An apparatus for converting at least one nitrogen oxide, such as NO, O₂, or N₂O converts oxide in the presence of a catalyst supported on a metal mesh-like structure. The mesh-like structure is preferably fibrous formed of metal or ceramic fibers which may include knitted wire, sintered metal fibers and so on and has a porosity greater than about 85%. The mesh is formed into channels, preferably corrugations, and includes vortex generators, which generate turbulence to create a pressure differential across the mesh, to promote flow of fluids through the mesh pores which normally do not exhibit flow therethrough in the absence of such pressure differential. Preferred embodiments of structured packing and monoliths are disclosed each having a catalyst preferably in the mesh pores and/or coated on the fibers for converting the nitrogen oxide. In one embodiment, corrugated mesh-like sheets are arranged in series with a ceramic solid monolith structure with the corrugated sheets initially receiving the fluid with the at least one nitrogen oxide to be converted and which fluid then flows into the monolith structure to complete the conversion. In other embodiments, the mesh-like structure may have different configurations including a honeycomb arrangement and may include metal, metal and ceramic or ceramic and may be fibrous.