JP5112966B2 - Lean burn exhaust gas purification catalyst - Google Patents
Lean burn exhaust gas purification catalyst Download PDFInfo
- Publication number
- JP5112966B2 JP5112966B2 JP2008161173A JP2008161173A JP5112966B2 JP 5112966 B2 JP5112966 B2 JP 5112966B2 JP 2008161173 A JP2008161173 A JP 2008161173A JP 2008161173 A JP2008161173 A JP 2008161173A JP 5112966 B2 JP5112966 B2 JP 5112966B2
- Authority
- JP
- Japan
- Prior art keywords
- catalyst
- exhaust gas
- mesoporous
- lean burn
- nox
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
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Description
本発明は、リーンバーン自動車が排出する排ガスを浄化するための浄化用触媒に関する。 The present invention relates to a purification catalyst for purifying exhaust gas discharged from a lean burn automobile.
従来、ガソリン車の排ガスに含まれるＮＯｘ、一酸化炭素、及び炭化水素は、白金族元素から成る三元触媒によって浄化されている。三元触媒の主成分である白金族触媒は、酸素濃度が１％以下であるリッチバーン排ガス中のＮＯをＮＯ２に酸化する能力が高く、ＮＯ２を還元性物質によってＮ２Ｏ及び窒素に還元する能力も高く、又、還元性物質を酸素によって完全酸化する能力も高い。三元触媒は、通常、触媒支持体としてコージェライト製のモノリス成形体を用い、該成形体のガス流路内壁に数μｍ〜数十μｍの大きさの活性アルミナ粒子を塗布し、該塗布層に数十ｎｍ〜数百ｎｍの大きさの白金−パラジウム−ロジウム粒子を担持させた構造となっている。三元触媒による浄化方法は、空気：燃料の重量混合比である空燃比を理論空燃比（１４．７：１）近傍に制御することで（この燃焼はリッチバーンと呼ばれている）排ガスに含まれる酸素濃度を１％以下に維持できるので、排ガスに含まれる一酸化炭素及び炭化水素をＮＯｘの還元剤として利用できるという利点を持つが、排ガス中の酸素濃度が数％以上になると触媒の著しい酸化劣化が生じるという問題がある。 Conventionally, NOx, carbon monoxide, and hydrocarbons contained in exhaust gas from gasoline vehicles have been purified by a three-way catalyst made of a platinum group element. The platinum group catalyst which is the main component of the three-way catalyst has a high ability to oxidize NO in the rich burn exhaust gas having an oxygen concentration of 1% or less to NO 2 , and NO 2 is converted into N 2 O and nitrogen by a reducing substance. The ability to reduce is also high, and the ability to completely oxidize a reducing substance with oxygen is also high. The three-way catalyst usually uses a cordierite monolith molded body as the catalyst support, and the activated alumina particles having a size of several μm to several tens of μm are applied to the inner wall of the gas flow path of the molded body. In this structure, platinum-palladium-rhodium particles having a size of several tens of nm to several hundreds of nm are supported. The purification method using a three-way catalyst controls the air-fuel ratio, which is the air: fuel weight mixing ratio, to be close to the theoretical air-fuel ratio (14.7: 1) (this combustion is called rich burn). Since the oxygen concentration contained in the exhaust gas can be maintained at 1% or less, carbon monoxide and hydrocarbons contained in the exhaust gas can be used as a reducing agent for NOx. However, if the oxygen concentration in the exhaust gas exceeds several percent, There is a problem that significant oxidative degradation occurs.
また、軽油燃料で走行するトラック、バス等の大型ディーゼル車の排ガス処理は、触媒として遷移金属化合物及び白金族元素を用い還元剤として尿素水を用いる所謂尿素ＳＣＲ法が検討されている。この方法は、２００℃付近の比較的低温付近から６００℃付近の比較的高温に渡ってＮＯｘを効率的に浄化できるという利点を持つが、還元剤として高価な尿素水の搭載が必要であるという問題と２００℃付近以下の低温排ＮＯｘの多くが硝酸アンモニウムとして排出されるので水質環境汚染を招くという問題がある。
尿素水以外の還元剤を用いる方法としては、排ガスに少量含まれる炭化水素（エチレン、プロピレン、等の低級オレフィンが還元性を有する）を還元剤として用いるハイドロカーボンＳＣＲ法が古くから検討されている。しかし、この方法は、リッチバーン排ＮＯｘに対しては高い浄化率が得られるがリーンバーン排ＮＯｘに対しては２００℃近辺の非常に狭い温度領域でしか処理できないという問題があるので、実用化されていない。
In addition, a so-called urea SCR method using a transition metal compound and a platinum group element as a catalyst and urea water as a reducing agent has been studied for exhaust gas treatment of large diesel vehicles such as trucks and buses that run on light oil fuel. This method has the advantage of being able to efficiently purify NOx from a relatively low temperature around 200 ° C. to a relatively high temperature around 600 ° C., but it is necessary to mount expensive urea water as a reducing agent. There is a problem that many of the low temperature exhaust NOx below about 200 ° C. is discharged as ammonium nitrate, which causes water quality environmental pollution.
As a method using a reducing agent other than urea water, a hydrocarbon SCR method using hydrocarbons (lower olefins such as ethylene, propylene, etc. having a reducing ability) contained in a small amount of exhaust gas as a reducing agent has been studied for a long time. . However, this method has a problem that a high purification rate is obtained for rich burn exhaust NOx, but lean burn exhaust NOx can be processed only in a very narrow temperature range around 200 ° C. It has not been.
一方、ディーゼル乗用車等の小型ディーゼル車の排ＮＯｘ処理には三元触媒が使用できない。それは、空燃比がガソリンの数倍以上であるので（ディーゼル燃料の燃焼はリーンバーンである）ディーゼル排ガス中の酸素濃度が通常５％以上であり還元性物質がほとんど含まれていないためである。同様の理由でリーンバーンガソリン車の排ガスも三元触媒だけでは浄化が難しい。リーンバーンガソリン車及び小型ディーゼル車の排ＮＯｘ処理には、現在、触媒としての白金族触媒にＮＯｘ吸蔵剤（アルカリ金属化合物やアルカリ土類化合物であるアルカリ性物質が主体）を添加した所謂ＮＯｘ吸蔵還元触媒が検討されている（特許文献１参照）。この方法は、リーンバーンだけでは排ＮＯｘ浄化が困難であるので、リーンバーンとリッチバーンを交互に繰り返す燃焼方式を行なっている。この方法によるＮＯｘ浄化は、リーンバーン排ＮＯｘをＮＯｘ吸蔵剤で吸収し、吸収ＮＯｘをリッチバーン雰囲気下で放出させ、放出ＮＯｘをリッチバーン排ガス中に供給した燃料もしくはリッチバーン排ガス中に存在する多量の一酸化炭素、水素、炭化水素等の還元性物質を用いて白金族触媒上で還元処理するという考えに立脚している。リーンバーンとリッチバーンを交互に繰り返す燃焼方式とＮＯｘ吸蔵還元触媒を用いた浄化方法は、ガソリン乗用車の排ガス処理に用いられている三元触媒が使用できないような高濃度の酸素雰囲気中でも２５０℃付近から６００℃付近に渡ってＮＯｘを浄化できるという利点をもつが、２００℃付近以下でのＮＯｘ浄化は非常に困難であるという問題がある。また、排ガス中の水分及びＳＯｘによってＮＯｘ吸蔵剤が著しく劣化するので、通常７５０℃以上での定期的な高温処理による触媒再生が必要であり、再生処理によって触媒は著しく熱劣化するという問題がある。 On the other hand, a three-way catalyst cannot be used for exhaust NOx treatment of small diesel vehicles such as diesel passenger cars. This is because the air-fuel ratio is more than several times that of gasoline (diesel fuel combustion is lean burn), and the oxygen concentration in diesel exhaust gas is usually 5% or more and contains almost no reducing substances. For the same reason, it is difficult to purify the exhaust gas from lean-burn gasoline vehicles using only a three-way catalyst. For NOx treatment of lean-burn gasoline vehicles and small diesel vehicles, the so-called NOx occlusion reduction is currently performed by adding a NOx occlusion agent (mainly alkaline substances such as alkali metal compounds and alkaline earth compounds) to platinum group catalysts as catalysts. Catalysts have been studied (see Patent Document 1). In this method, since it is difficult to purify exhaust NOx only by lean burn, a combustion method in which lean burn and rich burn are alternately repeated is performed. NOx purification by this method absorbs lean burn exhaust NOx with the NOx storage agent, releases the absorbed NOx in a rich burn atmosphere, and supplies the released NOx into the rich burn exhaust gas or a large amount present in the rich burn exhaust gas It is based on the idea of reducing the catalyst over a platinum group catalyst using a reducing substance such as carbon monoxide, hydrogen, or hydrocarbon. The combustion method that repeats lean burn and rich burn alternately and the purification method using NOx occlusion reduction catalyst are around 250 ° C even in a high-concentration oxygen atmosphere where the three-way catalyst used for exhaust gas treatment of gasoline passenger cars cannot be used. However, there is an advantage that NOx can be purified over about 600 ° C., but NOx purification at about 200 ° C. or lower is very difficult. Further, the NOx storage agent is significantly deteriorated by moisture and SOx in the exhaust gas. Therefore, it is necessary to regenerate the catalyst by regular high-temperature treatment usually at 750 ° C. or higher, and there is a problem that the catalyst is significantly deteriorated by the regeneration treatment. .
また、上記リーンバーンとリッチバーンを交互に繰り返す燃焼方式は、内燃機関における燃焼と酸素濃度を精密に制御するための高度で複雑な制御システムの搭載を必要とするので、リーンバーンだけの排ガスでもＮＯｘ浄化ができるのであれば上記の制御システムが簡略化できるので、このようなリーンバーン専用車のためのＮＯｘ浄化用触媒が望まれている。
このような事情から、最近、リーンバーンとリッチバーンを交互に繰り返す燃焼方式に代わる方法として、還元剤の少ないリーンバーンと還元剤の豊富なリーンバーンを交互に繰り返す燃焼方式が検討されはじめた。これはポストインジェクション方式と呼ばれる方式であり、通常のリーンバーンから次のリーンバーンに移行するまでの非常に短い時間に少量の燃料噴射をする燃焼方式である。この方式は、リーンバーン直後のまだ高温状態にある燃焼室に少量の燃料噴射をすることによって該噴射燃料を熱分解し、それによって低級炭化水素を生成させる方式である。この方式を行なえば、低級炭化水素が豊富に存在するリーンバーン排ガスを発生させることができる。ただし、この低級炭化水素は、一酸化炭素やエチレン、プロピレン等の還元性能の高い不飽和低級炭化水素と違って還元性能の低いＣ６〜Ｃ１２の飽和及び不飽和炭化水素が主成分であるので、従来の三元触媒やＮＯｘ吸蔵還元触媒等の公知の排ガス浄化用触媒では、ポストインジェクション方式で排出される排ガスのＮＯｘ処理が困難である。
In addition, the combustion method in which the lean burn and the rich burn are alternately repeated requires the installation of an advanced and complicated control system for precisely controlling the combustion and oxygen concentration in the internal combustion engine. Since the above control system can be simplified if NOx purification is possible, a NOx purification catalyst for such a lean burn vehicle is desired.
Under such circumstances, recently, as a method of replacing the combustion method in which lean burn and rich burn are alternately repeated, a combustion method in which lean burn with a small amount of reducing agent and lean burn with a large amount of reducing agent are alternately started to be studied. This is a so-called post-injection method, which is a combustion method in which a small amount of fuel is injected in a very short time from the normal lean burn to the next lean burn. This system is a system in which a small amount of fuel is injected into a combustion chamber still in a high temperature state immediately after lean burn to thermally decompose the injected fuel, thereby generating lower hydrocarbons. If this method is performed, lean burn exhaust gas rich in lower hydrocarbons can be generated. However, this lower hydrocarbon is mainly composed of C 6 -C 12 saturated and unsaturated hydrocarbons having low reducing performance unlike unsaturated lower hydrocarbons having high reducing performance such as carbon monoxide, ethylene and propylene. Therefore, with known exhaust gas purification catalysts such as conventional three-way catalysts and NOx occlusion reduction catalysts, it is difficult to treat NOx of exhaust gas discharged by the post-injection method.
また、活性炭は大気汚染物質や水質汚染物質を吸着する性質や触媒活性を有するので、その性質を排ガス処理のために利用することが一時期検討されたことがある。例えば、排ガスの主成分であるＮＯをＮＯ２に酸化する性質を利用して、排ガス中のＮＯをＮＯ２に酸化し、生成したＮＯ２を水存在下で硝酸イオンとしてトラップし、アルカリ性物質と反応させて硝酸塩に変換する方法が提案された（非特許文献１〜３参照）。しかしながら、活性炭が持つこのような性質は、温度上昇とともに急減し１００℃以上では殆どみられなくなるという原因のために、自動車排ガス処理用の浄化触媒としては使用されることはなかった。
ところで、国内で走行しているディーゼル自動車の殆どはトラック、バス等の大型自動車であり、乗用車は殆ど皆無であるので、ディーゼル自動車といえば、実質的に大型自動車のことをいうのであるが、排出する排ガスの温度は過渡走行時でおよそ１００℃〜２００℃であり安定走行時でおよそ２００℃〜４００℃であり、排出されるＮＯｘの約８０％が過渡走行時に排出されている。
以上のことから、ディーゼル自動車の排ガス処理に要求される触媒は、リーンバーンだけの燃焼方式で排出される排ガスに対して、上記１００℃〜２００℃の低温領域のリーンバーン排ＮＯｘに対して高活性であり尚且つ２００℃〜４００℃の中温領域のリーンバーン排ＮＯｘに対しても高活性であることが望まれているが、現在のところ２５０℃以下のリーンバーン排ＮＯｘに対して有効なＮＯｘ浄化用触媒は見出されていない。
Moreover, since activated carbon has the property of adsorbing air pollutants and water pollutants and catalytic activity, it has been studied for a while to use the properties for exhaust gas treatment. For example, using the property of oxidizing NO, which is the main component of exhaust gas, to NO 2 , NO in exhaust gas is oxidized to NO 2 , and the generated NO 2 is trapped as nitrate ions in the presence of water, A method of reacting and converting to nitrate has been proposed (see Non-Patent Documents 1 to 3). However, such properties of activated carbon have not been used as a purification catalyst for automobile exhaust gas treatment due to the fact that it suddenly decreases with increasing temperature and is hardly seen at 100 ° C. or higher.
By the way, most of the diesel cars that run in Japan are large cars such as trucks and buses, and there are almost no passenger cars. Therefore, a diesel car is essentially a large car. The temperature of the exhaust gas is about 100 ° C. to 200 ° C. during transient running, and is about 200 ° C. to 400 ° C. during stable running, and about 80% of the NOx discharged is discharged during transient running.
From the above, the catalyst required for exhaust gas treatment of diesel automobiles is higher than the exhaust gas discharged by the lean burn only combustion method with respect to lean burn exhaust NOx in the low temperature range of 100 ° C. to 200 ° C. Although it is desired to be active and highly active against lean burn NOx in the middle temperature range of 200 ° C. to 400 ° C., it is currently effective against lean burn NOx below 250 ° C. No catalyst for NOx purification has been found.
一般に、工業的な触媒は多孔性材料に担持した状態で使用されることが多い。多孔性材料の細孔は、ＩＵＰＡＣ（国際純正及び応用化学連合）によると、細孔直径が２ｎｍ以下のミクロ細孔、２〜５０ｎｍのメソ細孔、及び５０ｎｍ以上のマクロ細孔に分類されている。ミクロからメソの範囲に渡る広い分布を持つような単一の多孔性物質は活性炭以外には知られていない。
近年、細孔径が数ｎｍの細孔が規則的に配列し、比表面積が４００〜１１００ｍ２／ｇという非常に大きな値を有するシリカ、アルミナ、及びシリカアルミナ系のメソポーラス分子篩が開発された（特許文献２〜３参照）。これらの材料は、細孔の細孔配列があたかも結晶性物質の原子配列に類似していることから結晶性メソポーラス分子篩と命名されている。
また、最近、上記メソポーラス材料をテンプレートに用いてメソポーラスカーボンを製造する方法が数多く提案されている（例えば、非特許文献４参照）。
In general, industrial catalysts are often used in a state where they are supported on a porous material. The pores of the porous material are classified into micropores with a pore diameter of 2 nm or less, mesopores of 2 to 50 nm, and macropores of 50 nm or more according to IUPAC (International Pure and Applied Chemical Association). Yes. No single porous material other than activated carbon has a broad distribution ranging from the micro to meso range.
In recent years, silica, alumina, and silica-alumina-based mesoporous molecular sieves have been developed, in which pores having a pore size of several nanometers are regularly arranged and have a very large specific surface area of 400 to 1100 m 2 / g (patent) References 2-3). These materials are named crystalline mesoporous molecular sieves because the pore arrangement of the pores is similar to the atomic arrangement of the crystalline material.
Recently, many methods for producing mesoporous carbon using the mesoporous material as a template have been proposed (for example, see Non-Patent Document 4).
触媒反応は表面反応であるので、触媒の比表面積が大きいほど触媒活性が高い。また、触媒を担持するための担体は比表面積が大きいほど触媒活性を発現しやすい。このような観点から自動車用三元触媒をみると、支持体としてのモノリス成形体は成形体の断面が網目状で、軸方向に平行に互いに薄い壁によって仕切られたガス流路を設けている成形体であり、比表面積が０．２ｍ２／ｇ、担体としてのアルミナ粒子の比表面積が１１０〜３４０ｍ２／ｇであり、触媒の比表面積は粒径から２０〜４０ｍ２／ｇ程度であると推察される。したがって、従来の触媒粒子の粒径よりも一桁から二桁小さいナノサイズの触媒粒子を上記のようなメソポーラス材料の細孔内に担持することによって触媒の表面積は従来の三元触媒の１０２〜１０４倍大きくなるので、これをモノリス成形体に塗布することによって自動車排ガスに対する触媒活性の向上を図ることが考えられ、この考えは、例えば、特許文献４〜８に開示されている。しかし、ディーゼル自動車等が排出する１００℃〜２００℃付近の低温排ＮＯｘを効果的に除去することは困難であった。 Since the catalytic reaction is a surface reaction, the larger the specific surface area of the catalyst, the higher the catalytic activity. Further, the carrier for supporting the catalyst is more likely to exhibit the catalytic activity as the specific surface area is larger. From this point of view, when looking at a three-way catalyst for automobiles, the monolith molded body as a support has a mesh-shaped cross section and is provided with gas flow paths partitioned by thin walls parallel to each other in the axial direction. It is a molded body, the specific surface area is 0.2 m 2 / g, the specific surface area of alumina particles as a support is 110 to 340 m 2 / g, and the specific surface area of the catalyst is about 20 to 40 m 2 / g from the particle size. It is guessed. Therefore, the surface area of the catalyst is 10 2 of that of the conventional three-way catalyst by supporting the nano-sized catalyst particles that are one to two orders of magnitude smaller than the particle diameter of the conventional catalyst particles in the pores of the mesoporous material. since 10 becomes 4 times larger, which is believed to enhance the catalytic activity for automotive exhaust by applying monolith forming body, this idea, for example, disclosed in Patent Document 4-8. However, it has been difficult to effectively remove low-temperature exhaust NOx around 100 ° C. to 200 ° C. discharged by diesel vehicles and the like.
本発明の目的は、上記の事情に鑑み、従来困難であったリーンバーン排ガスを低温領域から中温領域に渡って効率的に浄化するための自動車用排ガス浄化用触媒を提供することである。具体的には、従来の三元触媒、尿素ＳＣＲ触媒、及びＮＯｘ吸蔵還元型触媒ではなくて、ＨＣ−ＳＣＲ型の触媒とＮＯｘ吸着性材料から成る触媒を提供することである。 In view of the above circumstances, an object of the present invention is to provide an automobile exhaust gas purification catalyst for efficiently purifying lean burn exhaust gas, which has been difficult in the past, from a low temperature region to an intermediate temperature region. Specifically, instead of the conventional three-way catalyst, urea SCR catalyst, and NOx occlusion reduction type catalyst, there is provided a catalyst comprising an HC-SCR type catalyst and a NOx adsorbing material.
本発明者らは、上記の目的を達成するために鋭意研究を重ねた結果、ＨＣ−ＳＣＲ型の触媒と活性炭から成る触媒がリーンバーン排ガスに対して非常に有効であることを見出し、この知見に基づいて本発明を完成させるに至った。
すなわち、本発明は、
１．メソポーラス材料に白金族元素を担持した触媒と活性炭から成ることを特徴とするリーンバーン自動車排ガス浄化用触媒。
２．活性炭が、黒鉛類似構造のメソポーラスカーボンであることを特徴とする上記１．に記載のリーンバーン自動車排ガス浄化用触媒に関する。
As a result of intensive studies to achieve the above object, the present inventors have found that a catalyst comprising an HC-SCR type catalyst and activated carbon is very effective against lean burn exhaust gas. Based on this, the present invention has been completed.
That is, the present invention
1. A lean-burn automobile exhaust gas purification catalyst comprising a catalyst having a platinum group element supported on a mesoporous material and activated carbon.
2. The activated carbon is a mesoporous carbon having a graphite-like structure. The lean burn automobile exhaust gas purification catalyst as described in 1.
本発明のリーンバーン自動車排ガス浄化用触媒（以下、本発明触媒ということがある。）は、排ガス中に含まれる極めて低濃度の炭化水素でも効果的に利用できるので、従来達成できなかったリーンバーン排ＮＯｘ処理を低温領域から中温領域に渡って極めて効率よく行なうことができる。例えば、三元触媒では、排ガス中に炭化水素が大量に存在しても酸素濃度１０％の雰囲気下ではＮＯｘはほとんど浄化できないが、本発明触媒を用いると、数万／ｈ以上の実用的な空塔速度（ＳＶ：ｓｐａｃｅ−ｖｅｌｏｃｉｔｙの略）の条件で、排ガス中に数百ｐｐｍ〜５００ｐｐｍ程度の炭化水素があれば、酸素濃度１０％のリーンバーン排ＮＯｘの８０％以上を１８０℃〜２００℃において浄化でき、４０％以上を２００℃〜３００℃にかけて浄化できる。また、同時に、炭化水素もほとんど浄化できる。 The lean burn automobile exhaust gas purification catalyst of the present invention (hereinafter also referred to as the catalyst of the present invention) can be effectively used even with extremely low concentrations of hydrocarbons contained in the exhaust gas, and thus has not been achieved in the past. Exhaust NOx treatment can be performed very efficiently from the low temperature region to the medium temperature region. For example, with a three-way catalyst, NOx can hardly be purified under an atmosphere with an oxygen concentration of 10% even if a large amount of hydrocarbon is present in the exhaust gas. If there is a hydrocarbon of several hundred ppm to 500 ppm in the exhaust gas under the condition of superficial velocity (SV: abbreviation for s space- v elocity), 80% or more of lean burn exhaust NOx with an oxygen concentration of 10% is 180 ° C. It can be purified at ˜200 ° C., and 40% or more can be purified at 200 ° C. to 300 ° C. At the same time, most hydrocarbons can be purified.
以下、本発明を詳細に説明する。
従来の三元触媒は、酸素濃度が１％よりも低い雰囲気で還元性物質がＮＯｘのモル濃度の１０倍程度あれば１６０℃〜６００℃に渡ってＮＯｘをほぼ１００％浄化できる。しかし、２５０℃以上では、酸素濃度が数％以上の雰囲気では還元性物質の含有量が酸素による酸化消費に見合う分以上の大過剰でない限りは、高温であるほどＮＯｘ浄化率が激減する。これは、三元触媒が酸素によって被毒されるという原因だけでは十分に説明できない。おそらく温度が２５０℃以上を超えたあたりから触媒上での還元性物質の完全燃焼反応がＮＯｘ還元反応よりも著しく早くなるからであろうと考えられている。
本発明触媒は、このような好ましくない触媒反応を改善するために工夫された触媒であり、これによってリーンバーン排ＮＯｘを低温領域から中温領域にかけて効率的に浄化するものである。
Hereinafter, the present invention will be described in detail.
The conventional three-way catalyst can purify NOx over a range of 160 ° C. to 600 ° C. if the reducing substance is about 10 times the NOx molar concentration in an atmosphere having an oxygen concentration lower than 1%. However, at 250 ° C. or higher, in an atmosphere with an oxygen concentration of several percent or higher, the NOx purification rate decreases drastically as the temperature increases, unless the content of the reducing substance is excessively large enough to meet the oxidation consumption by oxygen. This cannot be fully explained by the fact that the three-way catalyst is poisoned by oxygen. It is thought that the complete combustion reaction of the reducing substance on the catalyst is likely to be significantly faster than the NOx reduction reaction, probably after the temperature exceeds 250 ° C. or more.
The catalyst of the present invention is a catalyst devised to improve such an unfavorable catalytic reaction, and efficiently purifies lean burn exhaust NOx from a low temperature region to a medium temperature region.
すなわち、本発明は、リーンバーン排ガスを浄化するためのリーンバーン排ガス浄化用触媒であり、具体的には、メソポーラス材料に白金族元素を担持した触媒（以下、メソポーラス触媒と言う）と活性炭から成ることを特徴とするリーンバーン排ガス浄化用触媒である。本発明におけるリーンバーンとは、排ガス中の酸素イオン濃度が１％以上であることを言い、通常、酸素濃度が５％以上であることを言う。本発明におけるメソポーラス触媒の役割は、低温領域のリーンバーン雰囲気でＮＯをＮＯ２に酸化することである。また、活性炭の役割は、現在の所、まだ完全には解明できていないが、炭化水素を吸着することと排ガス中のＮＯ２の比率を高める方向にＮＯｘバランス（＝ＮＯとＮＯ２の量比）をシフトさせているものと推察している。その理由は、我々の実験では、活性炭の性質を調べると、活性炭そのものは、１００℃以上では空気存在下でもＮＯをＮＯ２に酸化する性質は全く無く（非特許文献１〜３の結果と同じであった）、炭化水素を供給してやると、逆にＮＯ２をＮＯに還元する性質が１５０℃あたりから現れるのであるが（炭化水素がなくてもＮＯ２をＮＯに還元するが、そのときの温度は３００℃以上である）、メソポーラス触媒に活性炭を添加するとＮＯ２の含有量が増大するという予想外の現象が見出されたからである。
本発明では、メソポーラス触媒と活性炭を配合する。通常は、両方の材料をミクロ分散させた混合物として用いるが、それぞれの材料を薄膜状の多層構造に積層して用いることもできる。混合物として使用するときの配合割合は、メソポーラス触媒に対して通常は同量から１０倍までの範囲で活性炭を配合するが、炭化水素の含有量に応じてメソポーラス触媒よりも少ない量を配合することもできる。
That is, the present invention is a lean burn exhaust gas purification catalyst for purifying lean burn exhaust gas, and specifically comprises a catalyst in which a platinum group element is supported on a mesoporous material (hereinafter referred to as a mesoporous catalyst) and activated carbon. This is a lean burn exhaust gas purification catalyst. The lean burn in the present invention means that the oxygen ion concentration in the exhaust gas is 1% or more, and usually means that the oxygen concentration is 5% or more. The role of the mesoporous catalyst in the present invention is to oxidize NO to NO 2 in a lean burn atmosphere in a low temperature region. Moreover, the role of activated carbon has not been completely elucidated yet, but the NOx balance (= the ratio of NO and NO 2 in the direction of adsorbing hydrocarbons and increasing the ratio of NO 2 in the exhaust gas) ). The reason is that, in our experiment, when the properties of activated carbon are examined, the activated carbon itself has no property of oxidizing NO to NO 2 even in the presence of air at 100 ° C. or higher (the same as the results of Non-Patent Documents 1 to 3). However, when hydrocarbon is supplied, the property of reducing NO 2 to NO appears from around 150 ° C. (NO 2 is reduced to NO even if there is no hydrocarbon, but at that time This is because an unexpected phenomenon has been found in which the content of NO 2 increases when activated carbon is added to the mesoporous catalyst.
In the present invention, a mesoporous catalyst and activated carbon are blended. Normally, both materials are used as a mixture in which the materials are micro-dispersed. However, the respective materials can be used by being laminated in a thin-film multilayer structure. When used as a mixture, the blending ratio is usually from the same amount to 10 times the mesoporous catalyst, but the blended amount is smaller than the mesoporous catalyst depending on the hydrocarbon content. You can also.
本発明に於ける活性炭とは、比表面積が大きく、吸着性の強い、大部分が炭素質の物質を云う。本発明における活性炭には、細孔径が２〜５０ｎｍの範囲にある高比表面積のカーボン、所謂メソポーラスカーボンも含まれる。比表面積は、２００〜４０００ｍ２／ｇであることが好ましく、４００〜２０００ｍ２／ｇであることがさらに好ましい。比表面積が２００ｍ２／ｇ未満では活性が乏しいので２００ｍ２／ｇ以上であることが好ましく、また、４０００ｍ２／ｇを越えると材料強度が低下するので４０００ｍ２／ｇ以下であることが好ましい。また、活性炭は可燃物であるので、実際の使用に際しては、活性炭の耐熱性と排ガス温度を考慮することが望ましい。黒鉛構造類似のメソポーラスカーボンは、耐熱性が高い活性炭であるので好ましい。例えば、一般に市販されている活性炭の耐熱性は約４００℃であり、黒鉛構造類似のメソポーラスカーボンの耐熱性は約６００℃であるので、通常の活性炭を使用する場合には排ガス温度が４００℃未満であるような条件で使用することが望ましいのであるが、黒鉛構造類似のメソポーラスカーボンを使用する場合には、排ガス温度が４００℃以上でも対応できる。本発明では活性炭に白金のような触媒活性成分を担持しない形で使用するのであるが、その理由は、白金のような触媒活性成分を担持させて用いると排ガス中の炭化水素の燃焼反応が急激に起きて大量の熱が発生し、それによって活性炭が燃焼するおそれがあるからである。本発明触媒の使用温度の上限は、通常６００℃であるが、活性炭の耐熱性を向上するために、活性炭に、不燃性の物質、例えば、シリコンカーバイド、窒化硼素、窒化珪素、炭化硼素、シリカなどで活性炭表面を修飾することは、使用温度の上限を上げるので好ましい。例えば、活性炭表面をシリコンカーバイドやメソポーラスシリカで被覆すると耐熱性が約８００℃に向上するので、使用温度の上限を上昇させることができるので好ましい。 The activated carbon in the present invention refers to a substance having a large specific surface area, strong adsorptivity, and mostly carbonaceous. The activated carbon in the present invention includes high specific surface area carbon having a pore diameter in the range of 2 to 50 nm, so-called mesoporous carbon. The specific surface area is preferably 200~4000m 2 / g, further preferably 400~2000m 2 / g. Since the activity is poor when the specific surface area is less than 200 m 2 / g, the activity is preferably 200 m 2 / g or more. When the specific surface area exceeds 4000 m 2 / g, the material strength decreases, and therefore it is preferably 4000 m 2 / g or less. Moreover, since activated carbon is a combustible material, it is desirable to consider the heat resistance and exhaust gas temperature of activated carbon in actual use. Mesoporous carbon having a similar graphite structure is preferable because it is activated carbon with high heat resistance. For example, the heat resistance of commercially available activated carbon is about 400 ° C., and the heat resistance of mesoporous carbon similar to the graphite structure is about 600 ° C. Therefore, when using normal activated carbon, the exhaust gas temperature is less than 400 ° C. However, when using mesoporous carbon similar in graphite structure, it is possible to cope with an exhaust gas temperature of 400 ° C. or higher. In the present invention, activated carbon is used in a form that does not support a catalytically active component such as platinum. The reason for this is that when a catalytically active component such as platinum is supported, the combustion reaction of hydrocarbons in the exhaust gas rapidly occurs. This is because a large amount of heat is generated and the activated carbon may burn. The upper limit of the use temperature of the catalyst of the present invention is usually 600 ° C., but in order to improve the heat resistance of the activated carbon, the activated carbon is made of a nonflammable substance such as silicon carbide, boron nitride, silicon nitride, boron carbide, silica. It is preferable to modify the activated carbon surface with, for example, to increase the upper limit of the use temperature. For example, it is preferable to coat the activated carbon surface with silicon carbide or mesoporous silica because the heat resistance is improved to about 800 ° C., so that the upper limit of the use temperature can be increased.
本発明に於けるメソポーラス触媒としては、メソポーラス材料に白金族元素を担持した触媒を用いる（但し、前記三元触媒は除く）。白金族元素とは、ルテニウム、ロジウム、パラジウム、オスミウム、イリジウム、及び白金の６元素の総称であるが、これらの中で、ロジウム、パラジウム、イリジウム、及び白金の中から選択される１種又は２種が好ましく、中でも白金が最も好ましい。通常、本発明では、白金主体の触媒を用いるが、その理由は、触媒の主成分である白金が排ＮＯｘの主成分であるＮＯをＮＯ２に酸化する能力が高く、還元剤によってＮ２Ｏ及びＮ２に還元する能力が高く、高濃度の酸素雰囲気中でも化学的に安定であるからであり、また、白金族元素の中では白金が比較的低温高活性であるからでもある。白金主体の触媒に異なる機能を有する助触媒的成分を添加することによってシナジー効果による触媒性能の向上をはかることもできる。このような成分として、例えば、遷移金属及びその酸化物を用いることができる。遷移金属元素としては、周期律表における３族〜１１族の元素が挙げられるが、これらの中でバナジウム、クロム、マンガン、鉄、コバルト、ニッケル、銅、亜鉛、モリブデン、タングステン、レニウム、が好ましく、中でも鉄、コバルト、タングステンが最も好ましい。これらの助触媒的成分の添加質量は、通常、白金の０．０１倍から１００倍程度であるが、必要に応じて１００倍以上であってもよい。 As the mesoporous catalyst in the present invention, a catalyst in which a platinum group element is supported on a mesoporous material is used (however, the three-way catalyst is excluded). The platinum group element is a generic name of six elements of ruthenium, rhodium, palladium, osmium, iridium, and platinum, and among these, one or two selected from rhodium, palladium, iridium, and platinum. Species are preferred, with platinum being most preferred. Usually, in the present invention, a platinum-based catalyst is used. The reason is that platinum, which is the main component of the catalyst, has a high ability to oxidize NO, which is the main component of exhausted NOx, to NO 2 , and N 2 O depending on the reducing agent. This is because it has a high ability to reduce to N 2 and is chemically stable even in a high-concentration oxygen atmosphere, and also because platinum is relatively active at a relatively low temperature among platinum group elements. By adding a co-catalytic component having a different function to the platinum-based catalyst, the catalyst performance can be improved by the synergy effect. As such a component, for example, a transition metal and its oxide can be used. Examples of the transition metal element include elements of Group 3 to Group 11 in the periodic table. Among these, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, molybdenum, tungsten, rhenium are preferable. Of these, iron, cobalt, and tungsten are most preferable. The added mass of these promoter components is usually about 0.01 to 100 times that of platinum, but may be 100 times or more as required.
触媒の担体として用いるメソポーラス材料は、高比表面積を有し細孔径がナノサイズであるので、そこに担持される触媒の比表面積を飛躍的に高められることと、触媒を細孔内に担持することで触媒粒子の再凝集を抑制し触媒の均一高分散を図れること、などの優れた効果がある。メソポーラス材料の比表面積は特別な事情がない限り高ければ高いほどよい。本発明に用いることができるメソポーラス材料の比表面積は１００〜４０００ｍ２／ｇであり、好ましくは２００〜３０００ｍ２／ｇ、さらに好ましくは、４００〜２０００ｍ２／ｇである。比表面積が１００ｍ２／ｇ未満では、触媒の担持量が少なくなるので担持触媒の触媒性能を引き出す上から１００ｍ２／ｇ以上であることが好ましい。一方、材料強度上の面からは比表面積が４０００ｍ２／ｇ以下であることが好ましい。また、本発明におけるメソポーラス材料の細孔の大部分は、細孔径（直径表示）が１〜５０ｎｍの範囲にあり、好ましくは２〜２０ｎｍの範囲にあり、より好ましくは２〜１０ｎｍの範囲にある。ここでいう細孔の大部分とは１〜５０ｎｍの細孔が占める細孔容積が全細孔容積の６０％以上であることをいう。細孔径が１ｎｍ未満であっても触媒の担持は可能であるが還元剤の流通や不純物等による汚染を考えると１ｎｍ以上が好ましい。５０ｎｍを越えると分散担持された触媒が水熱高温条件などによるシンタリング（＝焼結）によって巨大粒子に成長しやすくなるので５０ｎｍ以下が好ましい。メソポーラス材料の平均粒径は、０．１〜１０μｍ、好ましくは０．１〜１μｍ、特に好ましくは０．１〜０．５μｍである。メソポーラス材料の細孔に担持される触媒の粒径は、細孔径とほぼ同程度ないしそれ以下であるので、前記の細孔径の範囲は、高活性を発現する触媒の粒径範囲とも一致している。一般に、ナノサイズに微粒化された触媒粒子は、活性を示すエッジ、コーナー、ステップなどの高次数の結晶面を多量に持つので、触媒活性が著しく向上するだけでなく、バルクでは触媒活性を示さないような不活性金属でも予期しなかった触媒活性を発現する場合があることが知られている。したがって、触媒能力の観点からは触媒は小さいほど好ましいのであるが、反面、微粒化よる表面酸化、副反応などの好ましくない性質もでてくるので、微粒子の粒子径には最適範囲が存在する。本発明における目的のＮＯｘ浄化処理に対して効果的な活性を示す触媒の平均粒径は１〜５０ｎｍの範囲にあり、１〜２０ｎｍの範囲が好ましく、１〜１０ｎｍの範囲が特に好ましい。 The mesoporous material used as the catalyst support has a high specific surface area and a pore size of nano-size, so that the specific surface area of the catalyst supported on the mesoporous material can be dramatically increased and the catalyst is supported in the pores. Thus, there are excellent effects such as suppressing reaggregation of the catalyst particles and achieving uniform and high dispersion of the catalyst. The higher the specific surface area of the mesoporous material, the better, unless there are special circumstances. The specific surface area of mesoporous materials that may be used in the present invention is 100~4000m 2 / g, preferably 200~3000m 2 / g, and more preferably from 400~2000m 2 / g. The specific surface area of 100m less than 2 / g, it is preferable since the amount of supported catalyst is reduced from above to draw the catalytic performance of the supported catalyst is 100m 2 / g or more. On the other hand, in terms of material strength, the specific surface area is preferably 4000 m 2 / g or less. Further, most of the pores of the mesoporous material in the present invention have a pore diameter (diameter display) in the range of 1 to 50 nm, preferably in the range of 2 to 20 nm, more preferably in the range of 2 to 10 nm. . The majority of the pores referred to here means that the pore volume occupied by pores of 1 to 50 nm is 60% or more of the total pore volume. The catalyst can be supported even if the pore diameter is less than 1 nm, but it is preferably 1 nm or more in consideration of the flow of the reducing agent and contamination due to impurities. If it exceeds 50 nm, the dispersed and supported catalyst tends to grow into giant particles by sintering (= sintering) under hydrothermal high temperature conditions and the like. The average particle size of the mesoporous material is 0.1 to 10 μm, preferably 0.1 to 1 μm, particularly preferably 0.1 to 0.5 μm. Since the particle size of the catalyst supported in the pores of the mesoporous material is approximately the same as or smaller than the pore size, the above-mentioned pore size range coincides with the particle size range of the catalyst exhibiting high activity. Yes. Generally, catalyst particles atomized to nanosize have a large number of high-order crystal planes such as edges, corners, and steps showing activity, so that not only catalytic activity is remarkably improved but also catalytic activity is shown in bulk. It is known that even an inert metal such as this may exhibit unexpected catalytic activity. Therefore, the smaller the catalyst, the better from the viewpoint of catalytic ability. However, on the other hand, undesirable properties such as surface oxidation and side reactions due to atomization also appear, so there is an optimum range for the particle size of the fine particles. The average particle diameter of the catalyst exhibiting effective activity for the target NOx purification treatment in the present invention is in the range of 1 to 50 nm, preferably in the range of 1 to 20 nm, and particularly preferably in the range of 1 to 10 nm.
以上のような比表面積と細孔分布と平均粒径を併せ持ったメソポーラス材料としては、シリカ、アルミナ、チタニア、ジルコニア、マグネシア、カルシア、セリア、ニオビアなどを挙げることができ、シリカ、アルミナ、チタニア、ジルコニア、マグネシア、カルシア、セリア、ニオビア、及びこれらの複合材料が好ましく、シリカ、アルミナ、マグネシア、及びこれらの複合材料がさらに好ましい。メソポーラスシリカは、細孔が結晶格子のように規則正しく配列した結晶性メソポーラスシリカと、細孔配列に結晶のような規則性が殆どみられない非晶性のメソポーラスシリカに大別されるが、本発明の目的にとっては、水熱耐久性、耐熱性、ガス拡散性などに優れた非晶性のメソポーラスシリカのほうが優れている。
なお、上記吸脱着の比表面積は、吸脱着の気体として窒素を用いたＢＥＴ窒素吸着法によって測定される値であり、細孔径は、吸脱着の気体として窒素を用いた窒素吸着法によって測定される値であり、ＢＪＨ法によって求められる１〜２００ｎｍの範囲の細孔分布（微分分布表示）で示される。
メソポーラス材料に白金主体の触媒を担持する時の白金の担持量は０．０１〜２０質量％であり、好ましくは０．１〜１０質量％であるが、量的な問題がなければ、通常は、１ないし数％の担持量で用いる。担持量は２０質量％以上でも可能であるが、担持量が過剰になると反応にほとんど寄与しない細孔深部の触媒が増えるので２０質量％以下が好ましい。０．０１質量％未満では活性が十分ではないので０．０１質量％以上が好ましい。
Examples of the mesoporous material having both the specific surface area, the pore distribution, and the average particle size as described above include silica, alumina, titania, zirconia, magnesia, calcia, ceria, and niobia. Silica, alumina, titania, Zirconia, magnesia, calcia, ceria, niobia, and a composite material thereof are preferable, and silica, alumina, magnesia, and a composite material thereof are more preferable. Mesoporous silica is roughly classified into crystalline mesoporous silica in which the pores are regularly arranged like a crystal lattice, and amorphous mesoporous silica in which the regularity such as crystals is hardly observed in the pore arrangement. For the purpose of the invention, amorphous mesoporous silica excellent in hydrothermal durability, heat resistance, gas diffusibility and the like is superior.
The specific surface area for adsorption / desorption is a value measured by a BET nitrogen adsorption method using nitrogen as an adsorption / desorption gas, and the pore size is measured by a nitrogen adsorption method using nitrogen as an adsorption / desorption gas. It is shown by a pore distribution (differential distribution display) in the range of 1 to 200 nm obtained by the BJH method.
When the platinum-based catalyst is supported on the mesoporous material, the supported amount of platinum is 0.01 to 20% by mass, preferably 0.1 to 10% by mass. It is used at a loading of 1 to several percent. The supported amount can be 20% by mass or more, but if the supported amount is excessive, the catalyst in the deep pores that hardly contributes to the reaction increases, so 20% by mass or less is preferable. Since activity is not enough if it is less than 0.01 mass%, 0.01 mass% or more is preferable.
本発明で用いるメソポーラス材料の製造は、従来の方法である界面活性剤のミセルをテンプレートとして用いるゾル−ゲル法を応用することによって所用の材料を製造することができる。メソポーラス材料の前駆物質としては、メソポーラスシリカの場合、通常、テトラメトキシシラン、テトラエトキシシラン、テトラ−ｉ−プロポキシシラン、テトラ−ｎ−プロポキシシラン、テトラ−ｉ−ブトキシシラン、テトラ−ｎ−ブトキシシラン、テトラ−ｓｅｃ−ブトキシシラン、テトラ−ｔ−ブトキシシラン等のアルコキシドを用いる。メソポーラスシリカ、アルミナ、チタニア、ジルコニア、マグネシア、カルシア、セリア、ニオビアについても、通常、アルコキシドを用いて製造することができる。ミセル形成の界面活性剤は、例えば長鎖のアルキルアミン、長鎖の４級アンモニウム塩、長鎖のアルキルアミンＮ−オキシド、長鎖のスルホン酸塩、ポリエチレングリコールアルキルエーテル、ポリエチレングリコール脂肪酸エステル等のいずれであってもよい。溶媒として、通常、水、アルコール類、ジオールの１種以上が用いられるが、水系溶媒が好ましい。反応系に金属への配位能を有する化合物を少量添加すると反応系の安定性を高めることができる。このような安定剤としては、アセチルアセトン、テトラメチレンジアミン、エチレンジアミン四酢酸、ピリジン、ピコリンなどの金属配位能を有する化合物が好ましい。メソポーラス材料の前駆物質、界面活性剤、溶媒及び安定剤からなる反応系の組成は、メソポーラス材料の前駆物質のモル比が、０．０１〜０．６、好ましくは０．０２〜０．５、該前駆物質／界面活性剤のモル比が１〜３０、好ましくは１〜１０、溶媒／界面活性剤のモル比が１〜１０００、好ましくは５〜５００、安定剤／主剤（溶媒を除く反応系組成）のモル比が０．０１〜１、好ましくは０．２〜０．６である。反応温度は、２０〜１８０℃、好ましくは２０〜１００℃の範囲である。反応時間は５〜１００時間、好ましくは１０〜５０時間の範囲である。反応生成物は通常、濾過により分離し、十分に水洗、乾燥後、５００〜１０００℃の高温焼成によってテンプレートを熱分解除去し、メソポーラス材料を得ることができる。必要に応じて、焼成前に界面活性剤をアルコールなどで抽出することもできる。 The mesoporous material used in the present invention can be produced by applying a sol-gel method using a surfactant micelle as a template, which is a conventional method. As the precursor of the mesoporous material, in the case of mesoporous silica, usually, tetramethoxysilane, tetraethoxysilane, tetra-i-propoxysilane, tetra-n-propoxysilane, tetra-i-butoxysilane, tetra-n-butoxysilane An alkoxide such as tetra-sec-butoxysilane or tetra-t-butoxysilane is used. Mesoporous silica, alumina, titania, zirconia, magnesia, calcia, ceria and niobia can also be usually produced using alkoxides. Examples of micelle-forming surfactants include long-chain alkylamines, long-chain quaternary ammonium salts, long-chain alkylamine N-oxides, long-chain sulfonates, polyethylene glycol alkyl ethers, and polyethylene glycol fatty acid esters. Either may be sufficient. As the solvent, one or more of water, alcohols, and diols are usually used, and an aqueous solvent is preferable. When a small amount of a compound having a coordination ability to metal is added to the reaction system, the stability of the reaction system can be improved. As such a stabilizer, compounds having metal coordination ability such as acetylacetone, tetramethylenediamine, ethylenediaminetetraacetic acid, pyridine, and picoline are preferable. The composition of the reaction system comprising the precursor of the mesoporous material, the surfactant, the solvent and the stabilizer is such that the molar ratio of the precursor of the mesoporous material is 0.01 to 0.6, preferably 0.02 to 0.5, The precursor / surfactant molar ratio is 1-30, preferably 1-10, and the solvent / surfactant molar ratio is 1-1000, preferably 5-500, stabilizer / main agent (reaction system excluding solvent). The molar ratio of the composition is 0.01 to 1, preferably 0.2 to 0.6. The reaction temperature is in the range of 20 to 180 ° C, preferably 20 to 100 ° C. The reaction time is 5 to 100 hours, preferably 10 to 50 hours. The reaction product is usually separated by filtration, sufficiently washed with water, dried, and then thermally decomposed and removed from the template by high-temperature baking at 500 to 1000 ° C. to obtain a mesoporous material. If necessary, the surfactant can be extracted with alcohol or the like before firing.
メソポーラス材料に白金族元素を担持する方法は、例えば、イオン交換法又は含浸法によって行なうことができる。これらの２つの方法は、担体への触媒の沈着化について、イオン交換法が担体表面のイオン交換能を利用し、含浸法が担体のもつ毛管作用を利用しているという違いはあるが、基本的なプロセスはほとんど同じである。すなわち、メソポーラス材料を触媒原料の水溶液に浸した後、濾過、乾燥、必要に応じて水洗を行い、還元剤で還元処理することによって製造することができる。白金の触媒原料としては、例えば、Ｈ２ＰｔＣｌ４、（ＮＨ４）２ＰｔＣｌ４、Ｈ２ＰｔＣｌ６、（ＮＨ４）２ＰｔＣｌ６、Ｐｔ（ＮＨ３）４（ＮＯ３）２、Ｐｔ（ＮＨ３）４（ＯＨ）２、ＰｔＣｌ４、白金アセチルアセトナート等を用いることができる。必要に応じて、助触媒成分の硝酸塩、硫酸塩、炭酸塩、酢酸塩などの水溶性塩類を白金触媒原料に混合して同様にして製造することができる。触媒活性成分の還元剤としては、水素、ヒドラジン水溶液、ホルマリン等を用いることができる。還元は、それぞれの還元剤について知られている通常の条件で行なえばよい。例えば、水素還元は、ヘリウムなどの不活性ガスで希釈した水素ガス気流下にサンプルを置き、通常、３００〜５００℃で数時間処理することによって行なうことができる。還元後、必要に応じて不活性ガス気流下５００〜１０００℃で数時間熱処理してもよい。 The method of supporting the platinum group element on the mesoporous material can be performed by, for example, an ion exchange method or an impregnation method. These two methods are different in that the catalyst is deposited on the support, although the ion exchange method uses the ion exchange capacity of the support surface and the impregnation method uses the capillary action of the support. The general process is almost the same. That is, it can be produced by immersing the mesoporous material in an aqueous solution of the catalyst raw material, followed by filtration, drying, washing with water as necessary, and reduction treatment with a reducing agent. Examples of the platinum catalyst material include H 2 PtCl 4 , (NH 4 ) 2 PtCl 4 , H 2 PtCl 6 , (NH 4 ) 2 PtCl 6 , Pt (NH 3 ) 4 (NO 3 ) 2 , Pt (NH 3 ) 4 (OH) 2 , PtCl 4 , platinum acetylacetonate and the like can be used. If necessary, water-soluble salts such as nitrates, sulfates, carbonates, and acetates of the promoter component can be mixed with the platinum catalyst raw material and produced in the same manner. As the reducing agent for the catalytically active component, hydrogen, an aqueous hydrazine solution, formalin or the like can be used. The reduction may be performed under normal conditions known for each reducing agent. For example, hydrogen reduction can be performed by placing a sample in a hydrogen gas stream diluted with an inert gas such as helium and treating the sample at 300 to 500 ° C. for several hours. After reduction, if necessary, heat treatment may be performed at 500 to 1000 ° C. for several hours under an inert gas stream.
本発明で用いる活性炭は、通常、市販されている細孔分布が２〜２０ｎｍ、比表面積が８２０ｍ２／ｇ、平均粒径が５０μｍの活性炭であるが、細孔分布が２〜５０ｎｍのメソ領域である所謂メソポーラスカーボンであってもよい。
本発明触媒は、通常、自動車触媒用の担体として一般的に使用されているセラミックス製、あるいはメタル製のモノリス成形体のガス流路内壁に塗布した形状で用いるが、これらに限定されるものではない。上記モノリス成形体とは、成形体の断面が網目状で、軸方向に平行に互いに薄い壁によって仕切られたガス流路を設けている成形体のことであり、モノリス成形体に触媒を付着させて成る触媒を以下ではモノリス触媒という。成形体の外形は、特に限定されるものではないが、通常は、円柱形である。本発明触媒をモノリス成形体のガス流路内壁に付着させる時の触媒の付着量は、３〜３０質量％が好ましい。担体内部に存在する触媒内部へのガス拡散の面から３０質量％未満が好ましい。また、十分な触媒性能を引き出す上で３質量％以上が好ましい。
The activated carbon used in the present invention is usually activated carbon having a commercially available pore distribution of 2 to 20 nm, a specific surface area of 820 m 2 / g, and an average particle size of 50 μm, but a meso region having a pore distribution of 2 to 50 nm. It may be so-called mesoporous carbon.
The catalyst of the present invention is usually used in a shape applied to the inner wall of the gas flow path of a ceramic or metal monolith molded body generally used as a carrier for automobile catalysts, but is not limited to these. Absent. The above-mentioned monolith molded body is a molded body having a mesh-shaped cross section and provided with gas flow paths partitioned by thin walls parallel to each other in the axial direction. A catalyst is attached to the monolith molded body. Hereinafter, this catalyst is referred to as a monolith catalyst. Although the external shape of a molded object is not specifically limited, Usually, it is a cylindrical shape. The amount of the catalyst deposited when the catalyst of the present invention is deposited on the gas flow path inner wall of the monolith molded body is preferably 3 to 30% by mass. The amount is preferably less than 30% by mass from the viewpoint of gas diffusion into the catalyst inside the support. Further, 3% by mass or more is preferable in order to bring out sufficient catalyst performance.
上記のモノリス触媒は、自動車用三元触媒を付着したモノリス成形体の製造方法に準じて製造することができる。例えば、本発明触媒とバインダーとしてのコロイダルシリカ（シリカゾルとも言う）を、通常、１：（０．０１〜０．２）の質量割合で混合した混合物を調整し、これを水分散することによって通常１０〜５０質量％のスラリーを調整した後、該スラリーにモノリス成形体を浸漬してモノリス成形体のガス流路の内壁にスラリーを付着させ、乾燥後、窒素、ヘリウム、アルゴンなどの不活性雰囲気下、５００〜１０００℃で数時間処理することによって製造することができる。あるいは、あらかじめ、モノリス成形体にメソポーラス材料と活性炭の混合物を塗布した後、該モノリス成形体を触媒原料の水溶液に浸漬、取り出し、乾燥、還元処理、熱処理することによって製造することもできる。 Said monolith catalyst can be manufactured according to the manufacturing method of the monolith molded object which adhered the three-way catalyst for motor vehicles. For example, the mixture of the present invention catalyst and colloidal silica (also referred to as silica sol) as a binder is usually mixed at a mass ratio of 1: (0.01 to 0.2), and this is usually dispersed in water. After adjusting the slurry of 10 to 50% by mass, the monolith molded body is immersed in the slurry to adhere the slurry to the inner wall of the gas flow path of the monolith molded body, and after drying, an inert atmosphere such as nitrogen, helium or argon It can manufacture by processing at 500-1000 degreeC for several hours below. Alternatively, the monolith molded body can be produced by applying a mixture of a mesoporous material and activated carbon to the monolith molded body in advance, then immersing the monolith molded body in an aqueous solution of a catalyst raw material, drying, reducing treatment, and heat treatment.
コロイダルシリカ以外のバインダーとしては、メチルセルロース、アクリル樹脂、ポリエチレングリコールなどを適宜用いることもできる。モノリス成形体に付着させる本発明触媒の厚みは、前記のスラリーを付着させる方法では、通常、１μｍ〜１００μｍであるのが好ましく、１０μｍ〜５０μｍの範囲が特に好ましい。１００μｍを越えると反応ガスの拡散が遅くなるので１００μｍ以下が好ましい。触媒性能の劣化を抑制するためには１μｍ以上が好ましい。
本発明触媒は、自動車、特にディーゼル自動車及びリーンバーンガソリン自動車に搭載することによって、自動車が排出するリーンバーン排ガス中のＮＯｘ及び炭化水素類の有害物質を１６０℃〜４００℃の領域において極めて効果的に除去することができる。特に、トラックなどの大型車用の排ガス浄化方法として有効に用いることができる。
As a binder other than colloidal silica, methyl cellulose, acrylic resin, polyethylene glycol, or the like can be used as appropriate. The thickness of the catalyst of the present invention to be attached to the monolith molded body is usually preferably 1 μm to 100 μm, particularly preferably in the range of 10 μm to 50 μm, in the method for attaching the slurry. If it exceeds 100 μm, the diffusion of the reaction gas becomes slow, so that it is preferably 100 μm or less. In order to suppress deterioration of catalyst performance, 1 μm or more is preferable.
The catalyst of the present invention is extremely effective in mounting NOx and hydrocarbon harmful substances in lean burn exhaust gas emitted by automobiles in the range of 160 ° C to 400 ° C when mounted on automobiles, particularly diesel automobiles and lean burn gasoline automobiles. Can be removed. In particular, it can be effectively used as an exhaust gas purification method for large vehicles such as trucks.
以下に実施例などを挙げて本発明を具体的に説明する。
比表面積及び細孔分布は脱吸着の気体として窒素を用い、カルロエルバ社製ソープトマチック１８００型装置によって測定した。比表面積はＢＥＴ法によって求めた。細孔分布は１〜２００ｎｍの範囲を測定し、ＢＪＨ法で求められる微分分布で示した。製造したメソポーラス材料の多くは指数関数的に左肩上がりの分布における特定の細孔直径の位置にピークを示した。このピークを与える細孔直系が細孔径である。
自動車排ＮＯｘのモデルガスとして、ヘリウム希釈で希釈した一酸化窒素、酸素、プロピレンの混合ガスを用いた。減圧式化学発光法ＮＯｘ分析計（日本サーモ株式会社製造：モデル４２ｉ−ＨＬ及び４６Ｃ−Ｈ）によって処理前と処理後のガスに含まれるＮＯｘ（ＮＯとＮＯ２の合計））の濃度を測定し、Ｎｏｘ浄化率を式（１）によって算出した。
The specific surface area and pore distribution were measured with a Sorpmatic 1800 type apparatus manufactured by Carlo Elba using nitrogen as a desorption gas. The specific surface area was determined by the BET method. The pore distribution was measured in the range of 1 to 200 nm and indicated by a differential distribution obtained by the BJH method. Many of the produced mesoporous materials peaked at specific pore diameter positions in an exponentially increasing distribution. The pore diameter system giving this peak is the pore diameter.
As a model gas for automobile exhaust NOx, a mixed gas of nitric oxide, oxygen and propylene diluted with helium was used. The concentration of the NOx analyzer (NOx (total of NO and NO2)) contained in the gas before and after treatment is measured by a reduced pressure chemiluminescence NOx analyzer (manufactured by Nippon Thermo Co., Ltd .: models 42i-HL and 46C-H), The Nox purification rate was calculated by equation (1).
「製造例１」排ガス浄化用触媒としての三元触媒類似の触媒の合成
０．５３８ｇのＰｔＣｌ４・５Ｈ２Ｏ、０．２０６５ｇのＰｄＣｌ２・２Ｈ２Ｏ、及び０．４０５ｇのＲｈ（ＮＯ３）３・２Ｈ２Ｏを２０ｍｌの蒸留水に溶解した水溶液を蒸発皿に入れ、これに１０ｇの活性アルミナ（日揮株式会社製造 商品名：Ｎ−６１３Ｎ、：比表面積２５０ｍ２／ｇ、平均細孔径６．２ｎｍ、粒径２〜３μｍの微粒子）を加え、スチームバスで蒸発乾固した後、真空乾燥機に入れ１００℃で３時間真空乾燥を行なった。この試料を石英管に入れヘリウム希釈水素ガス（１０ｖ／ｖ％）気流下５００℃で３時間還元し、貴金属の含有量が約５質量％の触媒を合成した。これを三元触媒を模した貴金属触媒として比較実験に用いた。
“Production Example 1” Synthesis of a catalyst similar to a three-way catalyst as an exhaust gas purification catalyst 0.538 g of PtCl 4 .5H 2 O, 0.2065 g of PdCl 2 .2H 2 O, and 0.405 g of Rh (NO 3 ) 3 · 2H 2 O were placed an aqueous solution prepared by dissolving in distilled water of 20ml evaporation dish, to which 10g of activated alumina (JGC Corporation manufacturing trade name: N-613N ,: specific surface area of 250 meters 2 / g, average pore diameter 6.2 nm, fine particles having a particle diameter of 2 to 3 μm) were added and evaporated to dryness in a steam bath, and then placed in a vacuum dryer and vacuum dried at 100 ° C. for 3 hours. This sample was put into a quartz tube and reduced at 500 ° C. for 3 hours under a helium-diluted hydrogen gas (10 v / v%) stream to synthesize a catalyst having a precious metal content of about 5 mass%. This was used in a comparative experiment as a noble metal catalyst simulating a three-way catalyst.
「製造例２」メソポーラス触媒の製造
１リットルのビーカーに、蒸留水３００ｇ、エタノール２４０ｇ、及びドデシルアミン３０ｇを入れ、溶解させた。攪拌下でテトラエトキシシラン１２５ｇを加えて室温で２２時間攪拌した。生成物を濾過、水洗し、１１０℃で５時間温風乾燥した後、空気中で５５０℃−５時間焼成して含有するドデシルアミンを分解除去し、平均粒径が０．５μｍのメソポーラスシリカ材料を得た。該メソポーラスシリカ材料を小角Ｘ線回折測定した結果、１本のブロードな回折ピークを示した。また、透過型電子顕微鏡観察の結果、細孔の配列には規則的な配列が観測されず無秩序に分散している状態が観測された。これらの結果から、製造したメソポーラスシリカ材料は非晶性であることが確認された。また、細孔分布及び比表面積測定の結果、約２．５ｎｍの位置に細孔ピークがあり、比表面積が１１２３ｍ２／ｇ、細孔容積が０．８９ｃｍ３／ｇ、１〜５０ｎｍの細孔が占める容積は０．８９ｃｍ３／ｇであった。蒸留水２０ｇに塩化白金酸Ｈ２ＰｔＣｌ４・６Ｈ２Ｏを０．６６８ｇと硝酸ロジウムＲｈ（ＮＯ３）３・２Ｈ２Ｏを０．００４ｇ溶解した水溶液を蒸発皿に入れ、これに上記のメソポーラスシリカ材料５ｇを加え、スチームバスで蒸発乾固した後、真空乾燥機に入れ、１００℃−３時間真空乾燥を行なった。この試料を石英管に入れ、ヘリウム希釈水素ガス（１０ｖ／ｖ％）気流下５００℃で３時間還元し、メソポーラス触媒を合成した。メソポーラスシリカに担持された白金の担持量は約５質量％であり、ロジウムの担持量は約０．１５質量％であった。また、白金−ロジウム粒子の平均粒径は約２．５ｎｍであった。
[Production Example 2] Production of mesoporous catalyst In a 1 liter beaker, 300 g of distilled water, 240 g of ethanol, and 30 g of dodecylamine were added and dissolved. While stirring, 125 g of tetraethoxysilane was added and stirred at room temperature for 22 hours. The product is filtered, washed with water, dried in warm air at 110 ° C. for 5 hours, and then calcined in air at 550 ° C. for 5 hours to decompose and remove contained dodecylamine, and mesoporous silica material having an average particle size of 0.5 μm Got. As a result of small angle X-ray diffraction measurement of the mesoporous silica material, one broad diffraction peak was shown. Further, as a result of observation with a transmission electron microscope, a regular arrangement was not observed in the pore arrangement, and a disordered state was observed. From these results, it was confirmed that the produced mesoporous silica material was amorphous. Moreover, as a result of pore distribution and specific surface area measurement, there was a pore peak at a position of about 2.5 nm, a specific surface area of 1123 m 2 / g, a pore volume of 0.89 cm 3 / g, and a pore of 1 to 50 nm. The volume occupied by was 0.89 cm 3 / g. An aqueous solution of chloroplatinic acid H 2 PtCl 4 · 6H 2 O and 0.668g and rhodium nitrate Rh (NO 3) 3 · 2H 2 O were dissolved 0.004g of distilled water 20g were placed in an evaporating dish, to which the above mesoporous After adding 5 g of a silica material and evaporating to dryness with a steam bath, it was put in a vacuum dryer and vacuum-dried at 100 ° C. for 3 hours. This sample was put in a quartz tube and reduced at 500 ° C. for 3 hours in a helium-diluted hydrogen gas (10 v / v%) stream to synthesize a mesoporous catalyst. The supported amount of platinum supported on mesoporous silica was about 5% by mass, and the supported amount of rhodium was about 0.15% by mass. The average particle size of the platinum-rhodium particles was about 2.5 nm.
「製造例３」メソポーラス触媒と活性炭から成る触媒の製造
製造例２のメソポーラス触媒１０ｇと市販の活性炭（和光純薬株式会社製造 商品名：活性炭素、：液体クロマトグラフィー用のアルカリ処理活性炭、比表面積８３０ｍ２／ｇ）１０ｇを乳鉢に入れ均一に混合した。
[Production Example 3] Production of catalyst comprising mesoporous catalyst and activated carbon 10 g of mesoporous catalyst of Production Example 2 and commercially available activated carbon (manufactured by Wako Pure Chemical Industries, Ltd., trade name: activated carbon ,: alkali-treated activated carbon for liquid chromatography, specific surface area (830 m 2 / g) 10 g was put in a mortar and mixed uniformly.
「製造例４」メソポーラス触媒と活性炭から成る触媒の製造
製造例２のメソポーラス触媒１０ｇと市販の活性炭（和光純薬株式会社製造 商品名：活性炭素、：液体クロマオグラフィー用の酸処理活性炭、比表面積８２０ｍ２／ｇ）１０ｇを乳鉢に入れ均一に混合した。
“Production Example 4” Production of catalyst comprising mesoporous catalyst and activated carbon 10 g of mesoporous catalyst of Production Example 2 and commercially available activated carbon (manufactured by Wako Pure Chemical Industries, Ltd., trade name: activated carbon ,: acid-treated activated carbon for liquid chromatography, ratio 10 g of a surface area of 820 m 2 / g was placed in a mortar and mixed uniformly.
「製造例５」メソポーラス触媒とメソポーラスカーボンから成る触媒の製造
非特許文献４にしたがってメソポーラスカーボンを製造した。すなわち、製造例２で得たメソポーラスシリカ１００ｇにパラトルエンスルホン酸の０．５Ｍエタノール溶液を加え、含浸後、８０℃−１時間乾燥した。これにフルフリルアルコールで飽和した窒素ガスを７日間循環させた後、８０℃で２時間加熱重合させ、窒素気流下で８００℃に加熱し炭化させた。これを、３０％水酸化カリウム水溶液に入れ、メソポーラスシリカを溶解させ、減圧濾過、蒸留水洗浄、８０℃で１時間乾燥して、平均粒径が１μｍのメソポーラスカーボンを２ｇを得た。比表面積及び細孔径を測定した結果、それぞれ、１８００ｍ２／ｇ、及び３．０ｎｍであった。該メソポーラスカーボン２ｇと製造例２のメソポーラス触媒２ｇを乳鉢に入れ均一に混合した。
[Production Example 5] Production of catalyst comprising mesoporous catalyst and mesoporous carbon Mesoporous carbon was produced according to Non-Patent Document 4. That is, 0.5 M ethanol solution of paratoluenesulfonic acid was added to 100 g of the mesoporous silica obtained in Production Example 2, and after impregnation, it was dried at 80 ° C. for 1 hour. Nitrogen gas saturated with furfuryl alcohol was circulated for 7 days, followed by heat polymerization at 80 ° C. for 2 hours, and heating to 800 ° C. under a nitrogen stream to carbonize. This was put in a 30% aqueous potassium hydroxide solution to dissolve mesoporous silica, filtered under reduced pressure, washed with distilled water, and dried at 80 ° C. for 1 hour to obtain 2 g of mesoporous carbon having an average particle diameter of 1 μm. As a result of measuring the specific surface area and the pore diameter, they were 1800 m 2 / g and 3.0 nm, respectively. 2 g of the mesoporous carbon and 2 g of the mesoporous catalyst of Production Example 2 were placed in a mortar and mixed uniformly.
＜リーンバーン模擬ガスの浄化処理＞
「実施例１」、「実施例２」、「実施例３」、「比較例１」、「比較例２」
製造例１の触媒を８０ｍｇ、石英製の流通式反応管に充填した。反応管を外部ヒーターによって１６０℃〜４００℃までの任意の温度に調整した。次に、反応管に、プロピレン濃度の低いリーンバーン模擬ガスとプロピレン濃度の高いリーンバーン模擬ガスを交互に流通し、ＮＯｘ処理を行なった。プロピレン濃度の低いリーンバーン模擬ガスは、ヘリウムで濃度調整した一酸化窒素２５０ｐｐｍ、酸素１０％、プロピレン４００ｐｐｍ含有ガスであり、流量は毎分５００ｍｌとした。プロピレン濃度の高いリーンバーン模擬ガスは、ヘリウムで濃度調整した一酸化窒素２５０ｐｐｍ、酸素１０％、プロピレン２％含有ガスであり、流量は毎分５００ｍｌとした。そして、プロピレン濃度の低いリーンバーン模擬ガスとプロピレン濃度の高いリーンバーン模擬ガスとの供給サイクルは、１０分：１０分とした。排ガスをサンプリングし、プロピレン濃度の低いリーンバーン模擬ガスを流通している時のＮＯｘ浄化率とプロピレン濃度の高いリーンバーン模擬ガスを流通している時のＮＯｘ浄化率を測定した（比較例１）。
また、製造例２、３、４、及び５の触媒についても、充填量を１６０ｍｇとした以外は上記と同様にしてＮＯｘ処理を行ない、ＮＯｘ浄化率を測定した（実施例１〜３）。これらの結果を表１及び表２に示した。
<Purification of lean burn simulated gas>
“Example 1”, “Example 2”, “Example 3”, “Comparative Example 1”, “Comparative Example 2”
80 mg of the catalyst of Production Example 1 was packed in a quartz flow reaction tube. The reaction tube was adjusted to an arbitrary temperature of 160 ° C. to 400 ° C. by an external heater. Next, a lean burn simulated gas with a low propylene concentration and a lean burn simulated gas with a high propylene concentration were alternately circulated through the reaction tube to perform NOx treatment. The lean burn simulated gas having a low propylene concentration is a gas containing 250 ppm of nitric oxide, 10% of oxygen, and 400 ppm of propylene, the concentration of which is adjusted with helium, and the flow rate is 500 ml per minute. The lean burn simulated gas with a high propylene concentration was a gas containing 250 ppm of nitric oxide, 10% oxygen, and 2% propylene, the concentration of which was adjusted with helium, and the flow rate was 500 ml per minute. The supply cycle of the lean burn simulated gas having a low propylene concentration and the lean burn simulated gas having a high propylene concentration was 10 minutes: 10 minutes. The exhaust gas was sampled, and the NOx purification rate when the lean burn simulated gas with a low propylene concentration was distributed and the NOx purification rate when the lean burn simulated gas with a high propylene concentration was distributed were measured (Comparative Example 1). .
In addition, the catalysts of Production Examples 2, 3, 4, and 5 were also subjected to NOx treatment in the same manner as above except that the filling amount was 160 mg, and the NOx purification rate was measured (Examples 1 to 3). These results are shown in Tables 1 and 2.
表１に示すように、還元剤の含有量が少ないリーンバーン排ガスに対しては、本発明触媒は、従来の触媒に比べて、１６０℃〜４００℃において高いＮＯｘ浄化率を与え、特に、１８０℃〜２００℃に渡って８０％以上の浄化率が得られ、２００℃〜３００℃において４０％以上の浄化率が得られる。また、表２に示すように、還元剤の含有量が多いリーンバーン排ガスに対しては、本発明触媒は、従来の触媒があまり活性を示さないような１８０℃においても３０％以上の浄化率を与えた。
以上のことから、本発明浄化用触媒は、リーンバーン排ガスを低温領域から中温領域にわたって効率よく浄化できることがわかる。また、ポストインジェクション方式のようなリーンバーン排ガスに高濃度の還元剤が供給される排ガスに対しても低温から中温領域にかけて効率よく浄化できることがわかる。
As shown in Table 1, for lean burn exhaust gas with a low reducing agent content, the catalyst of the present invention gives a higher NOx purification rate at 160 ° C. to 400 ° C. than the conventional catalyst. A purification rate of 80% or more can be obtained over a temperature range of 200 ° C to 200 ° C, and a purification rate of 40% or more can be obtained at 200 ° C to 300 ° C. Moreover, as shown in Table 2, for lean burn exhaust gas with a high content of reducing agent, the catalyst of the present invention has a purification rate of 30% or more even at 180 ° C., where the conventional catalyst does not show much activity. Gave.
From the above, it can be seen that the purification catalyst of the present invention can efficiently purify lean burn exhaust gas from a low temperature region to a medium temperature region. Moreover, it turns out that it can purify | clean efficiently from low temperature to an intermediate temperature area | region also with respect to the exhaust gas by which a high concentration reducing agent is supplied to lean burn exhaust gas like a post injection system.
本発明浄化用触媒は、ディーゼル自動車排ガス浄化用触媒として有用である。 The purification catalyst of the present invention is useful as a diesel automobile exhaust gas purification catalyst.
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