A catalyst containment unit, includes two concentric perforate members defining an annular space in which a particulate catalyst may be placed, and process fluid-impermeable end members mounted on the ends of the perforate members, wherein one end member extends across the unit to provide a closed end and the other end member closes the annular space thereby providing an open end through which gases may enter or exit the unit, and suspending means attached to one or both end members for securing the unit within an ammonia oxidation vessel. The unit may be suspended, for example, below a precious metal ammonia oxidation gauze pack and may contain a particulate ammonia oxidation catalyst or nitrous oxide abatement catalyst. A catalyst combination includes a precious metal ammonia oxidation catalyst gauze on a supporting framework and a particulate catalyst disposed in a catalyst containment unit.