Exhaust gas purifying catalyst system

Abstract

Disclosed is an exhaust gas purifying catalyst system, which comprises an upstream catalyst disposed in an exhaust gas passage of an engine at a position on an upstream side with respect to a direction of an exhaust gas stream, and a downstream catalyst disposed in the exhaust gas passage at a position on a downstream side with respect to the direction of the exhaust gas stream. In the exhaust gas purifying catalyst system, the downstream catalyst includes a cerium (Ce)-containing rhodium-doped composite oxide and an Ni component, and the upstream catalyst includes a cerium (Ce)-containing composite oxide other than the rhodium-doped composite oxide, and a Ni component. A ratio of Ni to CeO 2 in the upstream catalyst is in the range of 15 to 20 mass %, and a ratio of Ni to CeO 2 in the downstream catalyst is in the range of 10 to 60 mass %. The exhaust gas purifying catalyst system of the present invention can reduce an amount of H 2 S emission while achieving high purification performance.

1 . An exhaust gas purifying catalyst system comprising an upstream catalyst disposed in an exhaust gas passage of an engine at a position on an upstream side with respect to a direction of an exhaust gas stream, and a downstream catalyst disposed in said exhaust gas passage at a position on a downstream side with respect to the direction of said exhaust gas stream, wherein:
 said downstream catalyst includes a cerium (Ce)-containing oxygen storage material consisting of a rhodium-doped composite oxide having rhodium arranged at a lattice position or interlattice position of a crystal structure thereof, and at least either one of nickel (Ni) and nickel oxide (NiO); and   said upstream catalyst includes a cerium (Ce)-containing oxygen storage material consisting of a composite oxide other than said rhodium-doped composite oxide, and at least either one of nickel (Ni) and nickel oxide (NiO),   wherein a ratio of Ni to CeO 2  in said upstream catalyst is in the range of 15 to 20 mass %, and a ratio of Ni to CeO 2  in said downstream catalyst is in the range of 10 to 60 mass %.   
   
   
       2 . The exhaust gas purifying catalyst system as defined in  claim 1 , wherein each of said respective composite oxides included in said upstream catalyst and said downstream catalyst includes zirconium and neodymium. 
   
   
       3 . The exhaust gas purifying catalyst system as defined in  claim 1 , wherein the ratio of Ni to CeO 2  in said upstream catalyst is greater than the ratio of Ni to CeO 2  in said downstream catalyst. 
   
   
       4 . The exhaust gas purifying catalyst system as defined in  claim 1 , wherein each of said respective composite oxides included in said upstream catalyst and said downstream catalyst supports a catalytic noble metal thereon. 
   
   
       5 . The exhaust gas purifying catalyst system as defined in  claim 4 , wherein:
 said catalytic noble metal supported on said composite oxide included in said upstream catalyst and said downstream catalyst is rhodium; said upstream catalyst includes palladium; and said downstream catalyst includes platinum.