JP5003954B2 - Exhaust gas purification oxidation catalyst, production method thereof, and exhaust gas purification method using exhaust gas purification oxidation catalyst - Google Patents
Exhaust gas purification oxidation catalyst, production method thereof, and exhaust gas purification method using exhaust gas purification oxidation catalyst Download PDFInfo
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Description
本発明は、低温での触媒活性に優れたＣｅＯ２を含む酸化触媒、その製造方法、および前記酸化触媒を用いた排ガスの浄化方法に関する。 The present invention relates to an oxidation catalyst containing CeO 2 having excellent catalytic activity at a low temperature, a method for producing the same, and a method for purifying exhaust gas using the oxidation catalyst.
従来より排ガス浄化用触媒として、排ガス中の一酸化炭素（ＣＯ）および炭化水素（ＨＣ）の酸化と窒素酸化物（ＮＯｘ）の還元とを同時に行なって浄化する三元系触媒が用いられている。このような三元系触媒としては、例えば、コージェライトなどからなる耐熱性ハニカム基材にγ−Ａｌ２Ｏ３などからなる担体層を形成し、その担体層に白金（Ｐｔ）やロジウム（Ｒｈ）などの触媒金属を担持させたものが知られている。 Conventionally, as a catalyst for exhaust gas purification, a three-way catalyst that purifies by simultaneously oxidizing carbon monoxide (CO) and hydrocarbon (HC) and reducing nitrogen oxide (NOx) in exhaust gas has been used. . As such a three-way catalyst, for example, a carrier layer made of γ-Al 2 O 3 or the like is formed on a heat-resistant honeycomb substrate made of cordierite or the like, and platinum (Pt) or rhodium (Rh) is formed on the carrier layer. And the like, on which a catalytic metal such as) is supported is known.
このような排ガス浄化用触媒に用いられる担体としては、比表面積が大きく耐熱性が高いものが好ましく、一般的には、Ａｌ２Ｏ３、ＳｉＯ２、ＺｒＯ２、またはＴｉＯ２などが用いられることが多い。また、排ガスの雰囲気変動を緩和するために助触媒としてＣｅＯ２が併用されることも多い。 As a carrier used for such an exhaust gas purification catalyst, a carrier having a large specific surface area and high heat resistance is preferable. In general, Al 2 O 3 , SiO 2 , ZrO 2 , TiO 2 or the like is used. There are many. Further, CeO 2 is often used as a co-catalyst in order to alleviate the atmospheric fluctuation of exhaust gas.
また、近年の排ガス規制の強化により、エンジン始動からごく短時間においても排ガスを浄化せしめる必要性が極めて高くなってきており、より低温で優れた酸化活性を示す触媒が求められている。このような低温での酸化活性に優れた触媒としてＣｅＯ２にＰｔを担持した触媒が知られているが、この触媒は、実際の排ガス中における耐久性が劣るものであり、また、熱によりＣｅＯ２がシンタリングするのに伴ってＰｔが粒子成長して触媒活性が低下するという問題があった。 Further, with the recent tightening of exhaust gas regulations, the necessity of purifying exhaust gas within a very short time after engine startup has become extremely high, and there is a need for a catalyst that exhibits superior oxidation activity at lower temperatures. As such a catalyst having excellent oxidation activity at a low temperature, a catalyst having Pt supported on CeO 2 is known. However, this catalyst is inferior in durability in an actual exhaust gas, and CeO is caused by heat. As 2 sinters, there is a problem that Pt grows and the catalytic activity decreases.
そこで、これらの問題を解決するため、例えば、特開２００３−２６５９５８号公報（特許文献１）には、ＣｅＯ２とＺｒＯ２とを主成分とする複合酸化物を含む担体に貴金属を担持した後、還元性雰囲気中、６００〜１０００℃の温度で熱処理されてなる排ガス浄化用触媒が開示されている。この排ガス浄化用触媒では、前記熱処理により担体のＣｅカチオンおよびＺｒカチオンの規則性が向上し、貴金属の粒子径が揃い、粒子成長が抑制されるため、耐久性が向上する。しかしながら、排ガス規制がより強化されるとエンジン始動からさらに短い時間においても排ガスの浄化が必要になり、より低温での酸化活性に優れた排ガス浄化用触媒が求められている。 In order to solve these problems, for example, Japanese Patent Application Laid-Open No. 2003-265958 (Patent Document 1) discloses a method in which a noble metal is supported on a support containing a composite oxide containing CeO 2 and ZrO 2 as main components. An exhaust gas purifying catalyst that is heat-treated at a temperature of 600 to 1000 ° C. in a reducing atmosphere is disclosed. In this exhaust gas-purifying catalyst, the regularity of the Ce cation and Zr cation of the carrier is improved by the heat treatment, the particle diameter of the noble metal is uniform, and particle growth is suppressed, so that the durability is improved. However, if exhaust gas regulations are further strengthened, exhaust gas purification is required even in a shorter period of time after engine start, and an exhaust gas purification catalyst having excellent oxidation activity at a lower temperature is required.
また、特開２００１−１８７３４３号公報（特許文献２）には、還元処理によって酸素欠損が導入された酸化物に貴金属を担持してなる常温浄化触媒が開示されている。この触媒により一酸化炭素などの環境負荷物質を浄化せしめることは可能であるが、この触媒は常温での浄化を目的とするものであり、２００℃以上で使用すると酸素欠損が消失して触媒活性が失われるため、ディーゼル排ガスの浄化など低温から高温の広い温度範囲において使用される触媒には不向きである。 Japanese Patent Application Laid-Open No. 2001-187343 (Patent Document 2) discloses a room temperature purification catalyst in which a noble metal is supported on an oxide into which oxygen deficiency has been introduced by reduction treatment. Although it is possible to purify environmentally hazardous substances such as carbon monoxide with this catalyst, this catalyst is intended for purification at room temperature. When used at 200 ° C. or higher, oxygen deficiency disappears and catalytic activity is lost. Therefore, it is not suitable for a catalyst used in a wide temperature range from low temperature to high temperature such as purification of diesel exhaust gas.
さらに、特開平９−１０３６８７号公報（特許文献３）には、触媒金属を担持した触媒担体に、酸化処理と非酸化処理とを交互に繰り返し施して、担持された前記触媒金属の粒子径を制御する方法が開示されている。この方法は、触媒金属が酸化雰囲気で担体上を移動しやすく凝集しやすいという性質と、還元雰囲気では担体上を移動しにくく粒子径が均一となりやすいという性質とを利用したものである。
本発明は、上記従来技術の有する課題に鑑みてなされたものであり、従来よりも低温域（１００〜１３０℃）においても排ガスなどの浄化が可能である低温活性に優れた酸化触媒を提供することを目的とする。 The present invention has been made in view of the above-described problems of the prior art, and provides an oxidation catalyst excellent in low-temperature activity that can purify exhaust gas and the like even in a lower temperature range (100 to 130 ° C.) than in the past. For the purpose.
本発明者らは、上記目的を達成すべく鋭意研究を重ねた結果、貴金属を担持した担体に還元処理と酸化処理と還元処理とを順次施して（以下、これらを「一連の還元酸化処理」という。）前記担体上に前記貴金属の粒子を形成させる場合に、担体としてＣｅＯ２を含む金属酸化物からなるものを使用し且つ特定の温度で前記一連の還元酸化処理を施すことにより、前記貴金属粒子の粒子径を小さく且つ均一に制御することができることを見出し、さらに粒子径が小さく且つ均一に制御された前記貴金属粒子を含有する触媒が低温での酸化活性に優れることを見出し、本発明を完成するに至った。 As a result of intensive studies to achieve the above object, the present inventors sequentially performed a reduction treatment, an oxidation treatment, and a reduction treatment on a support carrying a noble metal (hereinafter referred to as “a series of reduction oxidation treatments”). When the noble metal particles are formed on the carrier, the noble metal is obtained by using a metal oxide containing CeO 2 as a carrier and performing the series of reduction oxidation treatments at a specific temperature. It has been found that the particle size of the particles can be controlled to be small and uniform, and further, the catalyst containing the noble metal particles having a small and uniform particle size is excellent in oxidation activity at low temperature, and the present invention It came to be completed.
すなわち、本発明の排ガス浄化用酸化触媒（低温酸化触媒）は、ＣｅＯ ２ およびＺｒＯ ２ を含む複合酸化物からなる担体にＰｔ、ＰｄおよびＲｈからなる群から選択される少なくとも１種の貴金属を担持せしめ、該貴金属を担持せしめた担体に、先ず、還元性雰囲気下、６００〜８００℃の範囲の温度で還元処理を施し、次いで、該還元処理を施した担体に、酸化性雰囲気下、６００〜８００℃の範囲の温度で酸化処理を施し、さらに、該酸化処理を施した担体に、還元性雰囲気下、６００〜８００℃の範囲の温度で還元処理を施してなり、１００〜１３０℃においても排ガス中の炭化水素および一酸化炭素を酸化除去して該排ガスを浄化することが可能な排ガス浄化用酸化触媒であって、前記排ガス浄化用酸化触媒中の貴金属粒子のＣＯパルス法により測定した平均粒子径が２〜８ｎｍであり、前記貴金属粒子のＸＲＤにより測定した粒度分布における最大粒子径に対するＣＯパルス法により測定した平均粒子径の誤差が３５％以内である、ことを特徴とするものである。 That is, the oxidation catalyst for exhaust gas purification ( low-temperature oxidation catalyst ) of the present invention carries at least one noble metal selected from the group consisting of Pt, Pd, and Rh on a carrier composed of a composite oxide containing CeO 2 and ZrO 2. First, the support on which the noble metal is loaded is first subjected to a reduction treatment at a temperature in the range of 600 to 800 ° C. in a reducing atmosphere, and then the support that has been subjected to the reduction treatment is subjected to 600 to 600 ° C. in an oxidizing atmosphere. subjected to an oxidation treatment at a temperature in the range of 800 ° C., further the carrier subjected to oxidation treatment under a reducing atmosphere, Ri na subjected to reduction treatment at a temperature in the range of 600 to 800 ° C., at 100 to 130 ° C. Is an exhaust gas purifying oxidation catalyst capable of purifying the exhaust gas by oxidizing and removing hydrocarbons and carbon monoxide in the exhaust gas, wherein the precious metal particles in the exhaust gas purifying oxidation catalyst are CO. An average particle size of 2~8nm measured by pulse method, the error of the average particle diameter measured by a CO pulse method for the largest particle size in the particle size distribution measured by XRD of the noble metal particles is within 35%, that It is a feature.
前記排ガス浄化用酸化触媒（低温酸化触媒）中の貴金属粒子のＸＲＤにより測定した粒度分布における最大粒子径に対するＣＯパルス法により測定した平均粒子径の誤差は２０％以内であることが好ましい。 The average particle size error measured by CO pulse method for the largest particle size in the particle size distribution measured by XRD of the noble metal particles in exhaust gas purifying oxidation catalyst (low temperature oxidation catalyst) is not preferable to be within 20%.
本発明の排ガス浄化用酸化触媒（低温酸化触媒）の製造方法は、ＣｅＯ ２ およびＺｒＯ ２ を含む複合酸化物からなる担体にＰｔ、ＰｄおよびＲｈからなる群から選択される少なくとも１種の貴金属を担持せしめ、該貴金属を担持せしめた前記担体に、先ず、還元性雰囲気下、６００〜８００℃の範囲の温度で還元処理を施し、次いで、前記還元処理が施された前記担体に、酸化性雰囲気下、６００〜８００℃の範囲の温度で酸化処理を施し、さらに、前記酸化処理が施された前記担体に、還元性雰囲気下、６００〜８００℃の範囲の温度で還元処理を施す、ことを特徴とするものである。 The method for producing an exhaust gas purifying oxidation catalyst ( low-temperature oxidation catalyst ) of the present invention comprises at least one noble metal selected from the group consisting of Pt, Pd and Rh on a support composed of a composite oxide containing CeO 2 and ZrO 2. the carrier allowed, the carrier was allowed carrying the noble metal, firstly, a reducing atmosphere, and facilities the reduction treatment at a temperature in the range of 600 to 800 ° C., then, on the support, wherein the reduction process is performed, oxidizing atmosphere, and facilities oxidation treatment at a temperature in the range of 600 to 800 ° C., further to the support which the oxidation process is performed under a reducing atmosphere, subjected to a reduction treatment at a temperature in the range of 600 to 800 ° C., It is characterized by this.
本発明の排ガスの浄化方法は、排ガスを本発明の排ガス浄化用酸化触媒（低温酸化触媒）を用いて浄化せしめることを特徴とするものである。 The exhaust gas purification method of the present invention is characterized in that exhaust gas is purified using the exhaust gas purification oxidation catalyst ( low-temperature oxidation catalyst ) of the present invention.
なお、本発明の触媒の製造方法によって触媒に含まれる貴金属粒子が粒子径が小さく且つ均一に制御されたものとなる理由は必ずしも定かではないが、本発明者らは以下のように推察する。すなわち、貴金属を担持したＣｅＯ２を含む前記担体を、還元性雰囲気下、特定の温度で還元処理すると前記貴金属がある程度粒子成長するとともにＣｅＯ２に酸素空孔が形成される。次に、この担体を酸化性雰囲気下、特定の温度で酸化処理すると前記酸素空孔に酸素が導入されるとともに前記貴金属粒子は酸素原子を介してＣｅ原子と結合して分散し、貴金属の酸化物として微粒子化される。その後、２回目の還元処理を施すと、再度酸素空孔が形成されるとともに前記貴金属粒子が還元され、貴金属粒子となる。これにより、安定した酸素空孔が形成されるとともに、粒子径が小さく且つ均一に制御された貴金属粒子が形成されるものと推察する。 The reason why the noble metal particles contained in the catalyst are small and uniformly controlled by the catalyst production method of the present invention is not necessarily clear, but the present inventors infer as follows. That is, when the support containing CeO 2 supporting a noble metal is reduced at a specific temperature in a reducing atmosphere, the noble metal grows to some extent and oxygen vacancies are formed in CeO 2 . Next, when this support is oxidized at a specific temperature in an oxidizing atmosphere, oxygen is introduced into the oxygen vacancies, and the noble metal particles are bonded to and dispersed by Ce atoms via oxygen atoms. Fine particles as a product. Thereafter, when the second reduction treatment is performed, oxygen vacancies are formed again and the noble metal particles are reduced to become noble metal particles. This presumes that stable oxygen vacancies are formed, and noble metal particles having a small particle diameter and uniform control are formed.
本発明によれば、担体に、粒子径が小さく且つ均一に制御された貴金属粒子を担持することが可能となる。また、従来よりも低温域（１００〜１３０℃）においても排ガスなどを浄化せしめることが可能となる。 According to the present invention, it is possible to support precious metal particles having a small particle size and a uniform control on the carrier. Further, exhaust gas and the like can be purified even in a lower temperature range (100 to 130 ° C.) than in the past.
以下、本発明をその好適な実施形態に即して詳細に説明する。 Hereinafter, the present invention will be described in detail with reference to preferred embodiments thereof.
先ず、本発明の低温酸化触媒について説明する。本発明の低温酸化触媒は、ＣｅＯ２を含む金属酸化物からなる担体に貴金属を担持せしめた後、還元性雰囲気下、６００〜８００℃の範囲の温度で熱処理（還元処理）を施し、次いで酸化性雰囲気下、６００〜８００℃の範囲の温度で熱処理（酸化処理）を施し、さらに還元性雰囲気下、６００〜８００℃の範囲の温度で熱処理（還元処理）を施してなることを特徴とするものである。 First, the low temperature oxidation catalyst of the present invention will be described. In the low-temperature oxidation catalyst of the present invention, after a noble metal is supported on a support made of a metal oxide containing CeO 2 , heat treatment (reduction treatment) is performed in a reducing atmosphere at a temperature in the range of 600 to 800 ° C., followed by oxidation. Heat treatment (oxidation treatment) at a temperature in the range of 600 to 800 ° C. in a neutral atmosphere, and further heat treatment (reduction treatment) at a temperature in the range of 600 to 800 ° C. in a reducing atmosphere. Is.
本発明に用いる担体はＣｅＯ２を含む金属酸化物からなるものである。担体としてＣｅＯ２を含む金属酸化物からなるものを用いると、担体上に粒子径が小さく且つ均一に制御された貴金属粒子を形成することができる。前記担体中のＣｅ原子の含有率は、前記担体中の全金属原子量の１０原子％以上であることが好ましい。Ｃｅ原子の含有率が上記下限未満になると粒子径が小さく且つ均一に制御された貴金属粒子を形成することが困難になり、触媒の酸化活性が低下する傾向にある。 Carrier used in the present invention are those composed of a metal oxide containing CeO 2. When a support made of a metal oxide containing CeO 2 is used as a support, noble metal particles having a small particle size and a uniform control can be formed on the support. The Ce atom content in the carrier is preferably 10 atomic% or more of the total amount of metal atoms in the carrier. When the Ce atom content is less than the lower limit, it becomes difficult to form noble metal particles having a small particle size and uniformly controlled, and the oxidation activity of the catalyst tends to be lowered.
前記ＣｅＯ２を含む金属酸化物としては、ＣｅＯ２単独のもの、ＣｅＯ２と他の金属酸化物とを含む混合酸化物、ＣｅＯ２と他の金属酸化物との複合酸化物およびその混合物、ならびにＣｅＯ２と他の金属酸化物と前記複合酸化物との混合物などが挙げられる。これらのうち、触媒の耐熱性が向上し、また、比表面積の低下を抑制することができ、触媒の酸化活性の低下を抑制することができる点でＣｅＯ２と他の金属酸化物との複合酸化物を含むものが好ましい。 The metal oxide containing the CeO 2, CeO 2 alone ones, mixed oxide containing CeO 2 and other metal oxides, composite oxides and mixtures thereof with CeO 2 and other metal oxides, as well as Examples thereof include a mixture of CeO 2 , another metal oxide, and the composite oxide. Among these, the heat resistance of the catalyst is improved, the decrease in the specific surface area can be suppressed, and the decrease in the oxidation activity of the catalyst can be suppressed, so that the composite of CeO 2 and another metal oxide can be suppressed. Those containing oxides are preferred.
前記他の金属酸化物としては、ＺｒＯ２、Ａｌ２Ｏ３、ＴｉＯ２、ＳｉＯ２、Ｙ２Ｏ３、Ｌａ２Ｏ３、およびこれらの複合酸化物などが挙げられる。これらの他の金属酸化物は１種または２種以上含まれていてもよい。前記他の金属酸化物のうち、ＣｅＯ２と固溶して耐熱性を向上させることができるという観点ではＺｒＯ２がより好ましく、耐熱性に優れる金属酸化物を加えて複合酸化物の耐熱性を向上させることができるという観点ではＡｌ２Ｏ３がより好ましく、さらに、低温での酸化活性に優れた触媒が得られる点でＺｒＯ２とＡｌ２Ｏ３との複合酸化物が特に好ましい。したがって、これらの観点から本発明に用いるＣｅＯ２を含む金属酸化物としては、ＣｅＯ２とＺｒＯ２との複合酸化物が好ましく、ＣｅＯ２とＺｒＯ２とＡｌ２Ｏ３との複合酸化物が特に好ましい。 Examples of the other metal oxides include ZrO 2 , Al 2 O 3 , TiO 2 , SiO 2 , Y 2 O 3 , La 2 O 3 , and composite oxides thereof. One or more of these other metal oxides may be contained. Among the other metal oxides, ZrO 2 is more preferable from the viewpoint that it can be dissolved in CeO 2 to improve heat resistance, and a metal oxide having excellent heat resistance is added to improve the heat resistance of the composite oxide. From the viewpoint that it can be improved, Al 2 O 3 is more preferable, and a composite oxide of ZrO 2 and Al 2 O 3 is particularly preferable in that a catalyst having excellent oxidation activity at low temperatures can be obtained. Therefore, the metal oxide containing CeO 2 used in the present invention from these viewpoints, the composite oxide of CeO 2 and ZrO 2 are preferable, a composite oxide of CeO 2 and ZrO 2 and Al 2 O 3 is particularly preferable.
前記ＣｅＯ２と他の金属酸化物との複合酸化物において、前記複合酸化物中のＣｅ原子と他の金属原子との比（Ｃｅ／他の金属）は１／９〜９／１であることが好ましく、３／７〜７／３であることがより好ましい。前記原子比が上記下限未満になると粒子径が小さく且つ均一な貴金属粒子を形成することが困難になり、触媒の酸化活性が低下する傾向にあり、他方、上記上限を超えると前記複合酸化物の安定性が低下して触媒の酸化活性が低下する傾向にある。 In the composite oxide of CeO 2 and another metal oxide, the ratio of Ce atom to other metal atom (Ce / other metal) in the composite oxide is 1/9 to 9/1. Is preferable, and more preferably 3/7 to 7/3. When the atomic ratio is less than the above lower limit, it becomes difficult to form uniform noble metal particles having a small particle size, and the oxidation activity of the catalyst tends to decrease. The stability tends to decrease and the oxidation activity of the catalyst tends to decrease.
なお、本発明では、ＣｅＯ２と他の金属酸化物との複合酸化物には、ＣｅＯ２微粒子と前記他の金属酸化物の微粒子との凝集粒子であり且つＣｅＯ２と前記他の金属酸化物とのそれぞれ少なくとも一部が互いに固溶したものが包含される。 In the present invention, the composite oxide of CeO 2 and other metal oxides are agglomerated particles of CeO 2 fine particles and fine particles of the other metal oxides and the other metal oxides and CeO 2 In which at least a part of each is in solid solution with each other.
前記ＣｅＯ２と他の金属酸化物との複合酸化物がＣｅＯ２とＺｒＯ２と他の金属酸化物との三元系複合酸化物である場合、還元性雰囲気下での還元処理によりパイロクロア相などの規則相が形成され、ＣｅカチオンおよびＺｒカチオンのうちの少なくとも一部は規則的に配列する。これにより、前記他の金属酸化物がＣｅＯ２とＺｒＯ２と間に介在するため、還元性雰囲気下での還元処理においてＣｅＯ２−ＺｒＯ２複合酸化物と前記他の金属酸化物とが互いに障壁となって互いの粒子の成長が抑制される。また、格子の歪みが小さいため、酸素原子が放出されやすく、高い酸化活性を有する触媒が得られる。 When a composite oxide of the CeO 2 and other metal oxides are ternary composite oxide of CeO 2 and ZrO 2 and other metal oxides by reduction treatment under a reducing atmosphere pyrochlore phase, etc. And at least a part of the Ce cation and the Zr cation are regularly arranged. As a result, the other metal oxide is interposed between CeO 2 and ZrO 2 , so that the CeO 2 —ZrO 2 composite oxide and the other metal oxide have a barrier to each other in the reduction treatment in a reducing atmosphere. Thus, the growth of each other's particles is suppressed. Further, since the lattice distortion is small, oxygen atoms are easily released, and a catalyst having high oxidation activity can be obtained.
前記三元系複合酸化物において、他の金属原子とＣｅ原子およびＺｒ原子との比（他の金属／（Ｃｅ＋Ｚｒ））は１／５〜５／１であることが好ましく、１／３〜３／１であることがより好ましい。前記原子比が上記下限未満になると比表面積が小さくなり触媒の酸化活性が低下する傾向にあり、他方、上記上限を超えると相対的にＣｅＯ２の含有量が減少して粒子径が小さく且つ均一な貴金属粒子を形成することが困難になり、触媒の酸化活性が低下する傾向にある。 In the ternary composite oxide, the ratio of other metal atom to Ce atom and Zr atom (other metal / (Ce + Zr)) is preferably 1/5 to 5/1, and 1/3 to 3 / 1 is more preferable. When the atomic ratio is less than the lower limit, the specific surface area tends to be small and the oxidation activity of the catalyst tends to be reduced. On the other hand, when the upper limit is exceeded, the CeO 2 content is relatively reduced and the particle size is small and uniform. It is difficult to form noble metal particles, and the oxidation activity of the catalyst tends to decrease.
また、前記三元系複合酸化物において前記他の金属酸化物がＡｌ２Ｏ３を含むものである場合、前記他の金属酸化物にはさらに希土類元素酸化物が含まれていてもよい。前記担体中に含まれる希土類元素酸化物のうちの７０モル％以上はＡｌ２Ｏ３中に固溶していることが好ましく、９０モル％以上が固溶していることがより好ましい。これによりＡｌ２Ｏ３の耐熱性が向上して触媒の酸化活性の低下を抑制することができる。前記希土類元素としては、Ｌａ、Ｎｄ、Ｓｍ、およびＰｒなどが挙げられる。これらの希土類元素は１種単独で用いても２種以上を併用してもよい。これらのうち、前記観点からＬａが特に好ましい。 In the ternary composite oxide, when the other metal oxide contains Al 2 O 3 , the other metal oxide may further contain a rare earth element oxide. 70 mol% or more of the rare earth element oxide contained in the support is preferably dissolved in Al 2 O 3 , and more preferably 90 mol% or more is dissolved. Thereby, the heat resistance of Al 2 O 3 can be improved, and a reduction in the oxidation activity of the catalyst can be suppressed. Examples of the rare earth element include La, Nd, Sm, and Pr. These rare earth elements may be used alone or in combination of two or more. Of these, La is particularly preferred from the above viewpoint.
本発明に用いるＣｅＯ２を含む金属酸化物からなる担体は、特開２００３−２６５９５８号公報に記載された共沈法などの公知の方法により製造することができる。また、前記金属酸化物を安定させるために、貴金属を担持する前に前記担体に焼成処理を施すことが好ましい。この製造方法により得られる担体は、一般に平均粒子径が５０ｎｍ以下の金属酸化物微粒子が凝集したものであり、凝集粒子の平均粒子径は通常２０μｍ以下である。凝集粒子が複合酸化物粒子の場合、ＣｅＯ２と他の金属酸化物とは少なくとも一部が互いに固溶している。 Carrier comprising a metal oxide containing CeO 2 used in the present invention can be produced by a known method such as coprecipitation method described in JP-A-2003-265958. In order to stabilize the metal oxide, it is preferable that the carrier is subjected to a baking treatment before supporting the noble metal. The carrier obtained by this production method is generally an aggregate of metal oxide fine particles having an average particle size of 50 nm or less, and the average particle size of the aggregated particles is usually 20 μm or less. When the aggregated particles are composite oxide particles, CeO 2 and other metal oxides are at least partially dissolved in each other.
本発明に用いる貴金属としては、Ｐｔ、Ｐｄ、Ｒｈ、Ｉｒ、およびＲｕなどが挙げられる。これらの貴金属は１種単独で用いても２種以上を併用してもよい。これらのうち、酸化活性が高く、且つ劣化が進行しにくいという観点からＰｔ、Ｐｄ、およびＲｈが好ましい。前記貴金属を前記担体に担持せしめる方法としては吸着担持法、吸水担持法など公知の方法が挙げられる。貴金属担持量は特に制限はなく、触媒の用途、使用環境に応じて適宜設定することができる。 Examples of the noble metal used in the present invention include Pt, Pd, Rh, Ir, and Ru. These noble metals may be used alone or in combination of two or more. Among these, Pt, Pd, and Rh are preferable from the viewpoint that oxidation activity is high and deterioration does not easily proceed. Examples of a method for supporting the noble metal on the carrier include known methods such as an adsorption supporting method and a water absorbing supporting method. The amount of noble metal supported is not particularly limited, and can be appropriately set according to the use of the catalyst and the use environment.
このようにして貴金属を担持した担体に、先ず、還元性雰囲気下、６００℃〜８００℃の範囲の温度で１回目の還元処理を施す。前記還元性雰囲気としては、Ｈ２またはＣＯなどの還元性ガス雰囲気が挙げられる。前記還元性ガス雰囲気は、還元性ガスと窒素などの不活性ガスとの混合ガス雰囲気でもよい。 Thus, first, the support | carrier which carry | supported the noble metal is first reduced at the temperature of the range of 600 to 800 degreeC by reducing atmosphere. Examples of the reducing atmosphere include a reducing gas atmosphere such as H 2 or CO. The reducing gas atmosphere may be a mixed gas atmosphere of a reducing gas and an inert gas such as nitrogen.
前記１回目の還元処理により、担持された前記貴金属がある程度粒子成長するとともにＣｅＯ２に酸素空孔が形成される。前記１回目の還元処理の温度が上記下限未満になると、この酸素空孔が十分に形成されず、得られる触媒は酸化活性に劣るものとなる。他方、前記還元処理温度が上記上限を超えると貴金属粒子の粒子径が大きく且つ不均一になり、得られる触媒は低温での酸化活性に劣るものとなる。 By the first reduction treatment, the supported noble metal grows to some extent and oxygen vacancies are formed in CeO 2 . When the temperature of the first reduction treatment is less than the lower limit, oxygen vacancies are not sufficiently formed, and the resulting catalyst is inferior in oxidation activity. On the other hand, when the reduction treatment temperature exceeds the upper limit, the particle diameter of the noble metal particles becomes large and non-uniform, and the resulting catalyst is inferior in oxidation activity at low temperatures.
また、１回目の還元処理時間は、前記担体を十分に還元できる時間であれば特に限定されないが、１０分以上であることが好ましく、３０分以上であることがより好ましい。１回目の還元処理時間が上記下限より短いと酸素空孔が十分に形成されず、得られる触媒の酸化活性が劣る傾向にある。一方、過剰な粒成長を抑制するという観点から１回目の還元処理時間は１２０分以下であることが好ましい。 Further, the first reduction treatment time is not particularly limited as long as it can sufficiently reduce the carrier, but is preferably 10 minutes or more, and more preferably 30 minutes or more. If the first reduction treatment time is shorter than the lower limit, oxygen vacancies are not sufficiently formed, and the oxidation activity of the resulting catalyst tends to be inferior. On the other hand, from the viewpoint of suppressing excessive grain growth, the first reduction treatment time is preferably 120 minutes or less.
次に、前記還元処理が施された前記担体に、酸化性雰囲気下、６００℃〜８００℃の範囲の温度で酸化処理を施し、この酸化処理が施された担体に、還元性雰囲気下、６００℃〜８００℃の範囲の温度で２回目の還元処理を施す。 Next, the carrier that has been subjected to the reduction treatment is subjected to an oxidation treatment at a temperature in the range of 600 ° C. to 800 ° C. in an oxidizing atmosphere, and the carrier that has been subjected to the oxidation treatment is subjected to 600 ° C. in a reducing atmosphere. A second reduction treatment is performed at a temperature in the range of from 0C to 800C.
前記酸化性雰囲気としては、Ｏ２などの酸化性ガス雰囲気が挙げられる。また、酸化性ガス雰囲気は、酸化性ガスと窒素などの不活性ガスとの混合ガス雰囲気でもよい。前記還元性雰囲気としては前記１回目の還元処理において例示した還元性雰囲気と同様のものが挙げられる。 Examples of the oxidizing atmosphere include an oxidizing gas atmosphere such as O 2 . The oxidizing gas atmosphere may be a mixed gas atmosphere of an oxidizing gas and an inert gas such as nitrogen. Examples of the reducing atmosphere include the same reducing atmosphere as exemplified in the first reduction treatment.
前記酸化処理と２回目の還元処理とにより、前記担体上の貴金属粒子は、粒子径が小さく且つ均一なものとなり、この貴金属微粒子を備える触媒は低温での酸化活性に優れたものとなる。前記酸化処理および２回目の還元処理の温度が上記下限未満になると、前記担体上の貴金属粒子は、粒子径は小さいが不均一なものとなり、触媒は低温での酸化活性に劣るものとなる。他方、上記上限を超えると貴金属粒子の粒子径が大きく且つ不均一になり、触媒は低温での酸化活性に劣るものとなる。 By the oxidation treatment and the second reduction treatment, the noble metal particles on the carrier have a small and uniform particle diameter, and the catalyst including the noble metal fine particles has excellent oxidation activity at a low temperature. When the temperature of the oxidation treatment and the second reduction treatment is less than the lower limit, the noble metal particles on the support are small in size but non-uniform, and the catalyst is inferior in oxidation activity at low temperatures. On the other hand, when the above upper limit is exceeded, the particle size of the noble metal particles becomes large and non-uniform, and the catalyst is inferior in oxidation activity at low temperatures.
酸化処理時間は、前記担体を十分に酸化できる時間であれば特に限定されないが、１０分以上であることが好ましく、３０分以上であることがより好ましい。酸化処理時間が上記下限より短いとＣｅ−Ｏ−貴金属という結合が十分に形成されず、貴金属酸化物が十分に再分散されない傾向にある。一方、触媒調製に掛かるコストと時間の観点から酸化処理時間は３００分以下であることが好ましい。 The oxidation treatment time is not particularly limited as long as the carrier can be sufficiently oxidized, but is preferably 10 minutes or more, and more preferably 30 minutes or more. When the oxidation treatment time is shorter than the above lower limit, Ce—O—noble metal bonds are not sufficiently formed, and the noble metal oxide tends not to be sufficiently redispersed. On the other hand, the oxidation treatment time is preferably 300 minutes or less from the viewpoint of cost and time for catalyst preparation.
また、２回目の還元処理時間は、前記担体を十分に還元できる時間であれば特に限定されないが、１０分以上であることが好ましく、３０分以上であることがより好ましい。２回目の還元処理時間が上記下限より短いと前記酸化処理で生成した貴金属酸化物が十分にメタル状態に還元されない傾向にある。一方、貴金属の過剰な粒成長を抑制するという観点から２回目の還元処理時間は１２０分以下であることが好ましい。 Further, the second reduction treatment time is not particularly limited as long as the carrier can be sufficiently reduced, but is preferably 10 minutes or more, and more preferably 30 minutes or more. When the second reduction treatment time is shorter than the lower limit, the noble metal oxide generated by the oxidation treatment tends not to be sufficiently reduced to the metal state. On the other hand, from the viewpoint of suppressing excessive grain growth of the noble metal, the second reduction treatment time is preferably 120 minutes or less.
１回目の還元処理において形成された前記酸素空孔には前記酸化処理において酸素が導入されるが、２回目の還元処理において再び酸素空孔が形成される。この酸素空孔は気相中の酸素を活性化し、貴金属粒子上に吸着した一酸化炭素などの排ガスの酸化処理を促進する機能を備える。したがって、高い酸化活性を得るためには、１回目の還元処理において十分な酸素空孔を形成することが好ましい。 In the oxygen vacancies formed in the first reduction treatment, oxygen is introduced in the oxidation treatment, but oxygen vacancies are formed again in the second reduction treatment. These oxygen vacancies have a function of activating oxygen in the gas phase and accelerating the oxidation treatment of exhaust gas such as carbon monoxide adsorbed on the noble metal particles. Therefore, in order to obtain high oxidation activity, it is preferable to form sufficient oxygen vacancies in the first reduction treatment.
このようにして製造された触媒は粒子径が小さく且つ均一な貴金属粒子を備えるものである。本発明の低温酸化触媒では、前記貴金属粒子は、ＣＯパルス法により測定した平均粒子径が２〜８ｎｍのものであることが好ましい。また、ＸＲＤにより測定した粒度分布における最大粒子径に対するＣＯパルス法により測定した平均粒子径の誤差が３５％以内であることが好ましく、２０％以内であることがより好ましく、１５％以内であることが特に好ましい。このような粒子径が小さく且つ均一な貴金属粒子を備える酸化触媒は低温（１００〜１３０℃）での酸化活性に優れている。 The catalyst produced in this way has a small particle size and uniform noble metal particles. In the low-temperature oxidation catalyst of the present invention, the noble metal particles preferably have an average particle diameter of 2 to 8 nm measured by a CO pulse method. Further, the error of the average particle diameter measured by the CO pulse method with respect to the maximum particle diameter in the particle size distribution measured by XRD is preferably within 35%, more preferably within 20%, and within 15%. Is particularly preferred. Such an oxidation catalyst having a small particle diameter and uniform noble metal particles is excellent in oxidation activity at a low temperature (100 to 130 ° C.).
なお、前記誤差は下記式：
誤差［％］＝（最大粒子径−平均粒子径）／最大粒子径×１００
により算出される値である。
The error is expressed by the following formula:
Error [%] = (maximum particle diameter−average particle diameter) / maximum particle diameter × 100
Is a value calculated by.
したがって、エンジン始動から極短時間の低温時にも、本発明の低温酸化触媒に排ガスを接触せしめることによって、排ガスを浄化せしめることが可能となる。 Therefore, the exhaust gas can be purified by bringing the exhaust gas into contact with the low-temperature oxidation catalyst of the present invention even at a very low temperature from the start of the engine.
以下、実施例および比較例に基づいて本発明をより具体的に説明するが、本発明は以下の実施例に限定されるものではない。 EXAMPLES Hereinafter, although this invention is demonstrated more concretely based on an Example and a comparative example, this invention is not limited to a following example.
（実施例１）
純水に硝酸セリウムと硝酸ジルコニウムとをＣｅ／Ｚｒ＝５／１のモル比で溶解して水溶液を調製し、この水溶液にアンモニア水を攪拌しながら滴下して中和し、ＣｅＯ２−ＺｒＯ２複合酸化物を含むスラリーを得た。
Example 1
An aqueous solution is prepared by dissolving cerium nitrate and zirconium nitrate in pure water at a molar ratio of Ce / Zr = 5/1. Aqueous ammonia is added dropwise to the aqueous solution while stirring to neutralize it, and CeO 2 —ZrO 2. A slurry containing the composite oxide was obtained.
このスラリーを用いて見掛け容積３５ｃｍ３のコージェライト製ハニカム基材（セル密度：４００セル／ｉｎｃｈ２、セル壁厚さ：１１４μｍ）に前記ＣｅＯ２−ＺｒＯ２複合酸化物をウォッシュコーティングした。このウォッシュコーティングを、最終的なＣｅＯ２−ＺｒＯ２複合酸化物のコーティング量が前記ハニカム基材１Ｌ当たり約１００ｇとなるように繰り返した。その後、この基材を大気中、５００℃で１時間焼成してＣｅＯ２−ＺｒＯ２複合酸化物層（以下、「ＣｅＺｒ層」という。）を安定化させた。 Using this slurry, a cordierite honeycomb substrate (cell density: 400 cells / inch 2 , cell wall thickness: 114 μm) having an apparent volume of 35 cm 3 was wash-coated with the CeO 2 —ZrO 2 composite oxide. This wash coating was repeated so that the final CeO 2 —ZrO 2 composite oxide coating amount was about 100 g per 1 L of the honeycomb substrate. Thereafter, this base material was fired at 500 ° C. for 1 hour in the atmosphere to stabilize a CeO 2 —ZrO 2 composite oxide layer (hereinafter referred to as “CeZr layer”).
次に、前記ＣｅＺｒ層を備えた前記基材にジニトロジアミン白金（ＩＩ）硝酸水溶液を含浸させた後、この基材を大気中、３００℃で３時間焼成して前記ＣｅＯ２−ＺｒＯ２複合酸化物１００ｇ当たり約２ｇのＰｔを担持した。 Next, after impregnating the base material provided with the CeZr layer with a dinitrodiamine platinum (II) nitric acid aqueous solution, the base material is calcined in the atmosphere at 300 ° C. for 3 hours, and the CeO 2 —ZrO 2 composite oxidation is performed. About 2 g of Pt was loaded per 100 g of the product.
次に、Ｐｔ担持ＣｅＺｒ層（以下、「Ｐｔ−ＣｅＺｒ層」という。）を備えた前記基材を管状炉に入れ、Ｈ２／Ｎ２混合ガス（Ｈ２：５体積％、流量：１Ｌ／分）中、７００℃で１時間熱処理（還元処理）を施した。次いで、Ｏ２／Ｎ２混合ガス（Ｏ２：２０体積％、流量：１Ｌ／分）中、７００℃で１時間熱処理（酸化処理）を施し、さらにＨ２／Ｎ２混合ガス（Ｈ２：５体積％、流量：１Ｌ／分）中、７００℃で１時間熱処理（還元処理）を施し、Ｐｔ担持ＣｅＯ２−ＺｒＯ２複合酸化物からなる触媒層を備える基材を得た。 Next, the base material provided with a Pt-supported CeZr layer (hereinafter referred to as “Pt—CeZr layer”) is put into a tubular furnace, and a H 2 / N 2 mixed gas (H 2 : 5% by volume, flow rate: 1 L / L). Minute), heat treatment (reduction treatment) was performed at 700 ° C. for 1 hour. Next, heat treatment (oxidation treatment) was performed at 700 ° C. for 1 hour in an O 2 / N 2 mixed gas (O 2 : 20% by volume, flow rate: 1 L / min), and an H 2 / N 2 mixed gas (H 2 : 5% by volume, flow rate: 1 L / min) was subjected to heat treatment (reduction treatment) at 700 ° C. for 1 hour to obtain a base material provided with a catalyst layer made of Pt-supported CeO 2 —ZrO 2 composite oxide.
（実施例２）
純水に硝酸アルミニウムと硝酸セリウムと硝酸ジルコニウムとをＡｌ／Ｃｅ／Ｚｒ＝１／０．９／１のモル比で溶解して水溶液を調製し、この水溶液にアンモニア水を攪拌しながら滴下して中和し、Ａｌ２Ｏ３−ＣｅＯ２−ＺｒＯ２複合酸化物を含むスラリーを得た。
(Example 2)
An aqueous solution is prepared by dissolving aluminum nitrate, cerium nitrate and zirconium nitrate in pure water at a molar ratio of Al / Ce / Zr = 1 / 0.9 / 1, and ammonia water is added dropwise to the aqueous solution while stirring. Neutralization was performed to obtain a slurry containing Al 2 O 3 —CeO 2 —ZrO 2 composite oxide.
前記ＣｅＯ２−ＺｒＯ２複合酸化物を含むスラリーの代わりにこのＡｌ２Ｏ３−ＣｅＯ２−ＺｒＯ２複合酸化物を含むスラリーを用いた以外は実施例１と同様にしてＰｔ担持Ａｌ２Ｏ３−ＣｅＯ２−ＺｒＯ２複合酸化物からなる触媒層を備える基材を得た。なお、Ａｌ２Ｏ３−ＣｅＯ２−ＺｒＯ２複合酸化物のコーティング量は前記ハニカム基材１Ｌ当たり約１００ｇであり、Ｐｔ担持量はＡｌ２Ｏ３−ＣｅＯ２−ＺｒＯ２複合酸化物１００ｇ当たり約２ｇであった。 The CeO 2 -ZrO 2 composite oxide The Al 2 O 3 instead of the slurry containing -CeO 2 -ZrO 2 except for using a slurry containing a composite oxide in the same manner as in Example 1 Pt-supported Al 2 O 3 It was obtained -CeO 2 -ZrO 2 substrate with a catalyst layer comprising a composite oxide. Incidentally, the coating amount of Al 2 O 3 -CeO 2 -ZrO 2 composite oxide is the honeycomb substrate 1L per about 100 g, Pt support amount of about per 100g Al 2 O 3 -CeO 2 -ZrO 2 composite oxide 2g.
（比較例１）
実施例１と同様にして作製したＰｔ−ＣｅＺｒ層を備えた前記基材を管状炉に入れ、Ｈ２／Ｎ２混合ガス（Ｈ２：５体積％、流量：１Ｌ／分）中、７００℃で１時間の熱処理（還元処理）のみを施し、Ｐｔ担持ＣｅＯ２−ＺｒＯ２複合酸化物からなる触媒層を備える基材を得た。
(Comparative Example 1)
It said substrate having a Pt-CeZr layer prepared in the same manner as in Example 1 was placed in a tubular furnace, H 2 / N 2 mixed gas (H 2: 5% by volume, flow rate: 1L / min) in, 700 ° C. Then, only a heat treatment (reduction treatment) for 1 hour was performed to obtain a base material including a catalyst layer made of a Pt-supported CeO 2 —ZrO 2 composite oxide.
（比較例２）
前記２回の還元処理および前記酸化処理の温度をいずれも７００℃から５００℃に変更した以外は実施例１と同様にしてＰｔ担持ＣｅＯ２−ＺｒＯ２複合酸化物からなる触媒層を備える基材を得た。
(Comparative Example 2)
A base material provided with a catalyst layer made of a Pt-supported CeO 2 —ZrO 2 composite oxide in the same manner as in Example 1 except that the temperatures of the two reduction treatments and the oxidation treatment are both changed from 700 ° C. to 500 ° C. Got.
（比較例３）
前記２回の還元処理および前記酸化処理の温度をいずれも７００℃から９００℃に変更した以外は実施例１と同様にしてＰｔ担持ＣｅＯ２−ＺｒＯ２複合酸化物からなる触媒層を備える基材を得た。
(Comparative Example 3)
A substrate provided with a catalyst layer made of a Pt-supported CeO 2 —ZrO 2 composite oxide in the same manner as in Example 1 except that the temperatures of the two reduction treatments and the oxidation treatment are both changed from 700 ° C. to 900 ° C. Got.
（比較例４）
Ｌａを少量含有するＡｌ２Ｏ３粉末（以下、「Ｌａ安定化Ａｌ２Ｏ３粉末」という。）を純水に添加してスラリーを得た。前記ＣｅＯ２−ＺｒＯ２複合酸化物を含むスラリーの代わりにこのＬａ安定化Ａｌ２Ｏ３粉末を含むスラリーを用いた以外は実施例１と同様にしてＰｔ担持Ｌａ安定化Ａｌ２Ｏ３からなる触媒層を備える基材を得た。なお、Ｌａ安定化Ａｌ２Ｏ３のコーティング量は前記ハニカム基材１Ｌ当たり約１００ｇであり、Ｐｔ担持量はＬａ安定化Ａｌ２Ｏ３１００ｇ当たり約２ｇであった。
(Comparative Example 4)
An Al 2 O 3 powder containing a small amount of La (hereinafter referred to as “La-stabilized Al 2 O 3 powder”) was added to pure water to obtain a slurry. Comprising the CeO 2 -ZrO Pt supported La stabilized other than Al 2 O 3 using a slurry containing the La-stabilized Al 2 O 3 powder instead of the slurry in the same manner as in Example 1 containing 2 composite oxide A base material provided with a catalyst layer was obtained. The coating amount of La-stabilized Al 2 O 3 was about 100 g per 1 L of the honeycomb substrate, and the amount of Pt supported was about 2 g per 100 g of La-stabilized Al 2 O 3 .
＜触媒性能評価＞
実施例１〜２および比較例１〜４で得た触媒層を備える基材をそれぞれ固定床流通式評価装置に設置し、排気モデルガス（Ｏ２：６ｖｏｌ％、ＣＯ：０．８ｖｏｌ％、Ｃ３Ｈ６：０．１８ｖｏｌ％、ＣＯ２：１０ｖｏｌ％、Ｈ２Ｏ：１０ｖｏｌ％、残りはＮ２）を供給しながら前記触媒層を４００℃で１０分間加熱した。その後、前記触媒層を６０℃まで冷却した後、前記排気モデルガスを３０Ｌ／分の流量で供給しながら前記触媒層を昇温速度１５℃／分で６０℃から３００℃まで加熱し、浄化処理を実施した。このとき、供給した排気モデルガス中のＣ３Ｈ６およびＣＯが５０％浄化された温度（以下、「Ｔ５０」と表す。）を測定した。その結果を表１に示す。
<Catalyst performance evaluation>
The base material provided with the catalyst layer obtained in Examples 1 and 2 and Comparative Examples 1 to 4 was installed in a fixed bed flow evaluation device, respectively, and exhaust model gas (O 2 : 6 vol%, CO: 0.8 vol%, C 3 H 6 : 0.18 vol%, CO 2 : 10 vol%, H 2 O: 10 vol%, the rest being N 2 ), and the catalyst layer was heated at 400 ° C. for 10 minutes. Thereafter, the catalyst layer is cooled to 60 ° C., and then the catalyst layer is heated from 60 ° C. to 300 ° C. at a temperature rising rate of 15 ° C./min while supplying the exhaust model gas at a flow rate of 30 L / min. Carried out. At this time, the temperature at which C 3 H 6 and CO in the supplied exhaust model gas were purified by 50% (hereinafter referred to as “T50”) was measured. The results are shown in Table 1.
また、Ｘ線回折装置（（株）リガク製「ＲＩＮＴ−ＴＴＲ」）を用いて実施例１〜２および比較例１〜４で得た触媒層についてＸＲＤ分析を実施してＰｔ粒子の粒度分布を測定し、最大粒子径を求めた。また、触媒学会推奨のＣＯパルス法（「触媒を用いた測定法の標準化、２．ＣＯパルス法による金属表面積測定法」、触媒、（１９８６）、２８−４６頁を参照。）によって実施例１〜２および比較例１〜４で得た触媒層のＰｔ粒子の平均粒子径を求めた。これらの結果を表１に示す。 Further, XRD analysis was performed on the catalyst layers obtained in Examples 1 and 2 and Comparative Examples 1 to 4 using an X-ray diffractometer (“RINT-TTR” manufactured by Rigaku Corporation) to determine the particle size distribution of Pt particles. The maximum particle size was determined by measurement. In addition, Example 1 by the CO pulse method recommended by the Catalytic Society (see “Standardization of Measurement Method Using Catalyst, 2. Metal Surface Area Measurement Method by CO Pulse Method”, Catalyst, (1986), pages 28-46). The average particle diameter of Pt particles of the catalyst layers obtained in ˜2 and Comparative Examples 1 to 4 was determined. These results are shown in Table 1.
表１に示した結果から明らかなように、還元処理の後、酸化処理と還元処理とを施した本発明の触媒を用いた場合（実施例１）には、１回の還元処理のみを施した従来の触媒を用いた場合（比較例１）に比べて、Ｃ３Ｈ６およびＣＯの５０％浄化温度Ｔ５０が低く、本発明の触媒は低温活性に優れることが確認された。 As is apparent from the results shown in Table 1, when the catalyst of the present invention that has been subjected to oxidation treatment and reduction treatment after the reduction treatment (Example 1) is used, only one reduction treatment is performed. Compared with the case where the conventional catalyst was used (Comparative Example 1), the C 3 H 6 and CO 50% purification temperature T50 was low, and it was confirmed that the catalyst of the present invention was excellent in low-temperature activity.
また、触媒層のＰｔ粒子の粒子径について見ると、本発明の触媒層中のＰｔ粒子の最大粒子径と平均粒子径との誤差は１７％であり、比較例１のＰｔ粒子の場合（誤差：４６％）に比べて小さく、本発明の触媒には均一なＰｔ粒子が形成されていることが確認された。すなわち、還元処理の後、酸化処理と還元処理とを施すことによりＰｔ粒子の粒子径が均一になることが確認された。本発明者らは、このＰｔ粒子径の均一性が前記触媒の低温活性に大きく寄与しているものと推察する。 Further, regarding the particle diameter of the Pt particles in the catalyst layer, the error between the maximum particle diameter and the average particle diameter of the Pt particles in the catalyst layer of the present invention is 17%, and in the case of the Pt particles in Comparative Example 1 (error) : 46%), and it was confirmed that uniform Pt particles were formed in the catalyst of the present invention. That is, it was confirmed that the particle diameter of the Pt particles becomes uniform by performing oxidation treatment and reduction treatment after the reduction treatment. The present inventors presume that the uniformity of the Pt particle size greatly contributes to the low temperature activity of the catalyst.
還元酸化処理温度の影響について見ると、５００℃または９００℃で一連の還元酸化処理を施した場合（比較例２〜３）には、７００℃で一連の還元酸化処理を施した場合（実施例１）に比べて、触媒層中のＰｔ粒子の粒子径の均一性が劣り、Ｃ３Ｈ６およびＣＯについてのＴ５０が高く、触媒の低温活性も劣るものであった。 As for the influence of the reduction oxidation treatment temperature, when a series of reduction oxidation treatments were performed at 500 ° C. or 900 ° C. (Comparative Examples 2-3), a case of a series of reduction oxidation treatments at 700 ° C. (Examples) Compared with 1), the uniformity of the particle size of the Pt particles in the catalyst layer was inferior, the T50 for C 3 H 6 and CO was high, and the low-temperature activity of the catalyst was also inferior.
コーティング層がＡｌ２Ｏ３−ＣｅＯ２−ＺｒＯ２複合酸化物からなる層である場合（実施例２）には、ＣｅＯ２−ＺｒＯ２複合酸化物からなるコーティング層の場合（実施例１）に比べて、Ｃ３Ｈ６ についてのＴ５０がより低くなることが確認された。これは、コーティング層の耐熱性が向上し、一連の還元酸化処理におけるコーティング層の熱劣化が抑制されたことによるものと推察する。 In the case the coating layer is made of Al 2 O 3 -CeO 2 -ZrO 2 composite oxide (Example 2), in the case of a coating layer composed of CeO 2 -ZrO 2 composite oxide (Example 1) compared to, T50 about the C 3 H 6 it may turn lower was confirmed. This is presumably because the heat resistance of the coating layer was improved and the thermal deterioration of the coating layer in a series of reduction oxidation treatments was suppressed.
また、コーティング層が、Ｌａ安定化Ａｌ２Ｏ３からなり、ＣｅＯ２およびＺｒＯ２を含まない層である場合（比較例４）には、Ａｌ２Ｏ３−ＣｅＯ２−ＺｒＯ２複合酸化物からなるコーティング層の場合（実施例２）に比べて、触媒層中のＰｔ粒子の粒子径の均一性が劣り、Ｃ３Ｈ６およびＣＯについてのＴ５０が高く、触媒の低温活性も劣るものであった。すなわち、本発明にかかる一連の還元酸化処理は、ＣｅＯ２およびＺｒＯ２を含む触媒に有効であることが確認された。 When the coating layer is made of La-stabilized Al 2 O 3 and does not contain CeO 2 and ZrO 2 (Comparative Example 4), the coating layer is made of an Al 2 O 3 —CeO 2 —ZrO 2 composite oxide. Compared to the coating layer (Example 2), the uniformity of the particle diameter of the Pt particles in the catalyst layer was inferior, the T50 for C 3 H 6 and CO was high, and the low-temperature activity of the catalyst was also inferior. It was That is, it was confirmed that a series of reductive oxidation treatments according to the present invention are effective for a catalyst containing CeO 2 and ZrO 2 .
以上説明したように、本発明によれば、粒子径が小さく且つ均一な貴金属粒子を備える酸化触媒を製造することが可能となる。 As described above, according to the present invention, it is possible to produce an oxidation catalyst having noble metal particles having a small particle diameter and uniform.
したがって、本発明の酸化触媒は、低温での酸化活性に優れるため、エンジン始動からの極短時間においても排ガスを浄化せしめることが可能となり、排ガス規制の強化に対応した排ガス浄化用触媒などとして有用である。特に、酸素過剰雰囲気で炭化水素含有量が少なく、排ガス温度が低い（１００〜３００℃）ディーゼル排ガスの浄化用触媒として有用である。
Therefore, since the oxidation catalyst of the present invention is excellent in oxidation activity at low temperature, it is possible to purify exhaust gas even in a very short time after engine start, and it is useful as an exhaust gas purification catalyst that responds to stricter exhaust gas regulations. It is. In particular, it is useful as a catalyst for purifying diesel exhaust gas with a low hydrocarbon content in an oxygen-excess atmosphere and a low exhaust gas temperature (100 to 300 ° C.).
Claims (4)
前記排ガス浄化用酸化触媒中の貴金属粒子のＣＯパルス法により測定した平均粒子径が２〜８ｎｍであり、
前記貴金属粒子のＸＲＤにより測定した粒度分布における最大粒子径に対するＣＯパルス法により測定した平均粒子径の誤差が３５％以内である、
ことを特徴とする排ガス浄化用酸化触媒。 At least one kind of noble metal selected from the group consisting of Pt, Pd and Rh is supported on a support made of a complex oxide containing CeO 2 and ZrO 2 , and the support on which the noble metal is supported is first loaded in a reducing atmosphere. , subjected to a reduction treatment at a temperature in the range of 600 to 800 ° C., then, the carrier having been subjected to the reducing treatment, an oxidizing atmosphere, subjected to oxidation treatment at a temperature in the range of 600 to 800 ° C., further oxidation treatment the carrier having been subjected to, a reducing atmosphere, Ri na subjected to reduction treatment at a temperature in the range of 600 to 800 ° C., 100 to 130 exhaust gas hydrocarbons and carbon monoxide by removing oxidation even in the exhaust gas at ° C. An exhaust gas purifying oxidation catalyst capable of purifying
The average particle diameter measured by the CO pulse method of the noble metal particles in the exhaust gas purification oxidation catalyst is 2 to 8 nm,
The error of the average particle diameter measured by the CO pulse method with respect to the maximum particle diameter in the particle size distribution measured by XRD of the noble metal particles is within 35%.
An oxidation catalyst for exhaust gas purification characterized by the above.
該貴金属を担持せしめた前記担体に、先ず、還元性雰囲気下、６００〜８００℃の範囲の温度で還元処理を施し、
次いで、前記還元処理が施された前記担体に、酸化性雰囲気下、６００〜８００℃の範囲の温度で酸化処理を施し、
さらに、前記酸化処理が施された前記担体に、還元性雰囲気下、６００〜８００℃の範囲の温度で還元処理を施す、
ことを特徴とする請求項１または２に記載の排ガス浄化用酸化触媒の製造方法。 Supporting at least one kind of noble metal selected from the group consisting of Pt, Pd and Rh on a support made of a complex oxide containing CeO 2 and ZrO 2 ;
The carrier was allowed carrying the noble metal, firstly, a reducing atmosphere, and facilities the reduction treatment at a temperature in the range of 600 to 800 ° C.,
Then, the carrier said reduction treatment is performed under an oxidizing atmosphere, and facilities oxidation treatment at a temperature in the range of 600 to 800 ° C.,
Furthermore, the carrier subjected to the oxidation treatment is subjected to a reduction treatment at a temperature in the range of 600 to 800 ° C. in a reducing atmosphere .
The method for producing an exhaust gas purifying oxidation catalyst according to claim 1 or 2 .
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