Combination catalyst/adsorber system for treating an engine exhaust gas stream

A process for treating an engine exhaust gas stream is disclosed. The process involves arranging a catalyst bed and a molecular sieve bed side by side with a connecting pipe between and parallel to the two beds. When the engine is first started, the cool exhaust is flowed through the catalyst bed, then through the connecting pipe, then through the molecular sieve bed and finally discharged to the atmosphere. When the molecular sieve bed reaches a temperature of about 150 DEG C. to about 200 DEG C. the exhaust stream is diverted such that it is first flowed through the molecular sieve bed, to desorb the adsorbed hydrocarbons, and then through the catalyst bed and finally discharged to the atmosphere. Finally, when all the hydrocarbons are desorbed from the molecular sieve bed, the exhaust stream is diverted such that it is flowed through the catalyst bed and then discharged to the atmosphere.