Harmful ingredients such as carbon monoxide (CO), hydrocarbon (HC) and nitrogen oxide (NOx) are contained in an exhaust gas discharged from an automotive internal combustion engine such as a gasoline engine and diesel engine. Therefore, an exhaust gas purification system for decomposing and removing these harmful ingredients is generally provided in a vehicle, and the harmful ingredients are made harmless by an exhaust gas purification catalyst arranged in the exhaust gas purification system. Conventionally, a three-way catalyst capable of simultaneously performing oxidation of CO and HC and reduction of NOx in the exhaust gas has been used as the exhaust gas purification catalyst. More specifically, a catalyst obtained by supporting a noble metal, in particular a platinum group element such as platinum (Pt), rhodium (Rh) and palladium (Pd) on a porous oxide support such as alumina (Al2O3) is widely known as a three-way catalyst.
However, such a platinum group element is a very expensive rare metal, because it is produced in a small area and the production is concentrated in specific areas such as South Africa and Russia. In addition, the platinum group element is being used in increasingly larger amounts along with toughening of automotive emission controls. For this reason, depletion of resources is becoming a concern. Therefore, it is required to reduce the amount of use of the platinum group element and, in the future, to replace the platinum group element in role with other metals, etc. Thus, many studies are being made on a catalyst component to reduce the amount of use of the platinum group element or substitute for the platinum group element.
Japanese Unexamined Patent Publication (Kokai) No. 2005-185959 describes an exhaust gas purification catalyst comprising at least one kind of noble metal A selected from Ru, Rh, Pd, Ag, Ir, Pt and Au; a transition metal compound B including at least one kind of transition metal element selected from Mn, Fe, Co, Ni, Cu and Zn; and a porous carrier carrying the noble metal A and the transition metal compound B, wherein the noble metal A partially or entirely forms a composite with the transition metal compound B. For example, Japanese Unexamined Patent Publication (Kokai) No. 2005-185959 specifically discloses a combination of Pt and Co as the composite of the noble metal A and the transition metal compound B. Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 2005-185959 describes that according to the above exhaust gas purification catalyst, since the transition metal compound B develops catalytic activity, high catalytic activity can be maintained even if the amount of the expensive noble metal A is reduced.
Japanese Unexamined Patent Publication (Kokai) No. 2006-043634 describes an exhaust gas purification catalyst, comprising a noble metal; a transition metal compound partially or entirely forming a composite with the noble metal; a third component element coming into contact with the composite and having an electronegativity of 1.5 or less; and a porous support supporting the noble metal, the transition metal compound and the third component element and partially or entirely forming a composite oxide with the third component element. For example, Japanese Unexamined Patent Publication (Kokai) No. 2006-043634 specifically discloses a combination of Pt and Co as the composite of the noble metal and the transition metal compound. Furthermore, Japanese Unexamined Patent Publication (Kokai) No. 2006-043634 describes that according to the above exhaust gas purification catalyst, since the transition metal compound develops catalytic activity, it is possible to increase the catalytic activity of the exhaust gas purification catalyst while reducing the amount of noble metal used.
Japanese Unexamined Patent Publication (Kohyo) No. 2001-524030 describes a gold catalyst capable of simultaneously performing oxidation of carbon monoxide and hydrocarbon, reduction of nitrous oxide and decomposition of ozone, comprising an active complex containing a fine cluster formed of gold and a reducible transition metal oxide, and a porous support consisting of oxides of cerium and titanium. Also, Japanese Unexamined Patent Publication (Kohyo) No. 2001-524030 specifically discloses CO2O3 as the reducible transition metal oxide. Furthermore, Japanese Unexamined Patent Publication (Kohyo) No. 2001-524030 describes that according to the above gold catalyst, reduction of nitrous oxide and oxidation of carbon monoxide and hydrocarbon can be simultaneously performed at a low temperature as well as at a high temperature.