Exhaust purification system of internal combustion engine

Abstract

An internal combustion engine in which an SOx trap catalyst for trapping SOx contained in the exhaust gas is comprised of an upstream side catalyst and a downstream side catalyst into which the exhaust gas flowing out from the upstream side catalyst flows. The SOx storing material of the upstream side catalyst is mainly comprised of an alkali earth metal, while the SOx storing material of the downstream side catalyst is mainly comprised of an alkali metal.

1 . An exhaust purification system of an internal combustion engine in which an SO x  trap catalyst having an SO x  storing material is arranged in an engine exhaust passage and SO x  contained in an exhaust gas is stored in the SO x  storing material, wherein the SO x  trap catalyst is comprised of an upstream side catalyst and a downstream side catalyst into which the exhaust gas flowing out from the upstream side catalyst flows, the SO x  storing material of the upstream side catalyst is mainly comprised of an alkali earth metal, and the SO x  storing material of the downstream side catalyst is mainly comprised of an alkali metal. 
     
     
         2 . An exhaust purification system of an internal combustion engine as claimed in  claim 1 , wherein, in the SO x  trap catalyst, in the case where the SO x  storing material contains an alkali metal, when nitrates of the alkali metal become a molten state, the nitrates collect at a surface part of the SO x  trap catalyst and form a layer of sulfates of the alkali metal, which obstructs the storage of SO x , at the surface part of the SO x  trap catalyst. 
     
     
         3 . An exhaust purification system of an internal combustion engine as claimed in  claim 1 , wherein the SO x  storage material of the downstream side catalyst is comprised of an alkali metal and an alkali earth metal. 
     
     
         4 . An exhaust purification system of an internal combustion engine as claimed in  claim 1 , wherein the upstream side catalyst and the downstream side catalyst are comprised of a single monolithic catalyst which is integrally formed or is comprised of a pair of catalysts which are arranged at a distance from each other. 
     
     
         5 . An exhaust purification system of an internal combustion engine as claimed in  claim 1 , wherein a temperature of the downstream side catalyst is maintained at less than a melting point of nitrates of the alkali metal which is contained in the SO x  storing material of the downstream side catalyst. 
     
     
         6 . An exhaust purification system of an internal combustion engine as claimed in  claim 1 , wherein a precious metal catalyst is carried out the upstream side catalyst and the downstream side catalyst. 
     
     
         7 . An exhaust purification system of an internal combustion engine as claimed in  claim 1 , wherein the upstream side catalyst and the downstream side catalyst contain SO 2  adsorption-use oxides which can adsorb SO 2  contained in exhaust gas, and wherein at the upstream side catalyst and the downstream side catalyst, the SO 2  contained in the exhaust gas is adsorbed at the SO 2  adsorption-use oxides without being oxidized, and when a temperature of the catalyst becomes higher than an adsorbed SO 2  movement start temperature where the SO 2  adsorbed at the SO 2  adsorption-use oxides starts to move toward the corresponding SO x  storing material, the SO 2  adsorbed at the SO 2  adsorption-use oxides is oxidized and stored in the corresponding SO x  storing material in the form of sulfates, the temperatures of the upstream side catalyst and the downstream side catalyst being made to repeatedly change from the adsorbed SO 2  movement start temperature or less to the adsorbed SO 2  movement start temperature or more during engine operation. 
     
     
         8 . An exhaust purification system of an internal combustion engine as claimed in  claim 7 , wherein said SO 2  adsorption-use oxides are comprised of metal oxides, and the metal oxides are comprised of an oxygen absorbing and releasing material which changes in state of oxidation in the exhaust gas. 
     
     
         9 . An exhaust purification system of an internal combustion engine as claimed in  claim 8 , wherein a metal which forms the oxygen absorbing and releasing material is comprised of cerium Ce or iron Fe. 
     
     
         10 . An exhaust purification system of an internal combustion engine as claimed in  claim 7 , wherein when one or both of the temperature of the upstream side catalyst and the temperature of the downstream side catalyst do not exceed the adsorbed SO 2  movement start temperature for a predetermined period, a temperature elevation action of the upstream side catalyst and the downstream side catalyst is performed so that the temperatures of the upstream side catalyst and the downstream side catalyst exceed the adsorbed SO 2  movement start temperature. 
     
     
         11 . An exhaust purification system of an internal combustion engine as claimed in  claim 10 , wherein calculating means for calculating an adsorption amount of SO 2  adsorbed at the SO 2  adsorption-use oxides, and the temperature elevation action of the upstream side catalyst and the downstream side catalyst is performed when the calculated SO 2  adsorption amount exceeds a predetermined allowable limit value.