Catalytic reactor

A device for treating a medium in a fuel cell system has a reactor with a catalyst-containing region between a first part-chamber of the reactor and a second part-chamber of the reactor. The catalyst-containing region is arranged in the flow path of the medium, so that the medium flows through it. The first part-chamber is surrounded at least partially by the second part-chamber, and the catalyst-containing region is arranged as a partition between the first, inner part-chamber and the second, outer part-chamber. The partition is formed by a nonwoven provided with catalyst material.