Exhaust purifying device of internal combustion engine

Abstract

An upstream catalyst and a downstream catalyst are arranged in series and are housed in a common casing disposed in an engine exhaust passage. The upstream catalyst including a NOx storage-reduction catalyst that adsorbs NOx contained in incoming exhaust gas when the air-fuel ratio of the incoming exhaust gas is lean, and releases and reduces the adsorbed NOx when the air-fuel ratio of the incoming exhaust gas becomes rich, and the downstream catalyst includes a three-way catalyst. The upstream catalyst has a higher oxidizing capability than the downstream catalyst, and the downstream catalyst has a higher reducing capability than the upstream catalyst. The upstream catalyst has a multi-layer structure including an upper layer and a lower layer, and is prepared such that the upper layer has a higher oxidizing capability than the lower layer, and the lower layer has a higher reducing capability than the upper layer.

1 - 5 . (canceled) 
     
     
         6 . An exhaust purifying device for an internal combustion engine, comprising:
 an upstream catalyst and a downstream catalyst arranged in series with each other and housed in a common casing disposed in an engine exhaust passage, wherein:   the upstream catalyst comprises a NOx storage-reduction catalyst that absorbs NOx contained in incoming exhaust gas when the air-fuel ratio of the incoming exhaust gas is lean, and releases and reduces the absorbed NOx when the air-fuel ratio of the incoming exhaust gas becomes rich, and the downstream catalyst comprises one of a three-way catalyst and a NOx storage-reduction catalyst;   the upstream catalyst has a higher oxidizing capability than the downstream catalyst, and the downstream catalyst has a higher reducing capability than the upstream catalyst;   the upstream catalyst achieves a higher oxidizing capability than the downstream catalyst and achieves a lower reducing capability than the downstream catalyst by a multi-layer structure including an upper layer and a lower layer, each of the upper layer and the lower layer of the upstream catalyst contains both of a noble-metal catalyst and a NOx absorbent comprising at least one selected from alkali metals, alkaline earths, and rare earths,   the upper layer of the upstream catalyst contains, as the noble-metal catalyst, at least one selected from platinum (Pt), palladium (Pd), osmium (Os), and gold (Au);   the lower layer of the upstream catalyst contains, as the noble-metal catalyst, at least one selected from rhodium (Rh), iridium (Ir), and ruthenium (Ru),   the downstream catalyst has a multi-layer structure including an upper layer and a lower layer, and the upper layer has a higher reducing capability than the lower layer, and the lower layer has a higher oxidizing capability than the upper layer:   rhodium (Rh) is used as a noble-metal component of the upper layer of the downstream catalyst; and   platinum (Pt) is used as a noble-metal component of the lower layer of the downstream catalyst.   
     
     
         7 . The exhaust purifying device according to  claim 6 , wherein:
 the air-fuel ratio in the internal combustion engine is normally set to a lean air-fuel ratio that is larger than a stoichiometric ratio; and   when NOx stored in the upstream NOx storage-reduction catalyst is to be released and reduced, the air-fuel ratio of exhaust gas flowing into the upstream NOx storage-reduction catalyst is temporarily controlled to a rich air-fuel ratio that is smaller than the stoichiometric ratio.   
     
     
         8 . The exhaust purifying device according to  claim 7 , wherein the air-fuel ratio in the internal combustion engine is temporarily controlled to the stoichiometric ratio, depending on engine operating conditions. 
     
     
         9 . The exhaust purifying device according to  claim 6 , wherein:
 the amount of NOx absorbed per unit time by the upstream NOx storage-reduction catalyst is stored in advance in a memory as a function of engine operating conditions;   by integrating the NOx amount, a total value of the amount of NOx stored in the upstream NOx storage-reduction catalyst is calculated; and   each time the total value of the stored NOx amount exceeds an upper limit, a rich-mode operation is temporarily performed in which an air-fuel mixture having a rich air-fuel ratio is burned.   
     
     
         10 . The exhaust purifying device according to  claim 6 , wherein the air-fuel ratio of the incoming exhaust gas is enriched by supplying a reductant or secondary fuel into an exhaust passage upstream of the NOx storage-reduction catalyst.