Adsorbent for HC in exhaust gas, and process for producing the same

A process for producing an adsorbent for hydrocarbons (HC) in an exhaust gas, said adsorbent comprising an agglomerate of double-structure particles, each of which includes an HC-adsorbing zeolite core, and a ceramic coat wrapping said zeolite core and having a plurality of through-pores, each said through-pore being of a bottleneck shape with an opening narrowed down so that the HC is easy to flow into said through-pore and difficult to flow out of said through-pore, said process comprising the steps of:

• preparing a liquid mixture of an agglomerate of zeolite particles and a ceramic coat-forming precursor solution;
• and spraying said liquid mixture into a cylindrical flame and flying the mixture within said cylindrical flame;
• and
• during the flying of the mixture, carrying out the following steps in the mentioned order:
• forming at least one of a plurality of liquid drops each comprising a single of said zeolite particle wrapped with said precursor solution and a plurality of liquid drops each comprising a plurality of said zeolite particles wrapped with said precursor solution,
• precipitating said precursor by drying said liquid drops, and trapping at least one of a liquid and vapor into pores in said zeolite particles,
• forming a ceramic layer by pyrolysis of said precursor, and
• creating said through-pores by escaping of the trapped vapor through said ceramic layer.