Exhaust gas cleaner and method for removing nitrogen oxides

A method for removing nitrogen oxides from an exhaust gas containing oxygen in a proportion larger than the stoichiometric proportion of said oxygen relative to unburned components in said exhaust gas, comprising the steps of:

• (i) disposing an exhaust gas cleaner in a flow path of said exhaust gas, said exhaust gas cleaner consisting essentially of a first catalyst consisting essentially of 0.2-20 parts by weight on a metal basis of silver or silver oxide supported on 100 parts by weight of a porous inorganic oxide selected from the group consisting of γ-alumina and a composite oxide containing γ-alumina, and a second catalyst consisting essentially of of 1-50 parts by weight on an oxide basis of at least one oxide selected from the group consisting of tungsten oxide and vanadium oxide supported on 100 parts by weight of a porous inorganic oxide selected from the group consisting of titania, alumina and a composite oxide containing titania;
• (ii) introducing oxygen-containing inorganic compounds having two or more carbon atoms or a fuel containing said oxygen-containing organic compounds into said exhaust gas on an upstreamside of said exhaust gas cleaner;
• and
• (iii) bringing said exhaust gas into contact with said exhaust gas cleaner at a temperature of 150-650° C., thereby causing a reaction of said nitrogen oxides with said oxygen-containing organic compounds to remove said nitrogen oxides.