Exhaust gas purifying catalyst and process for preparing the catalyst

An exhaust gas purifying catalyst comprising:

• a monolithic support;
• a hydrocarbon adsorbent layer formed on the monolithic support, and containing zeolite as a main component and a second component comprising an alkaline metal and/or an alkaline earth metal;
• a metal-based catalyst layer formed on the hydrocarbon adsorbent layer, and containing at least one type of noble metal selected from the group consisting of palladium, platinum and rhodium, and the second component;
• wherein a weight ratio of the second component contained in the metal-based catalyst layer to the second component contained in the hydrocarbon adsorbent layer is set to 60:40 to 99:1;
• the metal-based catalyst layer is formed by a process comprising coating an aqueous slurry on the hydrocarbon adsorbent layer and baking the coated slurry, and
• the second component is contained in the hydrocarbon adsorbent layer and the metal-based catalyst layer by adding into the aqueous slurry in form of a water insoluble or water hardly-soluble alkaline metal and/or alkaline earth metal compound.

A process for preparing an exhaust gas purifying catalyst, comprising:

• forming a hydrocarbon adsorbent layer containing zeolite as a main component and a second component comprising an alkaline metal and/or alkaline earth metal on a monolithic support;
• preparing a slurry by mixing at least one noble metal salt selected from the group consisting of palladium, platinum and rhodium, and a salt of the second component, in which the salt of the second component is water insoluble or water hardly soluble;
• coating the slurry on the hydrocarbon adsorbent layer;
• and
• baking the coated slurry to obtain a metal-based catalyst layer, wherein after the baking process, a weight ratio of the second component contained in the metal-based catalyst layer to the second component contained in the hydrocarbon adsorbent layer is set to 60:40 to 99:1.