Exhaust Gas Purifying Apparatus and Exhaust Gas Purifying Method

Abstract

The apparatus comprises a lean burn engine, a Pd catalyst 1 placed in exhaust gas from the lean burn engine and formed by loading a high concentration of Pd in the range of 3% by weight or more on a porous oxide, and enriching means 32 for temporally making an exhaust gas atmosphere around the Pd catalyst a rich atmosphere. It is believed that NO is oxidized into NO 2 on the Pd catalyst 1 in the exhaust gas in a lean atmosphere and that NO 2 originally existing in the exhaust gas and NO 2 generated are adsorbed on Pd. This reaction occurs at low temperatures of about 200° C. to about 300° C., and thereby NO x are stored on the Pd catalyst 1. Therefore, NO x can be reduced and purified in a low temperature range from 200° C. to 300° C. without using a NO x storage material comprising an alkaline component.

1 - 7 . (canceled) 
   
   
       8 . An exhaust gas purifying apparatus, comprising:
 a lean burn engine operated at an oxygen-excess air to fuel ratio;   a Pd catalyst placed in exhaust gas from the lean burn engine, formed by loading a high concentration of Pd in the range of 3% by weight or more on a porous oxide and being free of an NO x  adsorbing material selected from alkali metals and alkaline earth metals; and   enriching means for temporally making an exhaust gas atmosphere around the Pd catalyst a rich atmosphere by supplying a high-boiling HC to the exhaust gas, wherein   the Pd catalyst is used in an atmosphere in which a lean atmosphere and the rich atmosphere are alternately repeated and reduces and purifies NO x  in the exhaust gas in a low temperature range from 200° C. to 300° C.   
   
   
       9 . The exhaust gas purifying apparatus recited in  claim 8 , wherein the amount of Pd loaded on the Pd catalyst is 5 to 10 g per liter of the volume of the Pd catalyst. 
   
   
       10 . the exhaust gas purifying apparatus recited in  claim 8  or  claim 9 , wherein the Pd catalyst further contains Pt and the weight ratio of Pt/Pd is 0.02 to 0.18. 
   
   
       11 . The exhaust gas purifying apparatus recited in  claim 8 , wherein an NO x  storage and reduction type catalyst loaded with a noble metal and an NO x  storage material is placed on the exhaust gas downstream side of the Pd catalyst. 
   
   
       12 . An exhaust gas purifying method for purifying NO x  in exhaust gas by placing the exhaust gas purifying apparatus recited in  claim 8  in an exhaust system of the lean burn engine operated at an oxygen-excessive air to fuel ratio, temporally making an exhaust gas atmosphere around the Pd catalyst a rich atmosphere by the enriching means, and repeating a lean atmosphere and the rich atmosphere thereafter.