Abstract: A ZnO--Pd composite catalyst wherein Pd is dispersed and fixed on the surface of ZnO is produced by ion adsorption process having the steps of soaking ZnO in an acid solution of Pd to adsorb Pd ions (Pd.sup.2+) onto the surface of ZnO; and reducing the adsorbed Pd ions to metallic Pd particles so that the metallic Pd particles are substantially evenly deposited on the surface of ZnO. The ZnO--Pd composite catalyst provides a stable and continuing CO oxidizing capability, without light irradiation, even when the air to be treated contains sulfur compounds such as H.sub.2 S and SOx.

Abstract: An air-purifying filter includes a carrier base and a TiO.sub.2 -Pd composite catalyst supported on the carrier base. CO in air treated by the filter is adsorbed onto the Pd surface and thus decomposed by oxidization to CO.sub.2, whereas sulfur compounds in the air are adsorbed onto the Pd surface and then transferred to the TiO.sub.2 surface where they are decomposed by oxidization to SO.sub.4.sup.-2 when the TiO.sub.2 is subjected to irradiation. Preferably, Pd exists in a relatively small quantity at a first side of the carrier base in the vicinity of a light source and in a greater quantity at a second side opposite the first side. The first side is preferably located upstream relative to flow of air treated by the filter. An air-purifier utilizing this filter is mounted in an automobile, preferably on a rear shelf or in a trunk compartment.