JP2005296759A - Catalyst for exhaust gas cleaning - Google Patents
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本発明は、自動車エンジン等の内燃機関から排出される排ガスを浄化するための、高温耐久性に優れた排ガス浄化用触媒に関する。 The present invention relates to an exhaust gas purifying catalyst excellent in high temperature durability for purifying exhaust gas discharged from an internal combustion engine such as an automobile engine.
従来より、自動車の排ガス浄化用触媒として、排ガス中のＣＯ及びＨＣの酸化とＮＯxの還元を同時に行って浄化する三元触媒が用いられている。このような三元触媒としては、例えばコージェライト等の担体基材にγ−アルミナからなる触媒担持層を形成し、この触媒担持層に白金（Ｐｔ）、パラジウム（Ｐｄ）、ロジウム（Ｒｈ）等の貴金属触媒を担持させたものが広く知られている。また、酸素貯蔵効果を有する酸化セリウムを併用し、排ガスの空燃比（Ａ／Ｆ）変動に対するウインドウ幅を広めた三元触媒も知られている。 Conventionally, as a catalyst for exhaust gas purification of automobiles, a three-way catalyst for purifying by simultaneously oxidizing CO and HC in exhaust gas and reducing NOx has been used. As such a three-way catalyst, for example, a catalyst support layer made of γ-alumina is formed on a support base material such as cordierite, and platinum (Pt), palladium (Pd), rhodium (Rh), etc. are formed on this catalyst support layer. Those having a noble metal catalyst supported thereon are widely known. There is also known a three-way catalyst which uses cerium oxide having an oxygen storage effect in combination and widens the window width against fluctuations in the air-fuel ratio (A / F) of exhaust gas.
ところで、近年の排ガス浄化用触媒には、800℃以上の高温下においても優れた排ガス浄化性能を有することが望まれており、耐熱性を向上させることが課題となっている。すなわち、連続した高温環境下では、白金はシンタリング（凝集）を起こし、排ガス浄化率が低下するという問題があった。 By the way, recent exhaust gas purification catalysts are desired to have excellent exhaust gas purification performance even at a high temperature of 800 ° C. or higher, and there is a problem of improving heat resistance. That is, in a continuous high temperature environment, platinum causes sintering (aggregation), resulting in a problem that the exhaust gas purification rate decreases.
貴金属のうち、白金及びパラジウムは主としてＣＯ及びＨＣの酸化浄化に寄与し、ロジウムは主としてＮＯxの還元浄化に寄与するとともに、ロジウムには白金のシンタリングを防止する作用がある。そこで、白金とロジウムを併用することにより、シンタリングにより性能低下が抑制され、耐熱性が向上することが知られている。 Of the noble metals, platinum and palladium mainly contribute to the oxidation and purification of CO and HC, rhodium mainly contributes to the reduction and purification of NOx, and rhodium has the action of preventing platinum sintering. Thus, it is known that the combined use of platinum and rhodium suppresses performance degradation by sintering and improves heat resistance.
ところが、ロジウムは非常に高価であるため、上記のような白金とロジウムを併用した排ガス浄化用触媒は高価なものとなってしまう。そこで、白金とパラジウムを併用し、白金を酸化セリウム上に、パラジウムをアルミナ上に担持させ、高温耐久性を向上させた排ガス浄化用触媒が提案されている（例えば、特許文献１参照）。 However, since rhodium is very expensive, the exhaust gas purifying catalyst using platinum and rhodium in combination as described above becomes expensive. Therefore, an exhaust gas purifying catalyst has been proposed in which platinum and palladium are used together, platinum is supported on cerium oxide, palladium is supported on alumina, and high temperature durability is improved (see, for example, Patent Document 1).
ところが、近年のエンジン性能の向上と高速走行の増加に伴い、排ガス温度が著しく上昇している。また、酸化セリウムは耐熱性が弱く、1000℃以上の高温に加熱されると粒成長が生じることが知られている。従って、より厳しい耐久条件下では、この酸化セリウムが粒成長し、この酸化セリウム上に担持された白金粒子が移動して、白金も粒成長し、シンタリングが生じて触媒性能が低下するという問題がある。 However, the exhaust gas temperature has risen remarkably with the recent improvement in engine performance and the increase in high-speed driving. Further, cerium oxide has low heat resistance, and it is known that grain growth occurs when heated to a high temperature of 1000 ° C. or higher. Therefore, under more severe durability conditions, this cerium oxide grows, platinum particles supported on this cerium oxide move, platinum grows, sintering occurs, and the catalyst performance decreases. There is.
本発明は、白金のシンタリングを一層抑制し、高温耐久性を一層向上させた排ガス浄化用触媒を提供することを目的とする。 An object of the present invention is to provide an exhaust gas purifying catalyst in which platinum sintering is further suppressed and high temperature durability is further improved.
上記課題を解決するために、１番目の発明によれば、金属酸化物からなる触媒担体に白金が担持された排ガス浄化用触媒において、前記金属酸化物をセリウムよりも電気陰性度の低い金属の酸化物としている。 In order to solve the above problems, according to a first invention, in an exhaust gas purifying catalyst in which platinum is supported on a catalyst carrier made of a metal oxide, the metal oxide is made of a metal having a lower electronegativity than cerium. It is an oxide.
上記課題を解決するために、２番目の発明によれば、１番目の発明において、前記金属酸化物としてＣａ、Ｓｒ、Ｂａ、Ｌａ、Ｌｉ、Ｎａ、Ｋ、Ｒｂ、Ｃｓ、及びＦｒより選ばれる少なくとも１種の金属の酸化物を用いている。 In order to solve the above problems, according to the second invention, in the first invention, the metal oxide is selected from Ca, Sr, Ba, La, Li, Na, K, Rb, Cs, and Fr. At least one metal oxide is used.
上記課題を解決するために、３番目の発明によれば、金属酸化物からなる触媒担体に白金が担持された排ガス浄化用触媒において、前記金属酸化物を、多孔質金属酸化物にセリウムよりも電気陰性度の低い金属の酸化物を複合してなるものとしている。 In order to solve the above problems, according to a third invention, in the exhaust gas purifying catalyst in which platinum is supported on a catalyst support made of a metal oxide, the metal oxide is more porous than the cerium. It is made of a composite of metal oxides with low electronegativity.
上記課題を解決するために、４番目の発明によれば、３番目の発明において、前記多孔質金属酸化物を、ＣｅＯ2、Ａｌ2Ｏ3、及びＺｒＯ2より選ばれる少なくとも１種としている。 In order to solve the above problems, according to a fourth aspect, in the third aspect, the porous metal oxide is at least one selected from CeO 2 , Al 2 O 3 , and ZrO 2 .
上記課題を解決するために、５番目の発明によれば、３番目の発明において、前記多孔質金属酸化物がＣｅＯ2であり、セリウムよりも電気陰性度の低い金属の酸化物の濃度を、触媒担体の内部から表面に向かって高めている。 In order to solve the above-mentioned problem, according to a fifth invention, in the third invention, the porous metal oxide is CeO 2 , and the concentration of the metal oxide having a lower electronegativity than cerium, It is raised from the inside of the catalyst support toward the surface.
上記課題を解決するために、６番目の発明によれば、３番目の発明において、触媒担持相が多孔質金属酸化物の一次粒子と、セリウムよりも電気陰性度の低い金属の酸化物の一次粒子が混合してなり、白金がセリウムよりも電気陰性度の低い金属の酸化物の一次粒子上に担持されている。 In order to solve the above problems, according to a sixth invention, in the third invention, the catalyst-supported phase is a primary particle of a porous metal oxide and a primary oxide of a metal oxide having a lower electronegativity than cerium. The particles are mixed, and platinum is supported on primary particles of a metal oxide having a lower electronegativity than cerium.
本発明の排ガス浄化用触媒では、触媒担持層の少なくとも一部を、セリウムよりも電気陰性度の低い金属の酸化物より構成することにより、高温における白金の移動を抑制し、シンタリングを防ぎ、結果として高温耐久性を向上させる。 In the exhaust gas purifying catalyst of the present invention, at least a part of the catalyst supporting layer is composed of an oxide of a metal having a lower electronegativity than cerium, thereby suppressing platinum migration at a high temperature and preventing sintering. As a result, high temperature durability is improved.
本発明の排ガス浄化用触媒は、触媒担体と、この触媒担体上に担持された白金から構成される。白金の担持量は、担体１リットルあたり0.1〜20gとすることが好ましい。0.1g未満では十分な触媒活性が得られず、20gを超えても活性向上はわずかであり、高価となるのみであるからである。 The exhaust gas purifying catalyst of the present invention is composed of a catalyst carrier and platinum supported on the catalyst carrier. The amount of platinum supported is preferably 0.1 to 20 g per liter of carrier. If the amount is less than 0.1 g, sufficient catalytic activity cannot be obtained, and if the amount exceeds 20 g, the activity is only slightly improved and only expensive.
本発明の排ガス浄化用触媒は、いわゆるペレット型触媒であってもよいが、一般には担体基材上に触媒担体をウォッシュコートしたモノリス型触媒として用いられる。担体基材としては、排ガス浄化用触媒に用いられている公知の基材を用いることができ、例えば、コージェライト、アルミナ、ジルコニア、炭化ケイ素等の、耐熱性を有するセラミックス材料や、ステンレス鋼等の金属からなるハニカム基材を用いることが好ましく、優れた耐熱性と低い熱膨張率を有するコージェライト製ハニカムを用いることが特に好ましい。このハニカム基材は、両端が開口した多数のセルを有するものが好ましい。この場合、ハニカム基材のセル密度は、特に制限されないが、２００セル／平方インチ程度のいわゆる中密度のハニカム、又は１０００セル／平方インチ以上のいわゆる高密度のハニカム基材を用いることが好ましく、セルの断面形状は、特に制限されず、円形、四角形、六角形、円形等であってよい。 The exhaust gas purifying catalyst of the present invention may be a so-called pellet type catalyst, but is generally used as a monolith type catalyst in which a catalyst carrier is wash-coated on a carrier substrate. As the carrier base material, known base materials used for exhaust gas purification catalysts can be used. For example, heat-resistant ceramic materials such as cordierite, alumina, zirconia, silicon carbide, stainless steel, etc. It is preferable to use a honeycomb substrate made of the above metal, and it is particularly preferable to use a cordierite honeycomb having excellent heat resistance and a low coefficient of thermal expansion. This honeycomb substrate preferably has a large number of cells open at both ends. In this case, the cell density of the honeycomb substrate is not particularly limited, but it is preferable to use a so-called medium-density honeycomb of about 200 cells / square inch or a so-called high-density honeycomb substrate of 1000 cells / square inch or more, The cross-sectional shape of the cell is not particularly limited, and may be a circle, a rectangle, a hexagon, a circle, or the like.
本発明の第１の態様によれば、この触媒担体がセリウムよりも電気陰性度の低い金属の酸化物より構成される。このセリウムよりも電気陰性度の低い金属としては、好ましくはＣａ、Ｓｒ、Ｂａ、Ｌａ、Ｌｉ、Ｎａ、Ｋ、Ｒｂ、Ｃｓ、及びＦｒが例示される。 According to the first aspect of the present invention, the catalyst support is composed of a metal oxide having a lower electronegativity than cerium. Preferred examples of the metal having a lower electronegativity than cerium include Ca, Sr, Ba, La, Li, Na, K, Rb, Cs, and Fr.
このようなセリウムより電気陰性度の低い金属の酸化物から触媒担持層を構成することにより白金のシンタリングが抑制されるのは、かならずしも明確ではないが、これらの金属の酸化物が白金に対して親和性が高いためであろうと考えられる。すなわち、電気陰性度が低いほど、この金属の酸化物において酸素側の電子密度が高くなり、酸素における電子の結合エネルギーが低くなる。その結果、白金はこの金属の酸化物と複合酸化物及び金属酸化物を形成し、安定化するため、担体上の移動が抑制され、その結果シンタリングが抑制される。 Although it is not always clear that the sintering of platinum is suppressed by forming the catalyst support layer from an oxide of a metal having a lower electronegativity than cerium, the oxide of these metals is less than that of platinum. This is probably due to the high affinity. That is, the lower the electronegativity, the higher the electron density on the oxygen side of this metal oxide, and the lower the electron binding energy in oxygen. As a result, platinum forms a complex oxide and metal oxide with this metal oxide and stabilizes, so that movement on the support is suppressed, and as a result, sintering is suppressed.
本発明の第２の態様によれば、触媒担体を構成する金属酸化物を、多孔質酸化物にセリウムより電気陰性度の低い金属の酸化物を複合したものから構成する。多孔質酸化物としては、熱的安定性の高い金属酸化物、好ましくはＣｅＯ2、Ａｌ2Ｏ3、及びＺｒＯ2を用いる。また、この多気孔質酸化物は、数ｎｍから数十ｎｍの一次粒子が集った５０〜５００ｎｍの二次粒子の集合体であることが好ましい。 According to the second aspect of the present invention, the metal oxide constituting the catalyst carrier is composed of a composite of a porous oxide and a metal oxide having a lower electronegativity than cerium. As the porous oxide, a metal oxide having high thermal stability, preferably CeO 2 , Al 2 O 3 , and ZrO 2 is used. Further, this multiporous oxide is preferably an aggregate of secondary particles of 50 to 500 nm in which primary particles of several nm to several tens of nm are collected.
セリウムより電気陰性度の低い金属の酸化物は上記のように白金との親和性が高く、白金のシンタリングを抑制できるものの、必ずしも熱的安定性が高いとはいえず、また、触媒担体としての十分な多孔性を有していない場合もある。そこで、熱的安定性の高い多孔質酸化物と複合することにより、触媒担体として十分な熱的安定性及び多孔性を確保することができる。 Although the metal oxide having a lower electronegativity than cerium has a high affinity with platinum as described above and can suppress platinum sintering, it cannot necessarily be said to have high thermal stability, and as a catalyst support. In some cases, it does not have sufficient porosity. Therefore, by combining with a porous oxide having high thermal stability, sufficient thermal stability and porosity as a catalyst carrier can be ensured.
ここで複合とは、多孔質酸化物とセリウムより電気陰性度の低い金属の酸化物が混合されていることのみならず、多孔質酸化物を構成する金属とセリウムより電気陰性度の低い金属とが複合酸化物を構成していてもよいことを意味する。この触媒担体において、セリウムより電気陰性度の低い金属の酸化物は、触媒担体に対して１〜５０mol％含まれていることが好ましい。 Here, the composite means not only that a porous oxide and a metal oxide having a lower electronegativity than cerium are mixed, but also a metal constituting the porous oxide and a metal having a lower electronegativity than cerium. Means that it may constitute a composite oxide. In this catalyst support, the metal oxide having a lower electronegativity than cerium is preferably contained in an amount of 1 to 50 mol% based on the catalyst support.
白金は、主に触媒担体の表面に担持され、さらに白金のシンタリングは触媒担体の表面において進行するため、多孔質酸化物にセリウムより電気陰性度の低い金属の酸化物を複合した触媒担体においては、セリウムより電気陰性度の低い金属の酸化物の濃度を触媒担体の内部から表面に向かって高めることが好ましい。 Platinum is mainly supported on the surface of the catalyst support, and platinum sintering proceeds on the surface of the catalyst support. Therefore, in a catalyst support in which a porous oxide is combined with a metal oxide having a lower electronegativity than cerium. It is preferable to increase the concentration of the metal oxide having a lower electronegativity than cerium from the inside to the surface of the catalyst support.
このように触媒担持層にセリウムより電気陰性度の低い金属の酸化物の濃度分布を設けるためには、例えば、触媒担持層を、多孔質酸化物を用いて形成した層とセリウムより電気陰性度の低い金属の酸化物を用いて形成した層からなる多層構造とする。具体的には、触媒担持層の内部を多孔質酸化物の層とし、排ガスの流路側の表面側をセリウムより電気陰性度の低い金属の酸化物の層とする。あるいは、多孔質酸化物から形成した一次粒子とセリウムより電気陰性度の低い金属の酸化物から形成した一次粒子の組み合わせより触媒担持層を形成し、セリウムより電気陰性度の低い金属の酸化物から形成した一次粒子の量を、触媒担持層中において内部に比べて表面に向かって相対的に多くすることにより濃度分布を設けることもできる。 Thus, in order to provide the catalyst support layer with a concentration distribution of a metal oxide having a lower electronegativity than cerium, for example, a catalyst support layer is formed by using a porous oxide and a cerium. A multi-layered structure composed of layers formed using a low-oxide metal oxide. Specifically, the inside of the catalyst support layer is a porous oxide layer, and the surface side of the exhaust gas flow path is a metal oxide layer having a lower electronegativity than cerium. Alternatively, a catalyst-supporting layer is formed from a combination of primary particles formed from a porous oxide and primary particles formed from a metal oxide having a lower electronegativity than cerium, and from a metal oxide having a lower electronegativity than cerium. It is also possible to provide a concentration distribution by increasing the amount of primary particles formed in the catalyst support layer relative to the surface as compared with the inside.
触媒担持層は、多孔質酸化物から形成した一次粒子とセリウムより電気陰性度の低い金属の酸化物から形成した一次粒子の組み合わせより形成した二次粒子の集合体としてもよいが、この場合、白金は、そのシンタリングを防止するため、セリウムより電気陰性度の低い金属の酸化物から形成した一次粒子上に担持させることが好ましい。この一次粒子の粒径は好ましくは５ｎｍ〜１００ｎｍであり、二次粒子の粒径は好ましくは５０ｎｍ〜５００ｎｍである。 The catalyst support layer may be an aggregate of secondary particles formed from a combination of primary particles formed from a porous oxide and primary particles formed from a metal oxide having a lower electronegativity than cerium. In order to prevent the sintering, platinum is preferably supported on primary particles formed from a metal oxide having a lower electronegativity than cerium. The particle size of the primary particles is preferably 5 nm to 100 nm, and the particle size of the secondary particles is preferably 50 nm to 500 nm.
この触媒担持層は一般的な金属酸化物からなる触媒担体の製造方法によって形成することができる。すなわち、金属酸化物又は炭酸塩、水酸化物等のその前駆体の粉末を混合して焼成する粉末焼成法、金属無機塩の水溶液にアルカリを添加して中和し、酸化物又は水酸化物のコロイド分散液を生成する共沈法、有機溶媒に溶解した金属アルコキシドに水を添加して加水分解するアルコキシド法、等により形成することができる。 This catalyst support layer can be formed by a method for producing a catalyst carrier made of a general metal oxide. That is, a powder firing method in which powders of precursors thereof such as metal oxides or carbonates and hydroxides are mixed and fired, an alkali is added to an aqueous solution of a metal inorganic salt to neutralize the oxides or hydroxide For example, a coprecipitation method for producing a colloidal dispersion of the above, an alkoxide method in which water is added to a metal alkoxide dissolved in an organic solvent to hydrolyze, and the like.
実施例１（触媒担体：Ｌａ2Ｏ3）
10gの硝酸ランタン６水和物（Ｌａ(ＮＯ3)3・６Ｈ2Ｏ）を100gのイオン交換水に溶解した（溶液Ａ）。この溶液Ａに溶解したＬａ(ＮＯ3)3・６Ｈ2Ｏの硝酸根に対し、モル比で1.1倍のＮａを含む炭酸ナトリウムを50gのイオン交換水に溶解した（溶液Ｂ）。溶液Ａに溶液Ｂを滴下し、Ｌａを含む沈殿物を得た。この沈殿物について80℃のイオン交換水による洗浄、濾過を５回繰り返した。洗浄後の沈殿物を120℃で一昼夜乾燥させ、500℃にて２時間焼成し、Ｌａ2Ｏ3粉末を得た。
Example 1 (Catalyst support: La 2 O 3 )
10 g of lanthanum nitrate hexahydrate (La (NO 3 ) 3 .6H 2 O) was dissolved in 100 g of ion-exchanged water (solution A). With respect to the nitrate radical of La (NO 3 ) 3 .6H 2 O dissolved in this solution A, sodium carbonate containing Na 1.1 times in molar ratio was dissolved in 50 g of ion-exchanged water (solution B). Solution B was added dropwise to solution A to obtain a precipitate containing La. This precipitate was washed with ion exchange water at 80 ° C. and filtered five times. The precipitate after washing was dried at 120 ° C. for a whole day and night and calcined at 500 ° C. for 2 hours to obtain La 2 O 3 powder.
こうして得たＬａ2Ｏ3粉末をイオン交換水に分散させ、白金担持量が２wt％となるようにテトラニトロ白金を加えた。２時間攪拌後、水分を乾燥除去させ、500℃にて２時間焼成し、排ガス浄化用触媒を得た。 The La 2 O 3 powder thus obtained was dispersed in ion-exchanged water, and tetranitroplatinum was added so that the platinum loading was 2 wt%. After stirring for 2 hours, the water was removed by drying and calcined at 500 ° C. for 2 hours to obtain an exhaust gas purifying catalyst.
実施例２（触媒担体：Ｃａ(10mol％)／ＣｅＯ2）
硝酸ランタン６水和物の代わりに０．５５ｇの硝酸カルシウム４水和物（Ｃａ(ＮＯ3)2・４Ｈ2Ｏ）と９．０３ｇの硝酸セリウム６水和物（Ｃｅ(ＮＯ3)3・６Ｈ2Ｏ）の混合物を用いたことを除き、実施例１と同様にして排ガス浄化用触媒を得た。
Example 2 (Catalyst support: Ca (10 mol%) / CeO 2 )
Instead of lanthanum nitrate hexahydrate, 0.55 g of calcium nitrate tetrahydrate (Ca (NO 3 ) 2 .4H 2 O) and 9.03 g of cerium nitrate hexahydrate (Ce (NO 3 ) 3 .6H 2 ) Exhaust gas purification catalyst was obtained in the same manner as in Example 1 except that the mixture of O) was used.
実施例３（触媒担体：Ｂａ(10mol％)／ＣｅＯ2）
硝酸ランタン６水和物の代わりに０．６０ｇの硝酸バリウム（Ｂａ(ＮＯ3)2）と９．０３ｇの硝酸セリウム６水和物の混合物を用いたことを除き、実施例１と同様にして排ガス浄化用触媒を得た。
Example 3 (Catalyst carrier: Ba (10 mol%) / CeO 2 )
As in Example 1, except that 0.60 g of barium nitrate (Ba (NO 3 ) 2 ) and 9.03 g of cerium nitrate hexahydrate were used instead of lanthanum nitrate hexahydrate. An exhaust gas purification catalyst was obtained.
実施例４（触媒担体：Ｌａ(10mol％)／ＣｅＯ2）
硝酸ランタン６水和物の代わりに１．００ｇの硝酸ランタン６水和物と９．０３ｇの硝酸セリウム６水和物の混合物を用いたことを除き、実施例１と同様にして排ガス浄化用触媒を得た。
Example 4 (Catalyst support: La (10 mol%) / CeO 2 )
Exhaust gas purifying catalyst as in Example 1, except that a mixture of 1.00 g of lanthanum nitrate hexahydrate and 9.03 g of cerium nitrate hexahydrate was used instead of lanthanum nitrate hexahydrate Got.
実施例５（触媒担体：Ｌａ(50mol％)／Ａｌ2Ｏ3）
硝酸ランタン６水和物の代わりに５．００ｇの硝酸ランタン６水和物と４．３３ｇの硝酸アルミニウム９水和物（Ａｌ(ＮＯ3)3・９Ｈ2Ｏ）の混合物を用いたことを除き、実施例１と同様にして排ガス浄化用触媒を得た。
Example 5 (Catalyst support: La (50 mol%) / Al 2 O 3 )
Except that instead of lanthanum nitrate hexahydrate, a mixture of 5.00 g lanthanum nitrate hexahydrate and 4.33 g aluminum nitrate nonahydrate (Al (NO 3 ) 3 · 9H 2 O) was used. In the same manner as in Example 1, an exhaust gas purification catalyst was obtained.
実施例６（触媒担体：Ｌａ(50mol％)／ＺｒＯ2）
硝酸ランタン６水和物の代わりに５．００ｇの硝酸ランタン６水和物と３．０９ｇのオキシ硝酸ジルコニウム２水和物（ＺｒＯ(ＮＯ3)3・２Ｈ2Ｏ）の混合物を用いたことを除き、実施例１と同様にして排ガス浄化用触媒を得た。
Example 6 (Catalyst support: La (50 mol%) / ZrO 2 )
Instead of lanthanum nitrate hexahydrate, a mixture of 5.00 g of lanthanum nitrate hexahydrate and 3.09 g of zirconium oxynitrate dihydrate (ZrO (NO 3 ) 3 .2H 2 O) was used. Except for the above, an exhaust gas purifying catalyst was obtained in the same manner as in Example 1.
実施例７（触媒担体：傾斜Ｃａ(10mol％)／ＣｅＯ2）
等電点のｐＨが５であるＣｅＯ2ゾルと、等電点のｐＨが10であるＣａＯゾルをイオン交換水に溶解し、水溶液のｐＨをＣｅＯ2ゾルとＣａＯゾルの中間になるように調整した。この溶液に、ＣｅＯ2ゾルの等電点のｐＨ５となるように硝酸を滴下し、主にＣｅＯ2を含む沈殿物を得た。次に、ＣａＯゾルの等電点のｐＨ10となるように水酸化ナトリウム水溶液を滴下し、主にＣａを含む沈殿物を、上記の主にＣｅＯ2を含む沈殿物上に形成させた。このようにして得た沈殿物に対し、80℃のイオン交換水で洗浄、濾過を５回行った。洗浄後の沈殿物を120℃にて一昼夜乾燥させ、500℃にて２時間焼成し、Ｃａ濃度が内部から表面に向かって高められている傾斜Ｃａ／ＣｅＯ2を得た。
Example 7 (Catalyst carrier: Gradient Ca (10 mol%) / CeO 2 )
A CeO 2 sol with an isoelectric point pH of 5 and a CaO sol with an isoelectric point pH of 10 are dissolved in ion-exchanged water, and the pH of the aqueous solution is adjusted to be between the CeO 2 sol and the CaO sol. did. Nitric acid was added dropwise to this solution so that the pH of the isoelectric point of the CeO 2 sol was 5 to obtain a precipitate mainly containing CeO 2 . Next, an aqueous sodium hydroxide solution was dropped so that the pH of the isoelectric point of the CaO sol was 10 and a precipitate mainly containing Ca was formed on the precipitate mainly containing CeO 2 . The precipitate thus obtained was washed with ion exchange water at 80 ° C. and filtered five times. The washed precipitate was dried overnight at 120 ° C. and calcined at 500 ° C. for 2 hours to obtain a gradient Ca / CeO 2 in which the Ca concentration was increased from the inside toward the surface.
こうして得たＣａ傾斜ＣｅＯ2をイオン交換水に分散させ、白金担持量が２wt％となるようにテトラニトロ白金を加えた。２時間攪拌後、水分を乾燥除去させ、500℃にて２時間焼成し、排ガス浄化用触媒を得た。 The Ca gradient CeO 2 thus obtained was dispersed in ion exchange water, and tetranitroplatinum was added so that the amount of platinum supported was 2 wt%. After stirring for 2 hours, the water was removed by drying and calcined at 500 ° C. for 2 hours to obtain an exhaust gas purifying catalyst.
実施例８（触媒担体：傾斜Ｂａ(10mol％)／ＣｅＯ2)
ＣａＯゾルの代わりに等電点のｐＨが10であるＢａＯゾルを用いたことを除き、実施例７と同様にして排ガス浄化用触媒を得た。
Example 8 (catalyst support: inclined Ba (10 mol%) / CeO 2 )
An exhaust gas purifying catalyst was obtained in the same manner as in Example 7 except that a BaO sol having an isoelectric point of
実施例９（触媒担体：傾斜Ｌａ(10mol％)／ＣｅＯ2)
ＣａＯゾルの代わりに等電点のｐＨが10であるＬａ2Ｏ3ゾルを用いたことを除き、実施例７と同様にして排ガス浄化用触媒を得た。
Example 9 (catalyst support: inclined La (10 mol%) / CeO 2 )
Exhaust gas purification catalyst was obtained in the same manner as in Example 7 except that La 2 O 3 sol having an isoelectric point of
実施例１０（触媒担体：傾斜Ｌａ(50mol％)／Ａｌ2Ｏ3)
ＣａＯゾルの代わりに等電点のｐＨが10であるＬａ2Ｏ3ゾルを用い、ＣｅＯ2ゾルの代わりに等電点のｐＨが５であるＡｌ2Ｏ3ゾルを用いたことを除き、実施例７と同様にして排ガス浄化用触媒を得た。
Example 10 (catalyst support: inclined La (50 mol%) / Al 2 O 3 )
Example 2 except that La 2 O 3 sol with an isoelectric point pH of 10 was used instead of CaO sol, and Al 2 O 3 sol with an isoelectric point pH of 5 was used instead of CeO 2 sol In the same manner as in Example 7, an exhaust gas purification catalyst was obtained.
実施例１１（触媒担体：傾斜Ｌａ(50mol％)／ＺｒＯ2)
ＣａＯゾルの代わりに等電点のｐＨが10であるＬａ2Ｏ3ゾルを用い、ＣｅＯ2ゾルの代わりに等電点のｐＨが５であるＺｒＯ2ゾルを用いたことを除き、実施例７と同様にして排ガス浄化用触媒を得た。
Example 11 (catalyst support: inclined La (50 mol%) / ZrO 2 )
Example 7 except that a La 2 O 3 sol having an isoelectric point pH of 10 was used instead of the CaO sol, and a ZrO 2 sol having an isoelectric point pH of 5 was used instead of the CeO 2 sol. Similarly, an exhaust gas purification catalyst was obtained.
実施例１２（触媒担体：Ｃａ(10mol％)／ＺｒＯ2)
硝酸ランタン６水和物の代わりに０．５５ｇの硝酸カルシウム６水和物と５．５６ｇのオキシ硝酸ジルコニウム２水和物を用いたことを除き、実施例１と同様にして排ガス浄化用触媒を得た。
Example 12 (Catalyst carrier: Ca (10 mol%) / ZrO 2 )
An exhaust gas purifying catalyst was prepared in the same manner as in Example 1 except that 0.55 g of calcium nitrate hexahydrate and 5.56 g of zirconium oxynitrate dihydrate were used instead of lanthanum nitrate hexahydrate. Obtained.
実施例１３（Ｐｔ選択担持Ｌａ(50mol％)／Ａｌ2Ｏ3)
Ｌａ2Ｏ3含有量５．００ｇのＬａ2Ｏ3ゾルにＰｔ含有量０．１３ｇのテトラニトロ白金を加え攪拌し、Ｐｔ担持Ｌａ2Ｏ3ゾルを得た。このＰｔ担持Ｌａ2Ｏ3ゾルをＡｌ2Ｏ3含有量１．５６ｇのＡｌ2Ｏ3ゾルと混合し、攪拌した。この混合物を加熱して水分を蒸発させ、Ｐｔ担持Ｌａ2Ｏ3とＡｌ2Ｏ3が一次粒子レベルで混合された粉末を得た。この粉末を空気中、500℃で焼成し、排ガス浄化用触媒を得た。
Example 13 (Pt selective support La (50 mol%) / Al 2 O 3 )
To a La 2 O 3 sol having a La 2 O 3 content of 5.00 g, tetranitroplatinum having a Pt content of 0.13 g was added and stirred to obtain a Pt-supported La 2 O 3 sol. The Pt-supported La 2 O 3 sol was mixed with Al 2 O 3 sol content of Al 2 O 3 1.56 g, and stirred. This mixture was heated to evaporate water, and a powder in which Pt-supported La 2 O 3 and Al 2 O 3 were mixed at the primary particle level was obtained. This powder was calcined in air at 500 ° C. to obtain an exhaust gas purifying catalyst.
実施例１４（Ｐｔ選択担持Ｌａ(50mol％)／ＺｒＯ2)
Ａｌ2Ｏ3ゾルの代わりにＺｒＯ2ゾルを用いたことを除き、実施例１３と同様にして排ガス浄化用触媒を得た。
Example 14 (Pt selective support La (50 mol%) / ZrO 2 )
Exhaust gas purifying catalyst was obtained in the same manner as Example 13 except that ZrO 2 sol was used instead of Al 2 O 3 sol.
実施例１５（Ｐｔ選択担持Ｃａ(50mol％)／ＺｒＯ2)
Ａｌ2Ｏ3ゾルの代わりにＺｒＯ2ゾルを用い、Ｌａ2Ｏ3ゾルの代わりにＣａＯゾルを用いたことを除き、実施例１３と同様にして排ガス浄化用触媒を得た。
Example 15 (Pt selective supported Ca (50 mol%) / ZrO 2 )
An exhaust gas purifying catalyst was obtained in the same manner as in Example 13 except that ZrO 2 sol was used instead of Al 2 O 3 sol and CaO sol was used instead of La 2 O 3 sol.
比較例１（触媒担体：Ａｌ2Ｏ3）
硝酸ランタン６水和物の代わりに８．６６ｇの硝酸アルミニウム９水和物を用いたことを除き、実施例１と同様にして排ガス浄化用触媒を得た。
Comparative Example 1 (Catalyst support: Al 2 O 3 )
An exhaust gas purifying catalyst was obtained in the same manner as in Example 1 except that 8.66 g of aluminum nitrate nonahydrate was used instead of lanthanum nitrate hexahydrate.
比較例２（触媒担体：ＣｅＯ2）
硝酸ランタン６水和物の代わりに１０．０３ｇの硝酸セリウム６水和物を用いたことを除き、実施例１と同様にして排ガス浄化用触媒を得た。
Comparative Example 2 (Catalyst support: CeO 2 )
An exhaust gas purification catalyst was obtained in the same manner as in Example 1 except that 10.03 g of cerium nitrate hexahydrate was used instead of lanthanum nitrate hexahydrate.
評価
実施例２〜１１の排ガス浄化用触媒について、担体表面のＣａ、ＢａもしくはＬａの濃度をＸＰＳにより測定し、下式により担体表面のＣａ、ＢａもしくはＬａの濃度％（α）を算出した。
この結果を図１及び図２に示す。実施例２と実施例７、実施例３と実施例８、実施例４と実施例９、実施例５と実施例１０、実施例６と実施例１１をそれぞれ比較すると、Ｌａ等の濃度が同じであっても、傾斜を設けた実施例７〜１１において、傾斜を設けない実施例２〜６よりも表面におけるＬａ等の濃度が高くなっている。 The results are shown in FIGS. When comparing Example 2 and Example 7, Example 3 and Example 8, Example 4 and Example 9, Example 5 and Example 10, Example 6 and Example 11, the concentrations of La and the like are the same. Even so, in Examples 7 to 11 in which the slope is provided, the concentration of La and the like on the surface is higher than in Examples 2 to 6 in which the slope is not provided.
次に、上記のそれぞれの排ガス浄化用触媒４ｇを排気量２Ｌのエンジンの排気系に取り付け、下記の組成のガスを、排気ガス温度800℃においてガス流量５Ｌ/minの条件で５時間流通させ、耐久試験を行った。 Next, 4 g of each of the above exhaust gas purifying catalysts is attached to the exhaust system of a 2 L engine, and a gas having the following composition is allowed to flow for 5 hours at an exhaust gas temperature of 800 ° C. under a gas flow rate of 5 L / min. A durability test was conducted.
そして、耐久後の白金の粒子径をＸ線回折により測定した。また、以下の組成のガスを流通下、500℃から100℃まで降温させ、そのときのＨＣの50％浄化温度を測定した。 And the particle diameter of platinum after durability was measured by X-ray diffraction. In addition, a gas having the following composition was lowered from 500 ° C. to 100 ° C. while flowing, and the 50% purification temperature of HC at that time was measured.
以上の結果を図３〜図６に示す。触媒担体をセリウムより電気陰性度の低い金属であるランタンの酸化物より構成することにより、又はカルシウム、バリウム、もしくはランタンを複合させることにより、耐久試験後における白金の粒子径が大きくなること、すなわちシンタリングを抑制し、さらに50％ＨＣ浄化温度を低くすることができる。また、ランタン等の濃度を表面において高くすることによりシンタリングを抑制し、50％ＨＣ浄化温度を低くすることができる。 The above results are shown in FIGS. By configuring the catalyst support from an oxide of lanthanum, which is a metal having a lower electronegativity than cerium, or by combining calcium, barium, or lanthanum, the particle diameter of platinum after the endurance test is increased. Sintering can be suppressed, and the 50% HC purification temperature can be further lowered. Further, by increasing the concentration of lanthanum or the like on the surface, sintering can be suppressed and the 50% HC purification temperature can be lowered.
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