JP4923412B2 - Exhaust gas purification catalyst - Google Patents

Exhaust gas purification catalyst Download PDF

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JP4923412B2
JP4923412B2 JP2005049740A JP2005049740A JP4923412B2 JP 4923412 B2 JP4923412 B2 JP 4923412B2 JP 2005049740 A JP2005049740 A JP 2005049740A JP 2005049740 A JP2005049740 A JP 2005049740A JP 4923412 B2 JP4923412 B2 JP 4923412B2
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義輝 矢澤
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トヨタ自動車株式会社
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本発明は、自動車等の内燃機関から排出される排ガスを浄化する排ガス浄化用触媒に関するものであり、詳しくは、窒素酸化物(NOx)を効果的に浄化する排ガス浄化触媒に関する。   The present invention relates to an exhaust gas purifying catalyst that purifies exhaust gas discharged from an internal combustion engine such as an automobile, and more particularly to an exhaust gas purifying catalyst that effectively purifies nitrogen oxides (NOx).

自動車分野において、リーンバーンエンジンの使用の拡大が図られる中、リーン領域でNOxの排出を低減できるNOx吸蔵還元触媒が実用化されている。NOx吸蔵還元触媒は、アルミナ等の担体に貴金属およびアルカリ金属元素やアルカリ土類金属元素(NOx吸蔵材)を担持させたものである。リーン状態では、貴金属上でNOxを酸化し、隣接するNOx吸蔵材と結合して硝酸塩を形成することにより、NOxを吸蔵する。一方、ストイキ〜リッチ状態では、硝酸塩が分解されて気相に放出されたNOxを貴金属上で還元性ガスと反応させて、または、放出される前に硝酸塩を分解して、窒素に還元する。NOxの吸蔵と還元とを繰り返すことにより、NOxが浄化される。   While the use of lean burn engines is being expanded in the automotive field, NOx storage reduction catalysts that can reduce NOx emissions in the lean region have been put into practical use. The NOx occlusion reduction catalyst is obtained by supporting a noble metal, an alkali metal element, or an alkaline earth metal element (NOx occlusion material) on a support such as alumina. In the lean state, NOx is oxidized on the noble metal and combined with the adjacent NOx storage material to form nitrate, thereby storing NOx. On the other hand, in the stoichiometric to rich state, the NOx decomposed and released into the gas phase is reacted with a reducing gas on the noble metal, or the nitrate is decomposed and reduced to nitrogen before being released. By repeating NOx occlusion and reduction, NOx is purified.

近年、さらなるエミッション規制に伴い、還元時(ストイキ〜リッチ状態で)のNOx排出量の低減が求められている。ところが、NOx吸蔵材に吸蔵されたNOxを還元する際には、NOx吸蔵材から気相に放出されたNOxが全て浄化しきれずに一部排出されるという問題がある。そこで、特許文献1には、NOx吸蔵還元触媒の下流側に三元触媒を配置して、NOx吸蔵還元触媒から排出されたNOxを還元する排気ガス浄化装置が開示されている。   In recent years, with further emission regulations, reduction of NOx emission during reduction (in a stoichiometric to rich state) has been demanded. However, when NOx occluded in the NOx occlusion material is reduced, there is a problem in that all of the NOx released from the NOx occlusion material into the gas phase is not completely purified but is partially discharged. Therefore, Patent Document 1 discloses an exhaust gas purification device that arranges a three-way catalyst on the downstream side of the NOx storage reduction catalyst to reduce NOx discharged from the NOx storage reduction catalyst.

また、Ptは、NOx吸蔵還元反応に有効な貴金属であるが、リーン雰囲気での耐熱性に劣る。そのため、リーン雰囲気で使用されると、Ptがシンタリングしやすいという問題がある。さらに、貴金属と併用されるNOx吸蔵材は塩基性物質であるため、Pt等の貴金属の活性を低下させるという問題がある。
特開平10−80620号公報
Pt is a noble metal effective for NOx occlusion reduction reaction, but is inferior in heat resistance in a lean atmosphere. Therefore, when used in a lean atmosphere, there is a problem that Pt is easily sintered. Furthermore, since the NOx occlusion material used in combination with the noble metal is a basic substance, there is a problem that the activity of the noble metal such as Pt is lowered.
Japanese Patent Laid-Open No. 10-80620

本発明は、上記問題点に鑑み、新規な構成により、ストイキ〜リッチ雰囲気においてNOx吸蔵材から気相に放出されるNOxの排出を効果的に抑制することができる排ガス浄化触媒を提供することを目的とする。   In view of the above problems, the present invention provides an exhaust gas purification catalyst that can effectively suppress the emission of NOx released from the NOx storage material into the gas phase in a stoichiometric to rich atmosphere with a novel configuration. Objective.

本発明の排ガス浄化触媒は、担体基材と、該担体基材に形成された触媒担持層と、該触媒担持層に担持されたPt、Pd、RhおよびNOx吸蔵材と、を有する排ガス浄化触媒であって、
排ガス流の上流側から下流側にかけて配置された上流部触媒と下流部触媒とを備え、
前記上流部触媒はPt、PdおよびNOx吸蔵材を含み、
前記下流部触媒はPtおよびPdを含まずRhおよびNOx吸蔵材を含み、Rhは前記上流部触媒よりも該下流部触媒に高濃度担持されていることを特徴とする。
The exhaust gas purifying catalyst of the present invention comprises a support base material, a catalyst support layer formed on the support base material, and a Pt, Pd, Rh, and NOx storage material supported on the catalyst support layer. Because
An upstream catalyst and a downstream catalyst arranged from the upstream side to the downstream side of the exhaust gas flow,
The upstream catalyst includes Pt, Pd, and NOx storage material,
Said downstream portion catalyst comprises a Rh and NOx-absorbing material does not contain Pt and Pd, Rh is characterized in that it is a high concentration carried to the downstream portion catalyst than the upstream portion catalyst.

これ以下、「上流部触媒」を単に「上流部」、「下流部触媒」を単に「下流部」、と略記することもある。  Hereinafter, the “upstream catalyst” may be simply abbreviated as “upstream part”, and the “downstream catalyst” may be simply abbreviated as “downstream part”.

なお、「高濃度担持」とは、担体基材の上流部と下流部とを比較した場合に、一方の担持濃度が他方の担持濃度よりも高いこと、さらには、ある成分の担持濃度が他の成分の担持濃度よりも高いこと、を示しており、担持濃度がある一定濃度以上であることを示す言葉ではない。   Note that “high concentration loading” means that when the upstream portion and the downstream portion of the carrier substrate are compared, the loading concentration of one component is higher than the loading concentration of the other, and the loading concentration of a certain component is the other. This is not a word indicating that the supported concentration is higher than a certain concentration.

本発明の排ガス浄化触媒によれば、Rhを担体基材の下流部で高濃度担持することにより、ストイキあるいはリッチ雰囲気で気相に放出されるNOxの還元活性が向上する。そのため、気相に放出されるNOxの排出を効果的に抑制することができる。   According to the exhaust gas purifying catalyst of the present invention, the reducing activity of NOx released into the gas phase in a stoichiometric or rich atmosphere is improved by supporting Rh at a high concentration in the downstream portion of the carrier substrate. Therefore, it is possible to effectively suppress the discharge of NOx released into the gas phase.

さらに、PtおよびPdを担体基材の上流部で高濃度担持することにより、リーン雰囲気でのPtの耐熱性が向上する。その結果、NOxの吸蔵量およびNOxの浄化性能が向上する。   Further, by supporting Pt and Pd at a high concentration in the upstream portion of the carrier base material, the heat resistance of Pt in a lean atmosphere is improved. As a result, the storage amount of NOx and the purification performance of NOx are improved.

本発明の排ガス浄化触媒は、担体基材と、担体基材に形成された触媒担持層と、その触媒担持層に担持されたPt、Pd、RhおよびNOx吸蔵材と、を有する。すなわち、本発明の排ガス浄化触媒は、リーン雰囲気時にNOxを吸蔵して、ストイキあるいはリッチ雰囲気時に吸蔵したNOxを還元するタイプの触媒である。   The exhaust gas purifying catalyst of the present invention has a support base, a catalyst support layer formed on the support base, and a Pt, Pd, Rh, and NOx occlusion material supported on the catalyst support layer. That is, the exhaust gas purifying catalyst of the present invention is a type of catalyst that stores NOx in a lean atmosphere and reduces NOx stored in a stoichiometric or rich atmosphere.

担体基材には、従来から用いられているセラミックス製や金属製のハニカム状の構造体などを用いればよい。担体基材に触媒担持層を形成するには、たとえば、ハニカム状の構造体に、多孔質酸化物を含むスラリーをウォッシュコートし、それを焼成して触媒担持層を形成し、さらに、その触媒担持層に吸着担持法あるいは吸水担持法でPt、Pd、RhおよびNOx吸蔵材を担持すればよい。多孔質酸化物としては、アルミナ、シリカ、チタニア、セリア、ジルコニア、あるいは、これらの複数種からなるセリア−ジルコニア固溶体などの複合酸化物が使用可能であり、これらのうち一種または複数種を用いることができる。もちろん、多孔質酸化物に予めPt、Pd、RhやNOx吸蔵材の一部または全部を担持させて、触媒担持層を形成してもよい。   For the carrier substrate, a conventionally used ceramic or metal honeycomb-like structure may be used. In order to form the catalyst support layer on the carrier substrate, for example, a honeycomb-shaped structure is washed with a slurry containing a porous oxide and fired to form a catalyst support layer. The Pt, Pd, Rh, and NOx storage material may be supported on the support layer by an adsorption support method or a water absorption support method. As the porous oxide, alumina, silica, titania, ceria, zirconia, or a composite oxide such as ceria-zirconia solid solution composed of a plurality of these can be used, and one or more of these can be used. Can do. Of course, part or all of Pt, Pd, Rh and NOx occlusion material may be supported on the porous oxide in advance to form the catalyst supporting layer.

この際、触媒層のコート量は、性能上、150〜270g/L(担体基材1L当たり:以下同様)であるのが好ましい。   At this time, the coating amount of the catalyst layer is preferably 150 to 270 g / L (per 1 L of the carrier substrate: the same applies hereinafter) in view of performance.

NOx吸蔵材は、アルカリ金属元素およびアルカリ土類金属元素から選ばれる少なくとも一種を用いることができる。具体的には、リチウム、ナトリウム、カリウム、ルビジウム、セシウム、フランシウム、バリウム、ベリリウム、マグネシウム、カルシウム、ストロンチウム、が挙げられる。NOx吸蔵材を触媒担持層に担持する際には、アルカリ金属塩、アルカリ土類金属塩の水溶液が利用できる。   As the NOx storage material, at least one selected from alkali metal elements and alkaline earth metal elements can be used. Specific examples include lithium, sodium, potassium, rubidium, cesium, francium, barium, beryllium, magnesium, calcium, and strontium. When the NOx storage material is supported on the catalyst support layer, an aqueous solution of an alkali metal salt or an alkaline earth metal salt can be used.

この際、NOx吸蔵材の担持量は、性能上、0.05〜1.6mol/L(担体基材1L当たり:以下同様)であるのが好ましく、さらに好ましくは、0.05〜1.5mol/Lである。   At this time, the supported amount of the NOx occlusion material is preferably 0.05 to 1.6 mol / L (per carrier substrate 1L: the same applies hereinafter), more preferably 0.05 to 1.5 mol in terms of performance. / L.

ところで、既に説明したが、リーン雰囲気時にNOx吸蔵材に吸蔵されたNOxは、ストイキ〜リッチ雰囲気時に、(1)硝酸塩が分解されて気相に放出されたNOxを貴金属上で還元性ガスと反応させて窒素に還元する、または、(2)気相に放出される前に硝酸塩を分解して窒素に還元する、という2つの方法によりNOxの浄化を行っている。そこで、本発明者は、(1)(2)の各方法に対して有効な貴金属(Pt、Pd、Rh)の担持構成を見出した。また、NOx吸蔵材に吸蔵されたNOxを還元する際に還元されなかったNOxが排出されるのは、NOx吸蔵還元触媒の貴金属の還元活性が不足するためであることに着目した。   By the way, as already explained, the NOx occluded in the NOx occlusion material in the lean atmosphere is, in a stoichiometric to rich atmosphere, (1) the NOx decomposed to the gas phase reacts with the reducing gas on the noble metal. NOx purification is performed by two methods of reducing to nitrogen, or (2) decomposing nitrate and reducing to nitrogen before being released into the gas phase. Therefore, the present inventor has found a supporting structure of noble metals (Pt, Pd, Rh) effective for the methods (1) and (2). Further, attention was paid to the fact that NOx that was not reduced when NOx occluded in the NOx occlusion material was reduced was due to the lack of noble metal reduction activity of the NOx occlusion reduction catalyst.

図3は、貴金属(Rh、Pd、Pt)がアルミナ(MI200)に担持されたペレットのNOx還元活性を示すグラフであって、縦軸のTOF[1/s]は、表面原子1個当たりの反応速度、すなわち、表面に露出した貴金属原子1個当たりの実質的な活性を評価している。各ペレットは、電気炉で750℃5時間耐久後に評価を行った。図3によれば、RhのNOx還元活性は、PdやPtに比べ、著しく高い値を示している。   FIG. 3 is a graph showing the NOx reduction activity of pellets in which noble metals (Rh, Pd, Pt) are supported on alumina (MI200), and the vertical axis TOF [1 / s] is the value per one surface atom. The reaction rate, ie the substantial activity per noble metal atom exposed on the surface, is evaluated. Each pellet was evaluated after durability at 750 ° C. for 5 hours in an electric furnace. According to FIG. 3, the NOx reduction activity of Rh shows a remarkably high value compared with Pd and Pt.

そこで、本発明の排ガス浄化触媒では、Rhが、排ガス流の下流側に配置される担体基材の下流部に高濃度担持されている構成とした。担体基材の下流部でRhを高濃度担持することにより、上流部のNOx吸蔵材から気相に放出されるが上流部では還元しきれずに下流側へ排出されるNOxを、下流部で還元することができる。下流部は、NOx還元活性の高いRhを上流部よりも高濃度で担持しているため、上流部のNOx吸蔵材から気相に放出されたNOxを効果的に還元することができる。したがって、NOxの還元時に排ガス浄化触媒から排出されるNOxの量が低減される。   Therefore, the exhaust gas purifying catalyst of the present invention has a configuration in which Rh is supported at a high concentration in the downstream portion of the carrier substrate disposed on the downstream side of the exhaust gas flow. By supporting a high concentration of Rh in the downstream part of the carrier base material, NOx released from the upstream NOx storage material into the gas phase but not fully reduced in the upstream part is reduced in the downstream part. can do. Since the downstream portion carries Rh having a higher NOx reduction activity at a higher concentration than the upstream portion, NOx released from the upstream portion of the NOx storage material into the gas phase can be effectively reduced. Therefore, the amount of NOx discharged from the exhaust gas purification catalyst when NOx is reduced is reduced.

Rhは、担体基材の上流部よりも下流部で高濃度となるように担持されていればよく、また、下流部において他の貴金属成分(Pt、Pd等)よりも高濃度で担持されていればよい。したがって、RhとPtやPdとを併用してもよいが、貴金属成分として、下流部にRhのみが担持されている(PtやPdを併用しない)状態が、NOx還元活性を高める上で、特に好ましい。   Rh is only required to be supported at a higher concentration in the downstream portion than the upstream portion of the carrier substrate, and is supported at a higher concentration than other noble metal components (Pt, Pd, etc.) in the downstream portion. Just do it. Therefore, although Rh may be used in combination with Pt or Pd, a state in which only Rh is supported in the downstream portion as a noble metal component (without using Pt or Pd in combination) is particularly effective for enhancing NOx reduction activity. preferable.

この際、PtおよびPdは、排ガス流の上流側に配置される担体基材の上流部で高濃度担持されているのが好ましい。PtとPdとを担体基材の上流側で併用することにより、Ptのリーン雰囲気での耐熱性が向上する。その結果、触媒作製時の加熱工程(乾燥や焼成)やリーン雰囲気での使用時に発生するPtのシンタリングが抑制され、NOx吸蔵性能およびNOx浄化性能が共に向上する。上流部のNOx吸蔵性能が上がれば、ストイキ〜リーン雰囲気時に気相に放出されるNOx量も多くなるが、本発明の排ガス浄化触媒では、下流部のNOx還元活性が高い(前述)。そのため、気相に放出されて上流部から下流部へ排出されたNOxは下流部で良好に浄化されるため、好ましい。   At this time, it is preferable that Pt and Pd are supported at a high concentration in the upstream portion of the carrier base disposed upstream of the exhaust gas flow. By using Pt and Pd in combination on the upstream side of the carrier substrate, the heat resistance of Pt in a lean atmosphere is improved. As a result, sintering of Pt generated during the heating process (drying and firing) during catalyst preparation and use in a lean atmosphere is suppressed, and both NOx occlusion performance and NOx purification performance are improved. If the upstream NOx occlusion performance is improved, the amount of NOx released to the gas phase in the stoichiometric to lean atmosphere increases, but the exhaust gas purifying catalyst of the present invention has high downstream NOx reduction activity (as described above). Therefore, NOx discharged into the gas phase and discharged from the upstream portion to the downstream portion is preferable because it is well purified in the downstream portion.

なお、図4は、貴金属(Pd、Pt)がアルミナ(MI200)に担持されたペレットのNOx吸蔵量を示すグラフである。各ペレットは、PtとPdの担持量が、それぞれ、Pt/Pd=2/0、0/5、1/2(重量比)となるように調製され、各ペレットを電気炉で750℃5時間耐久後のNOx吸蔵量を測定したものである。図4によれば、NOx吸蔵量は、PdとPtが併用された場合に高くなる。したがって、PtとPdを高濃度担持した上流部は、NOxの吸蔵性能に優れる。   FIG. 4 is a graph showing the NOx occlusion amount of pellets in which noble metals (Pd, Pt) are supported on alumina (MI200). Each pellet was prepared so that the supported amounts of Pt and Pd were Pt / Pd = 2/0, 0/5, and 1/2 (weight ratio), respectively, and each pellet was 750 ° C. for 5 hours in an electric furnace. The NOx occlusion amount after endurance is measured. According to FIG. 4, the NOx occlusion amount increases when Pd and Pt are used in combination. Therefore, the upstream portion carrying high concentrations of Pt and Pd is excellent in NOx storage performance.

PtおよびPdは、担体基材の下流部よりも上流部で高濃度となるように担持されていればよく、上流部において他の貴金属成分(Rh等)よりも高濃度で担持されていればよい。したがって、PtおよびPdとRhとを併用してもよい。   Pt and Pd are only required to be supported at a higher concentration in the upstream portion than the downstream portion of the carrier substrate, and are supported at a higher concentration than other noble metal components (Rh and the like) in the upstream portion. Good. Therefore, Pt and Pd may be used in combination with Rh.

そして、Pt、PdおよびRhのそれぞれの担持量は、性能上、0.1〜5g/Lであるのが好ましい。また、Pt、PdおよびRhを触媒担持層に担持する場合は、各種貴金属塩を使用する従来の方法を用いて担持すればよい。   And each load of Pt, Pd and Rh is preferably 0.1 to 5 g / L in view of performance. Further, when Pt, Pd, and Rh are supported on the catalyst support layer, they may be supported using conventional methods using various noble metal salts.

また、NOx吸蔵材は、担体基材の上流部で高濃度担持されているのが好ましい。NOx吸蔵材を、下流部よりも上流部に高濃度担持することにより、上流部でNOxの吸蔵を主体的に行わせ、上流部から放出されるNOxを下流部で効率よく浄化することができる。   The NOx occlusion material is preferably supported at a high concentration in the upstream portion of the carrier base material. By carrying the NOx occlusion material at a higher concentration in the upstream portion than in the downstream portion, NOx occlusion can be performed mainly in the upstream portion, and NOx released from the upstream portion can be efficiently purified in the downstream portion. .

また、担体基材の上流部の容量は、担体基材の下流部の容量以上であるのが好ましい。上流部の容量が大きいと、上流部でNOxを十分に吸蔵することができるので、効率よくNOxを浄化することができる。たとえば、上流部の容量:下流部の容量=1:1〜1.5:0.5であれば、NOxの排出を効果的に抑制することができる。   Moreover, it is preferable that the capacity | capacitance of the upstream part of a support base material is more than the capacity | capacitance of the downstream part of a support base material. If the capacity of the upstream portion is large, NOx can be sufficiently occluded in the upstream portion, so that NOx can be efficiently purified. For example, if the upstream capacity: downstream capacity = 1: 1 to 1.5: 0.5, NOx emissions can be effectively suppressed.

なお、本発明の排ガス浄化触媒は、上記の実施の形態に限定されるものではない。たとえば、本発明の排ガス浄化触媒の効果を損なわない程度であれば、他の機能を追加するために必要に応じて別の物質を添加してもよい。   The exhaust gas purifying catalyst of the present invention is not limited to the above embodiment. For example, as long as the effect of the exhaust gas purification catalyst of the present invention is not impaired, another substance may be added as necessary to add other functions.

以下、本発明の排ガス浄化触媒の実施例を比較例とともに具体的に説明する。   Examples of the exhaust gas purifying catalyst of the present invention will be specifically described below together with comparative examples.

[実施例1]
担体基材として、セル密度400cpsi、壁厚150μm、直径129mm、長さ75mmのコージェライト製ハニカム担体基材(1リットル)を準備した。
[Example 1]
A cordierite honeycomb carrier substrate (1 liter) having a cell density of 400 cpsi, a wall thickness of 150 μm, a diameter of 129 mm, and a length of 75 mm was prepared as a carrier substrate.

アルミナと、チタニア−ジルコニアと、ジルコニアと、OSC材と、を所定量の純水と混合し、ミリングしてスラリーS1 を調製した。また、アルミナと、チタニア−ジルコニアと、Rhを担持したジルコニアと、OSC材と、を所定量の純水と混合し、ミリングしてスラリーS2 を調製した。 Alumina, titania - and zirconia, mixed with zirconia, and the OSC material, and pure water a predetermined amount, the slurry S 1 was prepared by milling. Moreover, alumina, titania - mixed with zirconia, and zirconia carrying Rh, and the OSC material, and pure water a predetermined amount, was milled to prepare a slurry S 2.

次に、担体基材にスラリーS1 をウォッシュコートし、250℃で乾燥後500℃で1時間焼成し、触媒担持層を形成した。なお、触媒担持層のコート量は、270g/L(アルミナ100g/L、チタニア−ジルコニア100g/L、ジルコニア50g/L、OSC材20g/L)であった。そして、Ptが2g/L、Pdが4g/Lの担持量となるように調製したジニトロジアンミン白金溶液と硝酸パラジウム水溶液に触媒担持層が形成された担体基材を浸漬して、PtおよびPdを吸着担持させ、水溶液から引き上げて、250℃で乾燥後500℃で1時間焼成し、Pt、Pdを担持した。さらに、Baが0.3mol/L、Kが0.225mol/L、Liが0.15mol/Lの担持量となるように調製した硝酸バリウム水溶液、硝酸カリウム水溶液および硝酸リチウム水溶液に担体基材を浸漬して、Ba、KおよびLiを吸着担持させ、水溶液から引き上げて、マイクロウェーブ乾燥を経て、250℃で乾燥後500℃で1時間焼成し、NOx吸蔵材(担持量0.675mol/L)を担持した。得られた排ガス浄化触媒を、上流部触媒とする。 Then, the slurry S 1 was wash-coated on a carrier base, and baked for 1 hour after drying 500 ° C. at 250 ° C., to form a catalyst supporting layer. The coating amount of the catalyst support layer was 270 g / L (alumina 100 g / L, titania-zirconia 100 g / L, zirconia 50 g / L, OSC material 20 g / L). Then, a support base material having a catalyst support layer formed in a dinitrodiammine platinum solution and a palladium nitrate aqueous solution prepared so as to have a support amount of Pt of 2 g / L and Pd of 4 g / L is immersed in Pt and Pd. It was adsorbed and supported, pulled up from the aqueous solution, dried at 250 ° C., and calcined at 500 ° C. for 1 hour to carry Pt and Pd. Further, the carrier substrate is immersed in an aqueous solution of barium nitrate, potassium nitrate and lithium nitrate prepared so that the supported amount is Ba 0.3 mol / L, K 0.225 mol / L and Li 0.15 mol / L. Then, Ba, K and Li are adsorbed and supported, pulled up from the aqueous solution, subjected to microwave drying, dried at 250 ° C., and then baked at 500 ° C. for 1 hour, and the NOx occlusion material (supported amount 0.675 mol / L) Supported. Let the obtained exhaust gas purification catalyst be an upstream catalyst.

また、別の担体基材にスラリーS2 をウォッシュコートし、250℃で乾燥後500℃で1時間焼成し、触媒担持層を形成した。なお、触媒担持層のコート量は、270g/L(アルミナ100g/L、チタニア−ジルコニア100g/L、Rhを担持したジルコニア50g/L、OSC材20g/L)、Rhの担持量は、1g/Lであった。次に、Baが0.1mol/L、Kが0.0.075mol/L、Liが0.05mol/Lの担持量となるように調製した硝酸バリウム水溶液、硝酸カリウム水溶液および硝酸リチウム水溶液に担体基材を浸漬して、Ba、KおよびLiを吸着担持させ、水溶液から引き上げて、マイクロウェーブ乾燥を経て、250℃で乾燥後500℃で1時間焼成し、NOx吸蔵材(担持量0.225mol/L)を担持した。得られた排ガス浄化触媒を、下流部触媒とする。 Further, the slurry S 2 was wash-coated on another carrier substrate, dried at 250 ° C. and then fired at 500 ° C. for 1 hour to form a catalyst support layer. The coating amount of the catalyst supporting layer is 270 g / L (alumina 100 g / L, titania-zirconia 100 g / L, zirconia 50 g / L supporting Rh, OSC material 20 g / L), and the supporting amount of Rh is 1 g / L. L. Next, a carrier group was added to a barium nitrate aqueous solution, a potassium nitrate aqueous solution, and a lithium nitrate aqueous solution prepared so as to have a supported amount of Ba of 0.1 mol / L, K of 0.00.075 mol / L, and Li of 0.05 mol / L. The material is immersed to adsorb and support Ba, K, and Li, pulled up from the aqueous solution, subjected to microwave drying, dried at 250 ° C., and calcined at 500 ° C. for 1 hour. L) was supported. Let the obtained exhaust gas purification catalyst be a downstream catalyst.

そして、排ガス流の上流側から下流側にかけて、上流部触媒、下流部触媒、となるように2〜3mmの間隔を開けて順に並べたものを実施例1の排ガス浄化触媒とした。   Then, the exhaust gas purification catalyst of Example 1 was arranged in order from the upstream side to the downstream side of the exhaust gas flow with an interval of 2 to 3 mm so as to become an upstream catalyst and a downstream catalyst.

[比較例1]
担体基材として、セル密度400cpsi、壁厚150μm、直径129mm、長さ150mmのコージェライト製ハニカム担体基材(2リットル)を準備した。
[Comparative Example 1]
A cordierite honeycomb carrier substrate (2 liters) having a cell density of 400 cpsi, a wall thickness of 150 μm, a diameter of 129 mm, and a length of 150 mm was prepared as a carrier substrate.

担体基材にスラリーS2 をウォッシュコートし、250℃で乾燥後500℃で1時間焼成し、触媒担持層(コート量270g/L)を形成した。この際、Rhの担持量は、0.5g/Lであった。 Slurry S 2 was wash-coated on the support substrate, dried at 250 ° C. and then calcined at 500 ° C. for 1 hour to form a catalyst supporting layer (coat amount 270 g / L). At this time, the loading amount of Rh was 0.5 g / L.

そして、Ptが2g/Lの担持量となるように調製したジニトロジアンミン白金溶液に触媒担持層が形成された担体基材を浸漬して、Ptを吸着担持させ、水溶液から引き上げて、250℃で乾燥後500℃で1時間焼成し、Ptを担持した。さらに、Baが0.2mol/L、Kが0.15mol/L、Liが0.1mol/Lの担持量となるように調製した硝酸バリウム水溶液、硝酸カリウム水溶液および硝酸リチウム水溶液に担体基材を浸漬して、Ba、KおよびLiを吸着担持させ、水溶液から引き上げて、マイクロウェーブ乾燥を経て、250℃で乾燥後500℃で1時間焼成し、NOx吸蔵材(担持量0.45mol/L)を担持した。   Then, the support base material on which the catalyst support layer is formed is immersed in a dinitrodiammine platinum solution prepared so that the support amount of Pt is 2 g / L, and Pt is adsorbed and supported, and is lifted from the aqueous solution at 250 ° C. After drying, it was calcined at 500 ° C. for 1 hour to carry Pt. Further, the carrier substrate is immersed in an aqueous barium nitrate solution, an aqueous potassium nitrate solution and an aqueous lithium nitrate solution prepared to have a supported amount of Ba of 0.2 mol / L, K of 0.15 mol / L, and Li of 0.1 mol / L. Then, Ba, K, and Li are adsorbed and supported, pulled up from the aqueous solution, subjected to microwave drying, dried at 250 ° C. and then fired at 500 ° C. for 1 hour, and the NOx occlusion material (supported amount 0.45 mol / L) Supported.

[比較例2]
上流側触媒の貴金属担持量をPt:4g/L、Pd:0g/Lに変更した他は、実施例1と同様にして、比較例2の排ガス浄化触媒を得た。
[Comparative Example 2]
An exhaust gas purification catalyst of Comparative Example 2 was obtained in the same manner as in Example 1 except that the amount of noble metal supported on the upstream catalyst was changed to Pt: 4 g / L and Pd: 0 g / L.

実施例1、比較例1および比較例2の排ガス浄化触媒の構成を表1に示す。   Table 1 shows the configurations of the exhaust gas purification catalysts of Example 1, Comparative Example 1 and Comparative Example 2.

[評価]
実施例1、比較例1および比較例2の排ガス浄化触媒について、吸蔵還元反応の実機評価を行った。試験結果を図1および図2に示す。なお、図1は、各排ガス浄化触媒のNOx吸蔵プロファイルを示すグラフであって、経過時間(試験時間)に対する各触媒から排出される排出ガスに含まれるNOxの濃度を示す。また、図2は、各排ガス浄化触媒のNOx浄化性能を示すグラフである。
[Evaluation]
The exhaust gas purification catalysts of Example 1, Comparative Example 1 and Comparative Example 2 were subjected to actual evaluation of the occlusion reduction reaction. The test results are shown in FIG. 1 and FIG. FIG. 1 is a graph showing the NOx occlusion profile of each exhaust gas purification catalyst, and shows the concentration of NOx contained in the exhaust gas discharged from each catalyst with respect to the elapsed time (test time). FIG. 2 is a graph showing the NOx purification performance of each exhaust gas purification catalyst.

各評価試験は、実車80,000km相当の台上耐久後の排ガス浄化触媒に対して行った。耐久後の評価には、2,000ccのエンジン(1AZ−FSE)を用い、回転数;1600〜2400rpm、トルク;4.7〜8.2kgmの条件のもとで評価を行った。また、NOx吸蔵プロファイル(図1)では、触媒温度を300〜500℃とした。   Each evaluation test was performed on the exhaust gas purification catalyst after the on-board durability corresponding to an actual vehicle of 80,000 km. For the evaluation after endurance, a 2,000 cc engine (1AZ-FSE) was used, and the evaluation was performed under the conditions of the rotational speed: 1600 to 2400 rpm and the torque: 4.7 to 8.2 kgm. In the NOx occlusion profile (FIG. 1), the catalyst temperature was 300 to 500 ° C.

図2によれば、実施例1および比較例2では、NOx還元活性の高いRhを担体基材の下流部で高濃度となるように担持しているので、上流部で浄化しきれなかったNOxを下流部で効率よく還元できるため、NOx浄化率が高い。また、実施例1の排ガス浄化触媒は、PtおよびPdが上流部に担持されているため、Ptのシンタリングが抑制される。その結果、NOxの吸蔵量が増大し、効率よくNOxを浄化することができる。   According to FIG. 2, in Example 1 and Comparative Example 2, since Rh having a high NOx reduction activity is supported at a high concentration in the downstream portion of the carrier substrate, NOx that could not be purified in the upstream portion. Can be efficiently reduced in the downstream portion, so the NOx purification rate is high. Further, in the exhaust gas purifying catalyst of Example 1, Pt and Pd are supported on the upstream portion, so that sintering of Pt is suppressed. As a result, the storage amount of NOx increases and NOx can be purified efficiently.

そして、図1によれば、実施例1の排ガス浄化触媒は、リーン雰囲気でのNOx吸蔵性能が高いため、排出されるNOx濃度は低減される。リーン雰囲気で吸蔵された多くのNOxは、リッチスパイク時に気相へ放出されるが、多くのNOxが放出されても、Rhが高濃度で担持された下流部で効率よく還元されるため、リッチスパイク時のNOxの排出は抑制される。また、実施例1の排ガス浄化触媒では、試験開始からの積算量としてのNOx排出量も低減されている。   And according to FIG. 1, since the exhaust gas purification catalyst of Example 1 has high NOx occlusion performance in a lean atmosphere, the concentration of exhausted NOx is reduced. A lot of NOx occluded in the lean atmosphere is released to the gas phase during the rich spike, but even if a lot of NOx is released, Rh is efficiently reduced in the downstream portion where the high concentration is supported. NOx emissions during spikes are suppressed. Further, in the exhaust gas purification catalyst of Example 1, the NOx emission amount as the integrated amount from the start of the test is also reduced.

実施例1、比較例1および比較例2の排ガス浄化触媒のNOx吸蔵プロファイルを示すグラフであって、経過時間に対する排出ガスのNOx濃度を示す。It is a graph which shows the NOx occlusion profile of the exhaust gas purification catalyst of Example 1, Comparative Example 1, and Comparative Example 2, Comprising: NOx density | concentration of exhaust gas with respect to elapsed time is shown. 実施例1、比較例1および比較例2の排ガス浄化触媒のNOx浄化性能を示すグラフである。3 is a graph showing NOx purification performance of exhaust gas purification catalysts of Example 1, Comparative Example 1 and Comparative Example 2. 貴金属(Rh、Pd、Pt)のNOx還元活性を示すグラフである。It is a graph which shows NOx reduction activity of a noble metal (Rh, Pd, Pt). 貴金属(Pd、Pt)のNOx吸蔵量を示すグラフである。It is a graph which shows the NOx occlusion amount of a noble metal (Pd, Pt).

Claims (4)

担体基材と、該担体基材に形成された触媒担持層と、該触媒担持層に担持されたPt、Pd、RhおよびNOx吸蔵材と、を有する排ガス浄化触媒であって、
排ガス流の上流側から下流側にかけて配置された上流部触媒と下流部触媒とを備え、
前記上流部触媒はPt、PdおよびNOx吸蔵材を含み、
前記下流部触媒はPtおよびPdを含まずRhおよびNOx吸蔵材を含み、Rhは前記上流部触媒よりも該下流部触媒に高濃度担持されていることを特徴とする排ガス浄化触媒。
An exhaust gas purification catalyst having a support substrate, a catalyst support layer formed on the support substrate, and a Pt, Pd, Rh, and NOx storage material supported on the catalyst support layer,
An upstream catalyst and a downstream catalyst arranged from the upstream side to the downstream side of the exhaust gas flow,
The upstream catalyst includes Pt, Pd, and NOx storage material,
Said downstream portion catalyst comprises a Rh and NOx-absorbing material does not contain Pt and Pd, Rh exhaust gas purifying catalyst, characterized in that it is a high concentration carried to the downstream portion catalyst than the upstream portion catalyst.
前記NOx吸蔵材は、アルカリ金属元素およびアルカリ土類金属元素から選ばれる少なくとも一種を含む請求項1記載の排ガス浄化触媒。 The exhaust gas purifying catalyst according to claim 1 , wherein the NOx storage material contains at least one selected from an alkali metal element and an alkaline earth metal element . 前記NOx吸蔵材は、前記下流部触媒よりも記上流部触媒に高濃度担持されている請求項1記載の排ガス浄化触媒。 The NOx storage material of claim 1, wherein the exhaust gas purifying catalyst that is heavily supported before SL on the upstream portion catalyst than the downstream portion catalyst. 前記担体基材の上流部触媒の容量は、前記下流部触媒の容量以上である請求項1記載の排ガス浄化触媒。 Capacity of the upstream portion catalyst of the carrier substrate, according to claim 1, wherein the exhaust gas purifying catalyst before SL is greater than or equal to the capacity of the lower stream portion catalyst.
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