Abstract: A nitrogen-oxide-removing catalyst includes ? zeolite bearing a rare earth metal oxide, and titanium dioxide bearing a rare earth metal oxide; includes ?-zeolite bearing a rare earth metal oxide and iron oxide or iron hydroxide, and titanium dioxide bearing a rare earth metal oxide and iron oxide or iron hydroxide; or includes a carrier made of a ceramic or metallic material, and a layer of the nitrogen-oxide-removing catalyst supported on the carrier.

Abstract: A nitrogen-oxide-removing catalyst includes ? zeolite bearing a rare earth metal oxide, and titanium dioxide bearing a rare earth metal oxide; includes ?-zeolite bearing a rare earth metal oxide and iron oxide or iron hydroxide, and titanium dioxide bearing a rare earth metal oxide and iron oxide or iron hydroxide; or includes a carrier made of a ceramic or metallic material, and a layer of the nitrogen-oxide-removing catalyst supported on the carrier.

Abstract: A co-catalyst for purifying an exhaust gas which can be used for a loner period of time as an actual catalyst by using the cerium oxide in the conventional co-catalyst for purifying the exhaust gas as a cerium-containing complex oxide for elevating the resistance to heat and suppressing the performance reduction due to thermal deterioration and by making a specific surface area and an oxygen storage capacity over specified values. A co-catalyst for purifying an exhaust gas of the present invention includes a composite oxide including (a) cerium; and (b) at least one element selected from the group consisting of zirconium, yttrium, strontium, barium and a rare-earth element supported on a particulate aluminum oxide support; a specific surface area of the co-catalyst after sintering being not less than 40 m2/g; an oxygen storage capacity at 400° C. being not less than 10 &mgr;mols/g and an oxygen storage capacity at 700° C. being not less than 100 &mgr;mols/g.