Oxidation Catalyst and Exhaust-Gas Purification System Using the Same

Abstract

An oxidation catalyst that efficiently promotes oxidation of NO to NO 2 even in a low temperature range, and an exhaust-gas purification system and method that efficiently removes exhaust-gas components even in a low temperature range are provided. This invention provides an oxidation catalyst comprising platinum and palladium as catalytically active components, which promotes oxidation of nitrogen monoxide to nitrogen dioxide, wherein the oxidation catalyst comprises 1 to 55% by weight of the palladium relative to 100% by weight of the platinum.

1 . An oxidation catalyst comprising platinum and palladium as catalytically active components, which promotes oxidation of nitrogen monoxide to nitrogen dioxide, wherein the oxidation catalyst comprises 1 to 55% by weight of the palladium relative to 100% by weight of the platinum. 
     
     
         2 . The oxidation catalyst according to  claim 1 , wherein the catalytically active components are supported on a refractory inorganic oxide to form a catalytically active component-supporting inorganic oxide which is further supported on a three-dimensional structure. 
     
     
         3 . The oxidation catalyst according to  claim 1 , wherein the refractory inorganic oxide is at least one member selected from the group consisting of activated alumina, alumina-silica, zirconia, titania and zeolite. 
     
     
         4 . The oxidation catalyst according to  claim 1 , wherein the three-dimensional structure is a honeycomb structure. 
     
     
         5 . An exhaust-gas purification system, which comprises the oxidation catalyst set forth in  claim 1  in an exhaust-gas passage of an internal-combustion engine. 
     
     
         6 . The exhaust-gas purification system according to  claim 5 , which comprises a reducing agent supplying apparatus for supplying a reducing agent at the latter stage of the oxidation catalyst in the exhaust-gas passage, and further a NO X  reduction catalyst is installed at the latter stage of the reducing agent supplying apparatus in the exhaust-gas passage. 
     
     
         7 . The exhaust-gas purification system according to  claim 6 , which further comprises at least one member selected from the group consisting of a diesel particulate filter, an agent of suppressing slip of a reducing agent, a catalyst for hydrolyzing urea and another oxidation catalyst at the latter stage of the oxidation catalyst. 
     
     
         8 . The exhaust-gas purification system according to  claim 6 , wherein the reducing agent is at least one member selected among urea, ammonia, light oil, dimethyl ether, methanol, ethanol and propanol. 
     
     
         9 . A process for purifying an exhaust gas discharged from an internal-combustion engine, comprising (1) a step for converting nitrogen monoxide in the exhaust gas to nitrogen dioxide by bringing the above exhaust gas into contact with the oxidation catalyst set forth in  claim 1 , and (2) a step for reducing the nitrogen oxides in the exhaust gas by bringing the exhaust gas obtained in the step (1) into contact with a NO X  reduction catalyst in the presence of a reducing agent. 
     
     
         10 . The process according to  claim 9 , further comprising (3) a step for removing at least a part of particulate matters. 
     
     
         11 . A process for purifying an exhaust gas discharged from an internal-combustion engine, comprising (1) a step for converting nitrogen monoxide in the exhaust gas to nitrogen dioxide by bringing the above exhaust gas into contact with the oxidation catalyst set forth in  claim 2 , and (2) a step for reducing the nitrogen oxides in the exhaust gas by bringing the exhaust gas obtained in the step (1) into contact with a NO X  reduction catalyst in the presence of a reducing agent. 
     
     
         12 . A process for purifying an exhaust gas discharged from an internal-combustion engine, comprising (1) a step for converting nitrogen monoxide in the exhaust gas to nitrogen dioxide by bringing the above exhaust gas into contact with the oxidation catalyst set forth in  claim 3 , and (2) a step for reducing the nitrogen oxides in the exhaust gas by bringing the exhaust gas obtained in the step (1) into contact with a NO X  reduction catalyst in the presence of a reducing agent. 
     
     
         13 . A process for purifying an exhaust gas discharged from an internal-combustion engine, comprising (1) a step for converting nitrogen monoxide in the exhaust gas to nitrogen dioxide by bringing the above exhaust gas into contact with the oxidation catalyst set forth in  claim 4 , and (2) a step for reducing the nitrogen oxides in the exhaust gas by bringing the exhaust gas obtained in the step (1) into contact with a NO X  reduction catalyst in the presence of a reducing agent.