Hoos-u-dhimista catalytic Selective (SCR) ayaa loo isticmaalaa in lagu xakameeyo NOx ee qiiqa matoorka naaftada.NH3 ama urea (sida caadiga ah urea aqueous aqueous with mass ratio of 32.5%) ayaa loo isticmaalaa sidii walaxda yaraynta.Marka la eego shuruudaha ah in xoojinta O2 ay ka badan tahay laba amar oo ka sarreeya fiirsashada NOx, iyada oo la raacayo ficilka heerkulka qaarkood iyo kicinta, NH3 waxaa loo isticmaalaa in lagu yareeyo NOx ilaa N2 iyo H2O.Sababtoo ah NH3 waxay si xushmad leh u yaraynaysaa NOx iyada oo aan la falcelin O2 marka hore, Sidaa darteed, waxaa loo yaqaan "hoos u dhigista catalytic xulashada".