An object of the invention is to provide a catalyst for purifying sufficiently both NOx at high exhaust gas temperature and HC and CO at low exhaust gas temperature in using engines normally operated with excess air such as diesel or lean-burn engine.
The catalyst comprises the following two types of catalysts in the form of mixture or double layers supported on a honeycomb substrate; (a) a first type zeolite catalyst in which the active metal becomes supported on the zeolite by ion-exchange process and (b) a second type zeolite catalyst in which the active metal becomes supported on the zeolite by other process than ion-exchange. At high exhaust gas temperature, the first type zeolite catalyst converts paraffin type HC and/or olefin type HC into aromatic HC. The second type zeolite catalyst can reduce NOx into N 2 by making use of the aromatic HC as a reductant remaining in the gas even at high temperature.
At low exhaust gas temperature, the first type zeolite catalyst converts a medium size (medium carbon number ) aromatic HC into a small size olefin type HC (cracking ) and the second type zeolite catalyst can rapidly oxidize the small size olefin type HC to form H 2 O and CO 2 . As a whole the catalyst gains a performance improvement in both purification of NOx at high temperature and its activity at low temperature.