Selective catalytic reduction (SCR) e sebelisoa ho laola NOx ho tsoa ha enjene ea disele.NH3 kapa urea (hangata urea aqueous solution e nang le boima ba 32.5%) e sebelisoa e le ntho e fokotsang.Tlas'a boemo ba hore mahloriso a O2 a feta litaelo tse peli tsa boholo bo phahameng ho feta NOx concentration, tlas'a ketso ea mocheso o itseng le catalyst, NH3 e sebelisetsoa ho fokotsa NOx ho N2 le H2O.Hobane NH3 ka khetho e fokotsa NOx ntle le ho sebetsana le O2 pele, Ka hona, e bitsoa "khethollo ea catalytic e khethiloeng".