A ceramic microreactor for carrying out reactions having a large heat of reaction which has at least three interior spaces, with at least one interior space having internal buffers whose shape, number and positioning ensure homogeneous flow, is described. The microreactor is built up as a monolith from at least seven plate-shaped layers of inert ceramic material, preferably aluminium oxide, which form an upper heating/cooling space, a central reaction space and a lower heating/cooling space. One interior space has a coating of a catalyst comprising noble metal. The shape, number and positioning of the internal buffers is determined by means of flow simulation calculations; the internal buffers preferably have a lozenge shape. The microreactor displays very good selectivity in reactions having a large heat of reaction, in particular in heterogeneous gas-phase reactions, and is used in particular for hydrogen production and/or hydrogen purification in fuel cell technology.