Method of cleaning exhaust gas

A method of cleaning an exhaust gas containing oxygen, particulate matter, SO.sub.2, unburned hydrocarbons and CO, comprising passing said exhaust gas through an exhaust gas cleaner having a heat-resistant, porous ceramic-foam filter as a carrier, said filter being constituted by two portions consisting of a portion having a relatively low density and a high-density, thin layer portion having a thickness of 5-2,000 .mu.m and formed on one side of said low-density portion, said low-density portion supporting a catalyst consisting essentially of (a) at least one of alkali metal elements, and (b) one or more elements selected from the group consisting of Sn, Fe, Co, Ni, IB elements, IIB elements, V, Nb, Ta, Cr, Mo, W, Mn, and Re of the Periodic Table, and said high-density, thin layer portion supporting a catalyst consisting essentially of one or more of the elements selected from the group consisting of Ru, Rh, Pd, Os, Ir, and Pt, wherein said high-density, thin layer portion is located on the exhaust gas inlet side of said filter, and said low-density portion is located on the exhaust gas outlet side of said filter, so that particulate matter in said exhaust gas is burned or ignited, and unburned hydrocarbons and CO in said exhaust gas are oxidized in said high-density, thin layer portion, and particulate matter passing through said high-density, thin layer portion is burned in said low-density portion, thereby cleaning the exhaust gas while suppressing the formation of SO.sub.3.

A method of cleaning an exhaust gas containing oxygen, particulate matter, NO.sub.x, SO.sub.2, unburned hydrocarbons and CO, comprising passing said exhaust gas through an exhaust gas cleaner having a heat-resistant, porous ceramic-foam filter as a carrier, said filter being constituted by two portions consisting of a portion having a relatively low density and a high-density, thin layer portion having a thickness of 5-2000 .mu.m and formed on one side of said low-density portion, said low-density portion supporting a catalyst consisting essentially of (a) at least one of alkali metal elements, (b) one or more elements selected from the group consisting of Sn, Fe, Co, Ni, IIA elements, IB elements, IIB elements, V, Nb, Ta, Cr, Mo, W, Mn, and Re of the Periodic Table, and (c) at least one of rare earth elements;

• and said high-density, thin layer portion supporting a catalyst consisting essentially of one or more of the elements selected from the group consisting of Ru, Rh, Pd, Os, Ir, and Pt, wherein said low-density portion is located on the exhaust gas inlet side of said filter, and said high-density, thin layer portion is located on the exhaust gas outlet side of said filter, so that particulate matter, unburned hydrocarbons and NO.sub.x in said exhaust gas are removed simultaneously in said low-density portion, and the remaining hydrocarbons and CO in said exhaust gas are caused to react with remaining oxygen in said high-density, thin layer portion, thereby cleaning the exhaust gas while suppressing the formation of SO.sub.3.

A method of cleaning an exhaust gas containing oxygen, particulate matter, NO.sub.x, SO.sub.2, unburned hydrocarbons and CO, comprising passing said exhaust gas through an exhaust gas cleaner having a heat-resistant, porous ceramic-foam filter as a carrier, said filter being constituted by two portions consisting of a portion having a relatively low density and a high-density, thin layer portion having a thickness of 5-2,000 .mu.m and formed on one side of said low-density portion, said low-density portion supporting a catalyst consisting essentially of (a) at least one of alkali metal elements, (b) one or more elements selected from the group consisting of Sn, Fe, Co, Ni, IIA elements, IB elements, IIB elements, V, Nb, Ta, Cr, Mo, W, Mn, and Re of the Periodic Table and (c) at least one of rare earth elements;

• and said high-density, thin layer portion supporting a catalyst consisting essentially of one or more of the elements selected from the group consisting of Ru, Rh, Pd, Os, Ir, and Pt, wherein said high-density, thin layer, portion is located on the exhaust gas inlet side of said filter, and said low-density is located on the exhaust gas outlet side of said filter, so that particulate matter, unburned hydrocarbons and CO in said exhaust gas are removed by oxidation in said high-density, thin layer portion, and NO.sub.x is reduced by the remaining hydrocarbons and particulate matter in said low-density portion, thereby cleaning the exhaust gas while suppressing the formation of SO.sub.3.