタン系酸化物は、排ガス中にＳＯxが含まれている場合などにおいても、その影響を受けることがほとんどなく、ＳＯxに対する耐性が高く、硫酸塩などの形成がなく好適である。
【００１５】
特に、Ｔｉ−Ｓｉ複合酸化物を用いると、ＳＯ2酸化率が低く、かつ、排ガス浄化性能に優れるものが得られるので好ましい。
なお、チタン系酸化物の結晶構造としては、例えば、ルチル型やアナターゼ型などがあるが、なかでもアナターゼ型の結晶構造が好ましい。アナターゼ型の結晶構造を有するチタン系酸化物であれば、触媒活性が高いうえ比表面積も大きく、排ガス処理効率や処理性能をより一層向上させることができ、かつ、ＳＯ2酸化率を低く抑えることができる。
担体の製造は、通常の触媒材料となる金属酸化物からなる担体の製造技術が適用される。Ｔｉ−Ｓｉ複合酸化物の調製も、通常のＴｉ−Ｓｉ複合酸化物と同様の手段が採用でき、特開昭５３−１４６９９１号公報、特願２０００−９９５９３号の明細書等などに開示された技術や、硫酸チタンのような可溶性チタン化合物の溶液にシリカゾルを添加した混合溶液を８０〜その沸点の温度で加熱して加水分解（熱加水分解）して得られた固体生成物を水洗、乾燥後、２００〜８００℃で焼成する技術などが挙げられる。特に、本願特許出願人が先に特許出願している特願２０００−９９５９３号の明細書等に開示された技術が、好ましい技術として挙げられる。
【００１６】
担体となる金属酸化物を供給する原料として、予め用意された金属酸化物をそのまま使用するほかに、焼成によって酸化物を生成する材料が使用できる。具体的には、無機および有機のいずれの化合物でもよく、例えば、所定の金属を含む水酸化物、アンモニウム塩、アンミン錯体、シュウ酸塩、ハロゲン化物、硫酸塩、硝酸塩、炭酸塩、アルコキシドなどを用いることができる。
本発明の排ガス処理触媒で用いる担体は、例えば、担体材料となるチタン系酸化物粉体を押出し成形機などを用いて所望の形状にしたあと必要に応じて乾燥・焼成してなる一体成形型の形態であってもよいし、チタン系酸化物を別途所望の形状を有する骨格基材（例えば、非吸水性の耐熱基材）上に塗布して、コートした形態であってもよい。
【００１７】
＜触媒成分Ａ＞
触媒成分Ａとして、少なくとも１種の貴金属元素が使用できる。具体的には、例えば、Ｐｔ、Ｐｄ、Ｒｈ、Ｒｕ、ＩｒおよびＡｕ等が使用できる。
触媒成分Ａの供給原料としては通常の触媒製造などに利用されている材料が使用できる。具体的には、硝酸塩、ハロゲン化物、アンモニウム塩、アンミン錯体、水酸化物などが挙げられる。
担体への触媒成分Ａの担持手段としては、基本的には通常の貴金属担持金属酸化物触媒と共通する手段が採用できる。担体に触媒成分Ａを担持させる処理工程では、触媒成分Ａを担体の表面から内部方向にかけて均等に担持せることもできるし、担体の外表面近傍に偏在させて担持させることもできるが、担体の外表面近傍に触媒成分Ａを偏在させて担持させることが好ましい。
【００１８】
高い空間速度（ＳＶ）で触媒と排ガスを接触させて排ガス中の低濃度ＣＯを除去するような場合、触媒による浄化作用はその大部分が、触媒の表層部分で起こっていると考えられる。このような場合、排ガスが接触する触媒表層部分に触媒成分Ａを偏在させて担持させておくことで、触媒による排ガスの処理効率が高まるからである。
触媒成分Ａの担持量は、材料の組み合わせや担持処理の条件などによっても異なるが、通常は、触媒の全体量に対して触媒成分Ａを０．００５〜２．０重量％の範囲、好ましくは０．０１〜１．０重量％の範囲で用いる。触媒成分Ａの担持量が少なすぎると触媒活性が低くなる。触媒成分Ａの担持量が多すぎても、活性向上に対する効果は望めず、経済性を損なうのみであるからである。
【００１９】
触媒成分Ａは、通常、粒子の形態で担体に担持される。触媒成分Ａの粒径としては、平均粒子径３０ｎｍ以下のものが好ましく、さらに好ましくは２０ｎｍ以下である。触媒成分Ａの粒子径が小さく、高分散化された状態であるほど、活性が高くなる。
＜触媒成分Ｂ＞
触媒成分Ｂとして、周期律表第Ｉ〜ＩＩＩ族に含まれる少なくとも一種の元素が使用できるが、なかではＣａが好ましい。
【００２０】
触媒成分Ｂの供給原料としては、特に限定はされず、通常の触媒製造などに利用されている１種または２種以上の材料が使用できるが、好ましくは有機酸塩、アルコキシド、有機金属錯体など分子中に有機酸などの有機成分を含んでいるものを例示することができる。
触媒成分Ｂの担持方法も、特に限定はされず、通常の触媒製造に利用されている方法にて担持させることができる。
触媒成分Ｂの担持量は、触媒の全体量に対して０．０１〜２０重量％の範囲、好ましくは０．１〜１０重量％の範囲である。触媒成分Ｂの担持量が少なすぎるとその効果が得られず、上記範囲を超えて担持量を増加させても、活性向上に対する効果は望めず、逆に活性を低下させることもある。
【００２１】
担持順序に関しても特に限定されない。触媒成分Ａを担持した後に触媒成分Ｂを担持してもよいし、また、触媒成分Ａを担持する前に触媒成分Ｂを担持してもよいし、触媒成分Ａと触媒成分Ｂを同時に担持してよいが、触媒成分Ａと触媒成分Ｂを同時に担持するのが好ましい。
＜触媒成分Ｃ：その他触媒成分＞
触媒成分Ａ、または、触媒成分Ａおよび触媒成分Ｂを含む触媒に、Ｖ、Ｗ、Ｍｏ、Ｃｕ、Ｍｎ、Ｎｉ、Ｃｏ、ＣｒおよびＦｅよりなる群から選ばれる少なくとも１種の元素（触媒成分Ｃ）を、さらに含有させることができる。これら触媒成分Ｃが加わることで、低濃度ＣＯ排ガスに対する処理効率がさらに向上したり、脱硝機能を付与させたりすることができる。
【００２２】
触媒成分Ｃの供給原料としては、特に限定されず、通常の触媒製造に利用されている１種または２種以上の材料を使用することができる。担持方法に関しても、特に限定されず、通常の触媒製造に利用されている方法にて触媒へ担持させることができる。担持順序に関しても、特に限定されないが、触媒成分Ａを含む触媒の場合は、触媒成分Ａと触媒成分Ｃを同時に担持する、または、触媒成分Ａを担持した後に触媒成分Ｃを担持することが好ましい。触媒成分Ａおよび触媒成分Ｂを含む触媒の場合は、触媒成分Ａおよび触媒成分Ｂと触媒成分Ｃとを同時に担持する、または、触媒成分Ａと触媒成分Ｂとを担持した後に触媒成分Ｃを担持することが好ましい。なお、触媒成分Ｂ、Ｃは、触媒成分Ａと同様に触媒の表面に偏在させていてもよいし、偏在させなくてもよい。
【００２３】
〔触媒の全細孔容積〕
本発明の触媒は、通常、微細な細孔を有する多孔質構造である。細孔の量や大きさなどによって、排ガスの流通、触媒内部への排ガスの拡散、排ガス処理効率、機械的強度、触媒成分粒子の担持、触媒活性などに影響を与える。
触媒の全細孔容積は、大きい方が触媒内部への排ガスの拡散が促進されて排ガス処理効率が向上するため好ましいが、大きすぎると機械的強度が低下する。そのため、触媒の全細孔容積は、０．２０〜０．８０ｃｍ3／ｇの範囲であるのが良く、より好ましくは０．２５〜０．７５ｃｍ3／ｇ、さらに好ましくは０．３０〜０．６０ｃｍ3／ｇの範囲である。触媒の全細孔容積が０．２０ｃｍ3／ｇより小さいと、触媒内部への排ガスの拡散が促進されず、そのため排ガス処理効率が向上しにくくなるおそれがある。また、触媒の全細孔容積が０．８０ｃｍ3／ｇより大きいと、その増加量に見合う上記排ガス処理効率向上効果が得られない上に、触媒の機械的強度が却って低下するおそれがある。
【００２４】
本発明の触媒においては、触媒の平均細孔径についても、上述した触媒の全細孔容積と同様の理由で以下の範囲を満たすようにすることが好ましい。すなわち、触媒の平均細孔径は、０．０１０〜０．５０μｍの範囲であるのが良く、より好ましくは０．０１０〜０．０３０μｍ、さらに好ましくは０．０１５〜０．１０μｍの範囲である。触媒の平均細孔径が０．０１０μｍより小さいと、触媒内部への排ガスの拡散が促進されず、そのため排ガス処理効率が向上しにくくなるおそれがある。また、触媒の平均細孔径が０．５０μｍより大きいと、その増加量に見合う上記排ガス処理効率向上効果が得られない上に、触媒の機械的強度が却って低下するおそれがある。
【００２５】
本発明の触媒においては、触媒の全細孔容積と平均細孔径とが共に上記範囲を満たすようにすることが好ましい。
なお、触媒の全細孔容積および平均細孔径の値は、水銀圧入法で測定されるものである。
全細孔容積や平均細孔径が上記所望の範囲となるように、触媒を調製する方法としては、特に限定はされないが、例えば、▲１▼チタン系酸化物の粉体を用いて担体を製造する場合に、該粉体の粒径を適宜制御して調製しておく方法や、▲２▼チタン系酸化物を含む担体材料を混練する際に通常加える、澱粉等の成形助剤や、水あるいは触媒成分を含む溶液を担体材料と共に混練する場合にあっては該触媒成分を溶解させ得る溶媒などの添加物の添加量を制御する方法、▲３▼担体製造における焼成時や触媒成分担持後の焼成時に分解または揮発し得る樹脂等を、チタン系酸化物を含む担体材料の混練時に添加しておく方法などを挙げることができる。この▲１▼〜▲３▼等の方法においては、各種添加物の使用量や、粉体の粒子径、混練条件などの各種条件は、所望の全細孔容積や平均細孔径となるよう適宜設定すればよいが、好ましくは以下のとおりである。すなわち、▲１▼の方法においては、チタン系酸化物の粉体の平均粒子径を０．５〜５０μｍとすることが好ましく、より好ましくは１〜３０μｍであり、この範囲外であると所望の細孔（容積や平均細孔径など）を有する触媒が得られないおそれがある。▲２▼の方法においては、水や溶媒を用いる場合その全添加量を、チタン系酸化物を含む担体材料の重量に対して５〜２００質量％とすることが好ましく、より好ましくは１０〜１５０質量％であり、該添加量がこの範囲よりも少ないと所望の細孔（容積や平均細孔径など）を有する触媒が得られないおそれがあり、この範囲よりも多いと成形性が低下するおそれがある。▲２▼の方法における成形助剤や▲３▼の方法における樹脂を用いる場合その全添加量は、チタン系酸化物を含む担体材料の重量に対して０．５〜３０質量％とすることが好ましく、より好ましくは１〜２０質量％であり、該添加量がこの範囲よりも少ないと所望の細孔（全細孔容積や平均細孔径など）を有する触媒が得られないおそれがあり、この範囲よりも多いと焼成時の発熱量が大きくなり、活性成分のシンタリング等の問題が起こるおそれがある。
【００２６】
〔触媒の比表面積〕
触媒の比表面積も、性能に影響を与える。排ガス中にわずかしか含まれていないＣＯを触媒上で効率良く酸化・除去するには、担体に担持した触媒成分とＣＯとの接触を効率良く行わせる必要があり、そのためには、触媒成分をより高分散に担持させて触媒成分の露出面を多くすることにより、触媒成分とＣＯとの接触面積を増大させるのが好ましい。この点から、触媒の比表面積は、大きい方が好ましく、具体的には、２０ｍ2／ｇ以上が好ましく、３０ｍ2／ｇ以上がより好ましく、４０ｍ2／ｇ以上がさらに好ましい。触媒の比表面積が２０ｍ2／ｇ未満だと、触媒成分を分散性良く担持させることができないため、触媒成分とＣＯとの接触効率（排ガス処理効率）が向上しにくく、充分な触媒活性が得られないので、好ましくない。他方、比表面積が大き過ぎると、触媒成分とＣＯとの接触効率（排ガス処理効率）や触媒活性はそれほど向上しないのに、触媒被毒成分の蓄積が増加したり触媒寿命が低下したりするなどの弊害が生じる恐れがある。この点から、触媒の比表面積の上限は、特に限定されるわけではないが、好ましくは３００ｍ2／ｇ、より好ましくは２５０ｍ2／ｇ、さらに好ましくは２００ｍ2／ｇである。
【００２７】
なお、比表面積の値は、ＢＥＴ法で測定されるものである。
触媒の比表面積が所望の範囲となるように調製する方法としては、特に限定はされないが、例えば、▲１▼担体製造における焼成時や触媒成分担持後の焼成時の焼成温度や焼成時間を制御する方法や、▲２▼担持する触媒成分の量を制御する方法などを挙げることができる。この▲１▼、▲２▼等の方法においては、焼成温度、焼成時間、担持量などの各種条件は、所望の比表面積となるよう適宜設定すればよいが、好ましくは以下のとおりである。すなわち、▲１▼の方法においては、焼成温度は２００〜６５０℃とすることが好ましく、より好ましくは３００〜６００℃であり、焼成温度がこの範囲よりも高いと比表面積が所望の範囲より小さくなるおそれがあり、焼成温度がこの範囲よりも低いと十分な触媒活性が得られないおそれがある。また、▲１▼の方法においては、焼成時間（所定の焼成温度に達してからの保持時間）は１〜２０時間とすることが好ましく、より好ましくは２〜１０時間であり、焼成時間がこの範囲よりも長いと比表面積が所望の範囲より小さくなるおそれがあり、焼成時間がこの範囲よりも短いと十分な触媒活性が得られないおそれがある。
【００２８】
〔触媒〕
多孔質担体へ触媒成分を担持させた触媒は、多孔質担体と同様に多孔質構造を有している。本発明の触媒は、担体と触媒成分とのみで構成され、チタン系酸化物からなる担体層を支持する金属製等の基材などを構成材料として含まないモノリス構造体であることが好ましい。
触媒の形状寸法は、基本的には、担体の形状寸法とほぼ同じである。
触媒成分の種類や担持量、担持処理条件によって、担体と触媒とで、違ってくる性状や特性もあるが、多くの性状、特性については、担体で測定された値と触媒で測定された値との間に、実質的な違いは生じない。
【００２９】
例えば、前述した全細孔容積、平均細孔径、比表面積などは、触媒成分の担持量等によって担体と触媒とで異なる場合もあるが、通常、担体と触媒とでほぼ同じである。前記した触媒における数値範囲条件は、担体についても当てはまる。逆に、担体についての規定条件は、触媒についての規定条件ともなる。
〔触媒の使用形態〕
触媒形状については、特に制限はなく、ハニカム状、板状、網状、円柱状、円筒状、波状（コルゲート）状、パイプ状、ドーナツ状等多様な形状にて使用できる。なお、本発明の触媒は、チタン系酸化物からなる担体に触媒成分を担持してなる形態であるため、上記した多様な触媒形状は、予め調製したチタン系酸化物担体の形状・形態に依存する場合が多く、例えば、チタン系酸化物粉体を押出し成形機などを用いて所望の形状とした後、触媒成分を担持させたような触媒組成物のみからなる一体成形体であってもよいし、また、所望の触媒形状の骨格となる形状を有する非吸水性の耐熱基材上にチタン系酸化物を塗布して、コートし、次いで触媒成分を担持させたものであってもよいが、この場合、触媒の比表面積や全細孔容積等の測定は、上記非吸水性の耐熱基材等の骨格基材部分は除いて行う。上記非吸水性の耐熱基材としては、例えば、ステンレス鋼などの金属やコージェライト、ムライト、ＳｉＣ等のセラミックス、繊維状セラミックスを紙状素材に抄造したセラミックペーパーなどを、ハニカム状、板状、網状、円柱状、円筒状、波板（コルゲート）状、パイプ状、ドーナツ状、格子状、プレート状（波状プレートを複数積み重ねて隣り合うプレート同士の間に空間を設けるようにしてなる形状）、波状等の形状に加工したものを例示することができる。
【００３０】
触媒は、通常、金属などで構成された容器状の触媒反応器に収容して使用される。触媒反応器には、排ガスの導入口と排出口が設けられ、内部に収容された触媒に排ガスが効率的に接触できるような構造を備えておく。
〔排ガス処理方法〕
本発明の排ガス処理方法は、低濃度ＣＯ含有排ガスを処理する方法であって、ＣＯ濃度１００ｐｐｍ以下の低濃度ＣＯ含有排ガスを、前述した本発明の排ガス処理用触媒と接触させる工程（ａ）を含む方法である。
上記工程（ａ）を行うにあたっては、基本的には、通常の貴金属担持金属酸化物触媒を用いた排ガス処理技術が適用される。
【００３１】
通常は、触媒が収容された触媒反応器を、排ガスなどの排出経路の途中に設置しておくことにより、排ガスが触媒反応器を通過する際に触媒の表面と接触させることができ、所定の触媒作用を受けるようにすることができる。
本発明の触媒は、排ガスに含まれる１００ｐｐｍ以下の低濃度ＣＯと未燃の揮発性有機化合物とを同時に処理することができる。
燃焼排ガスの温度や空間速度などの条件を適切に設定することで、触媒による排ガス処理の効率が向上する。例えば、ガス温度２５０℃〜５００℃、空間速度３０，０００Ｈ-1〜１，０００，０００Ｈ-1の燃焼排ガスを処理することが好ましい。より好ましくは、ガス温度３００℃〜４５０℃が採用でき、空間速度５０，０００Ｈ-1〜５００，０００Ｈ-1が採用できる。さらに、ＬＶ＝０．１ｍ／ｓ（Ｎｏｒｍａｌ）以上、あるいは、ダスト１０ｍｇ／ｍ3（Ｎｏｒｍａｌ）以下の処理条件が好ましい。
【００３２】
本発明の排ガス処理方法でいう工程（ａ）の前や後に、別の排ガス処理工程を組み合わせることもできる。別の排ガス処理工程としては、本発明の触媒では処理し難い成分を効率的に処理できる工程が好ましい。例えば、脱硝触媒による排ガス処理工程を上記工程（ａ）の前に組み合わせれば、あらかじめ脱硝触媒で窒素酸化物を効率的に処理しておくことができる上、工程（ａ）において１００ｐｐｍ以下の低濃度ＣＯおよび未燃の揮発性有機化合物をさらにより効率的に処理することが可能になる。
上記の脱硝触媒による排ガス処理技術としては、本件特許出願人が先に特許出願している特開平１０−２３５２０６号公報に開示された技術が適用できる。この技術で使用する脱硝触媒は、触媒成分ａ（チタン酸化物）と、触媒成分ｂ（バナジウムまたはタングステンからなる金属の酸化物）とを組み合わせ、触媒成分ａに触媒成分ｂを担持させた構造を有する。
【００３３】
特開昭５３−１４６９９１号公報、特開昭６２−６５７２１号公報、特公平６−４１２６号公報などに記載された公知の排ガス処理方法を、本発明の排ガス処理方法と組み合わせることもできる。
本発明では、排ガス中に含まれる低濃度ＣＯを処理する触媒として、チタン系酸化物を担体とし、少なくとも１種の貴金属元素からなる触媒成分Ａが担持されてなる触媒を用いているので、非常に高い触媒活性が得られ、かつＳＯ2酸化の抑制、耐ＳＯx性および耐熱性の向上が図られ長期間にわたり安定して高い処理効率を維持できる。さらに、該触媒が、前述した１）〜３）のうちの少なくとも１つの条件を満たすことによって、処理効率がさらに向上する。その結果、ガスタービンなどから排出される大風量の低濃度ＣＯ含有排ガスを処理する方法として、非常に有効な方法となる。
【００３４】
次に、本発明の排ガス処理触媒を具体的に製造し、その性能を評価した結果について説明する。
【００４３】
＜実施例１＞
メタタングステン酸アンモニウム水溶液（ＷＯ３として５０重量％含有）３．３ｋｇと２５重量％アンモニア水１１０ｋｇと水２．２ｋｇとを混合した液に、硫酸チタニルの硫酸水溶液（ＴｉＯ２として７０ｇ／リットル、硫酸濃度３１０ｇ／リットル）２１５リットルを良く撹拌しながら徐々に滴下して、ゲル状生成物を得た。得られた沈殿物をろ過、水洗し、続いて１００℃で１２時間乾燥し、これを４５０℃で３時間空気雰囲気下で焼成し、さらにハンマーミルを用いて粉砕し、チタン−タングステン酸化物粉体（Ｔｉ−Ｗ均密混合物）を作製した。このＴｉ−Ｗ均密混合物からなる粉体２０ｋｇに、フェノール樹脂（カネボウ（株）製、商品名ベルパール）１ｋｇおよび成形助剤として澱粉４００ｇを加えて混合し、二ーダーで混練した後、押出し成形機で、外形８０ｍｍ角、目開き２．１ｍｍ、肉厚０．４ｍｍ、長さ５００ｍｍのハニカム状に成形した。次いで、８０℃で乾燥した後、４５０℃で５時間空気雰囲気下で焼成することにより、ハニカム成形体を得た。
【００４４】
次いで、このハニカム成形体を、テトラアンミンパラジウムと酢酸カルシウムとの混合溶液に浸漬した後、乾燥させた。次いで、４５０℃で２時間、空気雰囲気下で焼成することにより、ハニカム成形体からなる担体に、触媒成分ＡとしてＰｄが担持された触媒Ｅを得た。
得られた触媒Ｅの組成を分析したところ、Ｔｉ−Ｗ均密混合物：Ｃａ：Ｐｄ＝９８．８：１：０．２（重量比）であった。
また、触媒Ｅについて、ＢＥＴ一点法により測定した比表面積は８３ｍ２／ｇであり、水銀圧入法により測定した全細孔容積は０．３０ｃｍ３／ｇ、平均細孔径は０．０１７μｍであった。
【００４６】
＜比較例１＞
実施例１において、市販のチタン酸化物粉体を気流粉砕機でさらに微細になるよう粉砕して用いるようにし、また、ハニカム成形体の成形時にフェノール樹脂を加えず、押出し成形機の前段に脱気槽を設置して練り物中の空気を除去した以外は、実施例１と同様にして、触媒Ｇを得た。
得られた触媒Ｇの組成を分析したところ、ＴｉＯ２：Ｐｔ＝９９．８：０．２（重量比）であった。
また、触媒Ｇについて、ＢＥＴ一点法により測定した比表面積は６ｍ２／ｇであり、水銀圧入法により測定した全細孔容積は０．１９ｃｍ３／ｇであった。
【００４９】
［性能評価］
上記実施例および比較例で得られた触媒を用いて以下の性能評価試験を行った。
＜ＣＯ除去試験＞
試験条件：
排ガス組成
＝ＣＯ：２０ｐｐｍ、Ｏ２：５％、Ｈ２Ｏ：１２％、Ｎ２：バランス
ガス温度＝３４０℃
空間速度（ＳＴＰ）９００００Ｈｒ−１
ＣＯ除去率算出式：
ＣＯ除去率（％）＝
［（反応器入口ＣＯ濃度）−（反応器出口ＣＯ濃度）］／（反応器入口ＣＯ濃度）×１００
上記性能評価試験の結果は、実施例がＣＯ除去率８９％、比較例がＣＯ除去率５８％であった。
【００５１】
以上の結果、実施例１で得られた排ガス処理触媒は、比較例１で得られた排ガス処理触媒に比べて、ＣＯ除去率について優れた性能が発揮できることが確認された。
【００５２】
【発明の効果】
本発明の排ガス処理触媒を用いれば、多量の酸素や水蒸気の存在下でも低濃度のＣＯや揮発性有機化合物を効率よく除去することができる。 [0001]
BACKGROUND OF THE INVENTION
The present invention relates to an exhaust gas treatment catalyst and an exhaust gas treatment method. Specifically, an exhaust gas treatment catalyst that efficiently treats low-concentration CO contained in exhaust gas discharged from boilers, gas turbines, diesel engines, gas engines, and other various combustion devices, and the exhaust gas treatment catalyst were used. The present invention relates to an exhaust gas treatment method.
[0002]
[Prior art]
The combustion exhaust gas discharged from various combustion devices such as boilers, gas turbines, diesel engines, and gas engines varies depending on the combustion device, operating conditions, etc., but generally, volatile organic compounds derived from unburned fuel, CO, NO X, SO X is contained as harmful components. These combustion apparatus, in order enhance the combustion efficiency or thermal efficiency and volatile organic compounds in the combustion exhaust gas, CO, for the purpose of reducing the NO X, the supply air amount at the time of combustion, the fuel gas is completely combusted In many cases, combustion is performed with more than the required theoretical air volume. Volatile organic compounds in the combustion exhaust gas due to such control of the combustion state, CO, NO X is has been reduced, nonetheless, CO of volatile organic compounds and trace has remained and therefore these Although it is necessary to treat harmful components, these combustion exhaust gases contain a large amount of oxygen corresponding to the amount of surplus air and water vapor generated as a result of combustion. In order to oxidize and treat CO, it is necessary to develop a treatment catalyst and a treatment method that can be effectively applied even if these are contained.
[0003]
Up to now, for purification of combustion exhaust gas discharged from a combustion apparatus that burns fuel under conditions close to the theoretical air ratio, this exhaust gas contains almost no oxygen, and NO x , CO, and unburned volatilization For example, a three-way catalyst such as a Pt, Rh / alumina catalyst is used and is practically used. This usage is carried out by passing the exhaust gas through Pt, Rh / alumina catalyst (honeycomb catalyst) under the condition of about 500 to 700 ° C., whereby NO x , CO and unburned volatile organics in the exhaust gas are used. The compound is removed simultaneously.
However, since the exhaust gas to be treated by this method is based on its origin, it contains almost no oxygen, and it is assumed that the treatment temperature is about 500 to 700 ° C. This CO oxidation removal method is immediately and effectively adapted as a method for oxidizing CO in exhaust gas containing a large amount of oxygen and water vapor, usually discharged at about 300 to 500 ° C., and containing low concentration of CO. It is not possible.
[0004]
On the other hand, methods for oxidizing and detoxifying low-concentration CO in exhaust gas exhausted at a temperature of about 300 to 500 ° C. and containing a large amount of oxygen and water vapor include, for example, JP-A-7-241467 and JP-A-7-241468. Describes a method for removing low-concentration CO oxidation in exhaust gas from a lean combustion gas engine. Japanese Laid-Open Patent Publication No. 7-241467 relates to a catalyst and method for oxidizing and detoxifying low-concentration CO in a lean combustion gas engine exhaust gas. This catalyst is a platinum / alumina catalyst supported on a honeycomb carrier. In addition, the platinum loading is 1.2 to 2.5 g / liter. On the other hand, Japanese Patent Application Laid-Open No. 7-241468 relates to a catalyst and method for oxidizing and detoxifying low-concentration CO in a lean combustion gas engine exhaust gas, and platinum-palladium / alumina supported on a honeycomb carrier as a catalyst. Alternatively, a platinum-rhodium / alumina catalyst is used.
[0005]
In the technique described in Japanese Patent Laid-Open No. 7-241467, in order to oxidize and remove low-concentration CO in exhaust gas discharged from a lean combustion gas engine stably and effectively over a long period of time, platinum supported together with alumina on a honeycomb carrier is used. In the case where the amount of platinum supported is reduced to 1 g / liter or less, the low concentration CO in the exhaust gas can be stably stabilized over a long period of time. It could not be effectively oxidized and removed. In the technique described in Japanese Patent Laid-Open No. 7-241468, the platinum / alumina catalyst supported on the honeycomb carrier is replaced with a platinum-palladium / alumina or platinum-rhodium / alumina catalyst. In order to oxidize and remove low-concentration CO in exhaust gas stably and effectively over a long period of time, the amount of platinum supported is increased without being restricted to a specific range. Can be selected. However, for that purpose, palladium and rhodium, which are expensive noble metals similar to platinum, must be supported in the same amount as platinum.
[0006]
With low-concentration CO removal catalyst loaded with noble metal, it is expected that the catalytic function will improve if the amount of noble metal supported is increased. It becomes. Moreover, when SO x is contained in the exhaust gas, the SO 2 → SO 3 conversion rate increases, and problems such as pipe corrosion due to SO 3 occur. In addition, SO x in the exhaust gas may affect the catalyst performance itself, and the catalyst for treating the low concentration CO-containing exhaust gas needs to have resistance to SO x in the exhaust gas. However, Japanese Patent Application Laid-Open Nos. 7-241467 and 7-241468 do not mention at all the influence of SO X when oxidizing and detoxifying low concentration CO in exhaust gas.
[0007]
[Problems to be solved by the invention]
The object of the present invention is to further improve the treatment efficiency of the above-described catalyst component-supported metal oxide catalyst for exhaust gas containing low concentration CO, and in particular, can achieve high treatment efficiency without increasing the amount of catalyst component supported, and over a long period of time. An object of the present invention is to provide an exhaust gas treatment catalyst capable of stably maintaining high treatment efficiency, and an exhaust gas treatment method using the exhaust gas treatment catalyst. Another object of the present invention is to provide a low-concentration CO removal catalyst that keeps the catalyst material cost low.
[0008]
[Means for Solving the Problems]
As a result of examination of various elements by the present inventor, when treating low-concentration CO contained in exhaust gas, a catalyst containing a catalyst component A composed of at least one noble metal element using a titanium-based oxide as a carrier, or The catalyst component A and, if necessary, a catalyst further comprising a catalyst component B composed of at least one element included in Groups I to III of the Periodic Table are further added to at least one of the following 1) to 3): It was found that the processing efficiency was further improved if one condition was satisfied. Further, it has been found that high treatment efficiency can be achieved and the high treatment efficiency can be stably maintained over a long period of time while keeping the catalyst material cost low without increasing the amount of the catalyst component supported.
[0009]
1) The total pore volume measured by the mercury intrusion method is 0.20 to 0.80 cm 3 / g.
2) The specific surface area measured by the BET method is 20 m 2 / g or more.
3) The titanium-based oxide includes a titanium-based oxide having an anatase type crystal structure.
The present invention has been completed based on these findings.
That is, the exhaust gas treatment catalyst according to the present invention is a catalyst for treating low-concentration CO-containing exhaust gas, comprising a titanium-based oxide as a carrier, and carrying a catalyst component A comprising at least one noble metal element, It satisfies at least one of the above conditions 1) to 3).
[0010]
More particularly, exhaust gas treatment catalyst according to the present invention is a catalyst for processing low density CO-containing flue gas, a titanium-based oxide composed of T i-W intimate mixture with the carrier, at least one noble metal element Catalyst component A made of Ca and catalyst component B made of Ca are supported, and the total pore volume measured by mercury porosimetry is 0.20 to 0.80 cm 3 / g.
The exhaust gas treatment method according to the present invention is a method of treating low-concentration CO-containing exhaust gas, and the low-concentration CO-containing exhaust gas having a CO concentration of 100 ppm or less is treated at a space velocity of 50,000 H −1 to 500,000 H −1 . A step (a) of contacting the exhaust gas treatment catalyst according to claim 1 under conditions.
[0011]
DETAILED DESCRIPTION OF THE INVENTION
[Low concentration CO-containing exhaust gas]
It can be applied to low-concentration CO-containing exhaust gas discharged from ordinary various industrial equipment and facilities. Specific examples include boilers, gas turbines, diesel engines, gas engines, heating furnaces, and other various industrial process combustion exhaust gases.
In the case of combustion exhaust gas, unburned volatile compounds that are components derived from fuel but have not been burned are included, and adverse effects on the environment are a problem. The exhaust gas treatment catalyst of the present invention is also effective for treating exhaust gas containing volatile compounds in addition to low concentration CO.
[0012]
The exhaust gas may have been subjected to various exhaust gas treatments before the treatment process using the exhaust gas treatment catalyst of the present invention. Therefore, the exhaust gas at the stage discharged from the supply source and the exhaust gas at the stage of exhaust gas treatment with the exhaust gas treatment catalyst of the present invention may have different components.
The present invention is effective for low-concentration CO-containing exhaust gas, which is difficult to perform efficiently with conventional exhaust gas treatment catalysts. Specifically, it is suitable for exhaust gas having a CO concentration of 100 ppm or less.
The temperature condition and speed of the exhaust gas change depending on the discharge condition from the supply source and the history until the exhaust gas treatment is performed.
[0013]
[Catalyst material]
Basically, the catalyst material can be selected from materials common to ordinary exhaust gas treatment catalysts.
<Carrier>
A titanium-based oxide is used as the carrier. The titanium-based oxide may be an oxide composed of only Ti (titanium oxide), or a composite oxide composed of Ti and at least one element selected from the group consisting of Si, Al, and Zr. Also good. Further, it may be a mixture of such a complex oxide and an oxide (titanium oxide) made of only Ti, or an oxide made of at least one element selected from the group consisting of Si, Al and Zr. It may be a mixture with an oxide (titanium oxide) made of only Ti.
[0014]
In the composite oxide or the mixture, the content of Ti is preferably 5 to 95 mol% with respect to the total number of moles of Ti and other elements in the composite oxide or the entire mixture, Preferably, it is 20-95 mol%.
The support made of a titanium-based oxide not only has a function of supporting a catalyst component, but is also suitable for the removal of low-concentration CO in the supported state of the catalyst component supported on the titanium-based oxide support. This contributes to maintaining the state and enhancing its exhaust gas treatment function. As a result, high CO removal performance can be obtained without increasing the amount of catalyst component supported. In addition, unlike alumina carriers that have been used in the past, titanium-based oxides are hardly affected even when SO x is contained in the exhaust gas, and have high resistance to SO x . This is preferable because there is no formation of sulfate.
[0015]
In particular, it is preferable to use a Ti—Si composite oxide because an SO 2 oxidation rate is low and an exhaust gas purification performance is excellent.
In addition, examples of the crystal structure of the titanium-based oxide include a rutile type and an anatase type. Among these, an anatase type crystal structure is preferable. Titanium-based oxides with anatase-type crystal structure have high catalytic activity and large specific surface area, can further improve exhaust gas treatment efficiency and treatment performance, and keep the SO 2 oxidation rate low. Can do.
For the production of the carrier, a production technology for a carrier made of a metal oxide as a normal catalyst material is applied. The preparation of the Ti—Si composite oxide can also be carried out in the same manner as the ordinary Ti—Si composite oxide, and disclosed in Japanese Patent Application Laid-Open No. 53-146991 and Japanese Patent Application No. 2000-99593. Washing and drying the solid product obtained by hydrolysis (thermal hydrolysis) by heating a mixed solution obtained by adding silica sol to a solution of technology or soluble titanium compound such as titanium sulfate at a temperature of 80 to its boiling point Then, the technique etc. which bake at 200-800 degreeC are mentioned. In particular, a technique disclosed in the specification of Japanese Patent Application No. 2000-99593, for which the applicant of the present patent application has previously applied for a patent, is cited as a preferred technique.
[0016]
In addition to using a metal oxide prepared in advance as a raw material for supplying a metal oxide serving as a carrier, a material that generates an oxide by firing can be used. Specifically, any of inorganic and organic compounds may be used. For example, hydroxides, ammonium salts, ammine complexes, oxalates, halides, sulfates, nitrates, carbonates, alkoxides containing a predetermined metal, etc. Can be used.
The carrier used in the exhaust gas treatment catalyst of the present invention is, for example, an integral molding die in which a titanium-based oxide powder as a carrier material is formed into a desired shape using an extrusion molding machine and then dried and fired as necessary. The form of this may be sufficient, and the form which apply | coated and coated the titanium type oxide on the frame | skeleton base material (for example, non-water-absorbing heat-resistant base material) which has a desired shape separately may be sufficient.
[0017]
<Catalyst component A>
As the catalyst component A, at least one noble metal element can be used. Specifically, for example, Pt, Pd, Rh, Ru, Ir and Au can be used.
As a feedstock for the catalyst component A, materials that are used for normal catalyst production can be used. Specific examples include nitrates, halides, ammonium salts, ammine complexes, hydroxides, and the like.
As a means for supporting the catalyst component A on the carrier, basically, the same means as that for a normal noble metal-supported metal oxide catalyst can be employed. In the treatment step of supporting the catalyst component A on the carrier, the catalyst component A can be evenly supported from the surface of the carrier to the inside, or can be unevenly distributed near the outer surface of the carrier. It is preferable that the catalyst component A is unevenly distributed in the vicinity of the outer surface.
[0018]
When the catalyst and exhaust gas are brought into contact with each other at a high space velocity (SV) to remove low-concentration CO in the exhaust gas, it is considered that most of the purification action by the catalyst occurs in the surface layer portion of the catalyst. In such a case, the catalyst component A is unevenly distributed and supported on the catalyst surface layer portion in contact with the exhaust gas, so that the exhaust gas treatment efficiency by the catalyst is increased.
The amount of the catalyst component A supported varies depending on the combination of materials and the conditions of the support treatment, but usually the catalyst component A is in the range of 0.005 to 2.0% by weight with respect to the total amount of the catalyst, preferably It is used in the range of 0.01 to 1.0% by weight. If the supported amount of the catalyst component A is too small, the catalyst activity is lowered. This is because even if the amount of the catalyst component A supported is too large, the effect on the activity improvement cannot be expected and only the economic efficiency is impaired.
[0019]
The catalyst component A is usually supported on a carrier in the form of particles. The particle size of the catalyst component A is preferably an average particle size of 30 nm or less, more preferably 20 nm or less. The smaller the particle size of the catalyst component A and the higher the dispersed state, the higher the activity.
<Catalyst component B>
As the catalyst component B, at least one element included in Groups I to III of the periodic table can be used , and Ca is preferable.
[0020]
The feedstock for the catalyst component B is not particularly limited, and one or two or more materials used for normal catalyst production can be used, but preferably an organic acid salt, an alkoxide, an organometallic complex, etc. The thing containing organic components, such as an organic acid, in a molecule | numerator can be illustrated.
The method for supporting the catalyst component B is also not particularly limited, and the catalyst component B can be supported by a method that is used for normal catalyst production.
The amount of catalyst component B supported is in the range of 0.01 to 20% by weight, preferably in the range of 0.1 to 10% by weight, based on the total amount of the catalyst. If the supported amount of the catalyst component B is too small, the effect cannot be obtained, and even if the supported amount is increased beyond the above range, the effect for improving the activity cannot be expected, and the activity may be decreased.
[0021]
There is no particular limitation on the loading order. The catalyst component B may be supported after the catalyst component A is supported, the catalyst component B may be supported before the catalyst component A is supported, or the catalyst component A and the catalyst component B are simultaneously supported. The catalyst component A and the catalyst component B are preferably supported simultaneously.
<Catalyst component C: Other catalyst components>
At least one element selected from the group consisting of V, W, Mo, Cu, Mn, Ni, Co, Cr and Fe (catalyst component C) is added to the catalyst component A or the catalyst containing the catalyst component A and the catalyst component B. ) Can be further included. By adding these catalyst components C, the treatment efficiency for low-concentration CO exhaust gas can be further improved, or a denitration function can be imparted.
[0022]
The feedstock for the catalyst component C is not particularly limited, and one or more materials that are used for normal catalyst production can be used. The supporting method is not particularly limited, and it can be supported on the catalyst by a method used for normal catalyst production. The order of loading is not particularly limited, but in the case of a catalyst including catalyst component A, it is preferable to load catalyst component A and catalyst component C simultaneously, or to load catalyst component C after loading catalyst component A. . In the case of a catalyst containing catalyst component A and catalyst component B, catalyst component A and catalyst component B and catalyst component C are supported simultaneously, or catalyst component C is supported after catalyst component A and catalyst component B are supported. It is preferable to do. The catalyst components B and C may be unevenly distributed on the surface of the catalyst similarly to the catalyst component A, or may not be unevenly distributed.
[0023]
[Total pore volume of catalyst]
The catalyst of the present invention usually has a porous structure having fine pores. The amount and size of the pores affect the flow of exhaust gas, diffusion of exhaust gas into the catalyst, exhaust gas treatment efficiency, mechanical strength, loading of catalyst component particles, catalyst activity, and the like.
A larger total pore volume of the catalyst is preferable because the diffusion of the exhaust gas into the catalyst is promoted and the exhaust gas treatment efficiency is improved, but if it is too large, the mechanical strength is lowered. Therefore, the total pore volume of the catalyst is good in the range of 0.20~0.80cm 3 / g, more preferably 0.25~0.75cm 3 / g, more preferably 0.30 to 0 The range is .60 cm 3 / g. When the total pore volume of the catalyst is smaller than 0.20 cm 3 / g, the diffusion of the exhaust gas into the catalyst is not promoted, so that the exhaust gas treatment efficiency may not be improved. If the total pore volume of the catalyst is larger than 0.80 cm 3 / g, the effect of improving the exhaust gas treatment efficiency corresponding to the increase amount cannot be obtained, and the mechanical strength of the catalyst may be lowered.
[0024]
In the catalyst of the present invention, it is preferable that the average pore diameter of the catalyst satisfies the following range for the same reason as the total pore volume of the catalyst described above. That is, the average pore diameter of the catalyst is preferably in the range of 0.010 to 0.50 μm, more preferably in the range of 0.010 to 0.030 μm, and still more preferably in the range of 0.015 to 0.10 μm. When the average pore diameter of the catalyst is smaller than 0.010 μm, the diffusion of the exhaust gas into the catalyst is not promoted, and therefore the exhaust gas treatment efficiency may be difficult to improve. If the average pore diameter of the catalyst is larger than 0.50 μm, the effect of improving the exhaust gas treatment efficiency corresponding to the increase amount cannot be obtained, and the mechanical strength of the catalyst may be lowered.
[0025]
In the catalyst of the present invention, it is preferable that both the total pore volume and the average pore diameter of the catalyst satisfy the above range.
The total pore volume and average pore diameter of the catalyst are measured by mercury porosimetry.
The method of preparing the catalyst so that the total pore volume and the average pore diameter are in the desired ranges is not particularly limited. For example, (1) producing a support using titanium oxide powder. In the case of the above, a method in which the particle size of the powder is appropriately controlled, (2) a molding aid such as starch, which is usually added when kneading a carrier material containing a titanium-based oxide, water, Alternatively, when a solution containing a catalyst component is kneaded with a support material, a method of controlling the amount of additive such as a solvent capable of dissolving the catalyst component, (3) during calcination in support production or after support of the catalyst component Examples thereof include a method of adding a resin or the like that can be decomposed or volatilized during firing of the carrier material containing the titanium-based oxide during kneading. In the methods (1) to (3) and the like, the amount of various additives used, the particle size of the powder, the kneading conditions, and other conditions are appropriately set so that the desired total pore volume and average pore size are obtained. Although it may be set, it is preferably as follows. That is, in the method (1), the average particle diameter of the titanium-based oxide powder is preferably 0.5 to 50 μm, more preferably 1 to 30 μm. There is a possibility that a catalyst having pores (volume, average pore diameter, etc.) cannot be obtained. In the method (2), when water or a solvent is used, the total addition amount is preferably 5 to 200% by mass, more preferably 10 to 150%, based on the weight of the support material containing the titanium-based oxide. If the amount added is less than this range, a catalyst having desired pores (volume, average pore diameter, etc.) may not be obtained, and if it exceeds this range, moldability may be reduced. There is. When the molding aid in the method (2) and the resin in the method (3) are used, the total amount added should be 0.5 to 30% by mass with respect to the weight of the carrier material containing the titanium-based oxide. Preferably, it is 1 to 20% by mass. If the amount added is less than this range, a catalyst having desired pores (total pore volume, average pore diameter, etc.) may not be obtained. If it exceeds the range, the calorific value at the time of firing becomes large, and there is a possibility that problems such as sintering of the active ingredient may occur.
[0026]
[Specific surface area of catalyst]
The specific surface area of the catalyst also affects performance. In order to efficiently oxidize and remove CO that is contained in the exhaust gas in a small amount on the catalyst, it is necessary to efficiently make contact between the catalyst component supported on the carrier and CO. It is preferable to increase the contact area between the catalyst component and CO by increasing the exposed surface of the catalyst component by carrying it in a higher dispersion. From this viewpoint, the specific surface area of the catalyst is preferably large, specifically, 20 m 2 / g or more is preferable, 30 m 2 / g or more is more preferable, and 40 m 2 / g or more is more preferable. When the specific surface area of the catalyst is less than 20 m 2 / g, the catalyst component cannot be supported with good dispersibility, so the contact efficiency (exhaust gas treatment efficiency) between the catalyst component and CO is difficult to improve, and sufficient catalyst activity is obtained. Since it is not possible, it is not preferable. On the other hand, if the specific surface area is too large, the contact efficiency (exhaust gas treatment efficiency) and catalytic activity between the catalyst component and CO are not improved so much, but the accumulation of catalyst poisoning components increases or the catalyst life decreases. May cause adverse effects. From this point, the upper limit of the specific surface area of the catalyst is not particularly limited, but is preferably 300 m 2 / g, more preferably 250 m 2 / g, and further preferably 200 m 2 / g.
[0027]
The value of the specific surface area is measured by the BET method.
The method for preparing the specific surface area of the catalyst to be in a desired range is not particularly limited. For example, (1) controlling the calcination temperature and calcination time at the time of calcination in the carrier production or after the catalyst component is supported. And (2) a method for controlling the amount of the catalyst component to be supported. In the methods {circle around (1)}, {circle around (2)}, various conditions such as the firing temperature, the firing time, and the supported amount may be appropriately set so as to obtain a desired specific surface area, but are preferably as follows. That is, in the method (1), the firing temperature is preferably 200 to 650 ° C., more preferably 300 to 600 ° C. If the firing temperature is higher than this range, the specific surface area is smaller than the desired range. If the calcination temperature is lower than this range, sufficient catalytic activity may not be obtained. In the method (1), the firing time (holding time after reaching a predetermined firing temperature) is preferably 1 to 20 hours, more preferably 2 to 10 hours. If it is longer than the range, the specific surface area may be smaller than the desired range, and if the calcination time is shorter than this range, sufficient catalytic activity may not be obtained.
[0028]
〔catalyst〕
A catalyst in which a catalyst component is supported on a porous carrier has a porous structure like the porous carrier. The catalyst of the present invention is preferably a monolith structure which is composed only of a carrier and a catalyst component and does not contain as a constituent material a metal substrate or the like that supports a carrier layer made of a titanium-based oxide.
The geometry of the catalyst is basically the same as the geometry of the support.
There are properties and characteristics that differ between the carrier and the catalyst depending on the type of catalyst component, the loading amount, and the loading treatment conditions, but for many properties and characteristics, the values measured with the carrier and the values measured with the catalyst. There is no substantial difference between
[0029]
For example, the total pore volume, the average pore diameter, the specific surface area, and the like described above may differ between the support and the catalyst depending on the amount of the catalyst component supported, but are generally the same between the support and the catalyst. The numerical range conditions described above for the catalyst also apply to the support. Conversely, the prescribed conditions for the support also become the prescribed conditions for the catalyst.
[Usage form of catalyst]
The catalyst shape is not particularly limited, and can be used in various shapes such as a honeycomb shape, a plate shape, a net shape, a columnar shape, a cylindrical shape, a corrugated shape, a pipe shape, and a donut shape. Since the catalyst of the present invention is a form in which a catalyst component is supported on a support made of a titanium oxide, the various catalyst shapes described above depend on the shape and form of the titanium oxide support prepared in advance. For example, it may be an integrally formed body composed only of a catalyst composition in which a catalyst component is supported after a titanium oxide powder is formed into a desired shape using an extrusion molding machine or the like. In addition, a titanium-based oxide may be coated on a non-water-absorbing heat-resistant substrate having a shape that becomes a skeleton of a desired catalyst shape, coated, and then a catalyst component may be supported. In this case, the measurement of the specific surface area, total pore volume, etc. of the catalyst is carried out excluding the skeleton substrate portion such as the non-water-absorbing heat-resistant substrate. Examples of the non-water-absorbing heat-resistant substrate include, for example, metals such as stainless steel, ceramics such as cordierite, mullite, SiC, and ceramic paper made from fibrous ceramics in a paper-like material, honeycombs, plates, Net shape, columnar shape, cylindrical shape, corrugated plate shape, pipe shape, donut shape, lattice shape, plate shape (a shape in which a plurality of corrugated plates are stacked to provide a space between adjacent plates), The thing processed into shapes, such as a wave shape, can be illustrated.
[0030]
The catalyst is usually used by being housed in a container-shaped catalyst reactor made of metal or the like. The catalyst reactor is provided with an exhaust gas inlet and an exhaust port so that the exhaust gas can efficiently come into contact with the catalyst accommodated therein.
[Exhaust gas treatment method]
The exhaust gas treatment method of the present invention is a method for treating low-concentration CO-containing exhaust gas, which comprises the step (a) of contacting a low-concentration CO-containing exhaust gas having a CO concentration of 100 ppm or less with the above-described exhaust gas treatment catalyst of the present invention. It is the method of including.
In performing the step (a), an exhaust gas treatment technique using a normal noble metal-supported metal oxide catalyst is basically applied.
[0031]
Normally, by installing a catalyst reactor containing a catalyst in the middle of a discharge path for exhaust gas, the exhaust gas can be brought into contact with the surface of the catalyst when passing through the catalyst reactor. It can be catalyzed.
The catalyst of the present invention can simultaneously treat low-concentration CO of 100 ppm or less and unburned volatile organic compounds contained in exhaust gas.
By appropriately setting conditions such as the temperature and space velocity of the combustion exhaust gas, the efficiency of exhaust gas treatment with a catalyst is improved. For example, it is preferable to treat combustion exhaust gas having a gas temperature of 250 ° C. to 500 ° C. and a space velocity of 30,000 H −1 to 1,000,000 H −1 . More preferably, a gas temperature of 300 ° C. to 450 ° C. can be employed, and a space velocity of 50,000 H −1 to 500,000 H −1 can be employed. Furthermore, a treatment condition of LV = 0.1 m / s (Normal) or more or dust 10 mg / m 3 (Normal) or less is preferable.
[0032]
Another exhaust gas treatment step can be combined before or after the step (a) in the exhaust gas treatment method of the present invention. As another exhaust gas treatment step, a step capable of efficiently treating components that are difficult to treat with the catalyst of the present invention is preferable. For example, if an exhaust gas treatment step using a denitration catalyst is combined before the step (a), nitrogen oxides can be efficiently treated beforehand with the denitration catalyst, and a low level of 100 ppm or less can be obtained in the step (a). It becomes possible to treat the concentration CO and unburned volatile organic compounds even more efficiently.
As the exhaust gas treatment technology using the above-described denitration catalyst, the technology disclosed in Japanese Patent Application Laid-Open No. 10-235206, which has been previously filed by the applicant of the present patent, can be applied. The denitration catalyst used in this technology has a structure in which a catalyst component a (titanium oxide) and a catalyst component b (metal oxide made of vanadium or tungsten) are combined, and the catalyst component b is supported on the catalyst component a. Have.
[0033]
Known exhaust gas treatment methods described in JP-A-53-146991, JP-A-62-65721, JP-B-6-4126, and the like can be combined with the exhaust gas treatment method of the present invention.
In the present invention, as a catalyst for treating low-concentration CO contained in exhaust gas, a catalyst using a titanium-based oxide as a carrier and supporting a catalyst component A composed of at least one noble metal element is used. In addition, high catalytic activity can be obtained, and SO 2 oxidation can be suppressed, SO x resistance and heat resistance can be improved, and high processing efficiency can be maintained stably over a long period of time. Furthermore, when the catalyst satisfies at least one of the conditions 1) to 3) described above, the processing efficiency is further improved. As a result, it becomes a very effective method as a method for treating exhaust gas containing low-concentration CO containing a large amount of air discharged from a gas turbine or the like.
[0034]
Next, the results of specifically producing the exhaust gas treatment catalyst of the present invention and evaluating its performance will be described.
[0043]
<Example 1 >
An aqueous solution of ammonium metatungstate (containing 50 wt% as WO 3 ), a mixture of 3.3 kg of 25 wt% aqueous ammonia (110 kg) and 2.2 kg of water was added to a sulfuric acid aqueous solution of titanyl sulfate (70 g / liter as TiO 2 , sulfuric acid). Concentration 310 g / liter) 215 liter was gradually added dropwise with good stirring to obtain a gel product. The obtained precipitate was filtered, washed with water, subsequently dried at 100 ° C. for 12 hours, calcined at 450 ° C. for 3 hours in an air atmosphere, and further pulverized using a hammer mill to obtain titanium-tungsten oxide powder. A body (Ti-W homogeneous mixture) was prepared. To 20 kg of powder composed of this Ti-W homogeneous mixture, 1 kg of phenol resin (trade name Belpearl, manufactured by Kanebo Co., Ltd.) and 400 g of starch as a molding aid are added and mixed, kneaded with a feeder, and then extruded. This was formed into a honeycomb shape having an external shape of 80 mm square, an aperture of 2.1 mm, a wall thickness of 0.4 mm, and a length of 500 mm. Next, after drying at 80 ° C., the honeycomb formed body was obtained by firing in an air atmosphere at 450 ° C. for 5 hours .
[0044]
Next, the honeycomb formed body was dipped in a mixed solution of tetraamminepalladium and calcium acetate and then dried. Then, 2 hours at 450 ° C., by firing in an air atmosphere, the carrier comprising the honeycomb molded body, P d as a catalyst component A was obtained supported catalyst E.
When the composition of the obtained catalyst E was analyzed, it was Ti—W homogeneous mixture: Ca: P d = 98.8: 1: 0.2 (weight ratio).
Further, the specific surface area of the catalyst E measured by the BET single point method was 83 m 2 / g, the total pore volume measured by the mercury intrusion method was 0.30 cm 3 / g, and the average pore diameter was 0.017 μm. .
[0046]
<Comparative Example 1>
In Example 1 , commercially available titanium oxide powder was pulverized with an airflow pulverizer to be finer and used, and phenol resin was not added during the formation of the honeycomb formed body, and it was removed before the extrusion molding machine. Catalyst G was obtained in the same manner as in Example 1 except that an air tank was installed to remove air in the kneaded product.
When the composition of the obtained catalyst G was analyzed, it was TiO 2 : Pt = 99.8: 0.2 (weight ratio).
Further, the specific surface area of the catalyst G measured by the BET single point method was 6 m 2 / g, and the total pore volume measured by the mercury intrusion method was 0.19 cm 3 / g.
[0049]
[ Performance evaluation]
The following performance evaluation tests were conducted using the catalysts obtained in the above Examples and Comparative Examples.
<CO removal test>
Test conditions:
Exhaust gas composition = CO: 20 ppm, O 2 : 5%, H 2 O: 12%, N 2 : Balance Gas temperature = 340 ° C
Space velocity (STP) 90000Hr −1
CO removal rate calculation formula:
CO removal rate (%) =
[(Reactor inlet CO concentration) − (Reactor outlet CO concentration)] / (Reactor inlet CO concentration) × 100
As a result of the performance evaluation test, the CO removal rate of the example was 89%, and the CO removal rate of the comparative example was 58%.
[0051]
As a result, the exhaust gas treating catalyst obtained in Example 1, as compared with the exhaust gas treating catalyst obtained in Comparative Example 1, excellent performance, it was confirmed that it exhibited the C O removal ratio.
[0052]
【The invention's effect】
If the exhaust gas treatment catalyst of the present invention is used, low-concentration CO and volatile organic compounds can be efficiently removed even in the presence of a large amount of oxygen and water vapor .
Claims (2)
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