The mechanism of selective catalytic oxidation (SCO) of NH3 over Ag/Al2O3 was studied by NH3 temperature-programed oxidation, O-2-pulse adsorption, and in situ DRIFTS of NH3 adsorption and oxidation. The essence which affects the low temperature activity of Ag/Al2O3 has been elucidated through the mechanism study. Different Ag species on Ag/Al2O3 Significantly influence O-2 uptake by catalysts; while different oxygen species affect the activity of NH3 Oxidation at low temperature. The activated -NH could react with the atomic oxygen (O) at low temperatures (<140 degrees C) however, the -NH could also interact with the O-2 at temperatures above 140 degrees C. At low temperatures (<140 degrees C), NH3 oxidation follows the -NH mechanism. However, at temperatures above 140 degrees C, NH3 oxidation follows an in situ selective catalytic reduction (iSCR) mechanism (two-step formation of N-2 via the reduction of an in situ-produced NOx species by a NHx species). (C) 2009 Elsevier Inc. All rights reserved.