Device for treating a medium in a reactor with a reaction section holding a catalyst

The invention relates to a device for treating a medium in a fuel cell system, comprising a reactor with a catalyst-containing region between a first subspace of the reactor and a second subspace of the reactor, wherein the catalyst-containing region is arranged in the flow path of the medium and is traversed by the medium, wherein the first subspace is at least partially surrounded by the second subspace, the catalyst-containing region is arranged as a boundary wall between the first, inner subspace and the second, outer subspace, and the boundary wall is formed by a nonwoven fabric provided with catalyst material.