A metal support catalyst is inserted into the exhaust pipe of a reciprocating piston engine. The catalyst comprises two monoliths spaced away from one another and surrounded by jacket tubes which are centered coaxially with respect to one another with a sliding fit in beads and crimpings of the outer jacket. At an air feeding point between the beads of the outer jacket, air is introduced between the two monoliths in the lower rotational speed range of the engine in order to achieve an afterreaction of the exhaust gases.