A reaction vessel for oxidation of ammonia to nitrogen monoxide in the presence of a catalyst is provided. The catalyst can become dislodged during the oxidation. The reaction vessel includes a reactor body having a top portion, a bottom portion, and a middle portion. The top and middle portions cooperate to define a cavity where the ammonia is catalytically oxidized to provide the nitrogen monoxide. The reaction vessel also includes an internal filter element. The internal filter element includes a filter cage that defines an interior volume and a filter medium disposed adjacent to the filter cage. The internal filter element collects the catalyst dislodged during the oxidation.