Three-way catalysts that contain at least one precious metal selected from platinum (Pt), palladium (Pd), and rhodium (Rh) are often used for the purification of the exhaust gas discharged from an internal combustion engine such as an automobile engine (Patent Literature 1 to 3). In a typical three-way catalyst structure, a catalyst coating layer made of alumina is formed on the surface of a highly heat-resistant ceramic substrate and at least one precious metal selected from Pd, Pt, and Rh is supported in this catalyst coating layer. Of these precious metals, Pd and Pt contribute mainly to a purification capacity (oxidative purification capacity) for carbon monoxide (CO) and hydrocarbon (HC) and Rh contributes mainly to a purification capacity (reductive purification capacity) for NOx. As a result, the harmful components in exhaust gas can be efficiently purified at one time by using Rh in combination with Pd or Pt.
In order to efficiently purify the aforementioned components in exhaust gas using such a three-way catalyst, that is, in order to convert these components by oxidation or reduction into H2O, CO2, or N2, the air/fuel ratio, which is the mixing ratio between the air and gasoline fed to the engine, must be near the theoretical air/fuel ratio (stoichiometric). A ceria-zirconia complex oxide has been widely used as the support for these precious metals in order to smooth out changes in the atmosphere from the air/fuel ratio at which the catalyst can effectively operate. This ceria-zirconia complex oxide functions to store the oxygen in the exhaust gas when the air/fuel ratio for the exhaust gas is lean (i.e., an excess oxygen atmosphere) and to release the stored oxygen when the air/fuel ratio for the exhaust gas is rich (i.e., an excess fuel atmosphere). As a consequence, a stable catalyst performance is obtained even when the oxygen concentration in the exhaust gas varies and the catalyst purification performance is improved.
In order to raise the purification performance still further, exhaust gas purification catalysts have been proposed in recent years in which the catalyst coating layer has a two-layer structure and the Pd and Rh are supported separately. All of the precious metal catalyst is not supported in a single support layer and the catalyst coating layer is formed into a layered structure having at least two layers, i.e., an upper layer and a lower layer. A support having a good affinity for Rh and a support having a good affinity for Pd can be selected by supporting the Pd with it separated into one layer and supporting the Rh with it separated in another layer. For example, ZrO2 is preferred as a support for Rh. Patent Literature 1 describes an exhaust gas purification catalyst having a two-layer structure formed of an upper layer in which Rh is supported on ZrO2 and a lower layer in which Pd is supported on a CeO2—ZrO2 complex oxide.
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