Technical Field
The present invention relates to a catalytic converter that is fixedly housed in a pipe of an exhaust system for exhaust gas.
Background Art
In a variety of industries, a variety of attempts has been made worldwide to reduce environmental impacts and burdens. In particular, in the automobile industry, development for promoting the spread of not only fuel-efficient gasoline engine vehicles, but also so-called eco-friendly vehicles, such as hybrid vehicles and electric vehicles, as well as for further improving the performance of such vehicles has been advanced day by day. In addition to the development of such eco-friendly vehicles, research about an exhaust gas purifying catalyst for purifying exhaust gas discharged from an engine has also been actively conducted. An exhaust gas purifying catalyst contains an oxidation catalyst, a three-way catalyst, a NOx, storage-reduction catalyst, and the like. A noble metal catalyst, such as platinum (Pt), palladium (Pd), or rhodium (Rh), exhibits catalytic activity in the exhaust gas purifying catalyst. The noble metal catalyst is typically used while being carried on a support that is made of porous oxide, such as alumina (Al2O3).
A catalytic converter for purifying exhaust gas is typically arranged in an exhaust system for exhaust gas that connects a vehicle engine and a muffler. The engine may sometimes discharge environmentally harmful substances, such as CO, NOx, and unburned HC and VOC. In order to convert such harmful substances into allowable substances, exhaust gas is passed through a catalytic converter in which a catalyst layer, which has a noble metal catalyst, such as Rh, Pd, or Pt carried on a support, is arranged on the cell wall surface of a substrate, so that CO is converted into CO2, and NOx is converted into N2 and O2, while HC and VOC are burned to generate CO2 and H2O.
As a support on which a noble metal catalyst is carried, a ceria-zirconia-based composite oxide (which is also referred to as a CeO2—ZrO2 solid solution, a CZ material, and the like) can be used. This is also referred to as a promoter, and is an essential component of the aforementioned three-way catalyst for concurrently removing CO, NOx, and HC that are harmful components in the exhaust gas. Examples of the essential component of the promoter include CeO2. CeO2 has a property that its oxidation number changes to Ce3+ or Ce4+, for example, depending on the partial pressure of oxygen in the exhaust gas to which CeO2 is exposed, and has a function of absorbing and releasing oxygen as well as a function of storing oxygen (OSC: Oxygen Storage Capacity) to compensate for the deficiency and excess of electric charges. In addition, CeO2 can absorb and mitigate fluctuations of the exhaust gas atmosphere and maintain the air/fuel ratio at a level around the theoretical air/fuel ratio in order to retain a purifying window of the three-way catalyst.
By the way, how to reduce the amount of a noble metal catalyst used in the aforementioned three-way catalyst is an important element to be considered from the perspective of cost competitiveness. However, when the amount of a noble metal catalyst in a three-way catalyst is significantly reduced, the catalytic activity also decreases significantly. Thus results in significantly degraded OSC performance and low-temperature activity as well as significantly degraded NOx purification performance under a high-temperature environment, and the like. This occurs because a significantly reduced amount of a noble metal catalyst results in a significantly reduced number of active sites, and a significantly reduced number of catalytic reaction sites results in significantly degraded purification performance.
Among noble metal catalysts, such as Pt, Pd, and Rh, that are particularly used for a three-way catalyst, Rh is the most excellent in the NOx purification performance, but is sold at the highest market price per unit weight. It has been known that when Rh is carried on a support that contains cerium oxide (ceria), higher OSC performance is exhibited. To the contrary, it has been also known that increasing the amount of cerium oxide in the support will degrade the NOx purification performance that is a characteristic of Rh. Thus, when using Rh as a noble metal catalyst for a three-way catalyst, producing a three-way catalyst that is optimal in both the OSC performance and the NOx purification performance is an urgent object to be achieved in the technical field.
Herein, Patent Document 1 discloses an exhaust gas purifying catalyst including a first oxygen storage material on which no noble metal is carried and that has a pyrochlore phase type regular array structure; and a second oxygen storage material that has a higher oxygen storage rate and a lower oxygen storage capacity than the first oxygen storage material, in which a platinum group noble metal is carried on the second oxygen storage material. According to such an exhaust gas purifying catalyst, it is possible to provide an exhaust gas purifying catalyst that has high NOx purification performance after endurance.
Meanwhile, Patent Document 2 discloses an exhaust gas purifying catalyst having a first catalyst layer and a second catalyst layer that are sequentially formed on a support substrate, the first catalyst layer having rhodium carried thereon, and the second catalyst layer having platinum and palladium carried thereon, in which the ratio of the carried amount of palladium (y) to the carried amount of platinum (x) (y/x; molar ratio) satisfies 0<y/x≦1.0. According to such an exhaust gas purifying catalyst, it is possible to provide an exhaust gas purifying catalyst of a NOx storage-reduction type that has more excellent NOx purification performance.
By the way, when rhodium is carried on ceria, metallization of the rhodium is hindered, and the NOx purification performance is thus degraded as described above. Meanwhile, when the amount of ceria is increased to increase the oxygen storage capacity, a pressure loss is increased.
That is, even when the exhaust gas purifying catalyst disclosed in Patent Document 1 or 2 is used, it is unclear whether or not a catalytic converter that is excellent in both the OSC performance and the NOx purification performance can be provided.