Method for cleaning exhaust gas

Claim 1

A method for removing nitrogen oxides by reduction, and unburned components of carbon monoxide and hydrocarbons by oxidation, from an exhaust gas containing nitrogen oxides and oxygen in an amount larger than the stoichiometric amount of said oxygen relative to said unburned components in said exhaust gas, which comprises: 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 a first porous inorganic oxide supporting 0.2-15 weight % of an Ag component, 0.01-10 weight % of a W component and 0.001-1 weight % of a Pt component, said weight percentages being expressed by metal basis, said Ag component being at least one selected from the group consisting of Ag and compounds thereof, said Pt component being at least one selected from the group consisting of Pt, Pd, Ru, Rh, and Ir, and said W component being at least one selected from the group consisting of W, V, Mn, Mo, Nb and Ta; introducing into said exhaust gas upstream of said exhaust gas cleaner (1) at least one reducing agent selected from the group consisting of hydrocarbons and oxygen-containing organic compounds having at least two carbon atoms or (2) a fuel containing said at least one reducing agent; and bringing the resulting exhaust gas containing said reducing agent into contact with said exhaust gas cleaner at 150°-650° C., thereby causing a reaction of said nitrogen oxides with said reducing agent to remove said nitrogen oxides by reduction and unburned components of carbon monoxide and hydrocarbons by oxidation from said exhaust gas.

Claim 2

A method for removing nitrogen oxides by reduction, and unburned components of carbon monoxide and hydrocarbons by oxidation, from an exhaust gas containing nitrogen oxides and oxygen in an amount larger than the stoichiometric amount of said oxygen relative to said unburned components in said exhaust gas, which comprises: 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 a first porous inorganic oxide supporting 0.2-15 weight % of an Ag component, 0.01-10 weight % of a W component and 0.001-1 weight % of a Pt component, said weight percentages being expressed by metal basis, said Ag component being at least one selected from the group consisting of Ag and compounds thereof, said Pt component being at least one selected from the group consisting of Pt, Pd, Ru, Rh, and Ir, and said W component being at least one selected from the group consisting of W, V, Mn, Mo, Nb and Ta; introducing into said exhaust gas upstream of said exhaust gas cleaner (1) at least one reducing agent selected from the group consisting of hydrocarbons and oxygen-containing organic compounds having at least two carbon atoms or (2) a fuel containing said at least one reducing agent; and bringing the resulting exhaust gas containing said reducing agent into contact with said exhaust gas cleaner at 150°-650° C., thereby causing a reaction of said nitrogen oxides with said reducing agent to remove said nitrogen oxides by reduction and unburned components of carbon monoxide and hydrocarbons by oxidation from said exhaust gas.